Latest Seasonal Assessment -
The seasonal drought outlook for April-June 2011 is based in part on climate anomalies associated with the final stages of La
Niña, stream flows, soil moisture conditions, observed precipitation anomalies, mountainous snow water content, and the CPC
monthly and seasonal temperature and precipitation outlooks. Abundant precipitation in parts of the mid-Atlantic region, the
Ohio, Tennessee, and middle Mississippi Valleys during the past month has erased most of the drought/dryness in these areas,
and resulted in substantial stream and river flooding. Recent squall line activity has brought significant rainfall to northern
portions of the Gulf Coast states. Drought and dryness are expected to persist and/or intensify across the southern Atlantic
region, southern portions of the central Gulf Coast, the southern and south-central Great Plains, and the Southwest, as late
winter gives way to spring. The spring season in particular is usually the driest time of year for the Southwest, prior to the
climatological onset of the summer monsoon. Drought relief has occurred in Hawaii over the last few months, which is typically
the case during La Niña winters with increased storm activity (Kona Lows). For April-June, prospects for significant rainfall
across western slopes of the Islands diminish as the northeasterly trade winds once again become established.
Tools used in the U.S. Drought Outlook (USDO)
included the official CPC precipitation outlook for April 2011 and the long lead forecast for
April - June 2011, 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, La Niña composites for the April - June season, the four-month
Palmer drought termination and amelioration probabilities, climatology, and initial conditions.
Recent heavy precipitation has reduced drought
across portions of the mid-Atlantic region. For the past 30 days, surplus precipitation has been concentrated both near and
west of the Appalachians, with deficit precipitation east of the mountains, from the tidewater area of southeastern Virginia
to Florida. These deficits were greatest (75-100 mm) in the Florida Panhandle and adjacent parts of southern Georgia. Most
stream flows in the Appalachians currently range between the 75th and 89th percentiles (which are categorized as above-normal
when compared to the corresponding historical distribution for the day of the year). Much below normal stream flows (lowest
ten percent of historical distribution) are indicated by USGS stream flow gauges across the Piedmont region of North
Carolina, most of South Carolina, and in north-central Florida. The CPC 30-day outlook for April calls for equal chances
(EC) of below, near, and above-normal temperatures across the middle and southern Atlantic States, and for continued
above-median precipitation from near the spine of the Appalachians westward, and below-median precipitation across Florida,
Georgia, South Carolina, and southern portions of North Carolina. The official CPC 90-day temperature outlook depicts low
probabilities for abnormal warmth from Florida to the Virginia-North Carolina border, and EC north of there across the
mid-Atlantic region. The 90-day precipitation outlook calls for EC over most of this region, with the exception of the
Florida peninsula where there are slightly enhanced odds of deficit rainfall.
Forecast confidence for the southern Atlantic States is moderate to high;
Forecast confidence for the middle Atlantic States is low to moderate.
Across the Southeast, April-June La Nina
precipitation composites favor dryness in Florida and most of the Gulf Coast region, with fairly high frequency. ENSO
composites for April-June are generally regarded as the final set of useable composites for a particular ENSO season. The
monthly temperature outlook indicates equal chances (EC) of below, near, and above-normal temperatures across the region,
though the seasonal temperature outlook calls for enhanced odds of abnormal warmth. CPC’s monthly rainfall outlook indicates
robust probabilities for deficit rainfall across Florida and the central Gulf Coast states, though probabilities diminish
rapidly northward, with EC anticipated over far northern portions of Mississippi, Alabama and Georgia. For the AMJ season,
modest chances of below-median rainfall are indicated near the immediate Gulf Coast.
Forecast confidence for the immediate Gulf Coast States is high;
Forecast confidence for the remainder of the Southeast is moderate.
