Synoptic Discussion - September 2016
Note: This Synoptic Discussion describes recent weather events and climate anomalies in relation to the phenomena that cause the weather. These phenomena include the jet stream, fronts and low pressure systems that bring precipitation, high pressure systems that bring dry weather, and the mechanisms which control these features — such as El Niño, La Niña, and other oceanic and atmospheric drivers (PNA, NAO, AO, and others). The report may contain more technical language than other components of the State of the Climate series.
The atmosphere was in an ENSO-neutral state during September 2016. Like the previous month, the weather over the contiguous United States (CONUS) this month was dominated by a complex interplay between subtropical high pressure systems, jet stream weather systems, and tropical cyclone rainfall. Troughs and lows in the jet stream flow funneled cold fronts and surface lows into the western U.S., giving the West a colder-than-normal month, while the Bermuda High and upper-level ridges kept the U.S. east of the Rockies warmer than normal. The fronts and lows brought precipitation to parts of the western and central CONUS, while tropical systems drenched parts of the East Coast, but the North Pacific High and Bermuda High kept much of the rest of the country drier than normal. The fronts and upper-level troughs provided the dynamics that triggered severe weather, but, again, the Bermuda High helped keep the number of tornadoes below average for the month. Drought expanded in the South and New England and contracted in parts of the West and Midwest, with the overall national drought footprint changing very little, shrinking slightly to about 19.4 percent of the CONUS. The upper-level circulation and temperature anomaly patterns suggested the atmospheric drivers originating in the North Pacific and Arctic Oceans had the greatest influence on the month's weather. See below for details.
Animation of daily upper-level circulation for the month.
Animation of daily surface fronts and pressure systems for the month.
In the Northern Hemisphere, September marks the beginning of climatological fall (autumn) which is the time of year when solar heating decreases as the sun angle decreases, and an expanding circumpolar vortex forces the jet stream to migrate southward. Polar air masses begin to influence the weather over the contiguous U.S. (CONUS) more, and the warm, dry subtropical high pressure belts influence the weather less.
The mid-latitude circulation was very active during September 2016, with numerous troughs and ridges moving in the jet stream flow. The circulation became extremely meridional over Canada near the end of the month, with large areas of north-flowing and south-flowing air. This pattern resulted in the storm track staying mostly along the U.S.-Canadian border, with deep troughs of low pressure frequently plunging deep into the CONUS. The North Pacific and Bermuda subtropical highs struggled to maintain their dominance over the U.S. in spite of the onslaught of jet stream troughs and upper-level lows. The troughs and upper lows dragged cooler Canadian air behind cold fronts along with them at the surface. Under the dominance of the Bermuda High, temperatures averaged warmer than normal for the month from the Plains eastward, while the dominance of upper-level troughs gave much of the West cooler-than-normal temperatures.
The North Pacific High kept much of the Far West drier than normal for September, while descending air associated with the Bermuda High inhibited precipitation across the central and eastern CONUS. The fronts and upper-level troughs parried the dry trend by bringing above-normal precipitation to parts of the Interior West to Northern Plains and Southern Plains to Upper Midwest. Moisture from the remnants of East Pacific tropical cyclones gave parts of the Southwest above-normal precipitation, and moisture from North Atlantic tropical cyclones resulted in a wetter-than-normal month along the coast from the Southeast to Mid-Atlantic states.
The combination of lifting along the frontal boundaries and dynamical support from the upper-level troughs and lows triggered severe weather, mostly east of the Rockies, but the overall dominance of the Bermuda High kept the tornado count below average. According to preliminary data, 36 tornadoes occurred during September 2016, which is about half of the September average of 74. Most occurred in the Southwest to Central Plains and Midwest in association with fronts and low pressure systems along the storm track.
