Synoptic Discussion - February 2014
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 long-wave circulation pattern for February 2014 over North America was a continuation of the overall pattern seen throughout the winter, but with some changes. It consisted of a dominant upper-level trough over eastern North America which extended into western Canada and the northwestern U.S., thus bisecting the upper-level ridge over the eastern North Pacific. Short-wave troughs undercut the western ridge, allowing Pacific weather systems to penetrate into the Pacific Northwest. This long-wave pattern reflects the influence of atmospheric modes of variability (East Pacific-North Pacific, West Pacific, and Tropical/Northern Hemisphere) associated with the North Pacific driver, whose teleconnections include colder-than-normal temperatures across much of the United States (especially east of the Rockies) and warmer-than-normal temperatures in parts of the West, although signs of the influence of other modes of circulation were evident in the precipitation anomaly pattern. The combination of cold and moist air masses produced frequent winter storm systems east of the Rockies, while the Pacific weather systems that broached the western ridge brought mostly rain in the warmer air over the West. The February precipitation was not enough to eliminate seasonal deficits, and the crucial mountain snowpack remained deficient in many areas, so drought conditions continued across much of the West, although the precipitation brought improvement to some areas. The western ridge continued in force over the Southwest into the Southern Plains, where the lack of precipitation saw expansion of drought. See below for details.
In the Northern Hemisphere, February is deep in the midst of climatological winter which is the time of year when the jet stream and circumpolar vortex are most active and furthest south, spreading polar and arctic air masses from the north across the United States. The upper-level circulation consisted of a long-wave ridge over the eastern North Pacific and western coast of North America, and a long-wave trough over eastern North America, with many wiggles, or short-wave troughs and ridges, migrating through the flow. The short-wave troughs were strong enough and frequent enough to undercut the long-wave western ridge, bringing beneficial precipitation to the Pacific Northwest. When integrated across the month, the circulation produced a pattern of below-normal 500-mb heights (stronger-than-normal long-wave trough) over eastern North America which extended well into the northern CONUS (contiguous U.S.) and western Canada. Above-normal 500-mb heights dominated Alaska and northwest Canada, as well as the southwestern CONUS extending into the Pacific Ocean, reflecting the bisected western long-wave ridge.
For February 2014, the jet stream was very active with several upper-level short-wave troughs bringing another month of cold and wet weather to parts of the country. Low pressure systems with strong cold fronts accompanied these short-wave troughs. A dry northwesterly flow across the Northern Plains funneled cold polar air masses into the country. These air masses flowed up to the Rocky Mountains and down to the Gulf of Mexico coast before moving into the Atlantic Ocean. This circulation pattern resulted in below-normal precipitation for much of the Great Plains, and a few large wildfires developed in the Southern Plains and Southeast near the end of the month. But low pressure systems along the southern edge of the frontal zone drew in moisture from the Gulf of Mexico to produce areas of precipitation east of the Rockies. Where the moisture was entrained into sub-freezing air, wintry precipitation occurred. Several snowstorms expanded snow cover across the Lower 48 States early in the month, but the polar front retreated slightly at mid-month, allowing warmer air to flow northward and melt some of the snow cover. The snow coverage peaked at 67.4 percent of the CONUS on February 7th, reached a low of 31.2 percent on the 19th, and ended the month at 36.9 percent, according to daily data from the National Weather Service's National Operational Hydrologic Remote Sensing Center. When integrated across the month, based on the 1967-2014 satellite snow cover record analyzed by the Rutgers University Global Snow Lab, February 2014 ranked as the ninth largest (most extensive) February snow cover for the CONUS and ninth largest for North America.
The short-wave trough on the 20th-21st triggered 40 tornadoes and other severe weather along its associated cold front and surface low in the Midwest and Southeast. The monthly total (preliminary) count of 41 tornadoes is slightly higher than the climatological average of 29 tornadoes for February.
