Top U.S. weather story of 2009: drought hits a 10-year low

The weather gods were unusually kind to the U.S. in 2009, as the nation had no hurricane landfalls, a relatively quiet tornado season, no billion-dollar floods, and the lowest drought footprint of the decade by year's end. According to insurance giant Munich Re, the four costliest 2009 weather disasters in the U.S., not including droughts, were:

$2.5 billion: February 10 - 13 Severe weather and tornado outbreak
$2.0 billion: June 10 - 18 Severe weather and tornado outbreak
$1.7 billion: April 9 - 11 Severe weather and tornado outbreak
$1.5 billion: March 25 - 26 Severe weather and tornado outbreak


Figure 1. Lake Whitney, Texas in February 2009 (left) and July 2009 (right), after exceptional drought conditions gripped South Central Texas. Image credit: wunderphotographer Icheney44.

Costliest U.S. weather disaster of 2009: the Texas drought
The costliest U.S. weather disaster in 2009 was the Texas drought. According to preliminary estimates, the agricultural losses from the Texas drought will cost close to $4 billion. The drought actually began in 2007 - 2008, and at the beginning of 2009 (Figure 2), much of Texas was already experiencing "exceptional drought"--the highest level of drought classified by the U.S. Drought Monitor. By summer, much below-average rainfall and scorching triple-digit heat caused the exceptional drought region to expand over a large region of South Central Texas. However, by September, the southern branch of the jet stream became more active, as it typically does during strong El Niño events in the Eastern Pacific, putting southern Texas in the path of a series of drought-busting rain storms that continued into the winter. By December, just a few spots of moderate to severe drought remained in Texas. While the short-term drought is over, longer-term drought remains in Texas. Area lakes have seen only modest rises, and will take months to show significant improvements. Lake Corpus Christi was just 33% of capacity on December 26, and other South Central Texas lakes and reservoirs were between 59% and 100% of capacity. With El Niño conditions expected to continue through winter and spring, there is a good chance that Texas will enter summer 2010 free of both short and long-term drought, though.


Figure 2. What a difference a year makes: Texas began the year with exceptional drought conditions over a small region, which expanded to cover a large portion of the state in the summer. Fall and winter rains broke the drought, and by December only a few small spots of drought remained over Texas. The Southeast U.S. also began the year with widespread drought conditions, which eased by summer. However, drought conditions developed over the Desert Southwest, including most of Arizona. Image credit: U.S. Drought Monitor.

Top U.S. weather story of 2009: drought at a 10-year low
As of December 2009, the U.S. was free of "exceptional drought" for the first time since June 2008, and had the lowest levels of the two highest drought categories--"extreme" and "exceptional"--since June 2005. According to the U.S. Drought Monitor archive, the 4.24% of the country experiencing severe or higher drought conditions in December 2009 was the lowest U.S. drought footprint of the decade (Figure 3). The previous low was 4.57%, in November 2005. This is very good news, since droughts typically cost the U.S. an average of $6 - $8 billion per year, and are our costliest weather-related disasters, according to FEMA. For comparison, Floods cause an average damage of $2.4 billion per year, and hurricanes, $1 - $5 billion per year.


Figure 3. Areal coverage of drought over the Contiguous U.S. from January 2000 to December 2009. Dark red colors are the highest level of drought, "exceptional"; bright red colors include the the next highest level of drought, "extreme (D3); orange colors include the next highest level of drought, "severe" (D2); light orange colors include next highest level of drought conditions, "moderate" (D1); and yellow colors include the lowest level of drought conditions, "Abnormally Dry" (D0). At the end of 2009, the Contiguous U.S. was experiencing its lowest drought footprint of the decade. Image credit: U.S. Drought Portal, National Integrated Drought Information System.

Drought is not increasing in the U.S.
Global warming theory predicts that although global precipitation should increase in a warmer climate, droughts will also increase in intensity, areal coverage, and frequency (Dai et al., 2004). This occurs because when the normal variability of weather patterns brings a period of dry weather to a region, the increased temperatures due to global warming will intensify drought conditions by causing more evaporation and drying up of vegetation. Increased drought is my number one concern regarding climate change for both the U.S. and the world over the next forty years. Two of the three costliest U.S. weather disasters since 1980 have been droughts--the droughts of 1988 and 1980, which cost $71 billion and $55 billion, respectively. The heat waves associated with these droughts claimed over 17,000 lives, according to the National Climatic Data Center publication, Billion-Dollar Weather Disasters. Furthermore, the drought of the 1930s Dust Bowl, which left over 500,000 people homeless and devastated large areas of the Midwest, is regarded to be the third costliest U.S. weather disaster on record, behind Katrina and the 1988 drought. (Ricky Rood has an excellent book on the Dust Bowl that he recommends in his blog post from January). However, despite significantly warmer temperatures over the U.S. over the past 40 years, drought has not increased, according to the U.S. Climate Extremes Index (blue bars, Figure 4). The portion of the U.S. experiencing abnormal drought and exceptionally wet conditions has remained nearly constant at 10% over the past century. A recent paper by Andreadis et al., 2006, summed up 20th century drought in the U.S.: "Droughts have, for the most part, become shorter, less frequent, and cover a smaller portion of the country over the last century. The main exception is the Southwest and parts of the interior of the West, where, notwithstanding increased precipitation (and in some cases increased soil moisture and runoff), increased temperature has led to trends in drought characteristics that are mostly opposite to those for the rest of the country especially in the case of drought duration and severity, which have increased."


Figure 4. The Climate Extremes Index for January through November for drought (the December stats are not yet available, but the Jan - Nov numbers will not be much different). The worst U.S. droughts on record occurred in the 1930s and 1950s. There has been no trend in the amount of the U.S. covered by drought conditions (blue bars) or by abnormally moist conditions (red bars) over the past century. About 10% of the U.S. is typically covered by abnormally dry or wet conditions (black lines). Image credit: National Climatic Data Center.

References
Andreadis, K. M. Lettenmaier, D. P., "Trends in 20th century drought over the continental United States", Geo. Res. Letters 33, 10, L10403, DOI 10.1029/2006GL025711

Dai A., K.E. Trenberth, and T. Qian, 2004: A global data set of Palmer Drought Severity Index for 18702002: Relationship with soil moisture and effects of surface warming", J. Hydrometeorol., 5, 11171130.

Gleason, K.L., J.H. Lawrimore, D.H. Levinson, T.R. Karl, and D.J. Karoly, 2008: "A Revised U.S. Climate Extremes Index", J. Climate, 21, 2124-2137.

Next post
Have a great rest of the decade, everyone, and I'll be back next decade with a new post on the top global weather story of 2009 (yes, I realize that the end of the decade is really on December 31, 2010, but I'll go with the flow on this! I want to thank all of you who helped out the Portlight charity cause this year, everyone who uploaded a wunderphoto, and everyone who participated in the great community we've built here at Wunderground! I look forward to 2010.

Jeff Masters