Synoptic Discussion - January 2017
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 Earth's ocean-atmosphere system transitioned into an ENSO-neutral state during January 2017. The weather over the contiguous United States (CONUS) this month was dominated by strong troughs and lows moving in the fast jet stream flow. Cold fronts and surface lows dragged along by the upper-level troughs brought areas of precipitation to much of the country. Precipitation was above normal in most areas of the CONUS, improving drought conditions. Only a few parts of the country experienced a drier-than-normal month. The troughs tended to congregate in the western CONUS and spread their associated cold fronts east across the country. The result was a colder-than-normal month from the Pacific Northwest to Northern High Plains, with near to below-normal temperatures in the Southwest and warmer-than-normal temperatures from most of the Plains to the East Coast. The upper-level circulation, temperature, and precipitation anomaly patterns suggested the atmospheric drivers originating in the Pacific Ocean 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, January is the middle month of climatological winter which is the time of year when solar heating is at its minimum due to the low sun angle, and an expanded circumpolar vortex results in the furthest southern extent of the jet stream. Polar air masses dominate the weather over the contiguous U.S. (CONUS), and the warm, dry subtropical high pressure belts have their least influence.
A strong westerly flow dominated the upper-level circulation across the CONUS during January 2017. Troughs and cutoff lows moving in the jet stream flow dragged cold fronts and surface low pressure systems with them which brought areas of precipitation to much of the country. Only a few areas of the CONUS had a drier-than-normal month and included parts of the Pacific Northwest, Mid-Mississippi Valley, Rio Grande Valley, and East Coast. The rest of the country was wetter than normal with nine states in the West, Central Plains, Upper Midwest, and Southeast having the tenth wettest, or wetter January in the 1895-2017 record.
The precipitation improved drought conditions across much of the CONUS, decreasing the national moderate-to-exceptional drought footprint from 22.5 percent at the end of December to 14.6 percent at the end of January (from 18.8 percent to 12.2 percent for all of the U.S.). January began with several large wildfires burning in the Southern Plains, mostly in Oklahoma (wildfire maps for January 1, 10, 13, 26, 31). Widespread rains at mid-month helped reduce the number of fires (precipitation anomaly maps for weeks 1, 2, 3, 4).
The upper-level troughs and lows tended to favor the western CONUS. Their associated cold fronts and moist Pacific air masses kept temperatures cooler than normal for most of the month (temperature anomaly maps for weeks 1, 2, 3, 4). For the month as a whole, temperatures were cooler than normal across much of the West into the Northern Plains, but warmer than normal in the Southern Plains and from the Mississippi River to the East Coast where downstream ridging in the jet stream contributed to a southerly flow which brought in milder Gulf of Mexico air. Twenty states from the Southern Plains to Northeast had a top ten warmest January. The REDTI (Residential Energy Demand Temperature Index) for January 2017 ranked ninth lowest in the 123-year record for January. This was due to above-normal temperatures in the high-population centers of the South, Midwest, and East which reduced heating demand. There were 5,849 record warm daily high (2,299) and low (3,550) temperature records. This was about two and a half times the 2,318 record cold daily high (1,266) and low (1,052) temperature records that were associated with the air masses behind the cold fronts. The combination of warm and cold extremes resulted in the 18th warmest January, nationally, in the 1895-2017 record.
In spite of the downstream ridging in the jet stream, cold fronts and low pressure systems still moved across the eastern CONUS. Some of the fronts and surface lows were very strong and combined with upper-level lows to create unstable air and dynamics favorable for severe weather, especially on the 2nd and 20th-22nd. Based on preliminary data, there were 138 tornadoes in January 2017 (through January 27), which is four times the January average of 35. Most occurred in the Southern Plains to Southeast where Gulf of Mexico moisture was drawn in to the powerful upper-level troughs and surface fronts and low pressure systems.
Below-freezing air behind the cold fronts and surface lows caused precipitation to fall as snow instead of rain. The national snow cover area hovered around 40 to 45 percent of the CONUS for much of the month, with generally below-freezing air maintaining the snow cover in the West and North. An especially potent front and low pressure system spread snow across the Southern Plains and Southeast early in the month, with the CONUS snow cover area peaking at about 66 percent on the 8th. Based on the 1966-2017 satellite record, January 2017 had the 13th largest January snow cover area when averaged across the month and across North America.