With the undercutting of the western upper-level long-wave ridge, more Pacific weather systems were steered into the western CONUS and Canada and away from Alaska. Most of the weather systems made landfall in the Pacific Northwest and moved across the Northern Rockies before drying out over the Northern Plains. But with predominantly above-normal temperatures over the West beneath the long-wave ridge, the precipitation fell mainly as rain instead of much-needed snow, with snow cover, snow pack and snow water content still well below normal (except in the Northern to Central Rockies). In spite of the beneficial February precipitation, significant wet season deficits remained. The storm track stayed well north of the Southwest and Southern Plains, where drought continued to intensify. Monthly streamflow recovered in the Pacific Northwest but remained below normal in the Southwest and especially in California and the Southern Plains. The Climate Extremes Index (CEI) for the West ranked second biggest in the 1910-2014 record for February due to a combination of several factors — the long-term dryness (most extreme drought area component), short-term wetness (12th most extreme 1-day precipitation component), cold nighttime temperatures (second most extreme cold minimum temperature component), and cold daytime temperatures (11th most extreme cold maximum temperature component). Temperature and drought extremes were so persistent that the West region had the fourth most extreme year-to-date (January-February), most extreme winter (December-February), and fourth most extreme September-February (last six months) CEI.
The northern storm track brought above-normal precipitation to the Pacific Northwest and Northern Rockies and parts of the Upper Midwest to Northeast, while the southern storm track moistened the Gulf Coast and Tennessee Valley to Mid-Atlantic Coast. The Southwest and much of the Great Plains into the Mid-Mississippi Valley were drier than normal. Coastal Alaska was drier than normal with the eastern interior regions wetter than normal.
When integrated across the country, February 2014 ranked as the 65th driest (56th wettest) February in the 1895-2014 record. Three states in the Southwest and Southern Plains had the tenth driest, or drier, February, and an additional eight other states (in the Southwest, Plains, and Southeast) ranked in the driest third of the historical record. Twenty-one states ranked in the wettest third of the historical record, including Montana and Wyoming which were in the top ten wettest category. The precipitation reduced drought in the Midwest, Northwest, and Gulf Coast, but February's dryness expanded drought in the Southern Plains and Southwest. On balance, the national drought footprint shrank to 30.1 percent of the U.S. as a whole (according to U.S. Drought Monitor statistics).
The long-wave circulation pattern (ridge over eastern North Pacific and western North America, trough over eastern North America) set the stage for cold weather by establishing a long northwesterly flow in the upper atmosphere which extended into the high northern Arctic regions. Each short-wave trough in the northwesterly flow dragged cold fronts along with it. These fronts spread colder-than-normal air into the U.S. east of the Rockies, with many of the fronts reaching as far south as Florida and into Mexico. The cold air masses could not oppose the predominant flow and traverse the Rocky Mountain barrier into the West, where warmer-than-normal temperatures dominated. The circulation and temperature patterns varied throughout February, as seen in weekly temperature anomaly maps (weeks 1, 2, 3, 4). The ridge/trough pattern was strong at the beginning of the month, with below-normal temperatures across most of the country. At mid-month the circulation flattened, allowing above-normal temperatures to spread across much of the country. But the ridge/trough pattern strengthened again at the end of the month, with below-normal temperatures dominating in the east. When integrated across the month, February 2014 averaged colder than normal across the eastern half of the CONUS and the northern parts of the Pacific Northwest, and warmer than normal across the rest of the West and parts of the Southeast. Temperatures in Alaska averaged warmer than normal in the west and cooler than normal in the east.
When the temperatures are integrated across the CONUS, February 2014 ranked as the 37th coldest February. Twenty-two states, mostly east of the Rockies and west of the Appalachians, ranked in the coldest third of the historical record, with three of them ranking in the top ten coldest category. Only six states (in the Southwest plus Florida) had February temperatures in the warmest third of the historical record, with three in the top ten warmest category. Likewise, there were 1.75 times as many record cold daily highs (2205) and lows (1276, or a total of 3481) as record warm daily highs (945) and lows (1043, or a total of 1988). The dominance of colder-than-normal temperatures over the high-population centers of the central and eastern U.S. resulted in a national Residential Energy Demand Temperature Index (REDTI) for February 2014 that tied with 1948 as the 33rd largest February REDTI in the 1895-2014 record. The persistent cold during this winter resulted in 91 percent of the Great Lakes being frozen by the beginning of March, according to NOAA's Great Lakes Environmental Research Laboratory. This was the second largest ice cover for the Great Lakes in the 1973-2014 record.
