Synoptic Discussion - September 2011
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.
The weather pattern over the contiguous United States during September 2011 was a battle between an upper-level high pressure ridge (which dominated the country during the summer) and strong upper-level low pressure systems moving in the seasonally-reinvigorated jet stream flow. The month began with warm high pressure pervasive across most of the country. A strong upper-level low pressure trough plunged into the Southeast early in the month, bringing cooler-than-normal weather in its wake, while warm high pressure built up in the West. This was followed by another ridge-West/trough-East pattern during the last third of the month (weeks 4 and 5). This west-east pattern dominated the upper-level circulation for the month and contributed to the third warmest September in the 117-year record in Oregon. Early in the month, heavy rains resulted when Tropical Storm Lee joined with the upper-level trough, cutting a wet swath from the mid-Gulf Coast to the Northeast. Pennsylvania had the wettest September on record with Maryland ranking second wettest and Ohio third wettest. The rains from Lee were compounded by moisture from Hurricane Irene the previous month, resulting in widespread flooding across Pennsylvania, Maryland, and Virginia. Over four dozen preliminary tornado reports were generated by Lee, contributing to a near-normal month for tornado activity. Rain-producing systems mostly missed the central and western regions, with five states in the Great Plains ranking in the top ten driest category for September.
Numerous temperature extremes at both ends of the scale occurred. More than 2300 daily high temperature records were tied or broken in September 2011 compared to over 2100 reports of daily high temperatures that were coldest on record. September had 151 reports of the hottest monthly maximum temperature on record and 5 reports of the coldest monthly maximum temperature on record. There were nearly 1950 reports of record warm daily minimum temperatures and roughly 1400 reports of record cold daily minimum temperatures. All-time record temperatures occurred as well, with 11 hottest maximums but no coldest minimums reported. The mixture of hot and cold temperature anomalies gave the Residential Energy Demand Temperature Index (REDTI) a value near the middle of the historical distribution, indicating that the national residential energy consumption was about average for September 2011. However, the April-September 2011 REDTI was the highest value for this season in the 117-year record, indicating that the national residential energy consumption was about 10.3 percent above the long-term average for the warm season.
The circulation pattern, with its numerous cold fronts that swept across the nation, brought rain to some areas, but most areas outside the influence of Tropical Storm Lee were drier than normal. Minnesota (5th driest), Montana and Texas (both 7th driest), and Kansas and South Dakota (both 9th driest) ranked in the top ten driest category for September 2011. The dryness has persisted for the last twelve months, with Texas ranking driest on record and New Mexico and Oklahoma second driest for October 2010-September 2011. Strong winds associated with Tropical Storm Lee fanned wildfires across Texas, but national wildfire statistics for September were near average. Moderate to exceptional drought covered about 29 percent of the contiguous U.S. (24 percent of the U.S. including Alaska and Hawaii) at the end of September, with two core areas being the Southern Plains (76 percent coverage) and Southeast (42 percent coverage). Nearly all (97 percent) of the Lone Star State was in extreme to exceptional drought (the worst categories), which is a record in the eleven years of the U.S. Drought Monitor, and four-fifths (79 percent) of Oklahoma was in extreme to exceptional drought. Two areas of the southern U.S. experienced the most severe drought in the 1900-present record, according to the Palmer Hydrological Drought Index (PHDI). The two regions having the most severe PHDI on record were eastern New Mexico into western Texas and southwest Oklahoma, and northwestern Louisiana into adjacent eastern Texas.
When averaged together, the mixture of temperature and precipitation extremes gave the U.S. the 11th warmest and 50th driest September in the 117-year record. Averaging extremes tends to cancel them out. But when extremes are combined cumulatively, like in the U.S. Climate Extremes Index (CEI), they tell a different story. The large total area of hot daytime temperatures, warm nighttime temperatures, and very wet or very dry conditions gave the U.S. the second highest July-September and April-September CEI, seventh highest January-September CEI, and tenth highest September CEI in the 1910-2011 record. On a regional basis, the CEI was record high for the Northeast region (September, July-September, and the warm season [April-September]) and South region (July-September and the warm season). The warm season was much more extreme than previous years for the Northeast and South regions, but for different reasons. Both the Northeast and South had very warm temperatures for April-September 2011, but the precipitation patterns were opposite — the Northeast was record wet while the South was second driest.
Cold fronts and low pressure systems moving in the storm track flow are influenced by the broadscale atmospheric circulation. Four such large-scale atmospheric circulation drivers were potentially influential during September. Ocean temperatures and atmospheric circulation anomalies indicated that the equatorial Pacific was in a weak, but strengthening, La Niña state. La Niña this time of year (July-September) is weakly associated with temperature and precipitation anomalies across the U.S. — temperatures are typically warmer than normal in the central U.S. and cooler than normal in the West, with drier than normal conditions dominating across the central regions and wet conditions in the Southeast to Mid-Atlantic. The Pacific/North American (PNA) pattern was negative early in September and again during the last third of the month (corresponding to the times when strong upper-level low pressure troughs dominated the eastern U.S.), and positive at mid-month. The PNA during this transitional time of year (between the times represented by July and October on the PNA teleconnection maps) typically has a weak relationship with temperature and precipitation throughout the Lower 48 States. The North Atlantic Oscillation (NAO) pattern was neutral to slightly positive for September. Like the PNA, the NAO during this transitional time of year (between the times represented by July and October on the NAO teleconnection maps) typically has a weak relationship with temperature and precipitation throughout the Lower 48 States. The Arctic Oscillation (AO) pattern was also slightly positive for September. A positive AO this time of year (July-September) is typically associated with warmer-than-normal temperatures in the Northern Plains, dryness from the Southern Plains to the Great Lakes and New England, and anomalous wetness in the Southeast.
The pattern of observed temperature anomalies for September 2011 and the last three months (July-September) corresponds slightly to the NAO pattern for July (warmer than normal in the Northwest, cooler than normal in the Southeast) and to the AO pattern for July-September (warmer than normal in the Northern Plains). The September 2011 and July-September 2011 precipitation patterns are a reasonable match for the La Niña and NAO patterns, especially from the Great Plains to Mid-Atlantic (La Niña) and Ohio Valley (NAO).