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December 2008 - February 2009

 

Latest Seasonal Assessment - Medium-range outlooks through mid-December 2008 are consistent in showing a wet pattern for the drought areas in the East, except in Florida. A number of storm systems may affect the region, dropping potentially heavy precipitation on the current drought areas stretching from the Southeast into the Ohio Valley and central Appalachia. Improvement is forecast for this entire region, though it should be noted that complete drought eradication is unlikely for most of this region given the entrenched hydrologic shortages affecting many areas, and the fact that the odds still slightly favor dryness for the December - February period as a whole. Farther south, drier conditions are expected across Florida, where existing moderate drought should persist and potentially expand to cover a large part of the peninsula by the end of February 2009. Conditions in Florida will need to be monitored closely late in the period as the fire season approaches. Drought should persist and expand slightly southward across central Texas, and persist through the western Great Lakes region as well, except in areas that typically receive substantial lake-effect snowfall. Drought should at least nominally improve in south-central Oklahoma and adjacent Texas, where current indicators are mixed, and limited improvement is also anticipated for the small areas of drought in the High Plains and central Colorado. Drought should persist in southwest Wyoming and part of adjacent Idaho, but at least limited improvement is expected through other areas west of the Rockies and outside the desert Southwest. Substantial improvement is expected in northern California, though it should be noted that complete alleviation of the moisture deficits that have accumulated over the last 2 years is unlikely, and improvement is also anticipated in northern Montana. For central and southwestern California, central and north Nevada, and adjacent parts of Oregon, Idaho, and Utah, limited improvement is anticipated since winter is a relatively wet time of year and typically a period of hydrologic recharge and increasing mountain snowpack. In the desert Southwest, drought is expected to persist, and by February to expand into much of the western half of Arizona. It should be noted, however, that significant precipitation fell on this region in late November, and if even one additional system drops heavy precipitation on the region at any point in the next 3 months, the drought situation could change markedly in a relatively short period of time. The region is among the most climatologically arid in the United States, and a few inches of precipitation represent a significant proportion of the annual normal. Finally, drought conditions should improve throughout Hawaii.

 
Discussion for the Seasonal Drought Outlook

Tools used in the Drought Outlook included the official precipitation outlook for December and December-February, 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, and climatology.

The Drought Outlook calls for improvement throughout the drought areas in the eastern United States, outside Florida. Medium range forecasts through mid-December 2008 have called consistently for substantial precipitation for several days now, prompting a sharp change in the forecast relative to last issuance. Still, the entrenched and protracted nature of the hydrologic drought in part of the region should preclude broadscale drought elimination during the period, and for the December 2008 - February 2009 period as a whole, the odds still tilt slightly toward below-normal precipitation, thus precluding a highly confident forecast because of the uncertainty of the late December 2008 - February 2009 period.
Confidence for the East: Moderate

In Florida, the preponderance of guidance and the climatologically drier time of year should preclude any significant, long-lasting improvement in the current drought region, and given odds favoring a drier than normal December 2008 - February 2009 period, it also appears as though drought could expand to cover a significant portion of the Florida Peninsula by the end of February. Late in the forecast period, this region will need to be closely monitored as the climatological fire season approaches.
Confidence for Florida: High

In the Great Lakes region, most precipitation tools donít particularly favor dryness or wetness over the next 3.5 months, although substantial lake-effect snowfall may continue to fall periodically on the Upper Peninsula of Michigan, especially early in the period when the Great Lakes are still relatively mild. Also, a wetter than normal December no longer appears to be favored, but odds do tilt slightly wet for the medium-range period. Still, the preponderance of the tools indicate that surface moisture conditions should improve little during the period, so drought persistence is forecast for most of the region, with some improvement expected in the Upper Peninsula of Michigan where lake-effect snowfall is typically robust. Our confidence is eroded a bit by the lack of any significant precipitation signal for the period as a whole.
Confidence for the Great Lakes region: Moderate

