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National Weather Service

Operations Manual

Part D, Chapter 25


1. Purpose. This Chapter provides general policies and procedures for National Weather Service (NWS) meteorological support to Federal Aviation Administration (FAA) Air Traffic Facilities. Specific guidelines are provided for NWS participation in jointly (FAA/NWS) operated weather service facilities. This NWS meteorological support is designed to improve aviation safety and enhance the efficient flow of air traffic by the continual forecasting and monitoring of adverse weather. Efficiency is effected by maintaining a close coordination with traffic managers whose decisions affect the flow of air traffic through the NAS.

2. Air Traffic Meteorological Concerns. Weather that adversely impacts aviation operations often increases demands on FAA Air Traffic resources providing for the safe and efficient use of airspace and airports. FAA personnel need the best available weather information to facilitate their mission of supporting aviation operations. Required weather information includes but is not limited to the following:

    a.  Convective weather including thunderstorm timing, tops,
        movement, intensity, and character such as broken and
        solid lines or large clusters;
    b.  Operationally significant ceilings/visibility;
    c.  Cloud tops;
    d.  Winds and temperatures, both surface and aloft;
    e.  Wind shear;
    f.  Operationally significant pressure changes;
    g.  Precipitation;
    h.  Turbulence;
    i.  Icing; and
    j.  Volcanic ash.

The specific operational situation dictates the significance of any particular aviation weather phenomenon.

3. Definitions of Terms. The following definitions provide the meaning of the listed terms as used in this Chapter.

NOTE: All references to distance in the location line of the CWA and MIS products are understood to be in nautical miles (NM). The body of the text products should include NM and not MI (miles) when referring to line and areal width. See examples in section 7.

Airman's Meteorological Advisories (AIRMET) AIRMET advisories may be of significance to any pilot or aircraft operator and are issued for all domestic airspace. Scheduled AIRMET issuances and amendments describe turbulence, icing, sustained surface winds greater than 30 knots, ceilings below 1,000 feet and/or surface visibilities below 3 statute miles, and mountain obscuration when of sufficient value or intensity and/or areal extent. AIRMETs are of particular concern to operators and pilots of aircraft sensitive to the phenomena described and to pilots without instrument ratings. AIRMETs are intended for dissemination to all pilots in the preflight and en route phases of flight.

Air Traffic Control (ATC) ATC is a service operated by appropriate Federal authority to promote the safe, orderly, and efficient flow of air traffic.

Hazardous In-flight Weather Advisory Service (HIWAS) HIWAS is a continuous recorded hazardous in-flight weather information broadcast to airborne pilots over selected navigational facilities.

Hub Airport A hub, as defined by the FAA, is an area, city, or standard metropolitan statistical area where at least 1 percent of all scheduled air carrier passengers in the United States board aircraft.

National Airspace System (NAS) The common network of U.S. airspace; air navigation facilities, equipment and services, airports or landing areas; aeronautical charts, information and services; rules, regulations and procedures, technical information, and manpower and material. Included are system components shared jointly with the military.

Severe Weather Avoidance Plan An approved plan to minimize the effect of severe weather on traffic flows in impacted terminal and/or air route traffic control center areas. The Plan is normally implemented to provide the least disruption to the ATC system when flight through portions of airspace is difficult or impossible because of severe weather.

Significant Meteorological Advisories (SIGMET) SIGMETs are in-flight advisories that warn of internationally specified weather phenomena of an intensity and/or extent that concerns pilots and operators of all aircraft. In the conterminous United States, SIGMETs have been separated into two types, convective (i.e., thunderstorm-related) and nonconvective.

Weather Service Operations Manual (WSOM) The directives system used by the NWS that records delegations of authority, sets policy, defines programs, procedures, and standards, and establishes quality control and augments training activities.

4. Supported Air Traffic Facilities.

Air Traffic Control System Command Center (ATCSCC). The coordinating and command center for the FAA Air Traffic Control System. The ATCSCC is an air traffic service located in Herndon, VA, outside the Washington, D.C. metropolitan area.

Air Route Traffic Control Center (ARTCC). ARTCCs are facilities established to provide ATC service to aircraft operating on Instrument Flight Rules (IFR) flight plans within controlled airspace, principally during the en route phase of flight. When equipment capabilities and controller work load permit, certain advisory and assistance services may also be provided to Visual Flight Rules (VFR) aircraft.

The Air Traffic Manager (ATM), or designee, of each ARTCC has operational responsibility for the collocated Center Weather Service Unit (CWSU). The ATM, or designee, oversees CWSU operations and brings any special local weather support requirements to the attention of the CWSU meteorologist in charge (MIC).

Traffic Management Unit (TMU). The TMU, in an FAA ARTCC, is responsible for the active management of facility air traffic. The TMU is usually under the direct supervision of an assistant manager for traffic management.

Airport Traffic Control Tower (ATCT). The ATCT is a terminal facility that uses air/ground communications, visual signaling, and other devices to provide ATC services to aircraft operating at and in the vicinity of an airport. The ATCT authorizes aircraft to land or take off at the airport controlled by the tower or to transit the associated airspace regardless of flight plan or weather conditions. A tower may also provide approach control services (radar or non-radar).

Approach Control Facility. The approach control facility is a terminal ATC facility that provides approach control service in a terminal area.

Flight Service Station (FSS). FSS is an air traffic facility providing such aviation services as:

    a.  Pilot weather briefing (PWB) (see WSOM Chapter D-26,
        Airport Weather Warnings and Pilot Weather Briefings);
    b.  En route communications;
    c.  VFR search and rescue services;
    d.  Assistance to lost aircraft and aircraft in emergency
    e.  Relay of ATC clearances; and
    f.  Preflight and in-flight advisory broadcasts, and other
        services to pilots, via air/ground communications facilities.  

Selected automated FSSs (AFSS) also provide En Route Flight Advisory Services (EFAS) that are specifically designed to exchange timely weather information directly with en route pilots.

