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.

The weather patternweather pattern over North America during September 2012 consisted of the seasonal battle between subtropical high pressure (High, or upper-level ridge) to the south and the polar jet stream and associated storm track to the north. The month began with the remnants of Hurricane Isaac entrained into a cold front draped across the Midwest to Northeast. After this system tracked out of the country, the jet stream frequently pushed upper-level troughs and cool fronts into the High over the eastern United States. When averaged over the month of September, this pattern resulted in warmer-than-average temperatures beneath a stronger-than-average upper-level ridge over the western U.S. and cooler-than-average temperatures beneath a stronger-than-average upper-level trough over the East. Showers and thunderstorms along the fronts and frontal lows dropped above-normal rainfall from the Southern Plains to the Midwest and parts of the Northeast, while summer monsoon showers brought above-normal precipitation to parts of the Southwest and frequent storms gave Alaska the fifth wettest September in the state's 1918-2012 record. But descending air ("subsidence") associated with the High dominated the West and Northern Plains, giving Montana, Minnesota, and the Dakotas the driest September in the 1895-2012 record. Numerous wildfires broke out in the hot, dry, windy weather across the West, especially in the Northwest, giving September 2012 near-record high acreage burned and near-record average size of fire. This weather pattern inhibited the formation of tornadoes, with the preliminary national count of 43 tornadoes being below the long-term average.

The movement of fronts across the South and East, and monsoon showers in the Southwest, can be seen in the weekly above-normal precipitation anomaly patterns (weeks 1, 2, 3, 4). The persistent dryness in the Northwest, Northern Plains, and Upper Midwest is also evident. The weather pattern during September shielded the U.S. mainland from most tropical activity. All of the four hurricanes and tropical storms active in the North Atlantic during September were steered away from the U.S. mainland. The remnants of Hurricane Isaac at the beginning of the month, and rains from subsequent frontal passages, helped shrink drought in the Ohio Valley where Kentucky and Ohio had the tenth wettest, or wetter, September in the 1895-2012 record. Ten states, from Wisconsin to the Pacific Northwest coast, had the tenth driest, or drier, September, with drought expanding in the Northern Plains and Upper Midwest. According to the end-of-September (October 2) U.S. Drought Monitor, 64.6% of the contiguous U.S. (54.0% of the U.S. including Alaska, Hawaii, and Puerto Rico) was affected by moderate to exceptional drought overall. These values are slightly higher than those at the end of August. According to the Palmer Drought Index, which goes back to the beginning of the 20th century, 51.9% of the contiguous U.S. was in moderate to extreme drought, a decrease of about 3 percent compared to last month. The 2012 Palmer Drought Index percent area values have been exceeded only by the droughts of the 1930s and 1950s.

The movement of the cool fronts can also be seen in the weekly temperature anomaly maps (weeks 1, 2, 3, 4). The persistence of warm anomalies in the West, and frequent excursions of cool air masses into the central and eastern U.S., gave four states in the West the tenth warmest, or warmer, September in the 1895-2012 record, while seven states in the Midwest and South had cooler-than-normal monthly temperatures. On a local basis, more than twice as many record warm highs and lows occurred than record cold highs and lows. Nearly 1050 daily high temperature records and 2150 record warm daily low temperatures were tied or broken. In comparison, about 750 record low temperatures and 630 record cool daily high temperatures were tied or broken. (These numbers are preliminary and are expected to increase as more data arrive.)

When averaged together, the mixture of temperature and precipitation extremes gave the U.S. the 23rd warmest and 48th driest September in the 118-year record. Averaging extremes tends to cancel them out (as in the case for national precipitation this month). But when extremes are combined cumulatively, like in the U.S. Climate Extremes Index (CEI), they may tell a different story. Nationally, the large spatial extent of very dry conditions ranked fourth largest for September 2012 (behind September 1934, 1954, and 1956). However, the CEI components for percent area with very warm maximum temperatures and days with no rain ranked only in the top 20, and the other components ranked even lower, giving the U.S. a September CEI that ranked only 39th largest. Regionally, the September 2012 CEI for the West North Central and West regions ranked sixth and seventh largest, respectively. The preponderance of unusual warmth and dryness for the last several months has ranked the national CEI largest for the warm season (April-September) and year-to-date (January-September), and second largest for the last twelve months (October-September).

Subtropical highs, and cold fronts and low pressure systems moving in the storm track flow, are influenced by the broadscale atmospheric circulation. Five such large-scale atmospheric circulation drivers were potentially influential during September:

Map of monthly temperature anomalies Map of monthly 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 and July-September 2012 temperature, precipitation, and circulation patterns, suggests that ENSO, PNA, and NAO had little influence on the observed weather patterns. The AO and EP-NP may have exerted some influence on the weather this past month and season. The temperature patterns over the last three months, especially the frequent movement of cold fronts across the U.S. east of the Rockies, and upper-level circulation patterns reasonably reflect the transition from a negative to a positive EP-NP. As noted above, some of the indices were near neutral values for part or much of the month. When the atmospheric circulation drivers are neutral or in a state of transition, their influence becomes 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, Monthly Synoptic Discussion for September 2012, published online October 2012, retrieved on March 5, 2024 from