In central Texas, drought is expected to persist, and expand somewhat to the south. Near- or below-normal precipitation is forecast through mid-December 2008, and both the official December monthly and December-February seasonal forecasts favor drier than normal weather. In addition, this is a relatively dry time of the year for the region, especially in the southern and western reaches.
Confidence for central Texas: High in western areas; moderate farther east

In southern Oklahoma and adjacent Texas, the official forecast for December-February favors enhanced precipitation, but the odds now seem to favor normal to below-normal precipitation for December. In addition, objective tools through February are contradictory, leading to a low-confidence forecast for some improvement, leaning on the official 3-month outlook for somewhat wetter than normal conditions.
Confidence for southern Oklahoma and adjacent Texas: Low

For the drought areas in the High Plains, one covering southeastern Colorado and northeastern New Mexico, and the other encompassing southwestern North Dakota and adjacent Montana, some improvement is anticipated. In the southernmost area, the official December-February outlook favors above-normal precipitation following a dry end to November. In the northern area, there is no precipitation signal for the winter season, and the odds tilt toward wetter than normal conditions for December 2008. Winter is very cold in this area, so what falls is typically snow which can build up over the course of the winter and provide a soil moisture boost as it melts during the spring. In both drought areas, these considerations are tempered by the fact that precipitation totals are quite low compared to other times of the year.
Confidence for the High Plains: Moderate

Recent indicators point toward improvement in the drought situation in north-central Montana, though not strongly. December 2008 will likely be wetter than normal in this region, but in the drought area across central Colorado, neither wetter nor drier than normal weather seems favored for the month, and in neither region are there substantial indicators one way or the other for the season as a whole. Still, given the time of year and the fact that winter is typically a period of hydrologic recharge from building mountain snowpack for these areas, forecasts for improvement in northern Montana (based primarily on the expected wet December) and more limited improvement in central Colorado (where December is uncertain) seem most prudent, though not with a lot of confidence.
Confidence for northern Montana and central Colorado: Moderate

Farther west, in the sprawling drought area extending from California and southern Oregon eastward through the Intermountain West and into the central Rockies, expected changes to the current drought situation vary with location. The December-February outlook doesnít favor dryness or wetness anywhere in the existing drought area, but does tilt dry in part of Arizona that has been drier than normal for at least a few months. Thus, even though widespread moderate to locally heavy precipitation fell on the region in late November, drought is expected to expand into this area by the end of February, though with only low confidence. As for the areas already experiencing full-fledged drought, improvement is anticipated in northern California, where December is expected to be wetter than normal and climatology indicates that normal precipitation is quite robust this time of year. For central and southwestern California, the Great Basin and adjacent areas, and southern Idaho, some improvement is expected. This is a relatively wet and cold time of year, especially in the higher elevations where building mountain snowpack, a primary source of water for the region, provides hydrologic recharge. With no indication as to how the season might progress relative to normal, climatology dictates a forecast for some improvement. Farther east, winter is actually one of the drier times of year in southwestern Wyoming, and again with nothing favoring either dryness or wetness for the next few months, climatological considerations imply persisting drought. The forecast for the desert Southwest is a difficult one. Drought is forecast to persist with some eastward expansion into Arizona, based essentially on the December-February forecast and the fact that, though the season is relatively wet in some areas, this is one of the driest regions in the country climatologically, and dryness has been entrenched in the area for some time. This points toward persistence as the most prudent forecast, but also introduces substantial uncertainty should additional significant precipitation fall on the region, as happened in late November. If any subsequent system drops significant precipitation on the region, it could result in marked changes to the drought situation, since only a few inches of precipitation would be a substantial proportion of the normal annual total for the drier, lower-elevation areas. Other parts of the West, where normal precipitation is more abundant, are not so sensitive to individual storm systems.
Confidence for the West: Low in the desert Southwest; moderate elsewhere

For the Hawaiian Islands, with the climatologically wet time of year approaching, and considering the late November heavy rains and flash flooding, drought improvement is forecast.
Confidence for Hawaii: High

 

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Page last modified: December 4, 2008
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