Normally, on the southern side of the North Pacific and North Atlantic Highs, the winds blow out of the east to northeast and toward the equator to produce the northeasterly Trade Winds. The Trade Winds steer tropical weather systems that are close to the equator, thus making the subtropical highs important drivers of tropical cyclone motion. ENSO conditions along the equatorial Pacific Ocean (El Niño or La Niña) can influence the development of tropical cyclones in both the Pacific and Atlantic. The tropical Pacific was in an ENSO-neutral state during September, so it's influence on tropical cyclones was muted this month.
Five named tropical cyclones (Tropical Storms Ian, Julia, Karl, and Lisa, and Hurricane Matthew) formed in the Atlantic Basin in September. Three tropical cyclones which formed in August (Hurricanes Gaston and Hermine and Tropical Depression 8) were also active to some degree into early September. The Bermuda High steered Gaston, Ian, Karl, and Lisa into the central North Atlantic, keeping them away from the CONUS. Tropical Depression 8 brushed the North Carolina coast before being deflected out to sea. Hermine tracked across northern Florida on September 2 after developing in the Gulf of Mexico in late August. It joined with a cold front along the Southeast coast and transformed into a frontal low, then tracked just offshore along the Eastern Seaboard during the next several days, dropping tropical moisture along the coast from Florida to the Mid-Atlantic States. Julia developed along the Atlantic Coast near Jacksonville, Florida in mid-September and slowly meandered just off the Florida to North Carolina coast for the next several days, occasionally interacting with a passing cold front. The languid motion of Hermine and Julia was influenced by a complex interaction between the Bermuda High, which wanted to swing them out into the North Atlantic, and troughs in the mid-latitude jet stream, which kept pulling them back to the coast. Matthew developed near the end of the month in the vicinity of the Lesser Antilles and was steered westward into the Caribbean by easterly Trade Winds, until being drawn north by an upper-level trough dipping southward from the westerlies.
Five named tropical cyclones (Tropical Storm Roslyn and Hurricanes Newton, Orlene, Paine, and Ulika) formed in the central and eastern equatorial Pacific Basin in September. Two tropical cyclones which formed in August (Hurricanes Lester and Madeline) were also active to some degree in early September. Most of these cyclones moved westward in the easterly Trade Winds, with Lester and Madeline sweeping by Hawaii. Madeline passed by close enough to drop heavy rains on the Big Island. Deep troughs in the jet stream over western North America helped draw Newton and Paine northward along Baja California, with their tropical moisture eventually making it into southern California and the Southwest U.S., enhancing summer monsoon rainfall.
The Intertropical Convergence Zone was active across the U.S.-Affiliated Pacific Islands (USAPI) in the western equatorial Pacific. Several tropical disturbances moved westward across Micronesia in the easterly Trade Winds and developed into named storms as they moved west of the Mariana Islands and into the Philippine Sea. These included Typhoons Meranti, Malakas, Megi, and Chaba. Other than heavy rainfall, the USAPI were largely spared damaging effects from these cyclones.
The subtropical highs allowed warmer tropical-type air masses to cover the CONUS more often than cooler polar air masses. As a result, September ended up with 3,715 record warm daily high (1,100) and low (2,615) temperature records. This was about five and a half times the 669 record cold daily high (422) and low (247) temperature records that were associated with the air masses behind the cold fronts. Nationally, September 2016 ranked as the ninth warmest September in the 1895-2016 record. The last eight Septembers have been consistently much warmer than average. The REDTI (Residential Energy Demand Temperature Index) for September 2016 ranked seventh highest in the 122-year record for September, due to much-warmer-than-normal temperatures which increased cooling demand in the heavily-populated East Coast, Midwest, and Southeast regions.
Above-normal precipitation from fronts and tropical systems helped offset the dryness associated with the North Pacific and Bermuda Highs, with the result being the 40th wettest September on record for the CONUS. Drought and abnormal dryness shrank where heavy rains fell, but it expanded and intensified in the South and Northeast where it was dry and much warmer-than-normal temperatures enhanced evapotranspiration. Drought contraction and expansion roughly balanced each other out, so that the national moderate-to-exceptional drought footprint decreased only slightly from 19.5 percent at the end of August to 19.4 percent at the end of September (from 16.3 percent to 16.2 percent for all of the U.S.).