5. Aviation Weather Support Responsibilities.

5.1 NWS National Centers.

National Centers for Environmental Prediction (NCEP). NCEP provides weather analyses and forecast guidance for use by field offices. It also provides routine aviation guidance forecasts of cloud cover, ceilings, visibility, turbulence, icing, and wind. This guidance includes clear air turbulence forecasts that can be refined through the timely receipt of pilot reports (PIREP). In addition, NCEP is responsible for initiating the Volcanic Ash Forecast Trajectory and Dispersion (VAFTAD) model to project volcanic ash cloud density and movement.

Storm Prediction Center (SPC). SPC is responsible for preparing and issuing advisories (Weather Watches) on expected severe local storms, including tornadoes. SPC products include messages (Alert Weather Watches) with information tailored for aviation operations. The Aviation Weather Center (AWC) alerts the ATCSCC when Severe Weather Watches are issued.

Aviation Weather Center. AWC issues Aviation Area Forecasts (FA), and In-flight Advisories (AIRMETs, SIGMETs, International SIGMETS, and Convective SIGMETs).

In addition, AWC shall provide information on significant atmospheric pressure and pressure trend events to the ATCSCC, for relay to the appropriate ARTCC Weather Coordinator when CWSU meteorologists are not on duty. This responsibility includes notifying ATCSCC whenever an area's surface pressure is forecast to be above 31.00 inches of mercury. Significant pressure events include whenever an area's surface pressure change is expected to equal or exceed 1/2 inch of mercury during an 8-hour period. This information is used to alert ATC to changes in the lowest usable flight level above 18,000 feet.

Hurricane/Typhoon Forecast Centers. The responsibility for forecasting the path and intensity of hurricanes, other tropical disturbances, and associated sea conditions, as well as responsibility for the issuance of international SIGMETs in tropical portions of U.S. oceanic airspace, is divided among three hurricane forecast centers. The centers and their areas of responsibility are the:

   a.  Tropical Prediction Center in Miami, Florida, for the Atlantic,
       Caribbean, Gulf of Mexico, and eastern Pacific;
   b.  Central Pacific Hurricane Center in Honolulu, Hawaii, for the
       central Pacific; and
   c.  Joint Typhoon Warning Center for the western Pacific.  The
       NWS office in Guam will assume the meteorological watch
       office responsibility when the joint hurricane/typhoon
       responsibilities are reassigned.

5.2 Alaska Aviation Weather Unit (AAWU). AAWU provides for Alaskan airspace the same services provided by AWC for the contiguous United States. In addition, AAWU will develop services to include PWB assistance, backup Terminal Aerodrome Forecasts (TAF) if requested, and the issuance of a suite of Alaskan aviation graphics.

5.3 Satellite Field Distribution Facility (SFDF). SFDFs provide satellite support to NWS facilities and are responsible for transmitting geostationary satellite and polar orbiter imagery. SFDFs provide a consultation service on a real-time basis for other NWS facilities, including NWS facilities serving ATC, to discuss developing weather and other aviation hazards viewed from satellites, such as volcanic ash.

5.4 National Climatic Data Center (NCDC). NCDC is an office of the National Environmental Satellite, Data, and Information Service. The NCDC maintains the NWS Service Records Retention System that is a repository for non-climatic weather information including aviation weather forecasts.

5.5 Weather Forecast Office (WFO). For the purposes of this document, the term WFO will refer to the expanding network of offices with the Doppler Weather Surveillance Radar 88D (WSR-88D). As the modernization and restructuring proceeds, NWS offices not now designated WFOs, will assume responsibility for their areas' aviation forecasts. When aviation and other responsibilities are taken over, those remaining offices will become WFOs.

NWS WFOs provide aviation weather services for a specified area of responsibility. WFOs provide direct meteorological support through advice and consultation to the CWSU (when the meteorologist is on duty) and to the TMU Weather Coordinator (WC) or designee at other times. Other FAA facilities, e.g., ATC towers, shall be supported directly, as required and specified in local agreements. The WFO support consists of:

   a.  Providing CWSU and/or FSS pre-shift briefings and PWBs, as
       long as PWBs remain a requirement in the NWS and an office's
       PWB count doesn't fall below 100 per month.  If the office is
       decertified to perform PWBs, then no formal PWBs shall be
       provided by that office;
   b.  Only on very rare situations, assisting the ARTCC during
       in-flight emergencies when a pilot could be involved in a
       critical weather situation (if the CWSU meteorologist staff
       is not on duty and if there is no time to reach AWC); and
   c.  Providing the ARTCC with critical weather updates, e.g., TAFs
       and other vital aviation products (if CWSU is not in operation).

Honolulu, Hawaii, WFO provides support equivalent to that provided by AWC by issuing FAs and AIRMETs, for its area of responsibility.

Alaska in-flight weather is coordinated and disseminated by the Alaska Aviation Weather Unit (AAWU).

5.5.1 Associated WFO Responsibilities.

   a.  All WFO MICs with a CWSU located in their County Warning
       and Forecast Area, or in Alaska, the Regional Director's
       designee, are the first line supervisors of the CWSU MIC
       and have the responsibility for providing administrative
       and training support to the NWS staff members of the CWSU.
   b.  The WFO MIC has the responsibility for providing technical
       guidance and support to the CWSU.  This responsibility includes
       supporting the technical exchange and coordination requirements
       between the CWSU and other NWS facilities.
   c.  To provide adequate support capability, the WFO MIC ensures
       that WFO forecasters are aware of CWSU services and ARTCC
       meteorological needs.  The WFO MIC's FAA contact at the ARTCC
       is the ATM or designee.
   d.  Open lines of communication must be maintained between FAA
       facilities and NWS aviation weather support units within
       the ARTCC's area to ensure the timely exchange of necessary
       weather information. The WFO MIC or WFO designee shall monitor
       and evaluate the various links between the relevant NWS and
       FAA facilities.  Service, product, data, or data exchange
       deficiencies shall be documented and forwarded to the NWS
       Region Meteorological Services Division (MSD) either as part
       of a station evaluation report (with appropriate distribution)
       or as a separate memorandum with copies to the supervisors of
       the NWS and FAA facilities or units involved.  Initial attempts
       to remedy any deficiencies should be made at the local level.
       Problems that cannot be resolved locally should be brought to
       the NWS Region or Headquarters level for resolution.
   e.  Semi-annual visits to the CWSU should be made by the WFO MIC
       or WFO designee, and are highly encouraged.  Forecaster
       exchanges are also encouraged. A written report of each visit
       shall be sent to the regional MSD with copies to the CWSU MIC
       and the ARTCC ATM.

