Synoptic Discussion - September 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.

In the Northern Hemisphere, September marks the beginning of climatological fall which is the time of year when the jet stream becomes more active and spreads cooler air masses from the north across the United States. For September 2013, the subtropical high pressure belt, also called the Bermuda High (or Azores High), dominated the weather during the first half of the month, with warmer-than-average temperatures holding sway across much of the country during the first two weeks (week 1, 2). The Bermuda High retreated during the last half of the month, with a series of upper-level weather systems in the jet stream flow bringing below-normal temperatures to the eastern and western sections of the country. Warmth dominated for the month as a whole, with September 2013 ranking as the sixth warmest September, nationally, in the 1895-2013 record. Seven states, mostly in the Northern Rockies and Northern Plains, ranked in the top ten warmest category for September. There were nearly six times as many record warm daily highs (980) and lows (2294, or a total of about 3300) than record cold daily highs (373) and lows (187, or a total of about 560).

Numerous fronts moved across the country during the month, but the circulation pattern inhibited rain-producing systems over portions of the Midwest and East and contributed to a below-normal tornado count for September (preliminary count of 16 compared to an average count of 74). Upper-level troughs brought above-normal precipitation to much of the West, with Washington, Oregon, and Colorado having a record wet September and six other states ranking in the top ten wettest category. Six states in the Midwest and nine states in the East ranked in the driest third of the historical record, with Delaware and Maryland having the tenth driest, or drier, September. The Colorado rains produced significant flooding and resulted from a combination of an unusually deep, moist flow of tropical moisture pulled in by a slow-moving upper-level low pressure system that consistently focused the moisture towards the Front Range and along a stalled front. Tropical cyclone activity in the North Atlantic basin during September was near normal in terms of the number of named storms and hurricanes, but below normal in terms of the duration and strength of the cyclones, with none of the cyclones making landfall on the U.S. mainland. The wet conditions in the West contributed to a national precipitation rank of twelfth wettest September and reduced drought coverage in the region. Drought also shrank in the Great Plains and Alaska, but expanded in Hawaii and slightly in the Midwest. The national drought footprint shrank to 37.2 percent of the U.S. as a whole.

Map of monthly temperature anomalies
Map of monthly temperature anomalies.
Map of monthly precipitation anomalies
Map of monthly precipitation anomalies.

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 September 2013 temperature, precipitation, and circulation patterns, suggest that the MJO, PNA, NAO, EP-NP, and PT drivers each exerted some influence on September weather. ENSO was neutral, and thus not a player, and the AO did not seem to be influential. Those drivers associated with the Pacific (MJO, EP-NP, and PT) appeared to exert a controlling influence on the temperature patterns. The Pacific drivers (PNA, EP-NP, and PT) also seemed to have a strong influence on the upper-level circulation. Precipitation is not as strongly correlated to any of the atmospheric drivers during the early fall as it is during the winter, with random convective processes playing a greater role in precipitation during September, but the PNA, NAO, and PT seemed to have a common influence on the Great Lakes to Midwest dryness, while the EP-NP was the likely driver influencing the Pacific Northwest wetness. This month illustrates how competing atmospheric drivers may work in phase to create a complex weather pattern.

Citing This Report

NOAA National Climatic Data Center, State of the Climate: Synoptic Discussion for September 2013, published online October 2013, retrieved on October 25, 2014 from http://www.ncdc.noaa.gov/sotc/synoptic/2013/9.