The Climate Extremes Index (CEI) aggregates temperature and precipitation extremes across space and time. Although temperature and precipitation extremes occurred during the month in many areas, only two regions had a CEI that ranked in the top ten category. The Northeast region had the fifth most extreme September CEI in the 1910-2016 record due to the second most extreme warm maximum and seventh most extreme warm minimum temperature components, and 13th most extreme Palmer drought component. The East North Central region had the tenth most extreme September CEI in the 1910-2016 record due to the second most extreme warm minimum temperature component and fourth most extreme Palmer wet spell component. When aggregated across the nation, September 2016 had the seventh most extreme warm minimum temperature component, but the other national components ranked lower, with the September 2016 national CEI ranking as only the 39th most extreme September CEI in the 107-year record.
The upper-level circulation pattern, averaged for the month, consisted of near to below-normal upper-level (500-mb) height anomalies over the western CONUS extending into western Canada, resulting from troughs frequently moving across and deepening over the West. Above-normal heights dominated over the eastern CONUS and extending into eastern Canada. Of the circulation indices usually discussed on this page, the teleconnections aggregated for a negative EP-NP and a positive AO most closely match the September 2016 500-mb circulation anomalies.
September was drier than normal across most of the Far West, from the Central Rockies to parts of the Central High Plains, from the Lower Mississippi Valley to Southern Appalachians, and across most of the Northeast. It was wetter than normal from the Interior West to Northern Plains, along a strip from the Southern High Plains to western Great Lakes, and along the coast from Florida to the Mid-Atlantic states. Alaska was wetter than normal except in the southeast. Hawaii was generally wetter than normal except on the Big Island and Kauai. Puerto Rico was wetter than normal in the northwest and drier than normal along the southern coast and in the east.
September temperatures averaged below normal roughly from the Rockies to the West Coast, and above normal from the Plains to the East Coast, with Ohio having a record-warm September. Temperatures were above normal in Alaska except in the interior where they were near to below normal.
Global Linkages: The upper-level (500-mb) circulation anomaly pattern over North America was part of a complex long-wave pattern that stretched across the Northern Hemisphere. There were several regional ridge (above-normal 500-mb height anomalies) / trough (below-normal heights) "couplets" in the monthly average pattern, but positive height anomalies (ridges) seemed to dominate. The most notable below-normal height departures (troughs) occurred over western North America, the northeastern North Atlantic (near the Icelandic Low), southwestern Russia, and the northwestern North Pacific. The most notable above-normal height departures (ridges) occurred over eastern North America, Europe, north central Eurasia, and the eastern North Pacific. The above-normal 500-mb heights were associated with upper-level ridging at the mid-latitudes; below-normal precipitation; and above-normal surface temperatures over the continents and oceans. The few areas of below-normal 500-mb heights were associated with upper-level troughing, near- to below-normal surface temperatures over the continents and oceans, and above-normal precipitation over southwestern Russia. With most of the continents having warmer-than-normal temperatures, and large portions of the Atlantic and Pacific Oceans having warmer-than-normal sea surface temperatures, the September 2016 global temperature was well above normal.
Subtropical highs, and fronts and low pressure systems moving in the mid-latitude storm track flow, are influenced by the broadscale atmospheric circulation. The circulation of the atmosphere can be analyzed and categorized into specific patterns. The Tropics, especially the equatorial Pacific Ocean, provides abundant heat energy which largely drives the world's atmospheric and oceanic circulation. The following describes several of these modes or patterns of the atmospheric circulation, their drivers, the temperature and precipitation patterns (or teleconnections) associated with them, and their index values this month:
El Niño Southern Oscillation (ENSO)
- Description: Oceanic and atmospheric conditions in the tropical Pacific Ocean can influence weather across the globe. ENSO is characterized by two extreme modes: El Niño (warmer-than-normal sea surface temperature [SST] anomalies in the tropical Pacific) and La Niña (cooler-than-normal SST anomalies), with the absence of either of these modes termed "ENSO-neutral" conditions.