6. Joint FAA/NWS Units. CWSUs are joint agency (FAA/NWS) meteorological support units. CWSUs are composed of NWS meteorologists and FAA TMU personnel in each ARTCC. These weather support units serve the NAS directly and provide information critical to the safe and efficient flow of air traffic.

6.1 Center Weather Service Unit. The CWSUs are joint FAA/NWS weather support units generally staffed 16 hours a day by NWS meteorologists, and 24 hours a day by FAA TMU personnel, to provide meteorological support to 21 ARTCCs. The CWSU is a component in the national air traffic system and deals primarily with conditions in the en route structure of the NAS. The CWSU meteorologists work as a team with FAA air traffic control specialists assigned as Weather Coordinators (WC). The purpose of the CWSU is to provide meteorological consultation, forecasts, and advice to managers and staff within the ARTCC, and other supported FAA facilities and activities, regarding the impact of weather on their missions, equipment outages and repairs, and FAA staffing. In addition, the CWSU provides advisories of hazardous weather conditions for airborne aircraft. The CWSU provides weather information dissemination services making designated weather products available to both internal FAA and external aviation users.

Weather support is accomplished through various products and verbal briefings describing weather conditions, either by way of forecasts or actual weather, that may affect air traffic flow or operational safety in the ARTCC's portion of the domestic NAS (the CWSU area of responsibility), and in other locally defined, special operations areas (e.g., offshore helicopter operations areas). The CWSU meteorologists must remain cognizant of FAA requirements and procedures to adequately perform these tasks. Products generated for local dissemination and use describing conditions outside the CWSU area of responsibility, as defined above, may be prepared if, in the meteorologist's judgment, sufficient information and resources are available.

The CWSU also provides liaison between FAA facilities and other NWS offices in its area, and meteorological training for ARTCC personnel. CWSU meteorologists may assist in the distribution of weather forecasts, advisories, and warnings issued by other NWS offices. CWSU meteorologists usually staff two shifts a day (see appendix B). Exhibits D-25-1 and 2, and D-25-3, respectively, describe the areas of responsibility, and support offices for each CWSU.

6.1.1 CWSU MIC Responsibilities. The CWSU MIC is the first line supervisor of the assigned meteorologists. The MIC is the NWS liaison to the supported ARTCC and is responsible for providing the range of CWSU services to the FAA. The CWSU MIC is responsible for CWSU service obligations, labor-management relations, meteorological training for CWSU staff, and specified training for the ARTCC staff. The MIC, in agreement with the ARTCC ATM (or a designee given responsibility for CWSU operational oversight) is responsible for establishing CWSU meteorologist duty hours, implementing the procedures and policies detailed in this Chapter and compatible or approved alternate procedures to meet special local requirements. The CWSU MIC ensures that these procedures and policies are reflected in the CWSU SDM.

On occasion, it may become necessary for the MIC to amend the duty hours of the CWSU as a result of an FAA request for support. If this occurs, the MIC shall ask for a written notice of the proposed action from the ARTCC Facility Manager. Coordination shall also be accomplished with the associated WFO to determine if resources can allow the change in hours. If there is agreement between all parties, then the hours can be changed. Follow up the change with a letter to W/OM12, Chief Integrated Hydrometeorological Services Core at NWS Headquarters.

6.1.2 CWSU Meteorologist Responsibilities. CWSU meteorologists are NWS forecasters who monitor, forecast, analyze, and interpret available weather information. They provide forecasts and advisories for weather conditions pertinent to aviation and air traffic operations, primarily, within the ARTCC area of responsibility, and other locally defined special areas. CWSU meteorologists provide information to ATC supervisors and specialists, directly or through the WC, and prepare written and verbal briefings to ATC personnel, and to the ATCSCC as needed, on weather conditions that may affect the safe and efficient flow of air traffic.

The CWSU meteorologists shall:

   a.  participate in discussions of weather conditions, within
       the CWSUs ARTCC airspace (unless in back-up operations)
       likely to affect aviation operations with ATC personnel as
       required, and with ATCSCC personnel as requested;
   b.  issue Center Weather Advisories (CWA) and Meteorological
       Impact Statements (MIS) as conditions warrant;
   c.  provide weather forecasts and briefings for appropriate
       ATC personnel as required.  Briefings usually include
       actual and/or forecast weather conditions at major hub
       airports and along airways throughout the appropriate ARTCC
   d.  solicit PIREPs through the ATC work force from areas where
       conditions meet/approach advisory criteria and for validation
       of forecast advisory products;
   e.  relay, as appropriate, reports of conditions meeting specific
       urgent PIREP criteria, i.e. wind shear, tornadoes and funnel
       clouds, and volcanic ash, to the appropriate WFO(s) and/or
       the AWC meteorologist.  You may also want to check on the status
       of current advisories or watches if your call is to the AWC.
   f.  relay reports, as time permits, of conditions meeting Severe
       Weather Warning criteria to the NWS office(s) with Local
       Warning responsibility;
   g.  relay, as time permits, wind reports meeting Wind and
       Temperature Aloft Forecast (FD) amendment criteria and suggest
       amendments to the appropriate WFO/National Center for action.
       Provide the ARTCC Flight Data Section with any suggested FD
       amendments for use by the ARTCC;
   h.  act as consultants to ATCSCC in situations where weather
       conditions impede the normal flow of traffic in its respective
       ARTCC area if that service is requested by the ATCSCC;
   i.  confirm, as time permits, that the WC is aware of the issuance
       of CWSU products and other specified pertinent weather information.
       This information is distributed to personnel within the ARTCC,
       relevant FAA facilities within the ARTCC area of responsibility,
       and to other users as required;
   j.  provide meteorological forecasts, information and briefings in
       support of ATC operations during weather-related emergencies;
   k.  provide special, on-request PWBs to U.S. Government units,
       e.g., Air Force One, and time permitting, courtesy PWBs to FAA
       pilots either within or in contact with the ARTCC as work load
       permits.  Briefings shall be conducted and documented in
       accordance with WSOM Chapter D-26, Aviation Weather Warnings and
       Pilot Weather Briefings.  En route weather briefings and advice
       (as opposed to formal PWBs) may be provided to pilots in contact
       with the ARTCC directly or through appropriate ARTCC personnel.
       Procedures for any direct forecaster-to-pilot communications must
       be clearly stated in the CWSU SDM.  Other formal PWB duties are
       not the function of the CWSU and shall be referred to FSSs or
       designated NWS offices still performing that function;
   l.  assist in providing backup for an adjacent CWSU if requested
       (see Appendix B); and
   m.  conduct weather training sessions for the ARTCC personnel
       as work load permits.

6.1.3 Weather Coordinator. The ARTCC Weather Coordinator position is the designated interface between the CWSU meteorologists and ARTCC controllers; FAA facilities within the ARTCC area of responsibility; and users to whom CWSU products are disseminated. The WC is assigned the primary responsibility for collection and dissemination of pertinent aviation weather information. (see Exhibit D-25-4)

7. CWSU Products and Briefings.

This section provides policy and guidelines for the products and briefings created during CWSU operations.

7.1 Duties Priorities. The CWSU WC and the CWSU meteorologist operate as a team with each providing special skills to enhance the safe and efficient flow of air traffic. In the event that assigned resources make it impossible to accomplish all of the assigned duties, the CWSU meteorologist staff shall use professional judgment and the CWSU Priority of Duties List, Exhibit D-25-4, as a guideline to determine the order of task performance. It is recommended that a copy of this list be posted in the CWSU work area as a ready reference for the staff and for the information of ARTCC personnel. It should be emphasized that this list is not a schedule of tasks nor must the listed order of duties necessarily be reflected in task schedules as determined at the local level. The WC position has been assigned primary responsibility for the duties indicated by the asterisk (*) in Exhibit D-25-4. The meteorologist may assist the WC in the accomplishment of these tasks if other duties permit.

7.2 Product Preparation. The CWSU shall issue and disseminate forecasts and products and conduct briefings as detailed in this Chapter, other applicable WSOM chapters, and in the SDM. The conditions described in these products will generally be restricted to those within the boundaries of ARTCC domestic airspace. Reference points used in CWAs to describe the areal location and extent of these conditions shall be the same as those used in SIGMETs/AIRMETs (see Exhibit D-25-5, In-flight Advisory Plotting Chart), or distances from those points.

Forecasters should use the minimum number of points needed to describe the area accurately. Points outside of the ARTCC area may be used, with appropriate coordination, to simplify the areal shape and reduce the number of points required to describe where the conditions are occurring or are forecast to occur. Advisories requiring broadcast to aircraft should be kept as brief and concise as possible.

References to radar echo intensities within CWSU products should be expressed in terms of the WSR-88D DbZ "NWS VIP" equivalent and/or its corresponding adjective modifier. (See Appendix C)

In order for CWSU products to be consistent both with ICAO international coding and with national aviation products issued by the AWC, abbreviations and contractions used in CWSU products should be consistent with FAA Handbook 7340.1, Contractions. Terms used should be consistent with WSOM Chapters D-20, Aviation Area Forecasts, and D-22, In-flight Aviation Weather Advisories. All times shall be expressed in Coordinated Universal Time (UTC or Z). The format of the communications header must be followed exactly if the CWSU product is to be disseminated through the FAA and other communications systems.

Scheduled briefings and products shall be developed locally in agreement with the ATM or designee. These briefings should normally be produced and presented as required by the host ARTCC.

All users of CWSU advisories, statements, forecasts, and briefings should be kept aware that these products are not available 24 hours a day. This may be accomplished by adding the remark "NO UPDTS ddtttt UNTIL ddtttt" to the end of those products that will be in effect when CWSU duty hours end. The notation "dd" is the day of the month, and "tttt" is the hour and minute in UTC. The first "ddtttt" is the scheduled end of meteorologist duty hours, and the second is 1 hour after the next scheduled start of duty hours.

7.3 Communications Headers. Communications headers for use by the NWS Automation of Field Operations and Services (AFOS) and the World Meteorological Organization (WMO) are added at the NWS Telecommunication Gateway (NWSTG) to ensure appropriate MIS and CWA dissemination. The MIS and CWA examples in Sections 7.5 and 7.6, respectively, do not include these headers.

The AFOS (line 1) header format for the MIS is NMCCWSzzz, where "zzz" is the ARTCC identifier (e.g., ZOA for Oakland). The WMO (line 2) header format for all MISs in the contiguous United States is FOUS31 Kzzz ddtttt. The WMO header format for Alaska MISs is FOUS31 PAZA ddtttt. For example:

   FOUS31 KZOA 041421


   FOAK31 PAZA 041400

The AFOS header format for the CWA is NMCCWAzz#, where "zz" is the last two characters of the ARTCC 3-letter identifier (e.g., OA for Oakland) and "#" is the CWA Phenomenon Number (see Section 7.6). The WMO header format is WLUS31 Kzzz ddtttt in the contiguous United States, and WLAK31 PAZA ddtttt for Alaska. For example:

   WLUS31 KZME 081300


   WLAK31 PAZA 150420

7.4 Briefings. A CWSU briefing shall consist of at least a discussion of current and forecast weather conditions relevant to the ARTCC area during the shift in which it is provided and an outlook extending into the following shift or through the overnight off-duty hours period. Each briefing should contain sufficient information for ATC and TMU managers to make decisions regarding weather impacts on the NAS and appropriate operational adjustments.

