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U.S. Seasonal Drought Outlook for May 17 to Aug. 31, 2012 (Text)

May 17, 2012

Following is the text of the U.S. Seasonal Drought Outlook as released by the National Weather Service in Camp Springs, Maryland:

Latest Seasonal Assessment - The Drought Outlook for May 3 - July 31, 2012 is based on short-, medium-, and long-range forecasts, initial conditions, and climatology. Since the release of the previous Drought Outlook issued on April 19, 2012, a slight expansion of drought occurred in the Tennessee Valley and lower Ohio Valley while drought conditions intensified across the interior Southeast. In contrast, drought improvement is noted across southern New England and southeast Florida. Since precipitation signals are weak on the seasonal time scale, this outlook is based primarily on short to medium range forecasts and climatology. Some improvement is forecast along the East Coast with improvement most likely across the south-central Florida peninsula. The protracted drought across Georgia and South Carolina is expected to persist. Persistence is also forecast in northeast Minnesota, but prospects for improvement increase farther south in southern Minnesota and northern Iowa. Some improvement can be expected across the drought areas of the central and southern high Plains, while drought is expected to persist or expand across parts of the West along with western and south-central Texas. A drier climatology supports persistence in Hawaii.

Discussion for the Seasonal Drought Outlook

Tools used in the U.S. Drought Outlook (USDO) included the official CPC temperature and precipitation outlooks for June 2012 and the long lead forecast for June through August 2012, various medium- and short-range forecasts and models such as the 6-10 day and 8-14 day forecasts, the most recent 180 and 384- hour GFS total precipitation amounts, the soil moisture tools based on the Constructed Analog on Soil (CAS) moisture, the Climate Forecast System (CFS, versions 1 and 2), the four-month Palmer drought termination and amelioration probabilities, climatology, and initial conditions.

An upper level disturbance brought locally heavy rainfall to southeastern Florida, with 30-day precipitation totals running 1.5 to 3 inches above normal easing drought conditions. Seabreeze generated thunderstorm activity is ongoing across the Florida peninsula, indicating a timely start to the summer rainy season. An additional 1-2 inches are forecast by HPC across central and southern Florida during the upcoming 5-day period, while CPC forecasts covering the 6-10 and 8-14 day periods both indicate enhanced probabilities of above-average rainfall. During the summer months, seabreeze driven convection provides a significant percentage of annual rainfall to Florida and the coastal areas of the Southeast. CPC’s monthly and seasonal forecasts maintain equal chances for near, below or above median rainfall, so based on the wet climatology for these areas, drought improvement is likely, especially across the Florida peninsula. Forecast confidence for Florida and coastal areas of the Southeast is moderate to high.

Extreme to exceptional drought current extends from northern Florida through central Georgia and parts of South Carolina and Alabama. While spotty showers and thunderstorms brought increased moisture to portions of the interior Southeast during the previous month, particularly across South Carolina, the drought remains firmly entrenched due to extreme long term deficits, which top 20 inches for the year in some locations. The CPC 6-10 day outlook brings increased odds of above normal rainfall to coastal areas of the Southeast, but for the inland drought areas, all guidance from short range through the CPC seasonal outlook indicates equal chances for near, below, or above median precipitation. Thunderstorm activity is common across the interior Southeast during the summer, but the climatological percent of annual rainfall received is not particularly high when compared to other seasons. Additionally, due to the long standing precipitation deficits in the drought areas, soil moisture content is extremely low, which can promote abnormal heat and suppress convection. Sea surface temperatures in the Gulf of Mexico are above average, however, which promotes enhanced evaporation, and may provide a mitigating source of moisture during periods of southerly flow. Based on these conflicting factors, this forecast was produced under the assumption of near normal summer rainfall, which would likely not be sufficient to ameliorate the long term drought conditions. Therefore, persistence is indicated across the Southeastern Piedmont. Tropical cyclone activity in the Southeast may bring improvement which is not indicated in this forecast. Forecast confidence for the remainder of the Southeast is low to moderate.

Several storm systems traversed the eastern seaboard during the previous month, bringing periods of beneficial rainfall to drought areas which previously had been developing across the mid-Atlantic states and the Northeast. Other than a small area in western Connecticut, 30 day precipitation totals are generally above average in this region, especially across New England, while deficits of 4-8 inches still exist over 90 days from eastern Maryland through southern New England. No additional significant rainfall is expected during the upcoming two weeks, which may erode some of the recent moisture gains. The CPC June monthly and June-August seasonal outlooks both call for equal chances of near, below, or above median rainfall. Scattered thunderstorm activity is expected across the Northeast during the summer, though periods of heat and dryness are also common. Based on the relatively shallow nature of the drought in the Northeast, some improvement is forecast based on climatology. Forecast confidence for the Northeast and mid- Atlantic states is moderate.