- Status: During September 2016, ENSO-neutral conditions continued as equatorial SSTs were near or below average in the east-central and eastern equatorial Pacific.
- Teleconnections (influence on weather): The NWS CPC has no teleconnections for ENSO-neutral conditions.
- Comparison to Observed: But there is some similarity of the September 2016 temperature and precipitation anomaly patterns to the teleconnections expected with La Niña (upper left panels for temperature and precipitation).
Madden-Julian Oscillation (MJO)
- Description: The MJO is a tropical disturbance or "wave" that propagates eastward around the global tropics with a cycle on the order of 30-60 days. It is characterized by regions of enhanced and suppressed tropical rainfall. One of its indices is a phase diagram which illustrates the phase (1-8) and amplitude of the MJO on a daily basis. The MJO is categorized into eight "phases" depending on the pattern of the location and intensity of the regions of enhanced and suppressed tropical rainfall. The MJO can enter periods of little or no activity, when it becomes neutral or incoherent and has little influence on the weather. Overall, the MJO tends to be most active during ENSO-neutral years, and is often absent during moderate-to-strong El Niño and La Niña episodes.
- Status: The MJO index began the month incoherent, then spent the last half of the month weakly in phases r and 5. Other modes of coherent subseasonal tropical variability influenced the tropical Pacific and global climate, including Kelvin and Rossby wave activity, tropical cyclones, the Indian Ocean Dipole (IOD), and the Pacific monsoon trough (also known as the ITCZ, or Inter-tropical Convergence Zone). These Rossby and Kelvin waves, IOD, tropical cyclones, and the monsoon trough in the western North Pacific may have masked the influence of the MJO (MJO updates for September 5, 12, 19, 26, and October 3).
- Teleconnections (influence on weather): The MJO's temperature and precipitation teleconnections to U.S. weather depend on time of year and MJO phase. To the extent teleconnections are known, the September (August-October) teleconnections for temperature are shown here and for precipitation are shown here.
- Comparison to Observed: The MJO is transitory and can change phases (modes) within a month, so it is more closely related to weekly weather patterns than monthly. In the case of September 2016, the MJO index peaked in phase 4 at mid-month. The monthly temperature anomaly pattern, and weekly temperature anomaly pattern for week 3, have some similarity to the teleconnection pattern for a phase 4 MJO. Precipitation has a more complicated pattern, but the monthly precipitation anomaly pattern, and weekly precipitation anomaly pattern for week 3, also have some similarity to the teleconnection pattern for a phase 4 MJO.
- The Pacific/North American (PNA) pattern
- Description: The PNA teleconnection pattern is associated with strong fluctuations in the strength and location of the East Asian jet stream. PNA-related blocking of the jet stream flow in the Pacific can affect weather downstream over North America, especially the West and especially in the winter half of the year.
- Status: The daily PNA index started the month positive, then quickly went neutral to slightly negative where it stayed for the rest of the month, averaging near zero (slightly positive) for the month as a whole. The 3-month-averaged index was near zero but slightly negative.
- Teleconnections (influence on weather): To the extent teleconnections are known, there are no teleconnections for a neutral PNA. A positive PNA for this time of year (October on the quarterly teleconnection maps) is associated with above-normal temperatures in Alaska and western Canada extending down the west coast of the CONUS; below-normal temperature anomalies in the southern Plains to Southeast; above-normal upper-level circulation anomalies across western North America; and below-normal upper-level circulation anomalies across the Southern Plains to Southeast and Gulf of Mexico. There are few precipitation teleconnections.
- Comparison to Observed: The September 2016 temperature anomaly pattern shows no correlation to that expected with a positive PNA, the upper-level circulation anomaly pattern is opposite, and the precipitation anomaly pattern has little to compare to (few correlations over the CONUS).