The tropical Pacific ocean-atmosphere system transitioned to ENSO-neutral conditions during January, but La Niña-like tropical convection patterns lingered. La Niña typically suppresses hurricane activity in the central and eastern Pacific basins, and enhances it in the Atlantic basin. However, the tropical cyclone season in the North Atlantic and eastern tropical Pacific oceans normally ends in November. During January 2017, no tropical systems developed in these two basins. But a couple tropical disturbances developed in the western tropical Pacific which brought abundant rainfall to U.S.-Affiliated Pacific Islands. None of these developed into tropical storms or hurricanes (typhoons).
The Climate Extremes Index (CEI) aggregates temperature and precipitation extremes across space and time. Temperature and precipitation extremes occurred during the month in many areas, and several regions had component extremes in the top ten category, but none of the regions had a top ten integrated regional CEI. The Northeast region had the fourth most extreme warm minimum temperature component and tenth most extreme warm maximum temperature component. The East North Central region had the fifth most extreme wet spell component and sixth most extreme warm minimum temperature component. The Central region had the sixth most extreme warm minimum temperature component and ninth most extreme days with precipitation component. The Southeast region had the eighth most extreme warm minimum temperature component. The South region had the seventh most extreme warm minimum temperature component. The Southwest region had the eighth most extreme days with precipitation component. The West region had the fourth most extreme days with precipitation component and seventh most extreme one-day heavy precipitation component. When aggregated across the nation, January 2017 had the fourth most extreme warm minimum temperature component and tenth most extreme days with precipitation component, but only the 24th most extreme national January CEI in the 108-year record.
The upper-level circulation pattern, averaged for the month, consisted of below-normal upper-level (500-mb) height anomalies over the western CONUS, resulting from troughs frequently moving across the area, and above-normal height anomalies over the eastern CONUS. The below-normal heights reflected a weakening of the Aleutian Low and an increase in cyclonic activity over western North America. Of the circulation indices usually discussed on this page, the teleconnections aggregated for a negative PNA and positive WP most closely match the January 2017 500-mb circulation anomalies.
Precipitation was above normal across much of the CONUS. Pockets of below-normal precipitation could be found in the Northwest, mid-Mississippi Valley, Rio Grande Valley, and East Coast. Hawaii was drier than normal and a mixed anomaly pattern was seen over Alaska and Puerto Rico.
January 2017 was cooler than normal across much of the West into the Northern Plains, and warmer than normal in the Southern Plains and from the Mississippi River to the East Coast. Alaska was warmer than normal in the north and cooler than normal in the south central coastal region.
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. East-west trough/ridge couplets could be seen (trough or below-normal heights over the western CONUS, southern Europe, western Siberia, and western North Pacific coupled with ridge or above-normal heights over eastern North America/North Atlantic, eastern Asia, northern North Pacific) as well as north-south couplets (southern Europe/northern Europe). The below-normal heights over the Arctic could conceivably be linked to several of the above-normal/below-normal height pairs to the south of the Arctic. These couplets illustrate the wave nature of the atmosphere. The above-normal 500-mb heights were associated with upper-level ridging at the mid-latitudes, below-normal precipitation (in Europe), below-normal snow cover (over parts of eastern Asia), and above-normal surface temperatures over much of Canada, the eastern CONUS, northern Europe, and eastern Asia, and in parts of the North Atlantic. The areas of below-normal 500-mb heights were associated with upper-level troughing, near- to below-normal surface temperatures over northwest CONUS and southern Europe, above-normal precipitation over the southwestern CONUS, and above-normal snow cover over the southwestern CONUS and southern to eastern Europe. With the Americas and much of Africa and Asia having warmer-than-normal temperatures, and large portions of the Atlantic and Pacific Oceans having warmer-than-normal sea surface temperatures, the January 2017 global temperature was above normal, but the cooler-than-normal temperatures over parts of Europe and North Africa and parts of the oceans had a moderating effect.
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: ENSO-neutral conditions returned during January 2017 with slightly below-average SSTs in the central equatorial Pacific and above-average SSTs increasing in the eastern equatorial Pacific, but lingering La Niña-like tropical convection patterns were still evident.
