May - July 2008

Latest Seasonal Assessment - The protracted drought across the interior Southeast and southern mid-Atlantic, continually depicted on the Drought Monitor since February 2007 in a few areas, has improved in recent months. The region should receive near or above normal rainfall through the first half of May 2008, but there is no discernable tilt of the odds concerning precipitation trends for late May - July 2008, and given the typical increase in water demand and evaporative losses as temperatures warm through late spring and summer, significant hydrologic drought relief is not expected through the end of July. In spite of the likelihood of continuing short term relief, substantial long-term precipitation deficits (and thus hydrologic drought) remain entrenched across the southern Appalachians, and are expected to persist through July 2008. The greatest likelihood for improvement exists in Virginia and the Tennessee Valley. Precipitation should average near normal through the first half of May 2008 in southwestern Florida, but the typical seasonal increase in precipitation during May - July is expected to improve conditions in the state's lingering area of drought. Farther west, a similar climatological trend toward wetter weather, in addition to the above-normal precipitation expected for the next week, should improve drought conditions in the northern and central high Plains. However, odds favor persistence or development across the southern high Plains, west and south Texas, and the Southwest. A small area of some improvement is forecast for south-central Texas due to rainfall expected during the upcoming week. Drought has recently developed in the foothills of Sierra and should persist due to a dry climatology. In Hawaii, the wet season has ended. Therefore, persistence or development is forecast for parts of Molokai, Maui, and the Big Island.

Tools used in the Drought Outlook included the official updated CPC precipitation outlook for May 2008 and the long lead forecast for May - July 2008, the four-month drought termination and amelioration probabilities, 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 moisture, the CFS seasonal precipitation forecasts, climatology, and to some extent La Niña composites.

Minor changes were made to the outlook issued 2 weeks ago, primarily based on the latest short- and medium-range forecasts, the updated monthly outlook from CPC, and a decline in the reliance on La Niña composites given weakening oceanic anomalies and the fact that La Niña has a less consistent impact on conditions across the country during this forecast period than during the colder times of the year.

Short- and medium-range forecasts all indicate near or wetter than normal across the interior parts of the Southeast and mid-Atlantic, but longer-term outlooks favor neither above- nor below-normal precipitation, and with the seasonal increase in temperatures comes an increase in water demand by residential, commercial, and agricultural interests, as well as an increase in soil moisture usage by all manner of plants, and an increase in evaporative water losses. For these reasons, improvement is expected on the northeast and northwest portion of the southeast drought area, mainly in the region currently experiencing D1 or D2 drought. For the remainder of the southeast drought area, some improvement is anticipated early in the period. In the absence of any discernible anomalous trends for the May and May - July 2008 periods, and given the aforementioned climatological considerations, it appears unlikely that the short-term precipitation will be sufficient to provide lasting, substantial relief for the areas currently experiencing protracted hydrologic drought.
Confidence: Moderate

Although little or no relief is expected in southwest Florida during May, the lingering drought should improve by the end of July courtesy of the onset of the rainy season.
Confidence: High

Wetter than normal conditions during the first two weeks of May along with climatological considerations lead to a forecast of improvement over the lower Mississippi Valley.
Confidence: High

Across the northern Rockies along with the northern and central high Plains, forecast of improvement is based on above-normal precipitation during the next week, along with the climatological increase in precipitation during the forecast period. Across the northern and central high Plains, widespread moderate to heavy precipitation will likely be ongoing at the beginning of the period. Across central Wyoming, some improvement is expected, primarily early in the period.
Confidence: Moderate

In Texas, substantial amounts of precipitation appear likely in early May, but the heaviest rainfall will likely occur outside of the drought areas. The monthly outlook for May calls for drier than normal conditions in west Texas, and below-normal precipitation is predicted for the May - July 2008 period in south Texas. With the seasonal increase in water demand and evaporative losses, drought is expected to persist in Texas and expand across Arizona and New Mexico where abnormal dryness is now indicated by the Drought Monitor. It should be noted that monsoonal rainfall is typically on the increase in late July across the western parts of this region, but with no indication at this point regarding the strength of the monsoon, and considering that late July is still very early in the typical monsoon season, relief as a result of monsoon-related rainfall was not considered in this forecast. A small area of some improvement is forecast for south-central Texas where medium range model guidance indicates the potential for heavy rainfall.
Confidence: Moderate

The Drought Monitor has recently shown improvement across much of the Intermountain West during the early spring based on a generally robust snowpack that accumulated during the winter, and the anticipation that as this snowpack melts, streamflows, reservoir storage, and soil moisture should all improve. However, changes in drought conditions over the next few months should be relatively small across the region with the snowfall season winding down, even at the higher elevations. In addition, the seasonal increase in water demand and evaporative losses during the forecast period, the typical seasonal decreases in precipitation across California and adjacent areas, and official forecasts generally favoring subnormal precipitation on most or all time scales led to a forecast of drought persistence or development through July 2008.
Confidence: High

In Hawaii, the wet season is ending. Therefore, drought persistence is forecast for the western half of Molokai. Drought development is likely in the upcountry area of Maui and the northwest slopes of the Big Island.
Confidence: High