- The Arctic Oscillation (AO) pattern
- Description: The AO teleconnection pattern relates upper-level circulation over the Arctic to circulation features over the Northern Hemisphere mid-latitudes and is most active during the cold season.
- Status: The daily AO index oscillated above zero throughout the month, averaging positive for the month. The 3-month-averaged index was positive.
- Teleconnections (influence on weather): To the extent teleconnections are known, a positive AO in September (August-October in the AO Composites table) is typically associated with dry conditions across much of the CONUS from the Plains to Northeast; wet conditions in the Coastal Southeast and Coastal Northwest; above-normal temperatures in the Central and Northern Plains to Great Lakes; and upper-level circulation anomalies which are above normal across the Central Plains to Northeast CONUS and southeastern Canada, above normal over the western North Pacific, and below normal across the Arctic into Alaska and extreme northwest Canada.
- Comparison to Observed: The September 2016 upper-level circulation anomaly pattern shows good agreement with the teleconnections expected with a positive AO over North America and the polar sector, although above-normal upper-level heights over eastern North America are a bit more expansive and the below-normal heights over the North Polar region are somewhat contracted. The September 2016 monthly precipitation anomaly pattern is similar in the coastal Northwest and east of the Rockies (except in the Upper Midwest). The monthly temperature anomaly pattern shows some agreement in the Plains to Great Lakes, but little elsewhere.
- The North Atlantic Oscillation (NAO) pattern
- Description: The NAO teleconnection pattern relates upper-level circulation over the North Atlantic Ocean to circulation features over the Northern Hemisphere mid-latitudes.
- Status: The daily NAO index was positive for most of the month and negative for the last third of the month, averaging positive for the month. The 3-month-averaged index was negative.
- Teleconnections (influence on weather): To the extent teleconnections are known, a positive NAO during September (October on the quarterly teleconnection maps) is associated with above-normal upper-level circulation anomalies over the eastern half of the CONUS, extending across the North Atlantic, and below-normal anomalies over Greenland. The precipitation teleconnections are very weak, with wetter-than-normal anomalies over the Ohio Valley, and the there are no temperature teleconnections over the CONUS.
- Comparison to Observed: The September 2016 upper-level circulation anomaly pattern agrees with that expected with a positive NAO where teleconnections exist, they appear to be displaced and expanded. There are no teleconnections to compare against the temperature anomaly pattern, while the September 2016 precipitation anomaly pattern shows little agreement where teleconnections exist.
- The West Pacific (WP) pattern
- Description: The WP teleconnection pattern is a primary mode of low-frequency variability over the North Pacific and reflects zonal and meridional variations in the location and intensity of the (East Asian) jet stream in the western Pacific.
- Status: The monthly WP index was negative and the three-month average WP index was also negative.
- Teleconnections (influence on weather): To the extent teleconnections are known, a negative WP during September (October on the quarterly teleconnection maps) is typically associated with below-normal temperatures across the Central and Northern Plains to Upper Midwest; above-normal precipitation in the Pacific Northwest, Upper Midwest, and Deep South; below-normal circulation anomalies over central Canada to the north central CONUS and across the western North Pacific; and above-normal circulation anomalies over the eastern North Pacific off the U.S. coast.
- Comparison to Observed: The September 2016 monthly precipitation anomaly pattern is similar over the Upper Midwest, but opposite that expected with a negative WP elsewhere. The temperature anomaly pattern is opposite that expected with a negative WP where teleconnections exist. The upper-level circulation anomaly pattern shows little agreement.
- The Pacific Transition (PT) pattern
- Description: The PT teleconnection pattern relates upper-level circulation over the central subtropical North Pacific to circulation features over North America. It is a significant factor in the earth's circulation during August and September.
- Status: The PT index was positive during September.
