Synoptic Discussion - January 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.


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Synoptic Discussion

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

January is 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 January 2013, meandering in large loops of troughs and ridges as it blew across the country, bringing warm and cold spells to different areas during the month (weeks 1, 2, 3, 4, 5). Winter storms moving in the jet stream flow tapped Gulf of Mexico moisture to drop above-normal precipitation over the Lower to Mid-Mississippi and Ohio valleys and southern Appalachians (weeks 1, 2, 3, 4, 5), while winter storms in the western trough brought welcomed precipitation over parts of the West. But the weather systems largely avoided the heart of the Central Plains and Southeast drought areas. Snow cover extent expanded and contracted with each passing storm (from 67 percent of the country snowcovered on January 1st, to 40 percent on the 11th, 49 percent on the 16th, 34 percent on the 20th, and 45 percent on the 31st), but ended up near average when integrated across the country and across the month. A spring-like storm system near the end of the month generated a tornado outbreak in the South to Midwest that raised the total preliminary tornado count to 64 tornadoes, which is above the long-term average for January. This is usually a quiet time of year for wildfires, but even so, 2013 had the second lowest number of wildfires for January in the 14-year record.

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). Beneficial rain and snow helped improve drought conditions over much of the country, with contraction of the drought area occurring in the Midwest, Southeast, South, West, and Hawaii. However, storm systems largely missed the High Plains, where the worst drought categories (D3-D4, extreme to exceptional drought) expanded from 60.3 percent of the region at the end of December to 61.3 percent by the end of this month. Five states (Michigan and Virginia to Louisiana) had the tenth wettest, or wetter, January in the 1895-2013 record, while three states (California, Florida, and Connecticut) ranked in the top ten driest category. Overall, the precipitation helped reduce the national moderate to exceptional drought footprint from 61.1 percent at the end of December to 57.7 percent at the end of January (based on U.S. Drought Monitor statistics). According to the Palmer Drought Index, which goes back to the beginning of the 20th century, 45.6 percent of the contiguous U.S. was in moderate to extreme drought at the end of January, 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 weather patternmigrating nature of the weather systems, with warm air ahead of the fronts and colder air behind them, as they cycled west to east. Averaged across the month, temperatures were below normal in the West and above normal in the East, with two states (Nevada and Utah) ranking in the top ten coldest category for January and two southeastern states (Florida and Georgia) ranking as eleventh warmest. On a local basis, two and a half times as many record warm highs and lows occurred than record cold highs and lows. Over 2350 daily high temperature records and 2900 record warm daily low temperatures were tied or broken. In comparison, about 700 record low temperatures and 1300 record cool daily high temperatures were tied or broken. (These numbers are preliminary and are expected to increase as more data arrive.) The warmth in the highly-populated eastern U.S. contributed to a national Residential Energy Demand Temperature Index (REDTI) for January 2013 that was below normal.

When averaged together, the mixture of temperature and precipitation extremes gave the U.S. the 39th warmest and 38th wettest January 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 January 2013, the nation had a large spatial extent of drought conditions (sixth largest PDSI component for January), but mediocre extent of heavy daily precipitation (33rd largest extremes in 1-day precipitation component), and extreme minimum (35th largest cold minimum component and 54th largest warm minimum component) and extreme maximum (43rd smallest warm maximum component and 47th largest cold maximum component) temperatures, compared to the 104-year history. This combined to give the U.S. a January USCEI that was only 62nd largest (43th smallest). But the mediocre performance of January 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 (February 2012-January 2013) still ranking as the largest on record for February-January.

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 January 2013 and November 2012-January 2013 temperature, precipitation, and circulation patterns, suggests that no single atmospheric driver dominated the weather during January, but the weather was influenced in part by several of the drivers. ENSO was neutral and, thus, not a player. The PNA, AO, and NAO oscillated near or around neutral, and the EP-NP was near neutral, thus making their signals weak. Nevertheless, there is some agreement between the January upper-level circulation anomalies and the teleconnections for the AO and NAO, but the TNH circulation teleconnections were the closest match. Precipitation relationships are more susceptible to variability than temperature or circulation, but there is some match with the PNA and TNH. The temperature anomaly pattern matched the NAO in the east and TNH in the west. To summarize: it seems the TNH dominated the overall circulation for the month, but it may have shared influence with NAO for temperature and with PNA for precipitation, and possibly with the MJO for both. 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 Climatic Data Center, State of the Climate: Synoptic Discussion for January 2013, published online February 2013, retrieved on September 15, 2014 from http://www.ncdc.noaa.gov/sotc/synoptic/2013/1.