Synoptic Discussion - August 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.

August: Normally, the subtropical high pressure belt (called the Bermuda High or Azores High) dominates the weather over the U.S. this time of year, and the polar jet stream normally retreats well into Canada during the summer. This month was a departure from this normal pattern. The weather pattern during August 2013 underwent a change part way through the month, flipping from a persistent abnormally strong upper-level trough over the central and eastern United States during the first 22 days of the month to a strong upper-level high pressure ridge over much of the country during the last week. The persistent jet stream trough funneled cool polar air masses into the country east of the Rockies, resulting in a large area of below-normal temperatures for the first three weeks of the month (weeks 1, 2, 3). Unusually hotter-than-normal temperatures accompanied the ridge the following week (weeks 4, 5), effectively neutralizing the earlier cold in the north central regions when temperatures are averaged for the entire month. There were slightly more record warm daily highs and lows (about 1800) than record cold daily highs and lows (about 1450), especially in the Upper Midwest. A ridge persisted across the West for much of the month, with warmer-than-normal temperatures occurring during most weeks (weeks 1, 2, 3, 4, 5), especially in the northern regions. The monthly temperature anomaly pattern across the country consisted of warmer-than-normal temperatures in the Northwest, across the Rockies, and into the Southern Plains, with Idaho and Wyoming ranking in the top ten warmest category. Cooler-than-normal temperatures dominated from parts of the Central Plains, spreading across the Midwest to the East Coast. When averaged across the month and across the nation, August 2013 ranked as the 28th warmest August in the 1895-2013 record.

Cool fronts, steered by the upper-level trough over the north central U.S., moved south and east, frequently stalling along the east slopes of the Rockies to Central Plains and Southeast. Gulf of Mexico moisture to the south of the frontal boundary fed frequent mesoscale convective systems that dumped copious amounts of rain along the front (weeks 1, 2, 3), giving Kansas and Georgia precipitation ranks of top ten wettest for August. Precipitation was above normal along the average position of the frontal boundary; precipitation was below normal in the dry polar air mass to the north of the frontal boundary; and high pressure kept precipitation below normal south of the boundary over the Southern Plains to Lower Mississippi Valley, with the lack of tropical systems from a relatively quiet Atlantic hurricane season contributing to the southern dryness. With a dry northerly flow dominating across much of the country east of the Rockies inhibiting the formation of tornadoes, the preliminary August count of 37 tornadoes was below normal. Troughs and cut-off lows in the upper-level flow frequently brushed the West Coast, bringing above-normal precipitation to the Northwest and below-normal temperatures to California. Monsoon showers, some supplied by moisture from Pacific tropical systems such as Tropical Storm Ivo, brought above-normal precipitation to parts of the Southwest (weeks 4, 5). Otherwise, the West had a drier-than-normal August, with the hot and dry weather contributing to the development of several dozen large wildfires. Nationally, August 2013 ranked as the 53rd wettest August in the 1895-2013 record.

The extremes in temperature and precipitation gave certain regions unusually high ranks of the Climate Extremes Index (CEI), which is an index that combines extreme weather indices to show their cumulative impact. The second highest rank for Palmer Index wetness, third highest for cool maximum temperatures, and eighth highest for number of days with precipitation gave the Southeast region its seventh largest August CEI in the 104-year record (the Southeast had the third largest July CEI just last month). The Northwest region and West North Central region had the second biggest and fifth biggest rank, respectively, for the warm minimum temperature component. The West region and Southwest region had the second biggest and ninth biggest rank, respectively, for the Palmer drought component. The Central region and East North Central region had the sixth biggest and eighth biggest rank, respectively, for the days without precipitation component.

Beneficial rain over the drought areas, brought by the active jet stream, helped reduce the intensity of drought in most areas, especially the Great Plains, with the percentage of the U.S. in extreme to exceptional drought decreasing from 9.9 percent last month to 8.2 percent at the end of August, according to U.S. Drought Monitor statistics. However, dry conditions in Hawaii and the Southern Plains, and especially prolonged summer dryness in the Midwest — where Illinois, Indiana, Iowa, and Minnesota had a top ten driest August — caused the overall area in drought to expand, from 41.5 percent of the U.S. in moderate to exceptional drought at the end of July to 45.3 percent at the end of this month.

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

Summer: The jet stream was very active throughout summer 2013, sending troughs and ridges rippling across the contiguous U.S., some with high amplitude which sent synoptic weather systems (fronts, low pressure systems) far into the Deep South. The subtropical high pressure system pushed the jet stream back, attempting to regain its normal seasonal dominance. This resulted in swings of temperature and precipitation to both extremes across much of the country. On balance, however, the seasonal temperature pattern reflected the dominance of troughs in the eastern U.S. and ridges in the West — most of the states from the Rockies westward had June-August 2013 temperatures in the top ten warmest category for summer, while eleven states from the Ohio Valley to Southeast ranked in the bottom third of the historical record. Unusually warm summer temperatures were the rule in the Northeast, as well.

The complicated mix of fronts and low pressure systems resulted in a chaotic pattern of summer precipitation. The predominant flow associated with the upper-level trough brought moisture-laden air to the eastern U.S., where fronts and low pressure systems moving in the jet stream flow wrung it out in frequent heavy downpours. The result was a record wet June-August for Florida, Georgia, South Carolina, and New York, with 15 other eastern and Midwestern states ranking in the top ten wettest category. Cool and dry air masses from the north were funneled into the central part of the country by the upper-level trough, but ripples in the jet stream flow drew in enough Gulf of Mexico moisture to trigger periods of intense rainfall over parts of the Plains. As a result, Nebraska, Iowa, Minnesota, Illinois, and Louisiana ranked in the driest third of the 1895-2013 historical record, while Oklahoma, Kansas, and South Dakota fell in the wettest third category. Monsoon rainfall benefited the Southwest, where Arizona had the ninth wettest June-August and New Mexico had a seasonal reprieve from its long-term drought, but the monsoon moisture did not reach further north where dry conditions prevailed beneath the ridge, with Idaho and Wyoming ranking in the driest third of the historical record.

When these extremes are aggregated together, as in the Climate Extremes Index, several regions rank high for the CEI and for some of its components. A detailed discussion of the regional components of the CEI is available here.

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.

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:


Examination of these circulation indices and their teleconnection patterns, and comparison to observed August 2013 and summer (June-August) 2013 temperature, precipitation, and circulation patterns, suggest that the MJO, AO, NAO, and EP-NP drivers each exerted some influence on August temperatures (the influence for temperature is stronger than for precipitation this time of year). ENSO was neutral, and thus not a player; the PNA was also neutral, and its influence is not as strong during the summer as it is in the winter. The positive NAO was a good match to the monthly northwest-to-southeast temperature pattern and to the upper-level circulation anomaly pattern. The transition of the EP-NP from positive to negative, coupled with influence from the MJO and AO, seem likely drivers for the switch from cooler-than-normal temperatures to hotter-than-normal temperatures east of the Rockies at the end of the month, while the NAO seemed to be the strongest driver for upper-level circulation. Precipitation is not as strongly correlated to the atmospheric drivers during the summer as it is during the winter, with random convective processes playing a greater role in precipitation during the summer. But in August, the weekly precipitation patterns might be traced to the influence of the AO driver. 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 August 2013, published online September 2013, retrieved on October 20, 2014 from http://www.ncdc.noaa.gov/sotc/synoptic/2013/8.