- Teleconnections (influence on weather): To the extent teleconnections are known, a positive PT is typically associated with warmer-than-normal temperatures for the western CONUS and western Canada, colder-than-normal temperatures for the eastern CONUS, and drier-than-normal conditions in the Midwest, Alaska, and parts of the Pacific Northwest to Northern Plains. The upper-level circulation anomalies for a positive PT are below-normal 500-mb geopotential heights over the eastern third of the CONUS into the North Atlantic, below-normal heights over the northeastern Pacific (including the Gulf of Alaska), and above-normal heights over the western half of the CONUS into south central Canada.
- Observed: The September 2016 monthly temperature, precipitation, and upper-level circulation anomaly patterns are generally opposite those expected with a positive PT.
- The East Pacific-North Pacific (EP-NP) pattern
- Description: The EP-NP teleconnection pattern relates SST and upper-level circulation patterns (geopotential height anomalies) over the eastern and northern Pacific to temperature, precipitation, and circulation anomalies downstream over North America. Its influence during the winter is not as strong as during the other three seasons.
- Status: The North Pacific SSTs cooled north of 40 degrees North latitude and warmed between 30 and 40 degrees North, with little change elsewhere. The net result in the SST anomaly pattern, compared August, was a decrease in the temperature gradient. The SSTs were still warmer than normal across the far northern North Pacific, and from the Mexican coast to Indonesia, but the cooler-than-normal band between these two warmer-than-normal regions shrank considerably. Sea level pressure and upper-level height anomalies were above normal across most of the North Pacific, coincident with the above-normal SSTs, indicating a stronger-than-normal North Pacific High. The September 2016 monthly EP-NP index and 3-month running mean were both negative.
- Teleconnections (influence on weather): To the extent teleconnections are known, a negative EP-NP index during September (October on the quarterly teleconnection maps) is typically associated with warmer-than-normal temperatures across the CONUS from the Great Plains to East Coast; colder-than-normal temperatures along the West Coast and over Alaska; below-normal upper-level circulation anomalies (weaker upper-level ridge) over Alaska and western Canada; and above-normal upper-level circulation anomalies across eastern North America (weaker upper-level trough) and over the central North Pacific. The precipitation teleconnections are weak but show above-normal precipitation in the Pacific Northwest.
- Comparison to Observed: The September 2016 upper-level circulation and temperature anomaly patterns are a very good match to those expected with a negative EP-NP. The precipitation anomaly pattern has few teleconnections to compare to, but shows some agreement in the coastal Pacific Northwest.
Examination of the available circulation indices and their teleconnection patterns, and comparison to observed September 2016 temperature, precipitation, and circulation anomaly patterns, suggest that the weather over the CONUS in September was traced mostly to atmospheric drivers originating over the North Pacific and Arctic Oceans. The PNA index was near zero (neutral) and there is little agreement between the September 2016 anomaly patterns and those expected with the PT, NAO, and WP drivers. The tropical Pacific Ocean was in an ENSO-neutral state. Rossby and Kelvin waves, the IOD, tropical cyclones, and the monsoon trough in the western North Pacific may have masked the influence of the MJO. Yet, despite what the ENSO and MJO indices are indicating, the observed temperature and precipitation anomaly patterns show some similarities to those expected with a phase 4 MJO and with a La Niña. This may be a coincidence. There is some similarity in the circulation and precipitation anomaly patterns expected with a positive AO and those observed for September 2016, which suggests the Arctic Ocean drivers may have had some influence. But the greatest agreement was with the EP-NP pattern. The temperature and circulation patterns for a negative EP-NP index are an excellent match for those observed during this September. The North Pacific High was stronger than normal, and upper-level troughs and low pressure systems in the jet stream flow originating over the Pacific strongly influenced the weather over North America. The combination of the EP-NP pattern and AO pattern accounts for the shape of the September 2016 upper-level circulation anomaly pattern over the North American sector. Most of the indices have poor teleconnections for precipitation; this reflects the random nature of warm season convection.
This month illustrates how the weather and climate anomaly patterns can be strongly influenced by atmospheric drivers (or modes of atmospheric variability) originating in the North Pacific and Arctic Oceans, with the Bermuda and North Pacific subtropical highs playing an important role.