A shift briefing product (alphanumeric or graphic) should contain a heading with the ARTCC designator (zzz); CWSU BRIEFING; date and time (UTC) issued; and ending date and time (UTC). For example:

   ZKC CWSU BRIEFING VALID 141900-151100

The following information should be included in each briefing when appropriate. Local requirements may determine the order of the items b-g:

   a.  Advisories in effect at the time of the briefing; e.g., SIGMETs,
       AIRMETs, local airport advisories, CWAs, MISs, etc.;
   b.  Synopsis - discussion of weather systems and their movements;
   c.  An outlook on en route flight conditions, e.g., weather, turbulence,
       icing, volcanic ash, etc.;
   d.  Terminal weather, i.e., heavy snow, freezing precipitation, low
       IFR ceiling and/or visibility, and/or operationally significant
       surface winds, for designated large airports;
   e.  Wind direction and speed aloft at key flight levels including
       jet stream location(s);
   f.  Freezing level; and
   g.  Locally required items affecting the ARTCC area of responsibility,
       e.g., altimeter settings forecast or observed below 29.92 inches or
       above 31.00 inches.

7.5 Meteorological Impact Statement (MIS). An MIS is an unscheduled flow control and flight-operations-planning forecast. It details weather conditions that are expected to adversely impact the flow of air traffic in the CWSU area of responsibility. MISs are valid during the period beginning 2 hours through 12 hours after issuance time. They are also valid for conditions existing at the time the briefing is issued or when CWSU operations begin if the adverse weather conditions will persist for at least 3 hours. The MIS is a forecast and briefing product intended for those personnel at ARTCC, ATCSCC, and large terminal air traffic control facilities such as TRACONS and ATCTs, responsible for making flow control and flow control-related decisions.

An MIS may be tailored to meet the unique requirements of the host ARTCC. These special requirements shall be coordinated between the host ARTCC and the CWSU and shall be documented in a Letter of Agreement and/or the SDM.

An MIS will enable air traffic control facility personnel to include the impact of specific weather conditions in their flow control decision making. As a minimum, an MIS shall be issued when:

   a.  any of the following conditions occur, are forecast to occur,
        if previously forecast, are no longer expected:
        (1)  Conditions meeting convective SIGMET criteria
               (see WSOM Chapter D-22);
        (2)  Icing - moderate or greater;
        (3)  Turbulence - moderate or greater;
        (4)  Heavy precipitation;
        (5)  Freezing precipitation;
        (6)  Conditions at or approaching low IFR (see WSOM
               Chapter D-31, Aviation Terminal Forecasts);
        (7)  Surface winds/gusts 30 knots or greater;
        (8)  Low level wind shear (within 2,000 feet of the surface);
        (9)  Volcanic ash, dust storms, or sandstorms; and
   b.  in the forecaster's judgment, the conditions listed above, 
       or any others, will adversely impact the flow of air traffic
	   within the ARTCC area of responsibility; and
   c.  the forecast lead time (the time between issuance and onset
       of a phenomenon), in the forecaster's judgment, is sufficient to
       make issuance of a CWA unnecessary or premature.

MIS phenomena forecasts should use the location reference point identifiers depicted on the In-flight Advisory Plotting Chart, and include the height, extent, intensity, and movement of the conditions. MIS product issuances shall be numbered sequentially beginning at midnight local time each day. The MIS is disseminated and stored as a "replaceable" product. Thus, each issuance shall contain the details of all pertinent, known conditions meeting MIS issuance criteria, including ongoing conditions described in previously issued MISs.

The MIS shall be distributed to ARTCC personnel, including TMU personnel, and disseminated via the FAA and NWS communications systems. If an MIS is included or issued concurrently with a CWSU briefing, then the meteorologist should ensure that the MIS portion of the briefing is disseminated to those supported facilities that do not normally receive the briefing contents. While alphanumeric MIS products shall continue to be issued, electronic graphic versions of MISs may be developed and used to augment or provide quick reference to ARTCC users. Graphic MISs, as a minimum, shall depict all hazards or expected hazards with boundaries clearly defined. They shall also show all of the associated information covered in the alphanumeric text, a valid period date and time group, and map backgrounds, as required by the local ARTCC and specified in the SDM.

If the MIS is distributed over the FAA Flight Data Entry Printout (FDEP) system, the system's message size restriction of 10 lines should be considered. If the FDEP line limit will be exceeded, then meteorologists may revise an already disseminated product for FDEP-only use.

MIS FORMAT (See section 7.3 for other communications headers.)

The FAA header line is zzz MIS ii VALID ddtttt-ddtttt; where:

      "zzz" is the ARTCC identification, e.g., ZJX
      "MIS" is the product type
      "ii" is the 2-digit sequential issuance number, and
      "ddtttt" is the valid beginning and ending date/time UTC.

Text Line(s):

      The MIS text should follow general text guidelines in use by
      products detailed in WSOM Chapter D-22.  Remarks, if appropriate,
      shall be added to the end of the text, e.g.:

      -   "...SEE CONVECTIVE SIGMET 8W";
      -   "NO UPDTS AVBL AFT 0230Z"; and
      -   Forecaster initials and/or facility identifier may be placed
        at the end of the MIS.

