March-May 2005

Latest Seasonal Assessment - Storms have continued to ease drought across the Southwest and Great Basin, and the latest seasonal drought outlook calls for additional improvement for lingering drought areas. Medium and long-range seasonal outlooks, including the latest official Climate Prediction Center outlook, show a tendency for above-normal precipitation over much of the region, while the latest water supply forecasts call for above-normal streamflows during the upcoming spring-summer season. Some large reservoirs remain below normal, but are starting to recover. Lake Mead's elevation rose 9 feet from January 1 through mid-February, although the lake was still some 80 feet below capacity. In sharp contrast, near record-low snowpack in many locations has resulted in drought expanding across the Northwest. With the latest CPC outlooks calling for above-normal temperatures and below-normal precipitation during March-May, drought should persist from Washington and Oregon into western Montana. Drought should also see little improvement across southern Montana and northern Wyoming. Seasonal rain and snow could provide limited drought relief across central and eastern Montana as well as southern Wyoming, the western Dakotas, and western Nebraska. Because drought has persisted so long over the High Plains, normal precipitation would tend to ameliorate conditions but not lead to full recovery. Above-normal precipitation would be needed for major improvement. An active storm track should bring improving drought conditions to southern Nebraska and northern Kansas. Elsewhere, rainfall has been below normal from the Carolinas into Florida, with portions of South Carolina and the Florida peninsula measuring less than one-half their normal rainfall since October. At this time, development of large-scale drought is not expected, but this area bears watching.

Tools used in the Drought Outlook included the official CPC long-lead precipitation outlook for March-May, the drought termination and amelioration probabilities for May, 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 Analogues for the season, and the latest western water supply forecasts.

The current drought outlook focuses on the March-May precipitation outlook and considers current mountain snowpack and the latest spring-summer western streamflow forecast from USDA and NOAA. Near-record low snowpack in much of the Northwest has contributed to recent drought development in Oregon, Washington, northern Idaho, and western Montana, while the long-term drought persists over southern Idaho, the rest of Montana and much of Wyoming. Above-normal temperatures this winter have aggravated the situation caused by the below-normal precipitation. With the CPC March-May outlook indicating odds favoring more of the same, the drought outlook shows persisting drought across the Northwest. Both medium and long-range dynamic models tend to favor an upper air circulation pattern that features a Rex-type blocking pattern featuring anomalous ridging near British Columbia and troughing to the south near the U.S. Southwest. This would keep unseasonably dry weather over the Northwest and a southern stream that would transport Pacific storms into the Southwest. With more heavy rains forecast across the Southwest during the first week after this Outlook's release, and the region already experiencing abnormally high snowpack, saturated soils, and high river levels due to previous storms, the Outlook's depiction of continued improvement in this region is quite straightforward. Nevertheless, the desert Southwest typically sees a dropoff in rain after March accompanied by an increase in temperatures, so much of the additional drought improvement should come early in the outlook period. With the Colorado Reservoir system only slightly more than half full, it will take a long time before reservoirs fully recover from the drought that began in the Colorado River Basin in late 1999. But the 9-foot rise in Lake Mead's elevation since January 1 of this year is encouraging, as is the forecast for inflow into the River reservoir system reaching 110 percent of normal this water year.

The most recent runs of the medium-range models and the seasonal Coupled Forecast System model show that parts of the Northwest may see near normal precipitation, while other seasonal models such as the Constructed Analogue Soil moisture model and the IRI Multi-Model Ensemble show below-normal precipitation for March-May, as does the latest CPC official precipitation outlook. The odds favor the drought to persist despite some breaks in the dry pattern because of the very low snowpacks. To the east, the seasonal outlook for precipitation is more uncertain, with the CPC forecast showing equal chances of wet or dry over the northern Rockies and the northern High Plains for March-May. Unlike the previous month's Outlook, limited improvement instead of persisting drought is indicated for much of the northern High Plains drought region due to the seasonal trend toward higher precipitation amounts during spring, as shown in the NCDC drought amelioration maps. Also, the latest CFS runs show a tendency toward above normal precipitation for this region during March-May, and this is especially pronounced in May, one of the wettest months of the year. In contrast, the CFS also shows that the odds favor above-normal temperatures for March-May, especially in the northern Wyoming area. Balancing the odds for wetness against the odds for warmth results in an area of persisting drought extending across northern Wyoming and adjacent Montana, as this area has the weaker wet signal, the stronger heat signal, and is historically more likely to see improvement later in the spring rather than during March-May. In southern Nebraska and northern Kansas, the storm track favors increased moisture for the first 2 weeks of the outlook period, and the region's climate would favor improvement by the end of May as well.

Erratic rains since October have left parts of the Southeast abnormally dry, with the Florida southern peninsula especially close to drought status as of mid-February, and little rain forecast for the first week of the outlook period. Some seasonal models show below-normal rainfall for Florida for March-May, so it is quite possible that parts of Florida will see drought development by May. However, the dryness signal was not strong enough to result in the official CPC precipitation outlook for March-May showing dryness in Florida or the Southeast, and medium-range models suggest a wetter pattern is on tap by early March, so no area of development is indicated in the drought outlook at this time.