Abnormal dryness and drought have expanded rapidly across the mid-Mississippi Valley, particularly in western Kentucky, southern Illinois, far southeastern Missouri, and northeastern Arkansas. 30 day deficits greater than 3 inches are common in this area, and 90 day deficits are 4 to 8 inches or greater. No significant precipitation is expected in the short term, and the CPC 8-14 day outlook predicts enhanced chances of below average rainfall, which may promote continued rapid deterioration. CPC’s monthly and seasonal outlooks both indicate equal chances of near, below, or above median rainfall and enhanced probabilities of above-median temperatures. The percent of annual rainfall received during the summer months in the lower and middle Mississippi Valley are slightly below the median, indicating that the season is slightly drier when compared to other 3-month periods. Based on the likelihood of short term drought expansion, persistence is indicated in the existing drought areas. Due to the potential for small scale heavy rain events such as tropical cyclones, this forecast does not include additional development areas. Forecast confidence for the mid- Mississippi Valley is moderate.

Across the southern Plains, copious rainfall fell across southern and western Texas, bringing significant drought relief, with widespread one category reductions in the U.S. Drought Monitor this week. In contrast, increasing rainfall deficits and hot weather promoted rapidly developing dryness across northeastern Texas and eastern Oklahoma. CPC’s 6-10 and 8-14 day outlooks both favor below average rainfall, which occurring during May, a relatively wet time of year, may promote drought development and expansion. CPC’s monthly and seasonal outlooks maintain equal chances for near, below, or above median precipitation. Based on the short term dryness and upcoming dry season, persistence is expected across the existing drought areas of the southern Plains. Forecast confidence for the southern Plains is moderate to high.

Widespread heavy rainfall fell across the upper Midwest during the previous month, with portions of southern Minnesota receiving 3-5 inch rainfall surpluses. The increased moisture has reduced drought across the northern Plains and upper Midwest. Additional rainfall is expected based on HPC’s 5-day accumulated precipitation outlook, and a continued wet pattern through week 2 is reflected in CPC’s 6-10 and 8-14 day outlooks. Further drought reduction is likely based on these short term forecasts, and the summer months tend to be wet in the summer. Therefore, improvement is indicated in this outlook for the remaining drought areas of the upper Midwest, except across areas near Lake Superior, which may miss out on the short range rainfall. Forecast confidence for the upper Midwest is moderate to high.

Severe to extreme drought is ongoing across southern Arizona and New Mexico. The Southwestern Monsoon onset typically occurs in July, with peak rainfall occurring during August. Monsoon thunderstorms have the potential to bring local drought relief, but there is quite a bit of uncertainty in the monsoon’s intensity and extent forecast. A lack of snow cover in the southern Rockies may promote an early development of the Southwestern heat low, which could bring monsoon rains as early as June, but some forecast tools indicate a below average monsoon signal, particularly in eastern locations. Even given an average monsoon, annual rainfall deficits are large across southern New Mexico and the Texas panhandle. Therefore, some improvement is forecast across the monsoon region of the Southwest, with a small area of improvement in southeastern Arizona and southwestern New Mexico. Forecast confidence for the monsoon regions of the Southwest is low.

Widespread moderate to severe drought covers much of the remainder of the Southwestern U.S., and a small area of moderate drought has developed in central Washington. While the Southwestern Monsoon can bring moisture throughout the Four Corners States, the summer is climatologically dry across the Great Basin, California, and the Northwest. The CPC 6-10 and 8- 14 day outlooks both favor abnormal dryness across the Southwest, while the monthly and seasonal outlooks indicate enhanced odds for below median precipitation in the Northwest. Based on these forecasts, persistence is expected across the drought areas of the western U.S. Redevelopment of drought is possible in southwestern CO and southwestern UT. Forecast confidence for the remainder of the West is high.

The summer months are climatologically dry in Hawaii. Therefore, little improvement in existing drought areas is expected through August. Forecast confidence for Hawaii is high

SOURCE: National Weather Service

To contact the reporter on this story: Stephen Rose in Washington at srose31@bloomberg.net

To contact the editor responsible for this story: Steve Stroth at sstroth@bloomberg.net


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