Synoptic Discussion - December 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.


NCDC transitioned to the nClimDiv dataset on Thursday, March 13, 2014. This was coincident with the release of the February 2014 monthly monitoring report. For details on this transition, please visit our public FTP site and our U.S. Climate Divisional Database site.


Summary

The December 2013 circulation pattern over North America consisted of a dominant upper-level ridge over the eastern North Pacific and western North America with a trough over eastern North America. This pattern is most closely associated with the positive phase of the Tropical/Northern Hemisphere mode of circulation, whose teleconnections include colder-than-normal temperatures across much of the contiguous United States (CONUS), drier-than-normal weather in the West, and wetter-than-normal weather in the Ohio to Tennessee valleys. The combination of cold and moist air masses produced frequent winter storm systems east of the Rockies, while precipitation and snow pack in the West lagged behind seasonal normals. See below for details.


Synoptic Discussion

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

In the Northern Hemisphere, December marks the start 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. When integrated across the month, the circulation produced a pattern of above-normal 500-mb heights (stronger-than-normal long-wave ridge) over the Gulf of Alaska in the northeast Pacific and along the west coast of North America, with above-normal heights also over the North Atlantic spreading into the southeast CONUS. Below-normal 500-mb heights (stronger-than-normal long-wave trough) dominated eastern and central Canada and into the Great Lakes and Northern Plains of the CONUS.

For December 2013, the jet stream was very active with several upper-level short-wave troughs and closed lows bringing cold and wet weather to parts of the country. These systems drew in moisture from the Gulf of Mexico to produce tracks of precipitation as they moved east of the Rockies. Where the moisture was entrained into sub-freezing air, wintry precipitation occurred. Consequently, several significant snowstorms expanded snow cover across the Lower 48 States:

The short-wave trough responsible for the snowstorm of the 23rd was especially violent, triggering 14 tornadoes along its associated cold front on the 20th and 21st. This brought the monthly total (preliminary) tornado count to 14, which compares to the climatological average of 24 for December. Integrated over the month, snow cover was above normal across much of the U.S., with December 2013 snow cover area ranking eighth largest (in the 1966-2013 satellite record for December) for the CONUS and seventh largest for North America.

The upper-level long-wave ridge over the eastern North Pacific created a northwesterly flow in the circulation which inhibited the formation of massive winter storms over the West. As a result, both total precipitation and the mountain snowpack over the western U.S. were well below normal, both for the month and the water year to date (which began October 1), and streamflow was well below normal, especially along the West Coast. The Climate Extremes Index (CEI) for the West ranked second biggest in the 1910-2013 record for December due to the dryness (second most extreme drought area component, fifth most extreme days without precipitation component) and cold temperatures (fourth most extreme cold minimum temperature component).

Map of monthly precipitation anomalies
Map of monthly precipitation anomalies.

The storm track resulted in three streaks of above-normal precipitation: in the north central States, the Ohio Valley to Great Lakes, and the Southeast to Mid-Atlantic Coast. Precipitation largely missed the West and Central Plains, as well as parts of the Southern Plains, Midwest, and Florida. Although weather systems moved across the West before intensifying east of the Rockies, the precipitation they dropped totaled well below normal for the month. South central Alaska was drier than normal, but the western, northern, and eastern sections were generally wetter than normal.

When integrated across the country, December ranked near the middle of the historical record at 54th driest. But the monthly dryness along the West Coast and in the Central Plains gave nine states a December precipitation rank in their driest third of the historical record, with three of those (California at second driest, Oregon at third driest, and Washington at sixth driest) ranking in the top ten driest category. Six states ranked in the top ten wettest category, with 17 others in the wettest third of the historical record. The precipitation east of the Rockies helped reduce drought in the East and parts of the Midwest and Southern Plains, but December's dryness expanded drought in the West and Central Plains. On balance, the national drought footprint expanded slightly to 26.0 percent of the U.S. as a whole (according to U.S. Drought Monitor statistics).

Map of monthly temperature anomalies
Map of monthly temperature anomalies.

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 them. 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. Some of the troughs and fronts moved across the western U.S., bringing below-normal temperatures west of the Rockies, most notably during the first half of the month (weeks 1, 2, 3, 4). Warmer, southerly air masses, associated with upper-level ridges, followed the troughs and cold fronts, especially in the Southeast. When integrated across the month, December 2013 averaged colder than normal across the Plains and Northeast and much of the West, but warmer than normal for the Southeast. Temperatures in Alaska averaged colder than normal in the central to southeastern areas and warmer than normal in the north and west.

When viewed in terms of the patterns across the Northern Hemisphere, the December 2013 temperature and circulation patterns over North America represent a simple shift of the circumpolar vortex from the polar region to North America and the North Atlantic. The upper-level circulation anomalies over the North Pole, Europe, and much of Asia were above normal, as opposed to below normal over North America. This was manifested at the surface in the temperatures — while much of North America was colder than normal, much of Eurasia was warmer than normal.

When integrated across the CONUS, December 2013 ranked as the 21st coldest December in the 1895-2013 record. Twenty-three states, mostly along and west of the Mississippi River and Great Lakes, ranked in the coldest third of the historical record, with two of them (Minnesota at eighth coldest and North Dakota at ninth coldest) ranking in the top ten coldest category. Eight states (in the Mid-Atlantic to Southeast) had December temperatures in the warmest third of the historical record, with Florida ranking sixth warmest. There were about one and a half as many record cold daily highs (1774) and lows (1197, or a total of 2971) as record warm daily highs (1085) and lows (935, or a total of 2020). In spite of the widespread cold temperatures (the area-averaged temperature rank for the U.S. was 21st coldest), most of the high-population centers of the East Coast had near- to above-average temperatures, so the national Residential Energy Demand Temperature Index (REDTI) for December 2013 was near the long-term average (only 42nd largest).


Atmospheric Drivers

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 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:


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 December 2013 temperature, precipitation, and circulation patterns, suggest that the circulation patterns associated with the TNH atmospheric driver had the greatest influence on December weather, but other drivers shared influence. ENSO was neutral, and thus not a player. The MJO was incoherent or weak for most of the month, but could have exerted some influence on temperature during the last two weeks and precipitation in the East during the third week of the month. The circulation and temperature teleconnections of the PNA and NAO could have some association with the warm anomalies in the Southeast; the PNA and AO have some association with wetness in the East; and the NAO has some association with dryness in the West. The upper-level circulation anomalies for December 2013 have some resemblence to the teleconnections associated with the AO. But the TNH had the best match for the December 2013 temperature, precipitation, and upper-level circulation anomaly patterns. This month illustrates how the anomaly patterns can be strongly represented by one atmospheric driver (TNH in this case) but also have elements of several other drivers (or modes of atmospheric variability).

Citing This Report

NOAA National Climatic Data Center, State of the Climate: Synoptic Discussion for December 2013, published online January 2014, retrieved on November 22, 2014 from http://www.ncdc.noaa.gov/sotc/synoptic/2013/12.