If the phenomenon described in an MIS is no longer expected, a cancellation MIS message shall be issued. The FAA header shall not contain an issuance number. However, the MIS text shall begin with "Cancel zzz MIS ii." A text explanation for the cancellation should follow. If the phenomenon described in the MIS is expected to continue beyond the operating hours of the CWSU, then the remark "NO UPDTS AVBL AFT ttttZ" (where "ttttZ" is the UTC closing time of the CWSU) shall be added at the text end.


      /D ZOA MIS 01 VALID 041415-041900
      OCNL BLO 1SM TIL 19Z.
      ZOA CWSU
      /D ZOA MIS VALID 041650
      ZOA CWSU
      /D ZID MIS 03 VALID 041200-142330

      AFT 18Z...MAX TOPS TO FL410...TS MOV FM
      OCNL MOD MXD ICG IN CLDS/PCPN 020-120...
      ZID CWSU

7.6 Center Weather Advisory (CWA). The CWA is an aviation weather warning for conditions meeting or approaching national In-flight Advisory (AIRMET, SIGMET or convective SIGMET) criteria (see WSOM Chapter D-22). The CWA is primarily for use by air crews to anticipate and avoid adverse weather conditions in the en route and terminal environments. It is not a flight planning product because of its short lead time and duration. The CWA should reflect conditions at the time of issuance and/or be a short range forecast.

CWAs shall be valid for up to 2 hours and may include forecasts of conditions expected to begin within 2 hours of issuance. If conditions are expected to persist beyond the valid period of the advisory, a statement to that effect should be included in the last line of the text. Additional CWAs shall subsequently be issued as appropriate. Notice of significant changes in the phenomenon described in a CWA shall be provided by a new CWA issuance for that phenomenon. If the forecaster deems it necessary, CWAs may be issued hourly for convective activity. This may improve the usefulness of the HIWAS recordings that include those CWAs.

The urgent CWA (UCWA) communications header is intended for those situations where unforecast weather conditions occur and have an immediate effect on the safe flow of air traffic within the ARTCC area of responsibility. It should only be used when the CWSU meteorologist believes any delay in dissemination to FAA facilities would impact aviation safety. Use the routine CWA header for subsequent issuances of the same phenomenon. CWAs may be issued for the same phenomena described in advisories and forecast products issued by WFOs and by the AWC or other NCEP centers.

There are three situations under which a CWA should be issued:

1. When existing or anticipated weather conditions do not meet national In-flight Advisory criteria (i.e., in terms of intensity or areal coverage) but current PIREPs or other weather information sources indicate that those conditions, in the judgment of the CWSU meteorologist, will adversely impact the safe flow of air traffic within the ARTCC area of responsibility.

2. As a supplement to an existing In-flight Advisory. The issuance of a CWA in this circumstance should be limited to occasions when, in the judgment of the CWSU meteorologist, a redefining statement or update, in advance of a new national advisory, is adequately supported by real-time information. The purpose of the CWA in this case is to improve upon or update the existing advisory's description of the phenomenon. These improvements may be to make the location more relevant to users within the ARTCC area or to be more precise in describing the location, movement, extent, or intensity of the phenomenon. For example, a CWA describing the specific area(s) of low IFR conditions (AIRMET criteria) within the ARTCC area would be a valid redefinition of the location and intensity relative to the ARTCC area and meeting documented requirements.

3. When an In-flight Advisory has not yet been issued but observed or expected weather conditions meet In-flight Advisory criteria based on current PIREPs and/or other sources of information. The CWSU meteorologist should call the appropriate forecaster at the AWC or AAWU to coordinate. If, in the judgment of the CWSU meteorologist, it is necessary to issue a CWA to allow lead time while the national In-flight Advisory is being prepared, a UCWA shall be issued indicating that an AIRMET or SIGMET will be issued shortly.

The first line of each CWA's FAA communications system header shall have an ARTCC identifier immediately followed by a Phenomenon Number (1-6) (see CWA format below). This CWA Phenomenon Number shall be assigned to each meteorologically distinct condition or group of conditions or to each set of similar condition(s) in distinctly separate areas. The first meteorological event of the local calendar day that requires the issuance of a CWA shall be assigned Phenomenon Number "1." The latest CWA issuance with this number can replace and update the previous issuance. This numbering makes it possible to disseminate CWAs for up to six unrelated events with each event issuance capable of being individually updated.

The first line shall also contain an issuance/beginning valid time. When a CWA is issued with some lead time, the time entered is the issuance time. The time that the meteorologist expects the conditions to begin should then be stated in the text. If there is no lead time, then the issuance time is considered the beginning time of the phenomena. In either case, CWAs are valid upon issuance.

On the second line, the product identifier "CWA" shall be followed by a three-digit number. The first digit is the Phenomenon Number and the second two digits are an Issuance Number. Issuance Numbers for a given phenomenon shall be issued sequentially beginning with "01." This shall be followed by the "VALID UNTIL" time. The valid period (issuance time to valid until time) shall not exceed 2 hours. If the meteorological conditions are expected to persist beyond the 2-hour period, then a remark to that effect shall be included at the end of the advisory text, and on subsequent CWAs when appropriate, e.g. (ACTVTY XPCD TO CONT BYD 20Z). If the phenomenon described in the CWA is expected to continue beyond the CWSU hours of operation, an additional remark, "NO UPDTS AVBL AFT tt(tt)Z," where "ttttZ" is the UTC closing time of the CWSU, shall be added as the last remark in the last CWA issuance.

Time permitting, any CWA overlapping into another center's airspace should be coordinated and a statement should be included in the text, e.g., "SEE ZOB CWA 201 FOR TS CONDS IN ZOB CTA" ("CTA" is "control area"). If issuance prior to coordination is necessary, a statement regarding the area(s) affected should be included in the text, e.g., "LN TS EXTNDS NWWD INTO ZOB CTA."

