Synoptic Discussion - February 2013

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.

Synoptic Discussion

Monthly upper-level circulation pattern and anomalies
Monthly upper-level circulation pattern and anomalies.

February is in the heart of the winter season when the cold polar air masses of the circumpolar vortex have the greatest likelihood of expanding south across the United States. The polar jet stream (which marks the edge of the circumpolar vortex and the boundary between the cold polar air masses to the north and the warmer sub-tropical air masses to the south) was weather patternvery active during February 2013, with a series of strong upper-level weather systems propagating across the country. These generated several winter storms which dumped heavy snow on coastal New England early in the month (raising national snow coverage to about 45 percent by the 11th) and on the Plains states later in the month (raising national snow coverage to about 58 percent by the 22nd). The South was also affected by these winter storms and frontal systems, with heavy rain easing drought conditions in the Southeast (precipitation anomaly maps for weeks 1, 2, 3, 4, 5) and severe weather affecting the Gulf Coast states, causing the preliminary tornado count to rise above normal for February. In addition to bringing precipitation of the liquid and frozen variety, these upper-level weather systems funneled cold Canadian air into the Lower 48 States, especially during the last half of the month (weeks 1, 2, 3, 4, 5). The weather systems east of the Rockies tapped Gulf of Mexico moisture to fuel their precipitation engines, but those moving over the West were moisture-starved, resulting in a generally drier-than-normal month from the Rockies westward.

Monthly precipitation anomalies
Monthly precipitation anomalies.

The movement of the weather systems can be seen in the weekly precipitation anomaly patterns (weeks 1, 2, 3, 4, 5). Hundreds of monthly records were set by daily precipitation (255 preliminary reports) and daily snowfall (172 preliminary reports) reports this month, mostly along the paths of the Southeast rain system and Plains to Midwest snow systems, and (for snow) in the Northeast. The beneficial rain improved drought conditions in the Southeast, where moderate to exceptional drought (D1-D4) contracted from about 43 percent at the end of last month to 27 percent this month and the worst categories (extreme to exceptional drought, D3-D4) disappeared. At least some improvement occurred in most areas — even in the Great Plains, where the February snowstorms nibbled at the edges of the D3-D4 drought area in spite of massive twelve-month precipitation deficits. Five states (Georgia, Alabama, South Carolina, Massachusetts, and Rhode Island) had their tenth wettest, or wetter, February in the 1895-2013 record, with Georgia having its wettest February on record. Two states in the West (California and Oregon) ranked in their top ten driest category, with Montana not far behind at twelfth driest. Overall, the precipitation helped reduce the national (contiguous U.S.) moderate to exceptional drought footprint from 57.7 percent at the end of January to 54.2 percent at the end of February (based on U.S. Drought Monitor statistics). According to the Palmer Drought Index, which goes back to the beginning of the 20th century, 39.6 percent of the contiguous U.S. was in moderate to extreme drought at the end of February, a decrease of about 6 percent compared to last month.

Monthly temperature anomalies
Monthly temperature anomalies.

The weekly temperature anomaly maps (weeks 1, 2, 3, 4, 5) show the transition from near- to above-normal temperatures at the beginning of the month to the dominance of colder-than-normal weather during the last half of the month as polar air masses swept into the country. Averaged across the month, temperatures were near to below normal for much of the country, with pockets of above-normal temperatures in the Northern Rockies and adjacent High Plains, New England, and coastal Deep South. Five Southwest states ranked in the cool third of the historical record for February while twelve ranked in the warm third. On a local basis, one and a half times as many record cold highs and lows occurred than record warm highs and lows. About 400 record low temperatures and 500 record cool daily high temperatures were tied or broken. In comparison, over 200 daily high temperature records and about 350 record warm daily low temperatures were tied or broken. (These numbers are preliminary and are expected to increase as more data arrive.) The lack of persistent and widespread extreme monthly temperature anomalies contributed to a national Residential Energy Demand Temperature Index (REDTI) for February 2013 that was near the long-term average.

When averaged together, the mixture of temperature and precipitation extremes gave the U.S. the 49th warmest and 58th driest February in the 119-year record. Averaging extremes tends to cancel them out. But when extremes are combined cumulatively, like in the U.S. Climate Extremes Index (USCEI), they may tell a different story. For February 2013, in spite of a large spatial extent of long-term drought conditions (seventh largest PDSI component for February) and wet days (13th largest "days with precipitation" component), the national USCEI ranked only 32nd smallest (73rd largest) out of 104 years of record. But the mediocre performance of the first two months of 2013 (46th smallest January-February) was not enough to counter the preponderance of unusual warmth and dryness for much of 2012, with the national USCEI for the last twelve months (March 2012-February 2013) still ranking as the largest on record for March-February.

Subtropical highs, and cold fronts and low pressure systems moving in the storm track flow, are influenced by the broadscale atmospheric circulation. The following describes several such large-scale atmospheric circulation drivers and their potential influence this month:

Map of three-month temperature anomalies
Map of three-month temperature anomalies.
Map of three-month precipitation anomalies
Map of three-month precipitation anomalies.

Upper-level circulation pattern and anomalies averaged for the last three months
Upper-level circulation pattern and anomalies averaged for the last three months.

Examination of these circulation indices and their teleconnection patterns, and comparison to observed February 2013 and December 2012-February 2013 temperature, precipitation, and circulation patterns, suggests that no single atmospheric driver dominated the weather during February, but the weather was influenced in part by several of the drivers. ENSO was neutral and, thus, not a player. The influence of the EP-NP driver is weak during the winter. The NAO transitioned from positive to negative, which could have masked its influence. In spite of its transition, the upper-level circulation pattern seemed to match the NAO teleconnection closest but was modified by the AO and TNH. The NAO, AO, and TNH drivers apparently conspired to dominate the temperature pattern. The TNH and PNA drivers appeared to account for the dryness in the West. The AO and PNA drivers are normally associated with below-normal precipitation over the eastern half of the U.S., but weather systems, likely driven by the MJO, brought a mixed pattern of above-normal precipitation east of the Rockies. This month illustrates how competing atmospheric drivers can result in a complex weather pattern and how, when the atmospheric circulation drivers are neutral or in a state of transition, their influence can become difficult to trace and can be overwhelmed by other competing forces, including random fluctuations in the atmosphere.

Citing This Report

NOAA National Centers for Environmental Information, State of the Climate: Synoptic Discussion for February 2013, published online March 2013, retrieved on January 19, 2018 from