Tuesday, August 20, 2013

The Wet Get Wetter and the Dry Get Drier

The lack of rain in the central U.S. this month is becoming a bit of concern as soils continue to dry and crops, gardens, and lawns are thirsty for rain.  Abnormal Dryness as depicted on the U.S. Drought Monitor has returned to Iowa, Missouri, Wisconsin, and Illinois after being absent in those areas for much of the summer.

Meanwhile, the rain doesn't seem to want to stop in the Gulf States and the Southeast. Until recently a strong upper low rotating over southeastern Canada kept the jet stream pushed further south than usual for summer, bringing cool dry weather to the central U.S.

The 500 millibar map (~20,000 ft) on Sunday, August 18. A trough persists over the eastern half
of the country, though much weaker than earlier in the week
In the meantime, waves of low pressure moving along the stalled frontal boundary across the Gulf states provided a trigger for showers and thunderstorms from the Louisianna coast through the Carolinas.

Surface map for 7:00 CDT Sunday, August 18.

The persistence of this pattern is readily seen in an animation of the daily CoCoRaHS maps for the last seven days.

CoCoRaHS precipitation maps for 8/14 through 8/20/2013
A number of locations in the Southeast are on pace for the wettest year on record (information from the Southeast Regional Climate Center with totals updated through 8/19).

                       Precip through        All time wettest
Asheville, NC         58.62                 64.91  1973

Roanoke, VA          41.52               58.87   1948

Macon, GA            55.79                67.80 (1929)

For both Asheville and Macon it's really not a question of if the records will be broken, but when.  Roanoke's precipitation is currently 17.35 inches from the record. Normal precipitation for September through December is 13.12 inches, so higher than normal precipitation will be needed there to break the record. An interesting note about Asheville's rainfall -the  2013 rainfall total represents the largest departure from normal out of 180 cities reviewed in a recent study by the National Climatic Data Center (coincidentally located in Asheville).  The current total pf 58.62 inches is 31.41 inches above normal.

The are a number of locations in the western Carolinas with precipitation for the year-to-date in excess of 80 inches. Here are precipitation accumulations over 80 inches recorded by CoCoRaHS observers in the southeast. These are stations that had at least 95 percent of possible days reported from January 1-August 20.

SC-PC-4          Cleveland 8.0 WSW             89.15
NC-WT-10        Blowing Rock 2.2 NE           86.82
NC-AV-9           Linville 2.5 ENE                   83.00
NC-WT-1          Blowing Rock 22.8 ENE       81.31

More rain is on tap for the southeast for this week with an inch or two additional rain from northern Georgia through the western Carolinas and northward into West Virginia. Dry weather will continue across much of the central U.S., although a cold front will likely produce scattered showers and thunderstorms across the northern Midwest.

72-hour Quantitative Precipitation Forecast for the period ending 7:00 p.m. CDT Friday, August 23

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