AIRMETs/SIGMETs being augmented by the CWA shall be referenced in a text remark, e.g. "...SEE CONVECTIVE SIGMET 8W". Each CWA shall normally be disseminated via the FAA communications system. A hard copy of each CWA shall be given to the TMU WC for dissemination to affected ARTCC sectors and terminal facilities and for broadcast on the appropriate radio frequencies. Dissemination times should be logged on the daily CWSU operations log sheet. If the WC or designee can receive and disseminate the CWA electronically, then delivering hard copies of the CWA to ARTCC sectors may not be needed.

The CWA text shall follow the guidelines for other In-flight Advisories as detailed in WSOM Chapter D-22. Graphic versions of CWAs may be created to augment the disseminated text versions and provide quick reference to product users within the ARTCC. Graphic CWAs shall depict, as a minimum, all hazards detailed in the text, with hazard boundaries clearly defined, appropriate descriptive alphanumeric text, date and time group, and map backgrounds as required by the local ARTCC and specified in the SDM. Forecasters should be aware that if the CWA is to be distributed over the FAA FDEP system, that system has a product length restriction.


Line one of FAA header:
        zzzp (U) CWA ddtttt (Note:  () = optional)
Line two of FAA header:
        zzz CWA pii VALID UNTIL ddtttt, where header elements are:
        zzz        ARTCC Identification, e.g., ZKC
        p          Phenomenon Number (single digit, 1-6).
        (U) CWA    Product Type (note:  UCWA is only used on line
                   one of the header.)
        ddtttt     Beginning and/or issuance UTC date/time.
        ii         Issuance Number (issued sequentially for each
                   Phenomenon Number).
        ddtttt     Ending valid UTC date/time.
Line one of text - Phenomenon Location):
        FROM aaa TO bbb TO ccc TO aaa         (polygons)
        or FROM aaa TO bbb                    (lines)
        or VC aaa                             (vicinity)
        or aaa                                (point)
        or nnnDDD aaa                         (point)

     a.  The location line should not exceed one line of text
         and must end without a period.

     b.  aaa, bbb, etc. are location identifiers depicted on
         the In-flight Advisory plotting chart (Exhibit D-25-5).
         These identifiers shall be used as area or
         line-defining points, or as all or part of a point
         reference (i.e., VC (n)nn DD(D) XXX). The "(n)nn" is
         distance and "DD(D)" is a 16-point compass
         direction (e.g., VC IAH or 40NNE LBB).

     c.  Polygon areas defined with the keyword "FROM" 
	     must have the same beginning and ending point.

     d.  The text for a line phenomenon must contain the
	 	 keywords "LINE" and "nn NM WIDE" 
		 where "nn" is the width of the line in nautical

     e.  The text for a phenomenon defined around a point must

	 	 contain the keywords "AREA" or "ISOLD".
		 The diameter of a "point" phenomenon, i.e.,
		 "DIAM nn NM" must also be specified.

     f.  If some or all of a CWA phenomenon is outside of the points
         depicted on the In-flight Advisory plotting chart, then
         describe the location by using latitude, longitude, or
         plain language geographic location.

Line two of text - Phenomenon Description: Text description shall include key phrases detailed in the notes above. Remarks, if appropriate, shall be added to the end of the text.

Note: The forecaster may choose to include initials, facility identifier, or the forecaster number at the end of the CWA.


    D ZME1 CWA 081300
    ZME CWA 101 VALID UNTIL 081500
    FROM 26025KT. TOPS TO FL450.
    (JLW) (optional)
    /D ZFW3 CWA 131945
    ZFW CWA 303 VALID UNTIL 132145
    TOPS TO FL500.
    /D ZLC3 CWA 271600
    ZLC CWA 302 VALID UNTIL 271800

    /D ZME1 CWA 040100
    ZME CWA 101 VALID UNTIL 040300
    VC MEM
    020.CONDS CONTG BYD 03Z..NO UPDTS 040200Z UNTIL 041000Z.
    /D ZNY1 UCWA 021400
    ZNY CWA 502 VALID UNTIL 021600
    /D ZNY1 UCWA 041500
    ZNY CWA 401 VALID UNTIL 041700
    /D ZLA1 CWA 160400
    ZLA CWA 101 VALID UNTIL 160600
    35ENE LAX
    160500 UNTIL 161300.
    /D ZHU2 UCWA 031955
    ZHU CWA 205 VALID UNTIL 032155
    45N IAH

    /D ZAN1 UCWA 150420
    ZAN UCWA 101 VALID UNTIL 150620
    OPAKE (53N/170E) BTWN 0300-0330 UTC. FL340-360 WINDS 240070. ASH EXPCTD

7.7 Forecast Coordination. Forecast products issued by AWC or other NCEP centers, WFOs, and the CWSU will often address the same event(s) within the same area and time period. Coordination between the responsible offices prior to product issuances is necessary to avoid user confusion and avert any impact on aviation safety. To make coordination easier, CWSU forecasters should discuss the products they are about to issue with the AWC, NCEP, or WFOs. This coordination is particularly important when those products concern unexpected or suddenly changing weather conditions. In situations where a CWA has been issued prior to coordination, notification to the appropriate offices, national center, or WFO should follow as soon as higher priority duties permit.

A CWA issuance for conditions not meeting In-flight Advisory criteria, while generally based on those criteria, is primarily because the forecaster recognizes that a condition is having an adverse impact on the safe flow of air traffic. Prior coordination with the AWC (or if in Alaska with the AAWU or Anchorage CWSU) in this situation should take place if the CWA will describe a trend toward an In-flight Advisory criterion. Other NWS offices whose products may be impacted by the CWA should also receive prior notification of the issuance, if time permits. In either case the CWSU forecaster has the final responsibility for issuing a CWA.

All users and forecasters should understand that every national In-flight Advisory does not require a CWA. Also, every product for which a related CWA is issued does not require updating by the originating office. The guidelines and authorizations in this Chapter do not affect the update/amendment instructions for various products contained in other WSOM Chapters.