The upper-level circulation over North America in February is an example of an interconnected and amplified meridional pattern — above-normal 500-mb heights over the north polar region shifted the circumpolar vortex (below-normal heights) southward over North America and the North Atlantic and into Western Europe. Since the atmospheric circulation is interconnected around the world, the meridional pattern can be seen in the global circulation anomalies — the regularly-spaced pattern of paired above-normal and below-normal height anomalies over North America, the North Atlantic/Western Europe, and Eurasia reflect an amplified meridional wave pattern at mid-latitudes around the world. This was manifested at the surface in the temperatures — over North America and central Asia, temperatures were colder than normal beneath the upper-level troughs and northwesterly flow aloft; and over Europe, western Russia, and eastern Siberia, temperatures were warmer than normal beneath the upper-level ridges and southwesterly flow aloft.
Subtropical highs, and cold fronts and low pressure systems moving in the 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 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: Ocean temperatures and atmospheric circulation anomalies indicated that the equatorial Pacific continued in an ENSO-neutral state during February.
- Teleconnections (influence on weather): To the extent teleconnections are known, while in a neutral state, ENSO normally is not a player in the month's weather. Historical data has been analyzed by NOAA to show typical temperature and precipitation patterns associated with El Niño and La Niña ENSO episodes. Teleconnections are not available for ENSO-neutral conditions at that NOAA web site.
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 was incoherent at the beginning of the month but strengthened during the last half of the month, transitioning to phases 6 through 8; this variability was in phase with slowly-evolving low frequency variations in tropical Pacific convection (as indicated by the MJO discussions for February 10, 17, and 24, and March 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 January-March teleconnections for precipitation are shown here and for temperature 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. The February 2014 monthly temperature and precipitation anomaly patterns do not match the teleconnections for MJO phases 6 through 8, but there is some correlation at weeks 3 and 4 for precipitation.
- 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 was negative for the first half of the month and mostly neutral (near zero) for the last half, averaging negative for the month as a whole.
- Teleconnections (influence on weather): To the extent teleconnections are known, the temperature teleconnection map for a negative PNA at this time of year (January on the maps) shows below-normal temperatures along the West Coast, Alaska, western Canada, and Northern Plains, and above-normal temperatures across the eastern third of the CONUS and in the Southern Plains. The precipitation teleconnection map for this time of year shows wetter-than-normal conditions over much of the eastern half of the CONUS and in the Pacific Northwest to Central Rockies. The upper-level circulation anomaly teleconnection for a negative PNA shows below-normal heights over the western half of North America and above-normal heights over the southeastern third of the CONUS. A positive PNA is associated with the opposite patterns.
- Comparison to Observed: The February 2014 temperature anomaly pattern does not agree with that expected with a negative PNA, while the precipitation anomaly pattern shows some resemblance, especially in the Pacific Northwest. The upper-level circulation anomaly pattern shows some consistency with that expected for a negative PNA in a few areas but, overall, does not match.
- 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 neutral for most of February, then turned negative at the end of the month, averaging near zero (neutral) for the month.
- Teleconnections (influence on weather): To the extent teleconnections are known, a neutral AO this time of year (January-March) is typically associated with wet conditions in the coastal Northwest and the Tennessee to Ohio valleys, dryness in the Central Plains and parts of the Gulf of Mexico Coast, above-normal temperatures from the southern Great Lakes to Southeast, and upper-level circulation anomalies which are below normal over western Canada but near normal across the rest of North America.
- Comparison to Observed: The February 2014 upper-level circulation anomaly pattern is similar to that expected with a neutral AO over western Canada, but does not agree over the rest of North America. The precipitation anomaly pattern agrees in the Northwest and Central Plains, but not further east. The monthly temperature anomaly pattern agrees in the extreme Southeast but not anywhere else.
- 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 throughout February, and averaged positive for the month.
- Teleconnections (influence on weather): To the extent teleconnections are known, a positive NAO during this time of year (January on the teleconnection maps) is associated with warmer-than-normal temperatures east of the Rockies (except the Central Plains), is weakly associated with drier-than-normal conditions in parts of the West, and is associated with positive upper-level circulation anomalies over most of the country (except the West Coast).
- Comparison to Observed: The February 2014 monthly temperature and precipitation anomaly patterns do not match those expected with a positive NAO. The upper-level circulation anomaly pattern matches over the North Atlantic Ocean, but not over North America.
- 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 for the month, with the three-month average WP index negative as well.
