May - July 2008

Latest Seasonal Assessment - The protracted drought across the interior Southeast and mid-Atlantic regions, continually depicted on the Drought Monitor since February 2007 in a few areas, has improved in recent months from a relatively short-term standpoint, significantly reducing surface moisture and agricultural concerns for the time being. However, substantial long-term precipitation deficits (and thus hydrologic drought) remain entrenched, and are expected to persist through July 2008. The region should receive above-normal rainfall through the last half of April 2008, but there is no discernable tilt of the odds concerning precipitation trends for May - July 2008, and given the typical increase in water demand and evaporative losses as the weather warms up through late spring and summer, it does not appear that the next two relatively wet weeks would be sufficient by themselves to engender significant hydrologic drought relief through the ensuing 3 months. In contrast, precipitation should average near- to below-normal through the last half of April 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 couple of weeks, should improve drought conditions in the northern Plains and Rockies. South of this area, a similar trend toward wetness through the last half of April 2008 should bring some improvement to the dry areas across Wyoming, the west-central Plains, and part of central Texas; however, the odds favor below-normal precipitation for May 2008 across the central and southern High Plains, and for May-July in the west-central Plains and Wyoming, so any improvement should be limited. Finally, with dry weather anticipated on most time scales ranging from the next 5 days to 3 months, drought is expected to persist in most of the southern High Plains, southern sections of Texas and New Mexico, and the remaining drought areas across the northern Intermountain West, the Great Basin, and the desert Southwest. In fact, drought conditions are forecast to expand somewhat northwestward in southeastern Arizona and New Mexico by the end of the period.

Tools used in the Drought Outlook included the official CPC long-lead precipitation outlook for May 2008 and 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.

Some substantial changes were made relative to the outlook issued 2 weeks ago (which only extended through June 2008), primarily based on climatological considerations, the latest short- and medium-range forecasts, the new monthly and seasonal outlooks from CPC, and a decline in the reliance on La Niña composites given weakening oceanic and atmospheric 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.

The short- and medium-range forecasts all look wetter than normal across the interior Southeast and the mid-Atlantic regions, 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, some improvement is anticipated early in the period. However, in the absence of any discernable 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 from the protracted hydrologic drought.
Confidence: Moderate

Although the next few weeks favor neither wetter nor drier than normal conditions across southwestern Florida, the lingering drought in the region should improve with the onset of the wetter time of year.
Confidence: High

Across Montana, North Dakota, and adjacent parts of Wyoming and South Dakota, the forecast of improvement is based on generally above-normal precipitation forecasts on most time periods through the next two weeks, along with the climatological increase in precipitation during the forecast period.
Confidence: Moderate

Across much of Wyoming and the west-central Plains, some improvement is expected, primarily early in the period, based on the climatological increase in precipitation during late spring and forecasts for near- to above-normal precipitation through the last half of April 2008. However, the odds slightly favor subnormal precipitation for May 2008 across the west-central Plains, and for the May - July 2008 period farther west, making substantial and widespread improvement that continues through the end of the forecast period unlikely.
Confidence: Moderate

In southern and western Texas, southern New Mexico, and the southern High Plains, decent amounts of precipitation appear likely in late April 2008 across the easternmost parts of the region, but the short- and medium-range forecasts keep substantial precipitation out of other parts of the region, the monthly outlook for May calls for drier than normal weather throughout the area (though with a relatively small tilt of the odds), and below-normal precipitation is also favored for the May - July 2008 period in northwestern parts of this region. Given all these factors, along with the seasonal increase in water demand and evaporative losses, a small area of limited early improvement has been made for central Texas, with drought expected to persist in other parts of the region and even expand northwestward in southeastern Arizona and in New Mexico by the end of July 2008. 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.
Confidence: Moderate

The Drought Monitor has recently shown improvements across much of the mountainous and Intermountain West south and west of Montana during the past few weeks based on a generally robust snowpack that accumulated during the colder time of year, and the anticipation that as this snowpack melts, streamflows, reservoir storage, and soil moisture should all improve. However, having already made adjustments for these considerations, 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 through July 2008.
Confidence: High