U.S. has a cool and very dry March; severe weather outbreak likely today

It was another relatively quiet month for weather extremes in the U.S. during March 2013, said NOAA's National Climatic Data Center (NCDC) in their latest State of the Climate report. The month ranked as the 43rd coolest March in the 119-year record for the contiguous U.S., and was the coolest March since 2002. This is in stark contrast to what happened in 2012, when the U.S. had its warmest March on record. Eleven states, mostly in the Southeast, experienced a top-ten coolest March on record in 2013. Remarkably, Florida, Georgia, Alabama, South Carolina, and North Carolina had March 2013 temperatures that were cooler than in January 2013. California, Nevada, and Arizona had a top-ten warmest March. The pattern of warmth in the west and cool conditions in the east was due to the fact that the jet stream was stuck in a large loop that allowed cold air to spill out of Canada into the Southeast U.S., and warm air to flow northward over the southwestern states. We can describe the jet stream behavior using the Arctic Oscillation (AO) Index, which is a measure of pressure patterns across the Arctic. During March 2013, the monthly-averaged AO index reached its most negative March value since records began in 1950.

According to NOAA's U.S. Climate Extremes Index (CEI), which tracks the percentage area of the contiguous U.S. experiencing top-10% and bottom-10% extremes in temperature, precipitation, and drought, the year 2013 has been below average for extremes. The CEI during January - March 2013 was 15%, and on average, about 20% of the contiguous U.S. experiences top-10% extreme weather as defined by the CEI.


Figure 1. Historical temperature ranking for the U.S. for March 2013. Eleven states, mostly in the Southeast, had a top-ten coldest March, and California, Nevada, and Arizona had a top-ten warmest March. Image credit: National Climatic Data Center (NCDC).


Figure 2. Historical precipitation ranking for the U.S. for March 2013. Although only two states had a top-ten driest March--Wyoming and Louisiana--just eight states were wetter than average, making March 2013 the 5th driest March on record for the U.S. as a whole. Image credit: National Climatic Data Center (NCDC).

Fifth driest March on record in the U.S.
Although only two states had a top-ten driest March in 2013--Wyoming and Louisiana--just eight states were wetter than average, making March 2013 the 5th driest March on record for the U.S. as a whole. Thanks to three major winter storms that crossed the Great Plains during the first half of April, there should be a notable improvement in drought conditions over much of the core U.S. drought region when Thursday's Drought Monitor report is issued, though. The most recent report from April 9 showed a big reduction of the area in the worst drought category, "Exceptional", from 5% to 3%, during the first week of April. About 51% of the U.S. is still in moderate or greater drought. According to data from the Rutgers Global Snow Lab, the March snow cover extent for the contiguous U.S. was the 10th largest March snow cover extent in the 47-year period of record. However, snowpack, an important water resource in the West, was below-normal in the Sierra Nevada Mountains as well as the Central and Southern Rockies.


Figure 3. Severe weather outlook for Wednesday, April 17, calls for a "Moderate Risk" of severe weather over much of Oklahoma, and portions of surrounding states. The main threat appears to be large hail, but there may also be a few strong EF-2 and EF-3 tornadoes. You can follow today's severe weather outbreak from our Severe Weather page, and wunderblogger Lee Grenci has a detailed post on today's severe weather threat.

Jeff Masters