- Teleconnections (influence on weather): To the extent teleconnections are known, a negative WP during this time of year (January on the maps) is typically associated with above-normal temperatures in the Southwest, below-normal temperatures in the Midwest to Northeast, drier-than-normal weather from the Mid-Mississippi to Ohio valleys, above-normal circulation anomalies over the western CONUS, and below-normal circulation anomalies over eastern North America.
- Comparison to Observed: The February 2014 monthly temperature anomaly pattern is a very good match to that expected with a negative WP, while the precipitation anomaly pattern shows some agreement in the Mid-Mississippi Valley and the upper-level circulation anomaly pattern shows no agreement.
- 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 positive during February.
- Teleconnections (influence on weather): To the extent teleconnections are known, a positive TNH is typically associated with cooler-than-normal temperatures for much of the country, drier-than-normal conditions in the West, and slightly wetter-than-normal conditions in the Tennessee Valley. The upper-level circulation anomalies for a positive TNH are below-normal 500-mb geopotential heights over the eastern two-thirds of Canada and the north central U.S., and above-normal heights over the northeastern Pacific and Gulf of Mexico into the western North Atlantic.
- Observed: The February 2014 monthly temperature anomaly pattern is consistent with a positive TNH everywhere teleconnections exist, except in the Southwest. The upper-level circulation anomaly pattern matches that expected with a positive TNH except in the northeast North Pacific. The monthly precipitation anomaly pattern shows some agreement in the Tennessee Valley but is opposite to that expected in the West.
- The East Pacific-North Pacific (EP-NP) pattern
- Description: The EP-NP teleconnection pattern relates SST and upper-level circulation patterns 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 SST pattern over the northeastern North Pacific during February 2014 saw a continuation of the warmer-than-normal SSTs in the northeastern North Pacific, but the magnitude of the warmth decreased slightly. The monthly EP-NP index has been positive since October, pulling the 3-month running mean well into positive territory. The February value was also positive, but weaker than the previous months. The positive phase of the EP-NP pattern is associated with above-average surface temperatures over the northeastern Pacific (thus, the negative phase is associated with below-normal SSTs).
- Teleconnections (influence on weather): To the extent these teleconnections are known, a positive EP-NP index during this time of year (between October and April on the maps, but closer to April than October) is typically associated with cooler-than-normal temperatures across the U.S. east of the Rockies, warmer-than-normal temperatures along the immediate West Coast and in Alaska, wetter-than-normal conditions in the vicinity of Wyoming (although the precipitation teleconnections are weak), below-normal upper-level circulation anomalies (stronger upper-level trough) over the eastern half of North America (especially eastern Canada), and above-normal upper-level circulation anomalies (stronger upper-level ridge) over western Canada and Alaska.
- Comparison to Observed: The February 2014 temperature anomaly pattern is a very good match over the CONUS and western Alaska. The precipitation anomaly pattern is a reasonable match in Wyoming. The upper-level circulation anomaly pattern matches over eastern North America and Alaska, but not over western Canada and the northeast North Pacific.
Examination of these circulation indices and their teleconnection patterns, and comparison to observed February 2014 temperature, precipitation, and circulation patterns, suggest that the weather over the CONUS in February was most closely related to the jet stream and ocean-atmosphere interactions over the North Pacific Ocean. ENSO was neutral, and thus not a player. The MJO was incoherent during the first part of the month, but may have exerted some influence on precipitation during the last half of the month. The teleconnection patterns for the NAO do not match February's weather patterns. The PNA and AO indices show some correlation with precipitation in some parts of the country, but essentially no significant correlation for temperature or upper-level circulation. It is the North Pacific indices which have the best correlations — EP-NP, WP, and TNH with the February temperature anomaly pattern, and EP-NP and TNH with the upper-level circulation anomaly pattern. The lack of agreement in the northeast North Pacific probably reflects the undercutting of the western North America ridge by moist short-wave troughs, and the extension of below-normal upper-level circulation anomalies across southern Canada and the northern CONUS reflects the influence of the extended circumpolar vortex over North America. The EP-NP, WP, and TNH show some agreement with the precipitation anomaly pattern where teleconnections exist, but so do the MJO, PNA, and AO. This month illustrates how the anomaly patterns can be strongly represented by the indices (EP-NP, WP, TNH) measuring one atmospheric driver (the North Pacific Ocean) for circulation and temperature but also have elements of several other drivers (or modes of atmospheric variability) for precipitation.