Latest Seasonal Assessment -
During March, drought has expanded across Idaho, western Montana, and western Wyoming. Although a wet pattern is expected at
the beginning of April, a continued low snowpack, a seasonal forecast of above-average temperatures, and a drier climatology
support drought persistence and development across the interior Pacific Northwest, northeast California, northwest Nevada, and
the northern Rockies. Improvement is forecast in Arizona where snow-water equivalent values are high and runoff from snow melt
is expected to provide above-average streamflows. With the updated April outlook calling for enhanced odds for above-median
precipitation, improvement is forecast for the small drought area in northern Colorado. Enhanced odds for above-median
precipitation during early April coupled with an increasingly wet climatology support some improvement in northern Wisconsin.
However, the protracted hydrological drought is expected to persist into the early summer.
Drought has expanded across the Hawaiian Islands this winter, consistent with El Niño. Drought can be expected to persist
across the leeward side of the Hawaiian Islands, while rainfall associated with the trade winds is expected to prevent
development along the windward areas of the Hawaiian Islands. Due to dry initial conditions and enhanced odds for above-average
temperatures from April – June, development is forecast in parts of central Alaska.
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).
Although moderate to severe drought continues in
northwest Wisconsin (has persisted during the past 21 months), official CPC mean forecasts have wet weather expected in the
short-term (6-10 and 8-14 day periods), and slight odds for above-normal precipitation in the April 2010 outlook. The
April-June 2010 seasonal outlook has equal chances for above or below-median, with the CAS and CFS A-M-J precipitation
forecasts a mixed bag (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).
Forecast confidence for Wisconsin is moderate.
Unlike most of this wet winter
(especially January), a drier pattern has prevailed across southern California and the interior Southwest during the past 2
weeks. An increasingly dry climatology from April–June, along with near- to drier-than-normal conditions in the
short-term (6-10 and 8-14 days), initially points to no immediate improvement in Arizona. However, with enhanced odds for
above-median precipitation in the monthly and seasonal outlooks and near-record high March 30 snow water content (SWC) and
water year-to-date (WYTD) precipitation still lingering across much of Arizona and southern Utah, improvement in Arizona
should continue. In central Nevada, with only equal chances of precipitation and near-normal SWC and WYTD precipitation, this
area may only see some improvement. In a change from the March 18 USDO, developing drought was removed from extreme northern
parts of Nevada and Utah, northwestern Colorado, and southwestern Wyoming, and improvement was made for north-central
Colorado, in line with increased odds for above-normal precipitation in the 1- and 3-month precipitation Outlooks. In the
short-term (6-10, 8-14 days), however, near- to subnormal precipitation is expected in the region.
Forecast confidence for California, Nevada, and Four Corners Region is low.
For a change, moderate to heavy precipitation
(1 to 4 inches, locally to 8) fell on most of the Pacific Northwest and northern Rockies during the final week of March,
quite a contrast from the subnormal precipitation that 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 winter dryness was temperatures averaging
slightly above-normal in the Northwest, contributing to subnormal snow pack and snow water content. With the recent bout of
precipitation, most of the basin-averaged WYTD precipitation and SWC held steady or slightly increased, but still averaged
between 60% and 80% of normal across the Northwest. With wet and cool weather forecast in the short (6-10 and 8-14 days) and
medium (April) term, along with some D0 reduction in the Cascades in the latest USDM, development (as drawn in the March 18
USDO) was removed in western sections of Washington and Oregon. Likewise, the March 18 USDO development in central Montana
and Wyoming was erased in response to predicted short- and medium-term wetness. In addition, a forecast of above-median
precipitation in the 8-14 days and April outlooks may make some improvement in northern California. The large area of drought
persistence, however, was kept across the interior Northwest in response to probabilities of only equal chances in the
monthly and seasonal precipitation outlooks, and the drier initial conditions (lower WYTD precipitation – 58 to 70% of
normal; March 30 SWC values – 53 to 66% of normal) as compared to areas farther to the west and east.
Forecast confidence for the interior Pacific Northwest and northern Rockies is low.
This winter, the El Niño event and its associated
lack of winter rainfall expanded drought across much of Hawaii. During March, however, a weakening El Niño and increased
trade winds resulted in frequent shower activity across windward portions of the Big Island, Maui, Molokai, and Oahu. In
response to the rains, moderate drought was alleviated on the windward sides (except in Kauai) on the USDM, although it
remained on the leeward sides. With the increasingly persistent trade winds, the windward sides should see sufficient
moisture to starve off drought impacts, including Kauai which was given a depiction of improvement. In contrast, the leeward
sides should experience persistence of drought, or development where D1 has yet to occur.
Forecast confidence for Hawaii is moderate.
Abnormal dryness currently exists in parts of
central Alaska where snow-water content values are 25-49% of normal and precipitation deficits since December 1, 2009 are
large. With above normal temperatures favored across much of Alaska, development is forecast in central Alaska.
Forecast confidence for Alaska is moderate.