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Climate Prediction Center


August - October 2013


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Latest Seasonal Assessment - Drought continued across central and southern sections of the U.S. from the Great Plains westward, the southwestern shores of the Hawaiian Islands, and part of interior Alaska as of mid-July 2013. Exceptional drought (D4) covers substantial portions of the central and southern High Plains, where only 35 to 65 percent of normal precipitation fell from October 2010 through June 2013, with isolated spots reporting less than that. The scattered areas of drought that had affected parts of the East have been removed over the last four weeks, but the steady pattern of slow improvement that had been evident across the eastern Plains and upper Mississippi Valley came to a halt during late spring. Drought has now re-established itself in eastern sections of Nebraska, Kansas, and Texas and the western half of Arkansas. These areas are one to several inches below normal for the past 30 to 45 days. Similar deficits have accumulated across much of Iowa, Missouri, eastern Oklahoma, eastern Arkansas, northern Mississippi, and southwestern Louisiana, where the conditions will need to be closely monitored for signs of further deterioration. In contrast, the last four weeks brought 2 to 6 inches of rain from central Texas westward through large parts of New Mexico, south-central Colorado, and eastern Arizona, partially the result of a fairly robust monsoon so far this season in central and eastern sections of the area typically affected. Farther north, similar totals were observed in scattered sections of the central High Plains and the northern reaches of the High Plains and Great Plains. The Drought Outlook valid through October 2013 calls for improvement or removal of drought across approximately the areas affected by the monsoon, specifically from central Arizona and south-central Utah eastward through most of southern Colorado, New Mexico, and western Texas. Existing drought is forecast to persist or worsen elsewhere in the central and western U.S., with expansion into the areas of the central Rockies, eastern Oklahoma, and western Arkansas currently not in drought. In Hawaii, drought should persist along the western shores and expand to the northeast, enveloping central and southern parts of the state and encroaching on northeastern areas. Across Alaska, drought should ease across lower northern Alaska but continue in central parts of the state through October 2013.

Discussion for the Seasonal Drought Outlook

Tools used in the U.S. Drought Outlook (USDO) included the official updated CPC temperature and precipitation outlooks for August 2013, the long lead forecast for August through October 2013, various short- and medium-range forecasts and models such as the 5-day and 7-day precipitation totals from the Weather Prediction Center, 6-10 day and 8-14 day forecasts, the NAEFS precipitation outlooks, the soil moisture tools based on the Constructed Analog on Soil (CAS) moisture, dynamical models (CFSv2, NMME, and IMME), the 384-hour total precipitation forecasts from several runs of the GFS, the four-month Palmer drought termination and amelioration probabilities, climatology, and initial conditions.

Drought is forecast to persist across the southern Great Plains and central sections of the Great Plains and High Plains through October 2013. Only a few tenths of an inch of precipitation are forecast into late July, and the odds generally favor below-normal rainfall from central Texas northward through southwestern South Dakota until the end of the month. Neither the 1- nor 3-month (August and August – October) forecasts indicate any tilt of the odds away from climatology, but historically precipitation totals decrease as summer progresses into autumn, and soil moisture (at least as approximated by models) declines more often than not from the Oklahoma Panhandle northward. Given current conditions, climatology indicates that the chances of the Palmer Hydrologic Drought Index (PHDI) climbing above -2.0 by the end of October range from under 10 percent to not quite 30 percent from central Texas northward through the central High Plains and western half of the Great Plains. In contrast, soil moisture increases more often than not during this 3-month period across southern Texas, and chances for the PHDI to reach -2.0 by the end of October are in the 40 to 50 percent range climatology, except lower in the southern tip of the state. Since there are no indications of substantial rainfall through the end of July and because the chances of a significant increase in the PHDI are less than 50 percent, persistence is also forecast here, though with less confidence.
Forecast confidence is moderate from central Texas northward through affected areas of the Plains, but low in southern Texas.

Improvement (removal where extant drought is moderate) is anticipated in southern sections of the High Plains and Rockies. Monsoonal precipitation wanes climatologically as the August – October period progresses, but is still active through the end of summer. As a result, typically 35% to 45% of annual rainfall is observed during this 3-month period across central and eastern Arizona, New Mexico, south-central Colorado, and the Big Bend of Texas, with 30 to 35% more typical in adjacent areas. Not surprisingly, soil moisture content is typically greater in late October than in mid-July. The increase exceeds 25% as often as not in central New Mexico. Official forecasts indicate a tilt of the odds toward above-normal precipitation for the 6- to 10-day and 8- to 14-day periods throughout the region, and also in the August 1-month and August – October 3-month outlooks in eastern Arizona and western New Mexico.
Forecast confidence is high from central Arizona eastward through central New Mexico, and moderate elsewhere.

Across the Intermountain West and the northern half of the Rockies, drought is forecast to persist. Through July 25, 1 to 2 inches of rain is anticipated in central Colorado, but little or none is forecast elsewhere. The 6- to 10-day period indicates enhanced chances for above-normal precipitation in central sections of the Rockies and the southern half of the Intermountain West, but below-normal amounts are favored in northern sections of the Intermountain West and Rockies. During the latter part of July, neither wetter nor drier than normal conditions are favored, as is the case in the August and August – October Outlooks. Historically, August – October is not particularly dry or wet relative to other seasons in the central and northern Rockies and southern Intermountain West, but leans dry across northern sections of Utah and Nevada, and throughout Idaho. Soil moisture also declines more often than not across the northern portions of this region, and forecasts favoring above-normal temperatures during August and August – October increase the likelihood of decreased soil moisture.
Forecast confidence is high in the northern Intermountain West and Great Plains, low to moderate farther south.

In the areas of drought along the West Coast, the wet season usually doesn’t begin until late in the period at best, and historically only 5% to 15% of the annual precipitation total falls during August – October (2% to 5% in central California). As a result, soil moisture content declined by 25% or more in half of the years during 1932 – 2011, and forecasts favoring above-normal temperatures on average for the next three months further enhances the chances of soil moisture decreases.
Given the strong climatological indicators, forecast confidence here is high.

Across Hawaii, below-normal precipitation is indicated during August – October, and given antecedent dryness, drought persistence and expansion is forecast. Drought is expected to cover central and southern sections of the state and push eastward into north-central areas by the end of October. The tilt of the odds toward dryness during the 3-month period is substantial, but not overwhelming.
Forecast confidence in Hawaii is moderate.

Seasonably cooler temperatures in Alaska should abet drought improvement. However, odds favor below-normal precipitation during August – October in southern parts of the drought area at least slightly, so improvement is less likely there.
Forecast confidence in Alaska is low.

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Page Author: Climate Prediction Center Internet Team
Page last modified: July 18, 2013
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