Synoptic Discussion - February 2012
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.
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Like much of this winter, the weather pattern over North America in February 2012 consisted of an active jet stream along the U.S.-Canadian border. Strong low pressure troughs frequently dug into the western U.S., brushing northern Mexico before moving to the Great Lakes or Atlantic coast and dragging cold fronts across the southeastern United States. Winter storms with some of these low pressure systems spread blankets of snow across parts of the country, with snow coverage reaching nearly 48 percent on February 14th. The snow would stay around for a few days under cold air behind the lows, but the snow cover melted quickly as warm southerly air blew into the snow area ahead of the next system. Northern states typically had below-normal snow amounts, with the monthly snow cover for the U.S. and North America ranking below normal for a third month in a row. Severe weather accompanied some of these systems from the Great Plains to the Midwest and Southeast, with dozens of tornadoes triggered near the end of the month.
The frequent development of cold upper-level systems over the West, and the lack of frequent and persistent cold outbreaks in the East, resulted in widespread above-normal temperatures across much of the country east of the Rockies. Twelve states from the Upper Mississippi Valley to Northeast ranked in the top ten warmest category for February 2012, including Massachusetts which had the warmest February in the 118-year record. A similar pattern for winter (December-February) had even more states in the top ten warmest category — 27 states from North Dakota to Missouri and Maine to North Carolina. This temperature pattern is consistent with a positive Arctic Oscillation. The storm track for February brought above-normal precipitation to the middle third of the country and below normal precipitation to the western and eastern thirds. Four states (South Dakota, Nebraska, Kansas, and Louisiana) ranked in the top ten wettest category for February, with the precipitation helping to reduce the drought in the Southern Plains. Seven states in the Northeast had the tenth driest, or drier, February. In spite of several weather systems, the West had another dry month, with four states (Montana, Oregon, Idaho, and California) having the tenth driest, or drier, winter (December-February). Moderate to exceptional drought expanded to cover 45 percent of the West by February 28th. Moderate to exceptional drought increased to affect about 39 percent of the contiguous U.S. by month's end, but the area affected by the worst drought categories (extreme and exceptional drought) shrank slightly.
The northern storm track and relative lack of frequent very cold outbreaks resulted in far more daily warm record temperatures than daily cold record temperatures. There were over 800 daily high temperature records and over 1200 record warm daily low temperatures set during February 2012 compared to about 200 daily record cold temperatures and about the same number of record cold daily high temperatures. When averaged together, the mixture of temperature and precipitation extremes gave the U.S. the 17th warmest and 40th driest February in the 118-year record. Averaging extremes tends to cancel them out. But when extremes are combined cumulatively, like in the U.S. Climate Extremes Index (CEI), they may tell a different story. The preponderance of record warm highs and record warm lows gave the U.S. the 30th largest warm maximum temperature and 21st largest warm minimum temperature components in the 103-year CEI record. February 2012 ties with February 1951 as the 18th largest for the combined very wet and very dry extremes component. But when all of the components are integrated together, the February 2012 national CEI ranks below average at 53rd largest. Regionally, however, the February CEI ranked in the top ten category for the Northeast, and for the winter (December-February) the CEI ranked as the ninth largest nationally, third largest for the Northeast and Central regions, and tenth largest for the East North Central and South regions.
Cold fronts and low pressure systems moving in the storm track flow are influenced by the broadscale atmospheric circulation. Four such large-scale atmospheric circulation drivers were potentially influential during February:
- Ocean temperatures and atmospheric circulation anomalies indicated that the equatorial Pacific continued in a La Niña state, but the La Niña was weakening. La Niña this time of year (December-February) is associated with temperature and precipitation anomalies across the U.S. — temperatures are typically warmer than normal from the Southern Plains to Mid-Atlantic Coast and colder than normal in the Northern Plains and parts of the West, while drier-than-normal conditions dominate the southern tier and East Coast States with wet conditions limited to the Pacific Northwest and Tennessee Valley.
- The Pacific/North American (PNA) pattern was positive for most of the month, then turned negative at the end of the month. A positive PNA this time of year (January on the teleconnection maps) typically is associated with cooler-than-normal temperatures from the Southern Plains to Northeast and warmer-than-normal temperatures in the Northwest, with drier-than-normal conditions in the Northwest to Central Rockies and across much of the country from the Mississippi River to the East Coast.
- The Arctic Oscillation (AO) pattern was negative during the first part of February but turned positive during the last two-thirds of the month. A positive AO this time of year (December-February) is typically associated with warmer-than-normal temperatures east of the Rockies, dryness in the eastern Gulf Coast states and Northeast, and wetter-than-normal conditions in the coastal Northwest and Tennessee Valley. A negative AO this time of year is typically associated with the opposite conditions — colder-than-normal weather east of the Rockies (except the Southern Plains) and drier-than-normal weather in parts of the Southwest and from the Southern Plains to Ohio Valley. The AO relationship to monthly average weather can be difficult to assess during a month when the AO flips from one sign to the other. The December-February averaged upper-level circulation anomalies for a positive AO consist of a below normal pattern over the Arctic, including much of Alaska, and above normal pattern across the eastern half of the contiguous United States.
- The North Atlantic Oscillation (NAO) pattern was neutral to positive throughout the month. A positive NAO during this time of year (January on the teleconnection maps) is typically associated with drier-than-normal conditions in the West while temperatures trend warmer than normal from the Plains to East Coast.
The February 2012 and winter averaged upper-level circulation anomalies are consistent with a positive AO pattern across the Lower 48 States. Over Canada and the Arctic, the winter averaged upper-level circulation pattern is consistent with a positive AO pattern but the February 2012 upper-air anomalies were positive, reflecting hints of a negative AO pattern. The pattern of observed temperature anomalies for February 2012 and the last three months (December-February) corresponds to the positive phase of the AO and NAO east of the Rockies and the La Niña pattern from the Southern Plains to the Southeast. The February 2012 and December-February precipitation patterns are a bit more complicated. The dryness in the West could be associated with the positive NAO and PNA patterns and, in the Southwest, also with the La Niña pattern. The drier-than-normal conditions in the Southeast to Northeast are a reasonable match for the La Niña and positive phase of the AO and, in the Northeast, also the positive phase of the PNA. February dryness in the Ohio and Tennessee Valleys matches the positive PNA correlations. However, wetness in the Southern Plains matches El Niño teleconnections, not La Niña teleconnections.