Latest Seasonal Assessment -
Heavy rains falling over the drought areas in Florida near the start of the forecast period were already having a
marked impact on rivers and groundwater, providing the most significant drought relief since Tropical Storm Fay
swept across the state last August. Additional rains are forecast to bring more relief during the first days of
the forecast period, while long-range forecasts indicate favorable odds for above-normal rainfall, supporting
further drought alleviation during the summer. Elsewhere, widespread rains from March to May reduced drought in
Texas and Oklahoma, but the bulk of the moisture stayed away from southern Texas, where severe to exceptional
drought persisted. The Outlook indicates that further improvement is on tap for central and northern parts of
Texas, but more limited improvement is expected for southern Texas. Farther west, improvement is forecast for the
Southwest, but the drought in California and Nevada should persist. In the Midwest, spring rainfall benefited
areas in northern Wisconsin and southeastern Minnesota affected by drought since last summer. Abundant rains
during May early in the forecast period are forecast to bring additional relief, but drier weather later in May
into early June, along with uncertainty for the season ahead, result in an outlook for some improvement with
ongoing drought. In Hawaii, where the dry season is established, drought is expected to persist on parts of the
Big Island, Maui, and Molokai, and expand into all of Lanai and the southwestern halves of Kauai and Oahu.
Tools used in the Drought Outlook included the
official CPC precipitation outlook for June 2009 and the long lead forecast for June - August 2009,
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, and climatology.
In Florida, a surface and upper-level low pressure
area brought heavy rains to much of the Peninsula from May 17 through May 20 preceding release of this
Outlook. Most of the Peninsula reported 2 to 10 inches of rain for the week ending May 20, with the 5-day
QPF for May 20-24 indicating another 1 to 7 inches. This had already resulted in much higher streamflows
across the state, with flood watches or warnings posted on May 20 from the Daytona Beach area inland.
Groundwater had already risen by 1 to 2 feet in response to the rains for several real-time USGS groundwater
sites in central and southern Florida. With the CPC 6-10 day and 8-14 day forecasts showing the odds favoring
normal to above-normal rainfall over Florida, and the June-August CPC seasonal outlook depicting enhanced
odds for above-normal rainfall over the Peninsula, the outlook for improving drought is one of the more
Forecast confidence for Florida is high.
For Texas, the demise of the La Niña and return to
neutral ENSO conditions, with the chance for development of an El Niño during the coming months, would seem
to increase the odds for additional improvement this summer. But the signal from the various analogue years
based on transitions out of La Niña to neutral or El Niño conditions is weak. In addition, the official CPC
June-August El Niño composites show above-normal rainfall for most of Texas but with relatively low
reliability. The NCEP Climate Forecast System seasonal model has a wet and cold bias for the summer months
over the U.S., but its tendency to show increased odds for above-normal rainfall going from southern Texas
toward northern Texas is consistent with some other dynamic models. The latest CPC 6-10 day and 8-14 day
forecasts show normal to above-normal rainfall for Texas, but there are big differences among the medium
range models. Unpublished Palmer Drought Index probability change maps for April to August indicate somewhat
better odds for improvement in South Texas than in areas to the north. Given this statistic, and the week 2
forecast showing above-normal odds for wetness along the Rio Grande, the small area of persisting drought
shown for the previous Outlook in the Brownsville area was removed and the area made part of the southern
Texas area showing some improvement.
Forecast confidence for southern Texas is low.
The improvement shown for the Southwest has been
largely based on climatology, with the summer monsoon rains generally reliable in easing drought conditions,
at least for soil moisture, grasslands, and wildfire danger. Short-term forecasts are showing abnormally wet
weather already kicking in during late May, and there are some indications for above normal June-August rains
for parts of the Southwest, as shown in the official CPC precipitation outlook.
Forecast confidence for Arizona, New Mexico, Colorado, and West Texas is high.
The persisting drought forecast for
California, Nevada, and parts of Oregon, Washington, and Idaho is based on climatology, as this is the dry
season and, in the Northwest, the CPC seasonal outlook tilting toward below-normal rainfall. The outlook for
southwestern Idaho changed from some improvement to persistence because of fewer indications for wetness over
the medium range out to 2 weeks. The small area of lingering drought in southwest Wyoming into southeast
Idaho continues to show some improvement due largely to robust rains expected during the first 5 days.
Forecast confidence for the West is high.
The Outlook for the drought area in northern
Wisconsin and southeastern Minnesota changed to some improvement from improvement due to drier indications
for the 6-10 day and 8-14 day period and a number of seasonal statistical and dynamic forecasts that are
ambivalent at best for June-August. Summers transitioning out of La Niña tend to tilt toward dryness in this
area. In contrast, over 1 inch of rain is forecast for much of the drought area during the first 5 days of
the period, and this should easily lead to short-term improvement.
Forecast confidence for the Wisconsin-Minnesota area is low.
In Hawaii, the dry season is in progress, making
this a difficult time of the year to relieve drought. In addition, the official seasonal forecast tilts the
odds toward below-normal rainfall. As a result, current drought areas on the southern islands are expected to
persist, while the odds favor drought developing over Lanai and the southwestern halves of Oahu and Kauai.
Forecast confidence for Hawaii is moderate.