For about the past 6 weeks, the Arctic Oscillation
(AO) and the North Atlantic Oscillation (NAO) indices have been near zero or positive. In association with this, above-normal
precipitation returned to the Tennessee and Ohio Valleys, easing prior drought and even causing some flooding. The AO/NAO
pattern is expected to be less influential in the upcoming spring than it was during the winter. Across the lower
Mississippi Valley, conditions range from abnormally dry (D0) to extreme drought (D3). The hardest hit areas include western
Louisiana and southwestern Arkansas, where CPC’s calculated soil moisture anomalies in February ranged from 120-140 mm below
average. NLDAS soil moisture anomalies in the top 1 meter of soil during the past week are in line with the CPC anomalies. In
the past week, squall line activity has brought heavy rain (75-125 mm) to the central Gulf Coast states, bringing significant
relief to that region, but unfortunately missing the areas in the lower Mississippi Valley most severely impacted by drought.
The 30-day outlook for April 2011 calls for enhanced chances of above-normal temperatures and below-normal precipitation
across most of the lower Mississippi Valley, and above-median precipitation for the Ohio and Tennessee Valleys. For the
April-June season, the precipitation signal is much weaker, with equal chances indicated for all but Louisiana and southern
Mississippi, where odds modestly favor below-normal rainfall.
Forecast confidence for the Ohio and Tennessee Valleys is high;
Forecast confidence for the lower Mississippi Valley is moderate to high.
In Texas, drier and warmer weather since early
February has led to deteriorating conditions, with moderate (D1) or worse drought now enveloping much of the state. Soil
moisture is well below average (120-160 mm) over eastern Texas, with less serious deficits elsewhere across the state. The
Constructed Analog Soil Moisture Outlook (CAS) for the end of June 2011 anticipates below average soil moisture across
eastern portions of the southern Plains. La Nina composites for AMJ, climatology, and CPC’s monthly and seasonal outlooks,
all favor the persistence and intensification of drought for most of Texas, though limited relief is expected over far
northern areas near the Red River Valley.
Forecast confidence for Texas is high.
During the past 3 months, observed precipitation
was near-normal across a large portion of the south-central and central Great Plains. The exceptions included central Kansas
and especially Oklahoma, where the latter reported as much as 90-150 mm deficits in southern and eastern portions of the
state. Recent (February 2011) calculated soil moisture anomalies in eastern Oklahoma reveal deficits of 100-120 mm. The
unusually large number of wildfires still in progress over eastern Oklahoma can be attributed (at least in part) to these
factors, and the recent period of strong, gusty winds. Stream flows in the southeastern quarter of the state are generally
running in the lowest quartile, when compared to historical stream flow for the day of the year. The drought/dryness over
the south-central and central Great Plains is unlikely to expand northward into the Dakotas, given very high (above the 90th
percentile) soil moisture values and deep snow cover in that area. The CPC 30-day temperature outlook depicts elevated odds
of above-normal temperatures over Colorado, Oklahoma and much of Kansas, while the 90-day outlook keeps abnormal warmth only
over Colorado. For precipitation, the south-central and central Great Plains are expected to receive subnormal precipitation
during April, though this deficit area is trimmed westward to include only the High Plains for the AMJ seasonal outlook.
Forecast confidence for the south-central and central Plains is moderate to high.
During the past 90-days, drier than normal
conditions have affected much of Arizona and New Mexico, with southern portions of these states experiencing deficits
between 30-90 mm. Precipitation deficits and depleted soil moisture are typical of La Nina winters due to the northward
displacement of the jet stream and associated storm track. The March 13th SNOTEL report indicates basin-averaged snow water
content (SWC) ranges between 25-50 percent of average over the southern half of both Arizona and New Mexico, though SWC
values are considerably closer to average in far northern New Mexico. Drought degradation is most noticeable in central New
Mexico, and this is consistent with AMJ La Nina precipitation composites. Decreasing precipitation climatologies for areas
already in drought strongly support CPC’s April and AMJ 2011 outlooks which call for enhanced chances of above-normal
temperatures and below-median precipitation across New Mexico, Arizona, southeastern Utah, and southwestern Colorado.
Forecast confidence for the Southwest is high.
Heavy rainfall this winter has alleviated drought
and dryness on Kauai and Oahu and diminished drought conditions across most of the rest of the central and western Hawaiian
Islands which is typical for a La Nina winter. For the April-June forecast period, it is becoming less likely that the
western sides of the Islands will see substantial improvement as trade winds become more dominant heading toward the warm
Forecast confidence for Hawaii is moderate.