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Climate Prediction Center


August - October 2011


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Latest Seasonal Assessment - Climate anomalies attributed to the Summer 2010 - Spring 2011 La Niña event promoted widespread drought development and intensification across the southern tier of the U.S. In many locations, significant deterioration occured, with the southern Great Plains experiencing some of the worst impacts. Apart from the relief afforded by isolated thunderstorms, or perhaps a tropical system, drought conditions are anticipated to persist over the south-central states through the August-October period. During the past few weeks, a persistent ridge of high pressure maintained hot, dry conditions across the core drought areas of the southern Great Plains. Heavy rain (2-3 inches) fell across portions of the central and northeastern Gulf Coast, bringing substantial relief, while daily wet-season thunderstorms throughout the Florida Peninsula continued to erode entrenched drought conditions. For most of the Southeast, substantial improvement is forecast for the August-October 2011 season. Additional drought improvement is anticipated across Arizona, western New Mexico, and southern Colorado, due to a fairly healthy monsoon circulation across the Southwest. However, eastern New Mexico and western Texas have yet to see comparably widespread, significant rainfall, and prospects for this area remain uncertain. Across Hawaii, leeward drought persistence is expected under the summer trade wind regime, while developing drought is possible in Alaska's Kenai Peninsula region.

Discussion for the Seasonal Drought Outlook

Tools used in the U.S. Drought Outlook (USDO) included the official CPC temperature and precipitation outlooks for August 2011 and the long lead forecast for August - October 2011, various medium- and short-range forecasts and models such as the 6-10 day and 8-14 day forecasts, the soil moisture tools based on the GFS model and the Constructed Analogue on Soil (CAS) moisture, the Climate Forecast System (CFS) seasonal precipitation forecasts, the four-month Palmer drought termination and amelioration probabilities, climatology, and initial conditions.

A persistent ridge of high pressure maintained dry conditions and much above normal temperatures across the central U.S., exacerbating the widespread drought entrenched across the southern Plains. Daily maximum temperatures in San Angelo, TX have broken 100 degrees Fahrenheit 61 times so far in 2011 (as of July 26th), and there have been 28 consecutive days of highs reaching 100 degrees or more so far (as of July 29th), according to the National Weather Service Office in San Angelo. Both of these events are highly unusual. Mean temperatures for the month of July have averaged as much as 7 degrees F above normal over much of the Red River Valley, including much of Oklahoma and portions of north-central Texas. Isolated thunderstorms across the region have provided little to no relief under the persistent heat and widespread dryness. Drier weather is typical across eastern Texas northeastward through the lower Mississippi Valley during August, with climatological rainfall increasing slightly towards the beginning of autumn. Climatological rainfall increases more substantially in early autumn across southern and southwestern Texas. Short, medium, and extended range forecasts, including the CPC 6-10 and 8-14 day outlooks all indicate enhanced chances for below-median rainfall across the southern Plains states. The CPC August outlook maintains elevated chances of below-median rainfall for most of the southern Great Plains and lower Mississippi Valley. The seasonal precipitation outlook for August-October 2011 calls for equal chances (EC) of below, near, and above-median precipitation for this region. The Constructed Analog on Soil moisture (CAS) outlook for the end of August and the end of October persist the dryness across the southern Great Plains. Based on these outlooks, continued drought persistence is likely.
Forecast confidence for the southern Plains is moderate (near the coast) to high (well inland).

During the previous two weeks, heavy rainfall overspread portions of the central Gulf Coast and north-central Florida, with surpluses of 2 to 3 inches in most locations, but close to double these amounts in portions of southeastern Louisiana, central Mississippi, and along the extreme upper Texas coast (AHPS precipitation analysis). The heavy rainfall substantially boosted streamflows and ameliorated the widespread exceptional drought (D4)conditions. However, the CPC 6-10 day and 8-14 day outlooks favor elevated chances of below-median rainfall over this area, while the 30-day outlook for August favors equal chances (EC). The seasonal outlook for August-October tilts the odds in favor of above-median precipitation for the eastern Gulf region, primarily due to long-term precipitation trends. The skill-masked Climate Forecast System (CFS, version 2) favors relatively weak and mixed signals over this region. Given the peak of the Atlantic hurricane season falls just before the midpoint of the outlook period, continued drought improvement is expected across the Gulf Coast.
Forecast confidence for the central and northeastern Gulf Coast is moderate.

