Synoptic Discussion - October 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.
A vigorous weather pattern dominated the contiguous United States during October 2011. Several strong upper-level low pressure systems brought areas of rain and snow with cold temperatures, while southerly winds swept warmer-than-normal air ahead of the storm systems. Numerous wildfires affected the West at the beginning of the month, contributing to a record wildfire month for October, but weather systems later in the month dampened the wildfire activity. In spite of the active weather pattern, tornado activity was below average for the month.
A winter storm early in the month dumped snow over the high elevations of the West, with up to 6 percent of the contiguous U.S. snowcovered by October 9th, but the rest of the month saw warmer temperatures with more rain than snow, so the West ended the month with a below-normal snowpack. Near the end of the month, winter storms buried Colorado and the Northeast with blankets of snow, respectively covering up to 8.3 percent (by October 28th) and 7.4 percent (by October 30th) of the country. Heavy snow from these early-season snowstorms tied or broke over 400 daily snowfall records for the month of October and over 100 monthly October snowfall records.
The fronts and low pressure systems brought above-normal precipitation to parts of the West, Central and Southern Plains, Florida, and parts of the Ohio Valley to Northeast, where New Hampshire and Rhode Island had the tenth wettest October on record and Massachusetts ranked eighth wettest. But large parts of the Southwest, South, and Mississippi Valley missed out on the precipitation. Iowa, Missouri, and Louisiana had the ninth driest October in the 1895-2011 record, with Alabama ranking 11th driest. Parts of the Southern Plains drought area received beneficial rainfall, but it had little effect on the year-long deficits. Drought expanded further into the Upper Mississippi Valley where little rain fell this month, with the national drought footprint rising to about 33 percent in moderate to exceptional drought.
Air flow at the surface, associated with the upper-level low pressure systems, caused numerous temperature extremes at both ends of the scale. More than 1400 daily high temperature records were tied or broken in October 2011 compared to over 1400 reports of daily high temperatures that were coldest on record. There were more than 1200 reports of record warm daily minimum temperatures and about 600 reports of record cold daily minimum temperatures. This synoptic brew of daily hot and cold weather systems cooked out into a monthly spatial pattern of below-normal temperatures in the Southeast and above-normal temperatures in the western and northern states.
When averaged together, the mixture of temperature and precipitation extremes gave the U.S. the 33rd warmest and 51st driest October 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 may tell a different story. The national CEI for October 2011 ranked near the long-term average. The large area of extremely wet and extremely dry conditions ranked seventh largest (wet) and ninth largest (dry) in the 1910-2011 CEI record, individually, but taken together they ranked second biggest.
Cold fronts and low pressure systems moving in the storm track flow are influenced by the broadscale atmospheric circulation. Three such large-scale atmospheric circulation drivers were potentially influential during October:
- Ocean temperatures and atmospheric circulation anomalies indicated that the equatorial Pacific was in a La Niña state. La Niña this time of year (August-October) is associated with temperature and precipitation anomalies across the U.S. — temperatures are typically warmer than normal east of the Rockies and cooler than normal in the West, with drier than normal conditions dominating the Southwest to Central Plains and parts of the Midwest and Southeast, and wet conditions in the Southern Plains.
- The Pacific/North American (PNA) pattern was positive early in October but mostly neutral for the rest of the month. A positive PNA during October typically is associated with warmer-than normal temperatures in the Northwest and cooler than normal temperatures in the Southeast, but the precipitation teleconnections are weak.
- The Arctic Oscillation (AO) pattern was positive at the beginning of October then fluctuated around neutral the rest of the month. A positive AO this time of year (August-October) is typically associated with warmer-than-normal temperatures in the Northern Plains, dryness from the Southern Plains to New England and the Upper Mississippi Valley, and anomalous wetness in the extreme Northwest and Southeast.
The pattern of observed temperature anomalies for October 2011 and the last three months (August-October) corresponds to the AO in the Northern Plains and the PNA in the Southeast. The October 2011 and August-October 2011 precipitation patterns are a reasonable match for the La Niña and AO patterns, especially over the West and parts of the Southeast (La Niña), the Southern Plains and Upper Mississippi Valley (AO), and Great Lakes (La Niña and AO).