Wednesday, June 3, 2015

A Weird, Wet May Across Much of the U.S.

The first tropical storm of the season, Ana, developed off of the southeastern U.S. nearly four weeks before the official start of the season. As Ana weakened and moved away, a late spring storm dropped from one to feet of snow on Colorado, Wyoming, and western Nebraska, with lighter but significant amounts in South Dakota and North Dakota.

California continued to suffer through another month of extreme drought, and the eastern U.S. along and east of the Appalachians was dry with less than 50 percent of average precipitation for most of the area. The notable exception was coastal North Carolina which benefited from the rains from the close approach of T.S. Ana. It was very warm across Alaska and many locations experienced a "top five" warmest May. In the eastern Interior and northern Southeast Alaska this was the warmest May of record. A high temperature of 91°F recorded in Eagle, AK on May 23 was the earliest in the season for 90°F or greater to be reached in Alaska.

In much of the U.S. between California and the Appalachians precipitation was much above normal, well more than twice normal from the Gulf Coast northward through the central Rockies.

The heaviest precipitation, however, was concentrated in Texas and Oklahoma, particularly in northeastern Texas through central Oklahoma.

May precipitation was very heavy from the Gulf Coast northward into eastern Kansas with amounts raining from 10 to almost 30 inches.

It was hard not to know what was going on during the three weeks from May 5 to May 26 if you paid any attention to the news. Oklahoma experienced its wettest month ever since 1895 with average statewide precipitation of 14.40 inches, 9.58 inches above normal and almost four inches higher than the previous record of 10.75 inches in October 1941. In addition, many locations in Oklahoma had their wettest month on record. Oklahoma City had 19.48 inches of rain during the month, beating the precious record of 14.66 inches set in  June 1989. Normal may precipitation for Oklahoma City is 4.65 inches.

CoCoRaHS observers were there to document the historic rainfall. Of 204 Oklahoma CoCoRaHS stations reporting in May,  an astounding 59 of them had 20 or more inches of rain during the month. There were 92 stations reporting on at least 28 days (90 percent) during the month, and of these the largest amount was 26.58 inches at OK-PG-7 (Krebs 0.3 WNW), with the lowest 4.48 inches at OK-CM-2 (Keyes 10.5 ESE)

A large portion of Texas experienced heavy rain during May, but the heaviest rain was concentrated in north-central Texas. Of the 1775 CoCoRaHS stations with reports in Texas during May, 672 reported a foot or more of rain during the month. The highest CoCoRaHS amount was 27.32 inches at TX-GA-9, Pottsboro 7.1 WNW in Grayson County along the Oklahoma-Texas border. A U.S. Cooperative observer in Gainesville, TX, (west of Pottsboro in Cooke County) reported a total of 28.90 inches for the month.

The heavy rain brought a dramatic end to the drought that plagued Oklahoma and Texas since 2010. As of the May 26 U.S. Drought Monitor only 5.4 percent of the state of Texas was in Moderate Drought or greater. The last time that number was this low was on May 11, 2010 when it was 3.51 percent. Five years ago this month there was no drought in Oklahoma, but the drought ramped up there in October 2010.  It peaked in the fall of 2011. The drought disappeared briefly except in the Oklahoma Panhandle and the western half of Texas by the spring of 2012, but then reestablished and intensified, persisting at various intensities until this spring.

The drought in Texas in September 2011 (left), when it was at its peak, and on May 26, 2015.

The drought in Oklahoma on September 2011 (left) and on May 26, 2015.

This particular form of drought relief had other consequences as well. Significant flooding continues to occur along many rivers in Texas, particularly the Trinity River and the Red River. A number of locations along these rivers are expected to remain at major flood stage into next week.

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