Over the past few weeks, daily thunderstorms associated with the seabreeze-driven rainy season, as well as a brief influx of moisture associated with a tropical wave, helped to ease the areas of entrenched drought over the Florida Peninsula. Keetch-Byram Drought Indices across the state remained generally between 300 and 500, indicating a diminished risk for wildfire spread. Additionally, no areas of exceptional drought (D4) remain in the state. As the rainy season continues across Florida, further drought reduction is likely. Tropical Storm Emily is expected to pass just offshore of Florida's east coast shortly after the release of this Drought Outlook, and could bring additional rainfall to the region, further easing drought conditions. Both the CPC monthly outlook for August and the seasonal outlook for August-October maintain enhanced chances for above-median rainfall across much of the Florida Peninsula. Based on these considerations, and the fact that the climatological peak of the Atlantic hurricane season falls near the midpoint of the August-October period, drought improvement is maintained.
Forecast confidence for the Florida Peninsula is high.

Across the remainder of the Southeast, below-median rainfall was observed during the previous two weeks across northern Mississippi, most of both Alabama and Georgia, as well as most of the Tennessee Valley, although scattered thunderstorms provided some local drought relief. In the Carolinas, several weak fronts stalled across the region, accompanied by increased coverage of showers and thunderstorms. The August-October climatology across the interior South trends slightly drier in the late summer, with many locations receiving only 15-20 percent of their annual rainfall during the period. The CPC August monthly outlook does not tilt the odds in favor of above or below median precipitation across the Southeast, with the exception of wetter-than-median rainfall expected in southern Florida, as noted earlier. For the August-October season, above-median precipitation amounts are forecast over much of the southern Atlantic region due to long-term precipitation trends, also noted earlier, with equal chances elsewhere. Moisture from tropical systems is also a possibility during the late summer and early autumn months. Based on these factors, at least some improvement is maintained for a large portion of the Southeast.
Forecast confidence for the Southeast is low to moderate.

In some parts of the Northeast, the North American Land Data Assimilation System (NLDAS) shows current, top 1-meter soil moisture deficits running between 25-50 mm. Stream flows generally rank within the lowest quartile of the historical record for this time of year. These areas of fairly recent abnormal dryness (D0) and moderate drought (D1) are likely to experience some improvement over the next few months from frontal passages, organized thunderstorm clusters (MCS), and potentially tropical systems.
Forecast confidence for the Northeast is moderate to high.

Monsoon rainfall commenced during the first half of July across portions of the Southwest, primarily in Arizona, Utah, and Colorado. In New Mexico, however, where drought was present across the entire state and nearly half of the state was experiencing exceptional (D4) drought conditions through August 1st, thunderstorm activity was more sparse. In recent weeks, the core of monsoonal moisture has been located near the Arizona-New Mexico border in the higher terrain of the southern Rockies, and consistently been missing eastern New Mexico and western Texas. Extended range predictions (6-10 day and 8-14 day) favor the continuation of monsoon rainfall along the same general path as has been observed in the past 1-2 weeks. This also favors suppressed thunderstorm activity across the eastern monsoon region of eastern New Mexico and western Texas. The CPC monthly outlook for August also supports enhanced odds of below-median rainfall across the eastern monsoon region. Equal chances of below, near, and above-median rainfall are indicated in the CPC seasonal outlook for most of the Southwest. Based on these forecasts, continued drought improvement is forecast for Arizona, western New Mexico, and southern Colorado, but improvement is considerably less certain across eastern New Mexico and western Texas, where the odds seem to favor persistence of drought conditions.
Forecast confidence for the Southwest is moderate.

In Hawaii, the summer trade wind regime, which favors dryness on the lee sides of the islands, favors persistence of the existing drought areas in the state.
Forecast confidence for Hawaii is moderate.

Abnormal dryness has persisted across south-central Alaska for several months. Streamflows across the Kenai Peninsula region in particular have continued to decline. Therefore, drought development across Alaska's Kenai Peninsula is possible during the upcoming three month period.
Forecast confidence for Alaska is low to moderate.

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Page Author: Climate Prediction Center Internet Team
Page last modified: August 4, 2011
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