- Teleconnections (influence on weather): The NWS CPC has no teleconnections for ENSO-neutral conditions. However, to the extent teleconnections are known, the typical temperature and precipitation patterns associated with La Niña during January include above-normal precipitation along the Pacific Northwest Coast, in the Northern Rockies, and Tennessee Valley to Great Lakes; below-normal precipitation from the Southwest to Southern Plains, and along the Gulf of Mexico coast and East Coast from Florida to New Jersey; above-normal temperatures across the Southern Plains to Southeast; and below-normal temperatures in the West and Northern Plains.
- Comparison to Observed: The January 2017 temperature anomaly pattern shows some agreement with that expected with La Niña, while the precipitation anomaly pattern generally does not agree.
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 was incoherent for the first half of the month, then transitioned through phases 1 to 3 during the second half. The background state (La Niña transitioning to ENSO-neutral) and a negatively-phased Indian Ocean Dipole (IOD) were more influential on the pattern of anomalous tropical convection for the first half of the month. Kelvin Wave and Rossby wave activity were also influential during the second half of the month (MJO updates for January 9, 16, 23, and 30).
- 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 January (December-February) 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 January 2017, the MJO index transitioned through phases 1-3 during the second half of the month. The monthly temperature anomaly pattern, and the weekly temperature anomaly pattern for week 4, both have a similar pattern to that expected with a phase 2-3 MJO, especially in the West and Southwest. The monthly precipitation anomaly pattern, and the weekly precipitation anomaly pattern for week 3, generally match the precipitation anomaly patterns expected with a phase 1-3 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 negative then transitioned to positive, averaging negative for the month as a whole. The 3-month-averaged index was slightly negative.
- Teleconnections (influence on weather): To the extent teleconnections are known, a negative PNA for this time of year (January on the quarterly teleconnection maps) is associated with above-normal temperatures from the Southern Plains to New England; below-normal temperature anomalies in Alaska, along the West Coast, and in the Northern Plains; above-normal precipitation from the Pacific Northwest to Central Rockies and across much of the CONUS east of the Mississippi; below-normal upper-level circulation anomalies across western North America; and above-normal upper-level circulation anomalies across the Southern Plains to East Coast and over the northern portions of the North Pacific.
- Comparison to Observed: The January 2017 temperature anomaly pattern is a good match for that expected with a negative PNA; the upper-level circulation anomaly pattern is a very good match; but the precipitation anomaly pattern agrees in some regions where teleconnections exist but not in others.
- 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 was positive throughout the month, averaging positive for the month. The 3-month-averaged index was also positive.
- Teleconnections (influence on weather): To the extent teleconnections are known, a positive AO in January (December-February in the AO Composites table) is typically associated with a few areas of dryness in the Southeast and Northeast; scattered wet conditions along the Pacific Northwest coast and in the Tennessee Valley; above-normal temperatures for most of the CONUS east of the Rockies; and upper-level circulation anomalies which are below normal across the Arctic into Alaska and northwest Canada, and above normal across the North Pacific and from the eastern CONUS to North Atlantic and Western Europe.
- Comparison to Observed: The January 2017 monthly temperature anomaly pattern is a very good match for that expected with a positive AO. The upper-level circulation anomaly pattern has some agreement over the North Pacific, but is a poor match over the CONUS, Arctic, and North Atlantic. The precipitation anomaly pattern has little agreement where teleconnections exist.
- 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 oscillated between positive and slightly negative to zero, averaging positive for the month. The 3-month-averaged index was slightly positive.
- Teleconnections (influence on weather): To the extent teleconnections are known, a positive NAO during this time of year (January on the quarterly teleconnection maps) is associated with above-normal upper-level circulation anomalies over all of the CONUS except the West Coast, and above-normal temperature anomalies across most of the CONUS east of the Rockies. The precipitation teleconnections are very weak, with only drier-than-normal anomalies from California to Nevada.
- Comparison to Observed: The January 2017 temperature anomaly pattern agrees with that expected with a positive NAO over the CONUS east of the Rockies. The upper-level circulation anomaly pattern agrees somewhat over the North Atlantic, but not over North America. The precipitation anomaly pattern disagrees in the West but there are very few teleconnections.
- 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 and three-month average WP index were both positive.
