Latest Seasonal Assessment -
In August and early September, tropical weather systems continued to eat away at drought in the South and Southeast, and
Hurricane Hanna’s forecast heavy rains appeared likely to further reduce drought on September 5-6. The rains from Hanna were
forecast to drench drought areas from the eastern Carolinas into southern New Jersey, but largely miss drought areas in the
interior. There was also a possibility that Hurricane Ike could deliver more moisture to the Southeast during the second week of
September, although Ike’s ultimate track was very uncertain as of September 3. The overall forecast through November depicted
overall improvement for drought across the Southeast, as well as several areas of drought in the Plains, and for
recently-developed dry areas in the Midwest. In the Southeast, the expected moisture from tropical storms and other weather
systems will boost soil moisture and streamflows, but a few of the larger reservoirs in the interior Southeast, such as Lanier
in Georgia, are unlikely to recover before winter. In the West, although Pacific storms will likely begin to ramp up moisture
levels in northern and central California by the end of November, most of the region should not see significant improvement
until later in the winter, when seasonal snows starts piling up.
Discussion for the Seasonal Drought Outlook
Tools used in the Drought Outlook included the
official precipitation outlook for September 2008, the
long lead forecast for September-November
2008, 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 CFS seasonal precipitation
forecasts, and climatology.
TS Fay influenced the drought improvement across the Southeast in
last month’s Outlook, and now the impact from Hanna, which is expected to strike the South Carolina or North Carolina coast
as a Cat 1 hurricane on September 6, is having an influence on the updated Drought Outlook. With 2-4 inches of rain forecast
from eastern South Carolina northward through the coastal mid-Atlantic states as of September 3, when this forecast was
prepared, Hanna should bring drought alleviation to eastern areas of North Carolina and Virginia, as well as Delaware and
southern New Jersey, but miss the more severe drought areas farther inland. Farther west, Gustav’s rains obliterated drought
in Louisiana and may benefit dry areas in the Great Lakes region during the first days of this forecast period. In contrast,
a core drought area between the rains from Gustav and Hanna will likely see little relief early on, and this includes much of
Tennessee, Kentucky, and western parts of the Carolinas and Virginia, as well as much of Georgia. Out in the Atlantic, TS Ike
is expected to become a hurricane shortly, and appears likely to be approaching the Florida-Cuba area by September 8. The
final track for Ike is very uncertain, so Ike’s rains are not explicitly considered in this drought Outlook. However, the
active tropical season seems to justify the continued broad-brush approach to drought improvement across the Southeast. Again,
it should be noted that hydrological drought indicators such as large reservoirs and groundwater will respond more slowly
than small streams, soil moisture, and pasture conditions. As water demands decrease later in the fall, reservoir levels
should show gradual improvement. As for now, Lake Lanier in northern Georgia is near all-time low storage for this time of
the year, and the rains from Fay only caused a blip upward in elevation.
Confidence for the Southeast and mid-Atlantic: High along the coast, but moderate over the interior
In Texas, western Oklahoma, and southeast Colorado, medium range
forecasts as well as climatology continue to support improvement for lingering drought. Climatology supports less chance for
improvement to the west in Colorado, resulting in the depiction of more limited improvement in that area.
Confidence for Texas, Oklahoma, and Colorado: Moderate
Dry weather in August contributed to patchy drought across the
Midwest. The outlook for these small areas is very uncertain given forecasts for equal chances wet or dry in the 30 and
90-day time frames by CPC and mixed messages from medium range indicators. Given moderate rains forecast for the first 5
days, with cool weather, the wet forecasts from some of the medium range models, and the continued tendency for the Coupled
Forecast System to lean wet in this area for the September-November 2008 period, improvement is forecast. Most of the rains
from Gustav are forecast to fall between the driest areas in the region, so relief may not be imminent. For the longer term,
the Constructed Soil Analogues maintain dryness in Wisconsin to year’s end, while the latest GFS model runs shows at least
3-4 inches of rain in the next 15 days. Such diverging indications keep confidence low.
Confidence for the Midwest: Low
There were minimal changes to the previous outlook for North Dakota
and eastern Montana, with greater improvement forecast to the east and less to the west. Although damp and cool weather
support improvement for this area in the first 5 days, medium-range forecasts are dry and climatology supports the idea of
relatively less improvement from extreme western North Dakota into eastern Montana. The area of development forecast in the
previous Outlook for western Montana has been dropped as recent rain, snow and cool weather made drought less likely.
Across the West, the beginning of the wet season in the fall
should lead toward some improvement across northern California and in eastern Washington. In California, although rain and
mountain snow increase later in the fall, it will take time to build a snow pack in the Sierras.
Confidence for the West: High
In Hawaii, no changes were made to the previous outlook, with
development forecast in some leeward areas and improvement forecast in windward sides of the Big Island.
In Puerto Rico, climatology and prospects for abundant tropical
rains in coming weeks continue to suggest drought improvement.