Synoptic Discussion - November 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

In the Northern Hemisphere, November marks the end of climatological fall (or autumn) which is the time of year when the active jet stream and circumpolar vortex expand south, spreading polar and arctic air masses from the north across the United States. For November 2013, the jet stream was very active with several upper-level troughs and closed lows bringing cool and wet weather to parts of the country. These systems drew in moisture from the Pacific which mixed with cold Canadian air to produce rain and snow in the West, and they entrained moisture from the Gulf of Mexico to produce widespread rain and snow as they moved east of the Rockies. Consequently, several significant winter snowstorms expanded snow cover across the Lower 48 States; however, warm southerly air associated with upper-level ridges behind the troughs quickly melted the snow. Four systems in particular expanded the snow cover — to 18 percent of the country on the 6th, 19 percent on the 12th, 13 percent on the 17th, and 38 percent on the 24th. The storm system on the 24th was a slow-moving, cut-off low which dropped snow from the Southwest to Central Plains and Midwest by the 24th, then across the Ohio Valley to the Northeast by the 26th. Integrated over the month, snow cover was above normal across parts of the South and some northern tier states and below normal across parts of the West and Midwest, but November 2013 snow cover area still ranked 12th largest (in the 1966-2013 satellite record for November) for the continental U.S. (CONUS) and third largest for North America. The trough and associated cold front on the 17th was especially violent, spawning over 100 tornadoes in the Ohio Valley to southern Great Lakes. The preliminary count of 111 tornadoes for November 2013 is nearly double the climatological average count of 58 for November.

Map of monthly precipitation anomalies
Map of monthly precipitation anomalies.

The storm track resulted in above-normal precipitation in the Southwest, but only patchy areas of above-normal precipitation east of the Rockies. Precipitation largely missed the Northern Plains, and the Deep South to Northeast was drier than normal for most of the month (weeks 1, 2, 3, 4). The slow-moving system, which moved out of the Southwest and swept across the South and up the East Coast during the last week of the month, produced enough precipitation to transform what could have been a very dry month (pre-storm Palmer Z Index and precipitation departures for the Southeast, Northeast, and U.S.) into a near-normal (or not-so-dry) month for much of the Deep South and Northeast, thus illustrating the importance of one wet weather system to the total precipitation for a month. But the rain from this system wasn't enough to extinguish several large wildfires which continued to burn along the Appalachian spine at the end of the month. Although weather systems moved across the West Coast and Northwest and into the Northern Rockies, the precipitation they dropped totaled well below normal for the month. Much of Alaska was wetter than normal, except for the southern stations which had a mixed precipitation anomaly pattern.

When integrated across the country, November ranked near the middle of the historical record at 51st driest. But the monthly dryness along the West Coast to Northern Plains gave eight states a November precipitation rank in their driest third of the historical record, with Wyoming ranking eleventh driest. Michigan ranked in the top ten wettest category, with nine others (most in the Southwest or Southeast) in the wettest third, most of which would have been dry if not for the end-of-month storm system. Not only did that weather system save many southern and eastern states from a dry November, it also helped reduce drought or abnormally dry conditions for the country. Drought contracted in all regions except the Northeast, where it expanded, and the Southeast, where it was introduced this month. The national drought footprint shrank to 25.7 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.

As the troughs in the upper-level circulation moved across the country, they 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. This was especially the case during the last week of the month (weeks 1, 2, 3, 4). Warm, southerly air masses, associated with upper-level ridges, followed the troughs and cold fronts. When integrated across the month, November 2013 averaged colder than normal from the Plains to the East Coast, warmer than normal for much of Florida and parts of the West, and had a mixed temperature anomaly pattern in Alaska.

When integrated across the country, November 2013 ranked as the 49th coldest November in the 1895-2013 record. Twenty-seven states, all east of the Rockies, ranked in the coldest third of the historical record, while four states (Florida, California, Nevada, and Arizona) had November temperatures in the warmest third of the historical record. There were nearly three times as many record cold daily highs (1539) and lows (699, or a total of 2238) as record warm daily highs (317) and lows (432, or a total of 749).

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

The upper-level circulation was generally westerly with many wiggles, or troughs and ridges, and an occasional cutoff low, 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, along the west coast of North America, and into the CONUS as far as the Central Plains. Below-normal 500-mb heights (stronger-than-normal long-wave trough) dominated the eastern half of Canada and into the Great Lakes and Northeast of the CONUS. The slow-moving cutoff low near the end of the month appears as a trough with below-normal heights off the coast of Baja California.

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:


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 November 2013 temperature, precipitation, and circulation patterns, suggest that the North Pacific atmospheric driver had the greatest influence on November weather. ENSO was neutral, and thus not a player; the MJO was incoherent for much of the month; the WP was neutral; and the AO, NAO, and PNA did not seem to be influential. The EP-NP North Pacific driver appeared to have a controlling influence on the circulation and temperature patterns in the CONUS. Precipitation is not as strongly correlated to any of the atmospheric drivers during the fall as it is during the winter, with random convective and synoptic processes playing a greater role in precipitation during November, but the EP-NP appeared to be influential in the dry pattern in the Pacific Northwest, and the MJO may have exerted influence in the Ohio Valley to Great Lakes at the very beginning of the month. This month illustrates how an atmospheric driver may influence some aspects of the weather while other aspects (such as precipitation) may be influenced by a combination of drivers and random atmospheric processes.

Citing This Report

NOAA National Climatic Data Center, State of the Climate: Synoptic Discussion for November 2013, published online December 2013, retrieved on September 1, 2014 from http://www.ncdc.noaa.gov/sotc/synoptic/2013/11.