Latest Seasonal Assessment -
Lingering moderate to severe (D1-D2) hydrologic drought is expected to persist across much of
Washington, Oregon, Idaho, Wyoming and western Montana. Late summer and early fall are
generally not the best times of the year for the significant, widespread precipitation needed to
ease long term drought. In the Southwest, a late start to the monsoon will result in a shortened
wet season. In areas dependant on the monsoon for the majority of the annual rainfall total, this
may trigger moderate drought. Soaking rains from Hurricane Emily will result in drought
improvement across deep southern Texas, while some improvement is anticipated across eastern
Texas, southeastern Oklahoma and the lower Mississippi Valley. Improvement is expected
across the Great Lakes as the pattern becomes more favorable for precipitation and eventually
cooler temperatures. Some improvement in the severe to extreme drought (D2-D3) is expected
over northern Illinois.
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Tools
used in the Drought Outlook included the official CPC
long-lead precipitation outlook for August-October, the drought
termination and amelioration probabilities for October, 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.
A wet
spring has resulted in long term drought improvement over most of the
Great Basin, northern Rockies and northern High Plains. However, the wet spring
came on the heels of several years of below normal precipitation and anomalously
warm temperatures which plagued much of Idaho, Montana and Wyoming. Snowfall
during the preceding winter was below to much below normal in the Bitterroot,
Absaroka and Salmon River Mountains. As a result, moderate to severe (D1-D2)
long term drought continues across the region. Further west, the wet spring was
preceded by a very dry winter. Record and near record low snowpacks were
observed across Oregon and Washington. Despite the much needed moisture from
abundant spring precipitation, the deficits incurred during the winter were too
large to be overcome by the wet spring. As a result, moderate to severe drought
continues over large portions of these states. Because the months of August,
September and October are relatively dry in the northern Great Basin and
northern Rockies, the long term drought will persist in these areas. The next
chance for improvement will come in the autumn and winter when Pacific storm
systems once again produce rain and mountain snow across the region. In the
northern High Plains and Black Hills, late summer showers and thunderstorms may
result in some improvement. However, long term drought will likely persist
through the period. The best chance for improvement will come in spring, when
melt water from the mountains to the west recharge area rivers and spring
precipitation falls on the Plains.
In the
southwestern U.S., abundant rain and snow so far this water year has
resulted in precipitation amounts that are 100 to 200 percent of normal in much
of eastern Arizona and western New Mexico. However, pockets of moderate (D1)
long term drought persist due to drought in previous years and localized areas
of lighter precipitation. Lingering drought will continue during the period in
these pockets. The monsoon is quite late in arriving to the Southwest. In
Tucson, Arizona, the monsoon started on July 18. This is the second latest start
since 1949. The monsoon was preceded by 35 consecutive days with 100+ degree
heat. Other years that have seen a very late start to the monsoon were 1997,
1987 and 1979. In these years, rainfall was 70 to 90 percent of normal for the
period June-September. Therefore, there is a greater than average probability
that rainfall will be below average for June-September 2005. This is consistent
with the CPC seasonal forecast for August-October 2005. However, the deficits
incurred should not be sufficient to initiate severe drought in the area.
Nevertheless, the short falls could be enough to trigger moderate drought in
areas of Arizona and New Mexico which typically net the majority of their annual
precipitation during the late summer. Therefore, possible drought development
has been indicated in eastern Arizona, southern New Mexico and adjacent parts of
far western Texas.
Soaking
rains from Hurricane Emily will result in drought improvement across the
southern most counties of Texas. As September and October approach, rainfall
amounts tend to increase across eastern Texas and southeastern Oklahoma, as well
as the chances for drought improvement. Although drought improvement is
possible, there is uncertainty about the late summer and early autumn rains in
these areas. Further north, some improvement appears likely across Arkansas,
Missouri and Illinois. However, lingering drought effects may persist. Across
the Great Lakes, a more active storm track, increased precipitation and cooler
temperatures will ease drought across Michigan, Wisconsin, northern Indiana and
northwestern Ohio as fall begins.
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