Synoptic Discussion - August 2012


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.

The weather patternweather pattern over North America during August 2012 consisted of a 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 jet stream frequently pushed upper-level troughs and cool fronts into the High over the eastern U.S., while the remnants of Hurricane Isaac moved up the Mississippi Valley at the end of the month and became absorbed into a cool front along the Ohio Valley at the beginning of September. When averaged over the month of August, this pattern resulted in warmer-than-average temperatures beneath an upper-level ridge over the western U.S. and cooler-than-average temperatures beneath an upper-level trough over the Southeast. Although summer monsoon showers brought above-normal precipitation to parts of the Southwest, descending air ("subsidence") associated with the High dominated the West and Great Plains, giving Nebraska, Washington, and Wyoming the driest August in the 1895-2012 record. Numerous wildfires broke out in the hot, dry, windy weather across the West and Plains, giving August 2012 a record high acreage burned. This weather pattern inhibited the formation of tornadoes, with the preliminary national count of 52 tornadoes being below the long-term average. Of the tornadoes that did occur, many were associated with Isaac.

The movement of fronts in the Midwest and East, and monsoon showers in the West, can be seen in the weekly above-normal precipitation anomaly patterns (weeks 1, 2, 3, 4, 5). The persistent dryness in the Northwest and much of the Plains is also evident. The weather pattern during August shielded the U.S. mainland from most tropical activity. Of the six Atlantic Basin tropical storms and hurricanes that formed during the month, only Isaac made landfall. And while significant flooding occurred with Isaac, especially along the Gulf coast (Louisiana and Mississippi had the second wettest August on record), its rains brought relief to some parts of the Midwest drought area. Beneficial rains contracted the drought area in the Southeast and rainfall from Isaac improved drought conditions in the Lower Mississippi and Ohio valleys. But drought expanded in the West and intensified in the Great Plains beneath the hot, dry ridge. According to the end-of-August (August 28) U.S. Drought Monitor, 62.9% of the contiguous U.S. (52.6% of the U.S. including Alaska, Hawaii, and Puerto Rico) was affected by moderate to exceptional drought overall. These values are about the same as the end of July. However, the areas affected by the worst drought categories (extreme and exceptional drought) increased, indicating that the drought has gotten more intense. According to the Palmer Drought Index, which goes back to the beginning of the 20th century, 55.1% of the contiguous U.S. was in moderate to extreme drought, a decrease of about 3 percent compared to last month. The percent area in severe to extreme drought increased to 39.0%, confirming that the drought has intensified. 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, 5). The persistence of warm anomalies in the West, and frequent excursions of cool air masses into the central and eastern U.S., resulted in the warmest August in the 1895-2012 record for Nevada and cooler-than-normal monthly temperatures for six states in the Midwest and Southeast. In total, twelve states (in the West and Northeast) had the tenth warmest, or warmer, August. On a local basis, twice as many record warm highs and lows occurred than record cold highs and lows. Nearly 2000 daily high temperature records and 2300 record warm daily low temperatures were tied or broken. In comparison, about 1100 record low temperatures and 1000 record cool daily high temperatures were tied or broken. (These numbers are preliminary and are expected to increase as more data arrive.) On balance, the warm and cold anomalies contributed to a national Residential Energy Demand Temperature Index (REDTI) for August 2012 that was above average but nowhere near a record.

When averaged together, the mixture of temperature and precipitation extremes gave the U.S. the 16th warmest and 57th driest August 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 August 2012 (behind August 1934, 1954, and 1936). However, the CEI components for percent area with very warm maximum and minimum temperatures ranked only in the top 20, and the other components ranked even lower, giving the U.S. an August CEI that ranked only 22nd largest. Regionally, the August 2012 CEI for the West and Southwest regions ranked fifth and seventh largest, respectively. The preponderance of unusual warmth and dryness for the last several months has ranked the national CEI largest for the last six months (March-August) and year-to-date (January-August), second largest for the last twelve months (September-August), and eighth largest for the summer (June-August).

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 August:

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 August and June-August 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. 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 Climatic Data Center, State of the Climate: Synoptic Discussion for August 2012, published online September 2012, retrieved on September 21, 2014 from http://www.ncdc.noaa.gov/sotc/synoptic/2012/8.