7.8 Dissemination of CWSU Products. PIREPs, generated by the CWSU, shall be relayed nationally, within the ARTCC, and to supported FAA facilities in the most prompt and efficient manner.

7.9 Station Duty Manual (SDM). All CWSUs shall maintain an SDM in accordance with existing NWS directives as specified in WSOM Chapter A-13, Station Duty Manual. The SDM should be developed in agreement with the ATM or designee and should contain those guidelines and instructions needed to meet local ATC requirements.

A copy of all CWSU SDMs should be on file with the NWS supervisor having administrative responsibility for the CWSU meteorologists. This official should review the SDM prior to implementation of a new or changed issuance. A historical SDM file shall be maintained at the WFO providing administrative support to the CWSU. Retention of superseded and/or cancelled portions of the SDM in this file shall be in accordance with WSOM Chapters A-13, Station Duty Manual, and D-90, Weather Support for Accident Investigations and Litigation.

Copies of approved changes to the SDM shall be forwarded, with a cover letter, to the NWS official for inclusion with the historical SDM file.

7.10 CWSU Facilities. CWSU products are time critical and must be relayed as soon as possible. CWSU personnel must also maintain an enhanced awareness of ARTCC and NAS operations. It is essential that the physical location of the CWSU be as close as possible to those with whom CWSU meteorologists have the greatest interaction, i.e., Traffic Management Coordinators/Supervisors, FAA Area Managers In Charge, and Military Operations Specialists. Full communications and ATC display capabilities need to be provided to the CWSU to facilitate the operational support specified by this directive.

7.11 Support to EFAS and FSS. The CWSU in each ARTCC is designated as the primary support facility for each associated EFAS facility. The CWSU should expect to be contacted by the EFAS specialist when meteorological conditions either impede or threaten to impede the normal flow of air traffic. Since the CWSU meteorologists are continuously aware of aviation weather problems, close cooperation should exist between the CWSU and EFAS staffs. The EFAS staff has access to additional sources of PIREP information which may be of use to the CWSU meteorologist. During the period when the CWSU is not available to provide consultation service to EFAS, WFOs are responsible for responding to EFAS facility requests regarding weather conditions within the WFOs area of responsibility.

Pre-shift briefings for FSS personnel should normally be done by the CWSU in accordance with FAA Order 7110.10K paragraph 4-71. Weather support, during the period when the CWSU is closed or not available, except if back-up services are in effect, is the responsibility of designated WFOs (in accordance with a FAA Order 7110.10K paragraph 4-74). This responsibility ensures that the link with an NWS facility capable of providing 24-hour support remains clear cut. Any requests for CWSU PWBs that cannot be entertained because of CWSU work load shall normally be referred to the FSS. (See Section 6.1.2.k.)

7.12 Handling of Weather Records. CWSU weather records and logs shall be retained per the instructions in WSOM Chapter D-90 and this Chapter. Electronically displayed products generated on the Meteorologist Weather Processor or other systems shall not be printed solely for retention purposes.

7.12.1 Retention. All written and hard copy graphic CWSU records shall be retained by the unit for 30 days. Records retention at the end of the 30 days shall be as follows:

    a.  A daily record of facility operation (FAA Form 7230-4 or equivalent)
        shall be retained per FAA directives.  FAA retention of copies of this
        record is the responsibility of the ATM.

    b.  All hard copy service products generated by CWSUs, i.e., briefing
        sheets, printed graphics that were generated and used by the CWSU
        and are not available through the NCDC as archived products, shall
        be transferred to the WFO supporting the CWSU and retained per
        WSOM Chapter D-90.  (NOTE: These records may be maintained at
        the CWSU instead of the WFO if a secure area is available and a letter
        authorization from the WFO is on file.)  Worksheets used to update
        briefings or to supplement other products need not be retained.

7.12.2 Protection of Records. All requests for copies of weather exhibits or written records prepared by the CWSU meteorologists shall be handled in accordance with procedures outlined in WSOM Chapter D-90. In the event of an aircraft mishap or accident within the area of responsibility of the ARTCC facility, retention procedures described above shall be followed unless otherwise requested by the Manager, Forensic Services, Customer Service Core, at NWS Headquarters. In the event of a major accident (defined in WSOM Chapter D-90), all relevant products prepared by the CWSU meteorologists, including the relevant observations, charts, and forecasts available to the CWSU meteorologists, should be forwarded to the appropriate WFO as soon as possible. These records shall be protected and retained in the WFO for at least 30 days to provide time for determining:

    a.  to what extent weather was a factor, and/or

    b.  what weather information will be required for investigation purposes.

After this period, normal retention procedures shall be followed unless the Manager, Forensic Services requests otherwise.

7.13 Statements. No written statements by CWSU meteorologists concerning a system incident, or an aircraft incident or accident, shall be provided to offices, agencies, organizations, or individuals (government or public) outside of the NWS without the approval of the Manager, Forensic Services. Any such statements shall concern only the meteorological facts and must be reviewed by the appropriate regional headquarters and the Manager, Forensic Services. When a written statement is prepared, the original and one copy shall be forwarded directly to the Manager, Forensic Services. One copy shall be forwarded to the appropriate NWS regional headquarters through the WFO.

The comments of CWSU meteorologists are not a matter of public record. There is no requirement that anyone other than members of a Government investigation team be allowed to question or interview personnel in connection with an aircraft accident. When an accident has occurred and the details are being discussed by persons outside of the NWS, or when being questioned or interviewed in connection with an accident, NWS personnel should attempt to determine that their verbal comments are not being recorded. Should a request to record such comments be made, the request shall be referred to the Manager, Forensic Services. Any such arrangements shall be made at the regional or NWS Headquarters level.

7.14 Back-Up of CWSU Operations On occasion, a CWSU may be closed for all or part of their duty day. At these times, back-up of the CWSU shall be accomplished in accordance with Appendix B.

HTML Conversion: A. W. Jarvi, NWS, Office of Systems Operations