- Teleconnections (influence on weather): To the extent teleconnections are known, a positive WP during this time of year (January on the quarterly teleconnection maps) is typically associated with above-normal temperatures across the Upper Midwest to Northeast; above-normal precipitation in the Mid-Mississippi to Ohio Valleys; above-normal circulation anomalies over eastern North America; and below-normal circulation anomalies over the western CONUS.
- Comparison to Observed: The January 2017 monthly upper-level circulation and temperature anomaly patterns generally show good agreement over North America, but not over eastern Siberia for the upper-level circulation. The precipitation anomaly pattern shows little agreement but there are few teleconnections.
- The Tropical/Northern Hemisphere (TNH) pattern
- Description: The TNH teleconnection pattern reflects large-scale changes in both the location and eastward extent of the Pacific jet stream, and also in the strength and position of the climatological mean Hudson Bay Low, and is dominant in the winter months. The pattern significantly modulates the flow of marine air into North America, as well as the southward transport of cold Canadian air into the north-central United States.
- Status: The TNH index was slightly negative during January.
- Teleconnections (influence on weather): To the extent teleconnections are known, a negative TNH is typically associated with warmer-than-normal temperatures for much of the country, wetter-than-normal conditions in the West, and slightly drier-than-normal conditions in the Tennessee Valley. The upper-level circulation anomalies for a negative TNH are above-normal 500-mb geopotential heights over the eastern two-thirds of Canada and the north central U.S., and below-normal heights over the northeastern Pacific and Gulf of Mexico into the western North Atlantic.
- Observed: The January 2017 monthly upper-level circulation anomaly pattern has some similarities with that expected with a negative TNH, except they are shifted. The temperature anomaly pattern agrees over the eastern CONUS but is opposite over the West. The precipitation anomaly pattern shows some agreement, but there are few teleconnections.
- 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 had a patchwork pattern of warming and cooling compared to the previous month, with the net result being a continuation of an SST anomaly pattern consisting of cooler-than-normal SST anomalies between 40 and 50 degrees North latitude that stretched from Asia almost to the North American coast. SSTs were warmer than normal north and south of this cool area. The SST anomalies next to the North American coast were near normal, a considerable change from two months ago. Sea level pressure and upper-level height anomalies were above normal across the North Pacific from the Sea of Okhotsk to Gulf of Alaska, reflecting a weaker-than-normal Aleutian Low. Sea level pressure was lower than normal across much of the CONUS, and upper-level height anomalies were lower than normal over western North America and the adjacent North Pacific, reflecting the frequent passage over the CONUS of upper-level troughs and surface low pressure systems. The January 2017 monthly EP-NP index and 3-month running mean were both slightly positive.
- Teleconnections (influence on weather): To the extent teleconnections are known, a positive EP-NP index during January (October or April on the quarterly teleconnection maps since CPC does not provide a January map) is typically associated with cooler-than-normal temperatures across the CONUS from the Great Plains to East Coast; warmer-than-normal temperatures along the West Coast and over western Alaska; above-normal upper-level circulation anomalies (stronger upper-level ridge) over Alaska and western Canada; and below-normal upper-level circulation anomalies across eastern North America (stronger upper-level trough) and over the central North Pacific. The precipitation teleconnections are weak.
- Comparison to Observed: The January 2017 temperature and upper-level circulation anomaly patterns are opposite those expected with a positive EP-NP over the CONUS. The precipitation anomaly pattern has few teleconnections to compare to.
Examination of the available circulation indices and their teleconnection patterns, and comparison to observed January 2017 temperature, precipitation, and circulation anomaly patterns, suggest that the weather over the CONUS in January was traced mostly to atmospheric drivers originating over the North Pacific and equatorial Pacific Oceans, with possibly some influence from the Arctic. The precipitation teleconnections for all of the circulation indices were either weak or not strongly correlated with the observed precipitation anomaly pattern. However, there was some agreement between the observed pattern and that expected with this month's MJO phases and with the PNA. The temperature anomaly pattern appears to be an integration of the patterns associated with the PNA, WP, and AO drivers, with influence from the MJO some weeks, and possible some residual influence from La Niña. The NAO temperature anomaly pattern agrees east of the Rockies, but that may have been coincidence since the upper-level circulation pattern isn't a good match. The circulation anomaly pattern fits very well as an integration of the patterns associated with the PNA and WP drivers.
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 Pacific Ocean, with Arctic drivers possibly playing a role.