US Drought Outlook
The Seasonal Drought Outlook is issued on the third Thursday of the month, and updated on the first Thursday. These outlooks analyze recent precipitation, weather forecasts, and expected trends to determine improvement or worsening of drought conditions.
Latest Seasonal Assessment - March 18, 2010
Following major drought reduction across California courtesy of an El Niño winter, additional drought relief is not expected in northern California. Improvement is forecast in Arizona where snow-water equivalent values are high and runoff from snow melt is expected to provide above-average streamflows. During the past month, drought has expanded across the interior Pacific Northwest and northern Rockies. Initial conditions consisting of a low snowpack, a seasonal forecast of above-average temperatures, and a drier climatology support drought persistence or development across the interior Pacific Northwest and northern Rockies. Due to an increasingly wet climatology by June, some improvement is forecast in northern Wisconsin. Drought has expanded across the Hawaiian Islands this winter, consistent with El Niño. Drought can be expected to persist or develop across Hawaii.
Discussion for the Seasonal Drought Outlook
Tools used in the U.S. Drought Outlook (USDO) included the official CPC precipitation outlook for April 2010 and the long lead forecast for April - June 2010, the four-month Palmer 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 (CAS) moisture, the Climate Forecast System (CFS) seasonal precipitation forecasts, El Niño precipitation and temperature composites for April - June 2010, climatology, and initial conditions (in the West, USDA/NRCS SNOTEL and Water Year to Date Precipitation).
Moderate to severe drought continues in northwest Wisconsin. The persistent drought in this area has lasted for at least 20 months (late Summer 2008), and any precipitation that has fallen during this period has not been enough to overcome long-term deficits. Official CPC mean forecasts out through the April-June 2010 season has equal chances for above or below-median, with the exception of below-median precipitation and above-normal temperatures expected for the April 2010 outlook, and subnormal precipitation in the 8-14 day period. CAS and CFS precipitation forecasts for the A-M-J season are a mixed bag, with CAS favoring wet conditions and CFS trending dry. However, with a wetter climatology during May and June, we expect some improvement in northern Wisconsin, although the drought is not expected to be eliminated due to the long-term hydrologic situation (very low lakes and rivers). The previous area of development (March 4 USDO) in northern lower Michigan was removed due to expected wetness in the short- and medium-range forecasts, and increasing precipitation climatology.
Forecast confidence for Wisconsin is moderate.
Similar to most of this winter, a wet pattern has generally prevailed across California and the interior Southwest during the past 3 weeks. This has led to additional reductions of drought and abnormal dryness in parts of Arizona and southern Nevada in the latest U.S. Drought Monitor (USDM) as precipitation and stream flows are generally at or above normal. Exceptions to this, however, included far northeastern California and adjoining northwestern Nevada where little relief has occurred. An increasingly dry climatology from April – June along with no signal for wetness at all forecasted time scales supports persistence in northeastern California. Enhanced odds for above-median precipitation indicated by the seasonal outlook combined with near-record high March 17 snow water content (SWC) and water year-to-date (WYTD) precipitation (since October 1, 2009) across much of Arizona and southern Utah, points to continued improvement in these 2 regions. In central Nevada, with only equal chances of precipitation and near-normal SWC and WYTD precipitation, this area was changed from improvement (March 4) to some improvement. Finally, developing drought was added to northern Utah, northwestern Colorado, and southwestern Wyoming despite the 3-month precipitation Outlook (odds tilted slightly wet). The development was kept after assessing current conditions that portrayed a poor WYTD season (61-87% of normal precipitation and 52-85% of normal March 17 SWC) that would require much-above normal precipitation where climatology totals are relatively low. Above-normal temperatures in both 1-month and 3-month Outlooks also favored development due to depletion of snowpack due to rapid snow melt.
Forecast confidence for California, Nevada, and Four Corners Region is moderate.
In the interior Pacific Northwest and northern half of the Rockies, subnormal precipitation has prevailed for much of the winter due to the southward displaced storm track and jet stream which is common during El Niño winters. Compounding the dryness was temperatures averaging slightly above-normal in the Northwest, contributing to subnormal snow pack and snow water content. As of March 17, most of the basin-averaged WYTD precipitation and SWC ranged between 55% and 80% of normal across the Northwest. Accordingly, some slight expansion of drought was made in the latest USDM where winter precipitation has been disappointing (southwest Montana, western Wyoming, southeastern Oregon and southwestern Idaho). With these values not expected to significantly improve in the next month or two, and 1- and 3-month temperatures forecasted to be above-normal (would lead to rapid snow melt), there will be less recharging of both soils and streams during the upcoming spring and related water supply issues. As a result, a large area of development was added across much of the Northwest, connecting the two large separate areas of persistent drought. However, some AMJ 2010 models (CAS, CSK, CFS, IRI) suggest a tilt in the odds away from developing drought for eastern Oregon, and with WYTD precipitation and current SWC close to normal, this area was excluded from development.
Forecast confidence for the interior Pacific Northwest and northern Rockies is moderate.
Due to El Niño and its associated lack of winter rainfall, drought has expanded across Hawaii. During early March, persistent trade winds resulted in an increase in rainfall across windward areas of the Big Island. Despite the recent increase in wetness, the precipitation deficit at Hilo is nearly 18 inches since January 1. Oahu remains the only major island in the State without drought but is classified as abnormally dry. With El Niño conditions expected to continue in the central Pacific at least through the spring, drought is forecast to persist or develop across the Hawaiian Islands.
Forecast confidence for Hawaii is high.
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