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National Drought Summary -- February 2, 2010

The discussion in the Looking Ahead section is simply a description of what the official national guidance from the National Weather Service (NWS) National Centers for Environmental Prediction is depicting for current areas of dryness and drought. The NWS forecast products utilized include the HPC 5-day QPF and 5-day Mean Temperature progs, the 6-10 Day Outlooks of Temperature and Precipitation Probability, and the 8-14 Day Outlooks of Temperature and Precipitation Probability, valid as of late Wednesday afternoon of the USDM release week. The NWS forecast web page used for this section is: http://www.cpc.ncep.noaa.gov/products/forecasts/.

The past week can be summarized by noting the active storm track along the southern contiguous United States.  A storm system that impacted the desert southwest at the start of the week developed into a vigorous low-pressure system across the southern plains by week’s end.  Heavy precipitation fell from Texas to the Mid-Atlantic with widespread damage from wintry precipitation felt as far south as Texas.  Light precipitation from weak impulses moving onshore across the Pacific Northwest brought rain to the coastal regions but below-average precipitation to the interior portions of the Intermountain West and Sierra Nevada range.  By month, another impulse in the southern stream linked up with one of the weak impulses moving across the northern Rockies.  As they moved eastward, the systems produced precipitation as snow from Chicago to the Mid-Atlantic and heavy rains down into Florida. 

The Southwest:  Most of the changes to the depiction were made in the Southwest.  As a result of a reassessment of the area of extreme drought (D3) in northeast Arizona, this area was removed. Standardized Precipitation Index (SPI) charts generated by the High Plains Regional Climate Center (HPRCC) for 30-days back through 6 months indicate that the worst conditions are representative of severe drought (D2) conditions.  Additionally, National Centers for Environmental Prediction-North American Land Data Assimilation System (NCEP-NLDAS) ensemble soil moisture values show conditions ranging between D0 (anomalous dryness) and D1 (moderate drought), while the objective blends also show complete relief of the drought.  According to preliminary PRISM rainfall estimates for the month of January, rainfall totals were at least 150 percent of normal for most of the state.  The areas with rainfall totals ranging from near normal to 130 percent of normal are where extreme drought conditions are retained.  Snowpack values across Navajo, Apache, and northern Coconino Counties in Arizona are well above normal, ranging from 130 to greater than 180 percent of normal).

Across southeastern Arizona, widespread rains (0.5-1.5 inches) allowed for some trimming of the moderate drought coverage across Pima, Pinal, Graham, and Cochise counties.  The areas improvement that resulted in the removal of all drought indications included southwest New Mexico and Greenlee County, Arizona, where rains approximately 1.0 inch fell during the period.  Additional improvements were made across southern and central New Mexico.  Mountain snowpack measurements indicate that much of the area has snowpack well above normal (180 percent of normal).  Reports indicate record high snow-water equivalent (SWE) values of 225 percent at the Sierra Blanca SNOTEL site. 

Abnormal dryness was also removed from La Paz County in southwestern Arizona to have the depiction in the U.S. Drought Monitor reflect 30-, 60-, 90-day, 12-month SPI charts (all positive which indicated wetness), soil moisture values from NCEP (70th-80th percentile) and 14-day percent of normal precipitation totals (400-800 percent of normal).

The Northern Rockies and Northwest:  A reassessment of the depiction across north-central Washington resulted in a decrease in the spatial extent of the area of severe drought conditions and a southward shift of the area.  The new area better aligns with the areas of below normal precipitation starting 12 months ago.  The northern edge of the previously indicated area had received above-normal rainfall for the 2009-2010 water year.  Omak, WA reported 5.91 inches compared to a normal of 5.27 inches for the current water year. The reported rainfall at Omak was 66 percent of normal for all of 2009, so the wetness is a rather recent change.  Across Douglas and Chelan counties, limited precipitation and warmer temperatures have aided field work, but done nothing to alleviate the drought conditions.

In Montana, a continued lack of major precipitation (0.25 inch or less for most of Montana) resulted in some improvements and some degradation.  The area of abnormal dryness was expanded in western Montana where snowpack ranged from 25-69% of normal and SPI values indicated D0-D1 conditions for timeframes from 30 days to 12 months.  Drought conditions near Blaine County have been persistent on the same timescales.  Near normal stream flows, as reported by the United States Geological Survey, are mitigating the expansion of drought conditions. 

Most of Wyoming and Northern Colorado appear to be some of the driest locations across the contiguous 48 states when considering percent of normal precipitation for the past 6 months.  Precipitation over the previous week was scattered and light (1.0 inch in the high terrain to 0.0 in the low country), prompting a further expansion of D0 across portions of Wyoming.  In Northern Colorado, snowpack and snow water content are ranging from 50 to 70 percent of normal, so borderline for inclusion in the area of abnormal dryness.  Reservoir storage in these areas is very good, so that mitigates the overall depiction of drought conditions.

A small area of improvement was made across Shasta and Tehama counties in Northern California.  Rainfall totals of 1.8 and 1.4 inches came in from two stations in this region.

Texas:   A 1 category improvement, based on rainfall totaling 1.0 to 2.8 inches, was made across southwestern Texas from Terrell County to LaSalle County.  The core area of the drought region was maintained as severe drought (D2) but reduced in size.  January was the 4th wettest at Del Rio, TX in a record stretching over 100 years while San Antonio logged the 10th wettest January since 1871.  The severe drought area was removed from the drought area near Corpus Christi in response to light rains (0.1-0.2 inch), good stream flow (near normal in the area indicated as D2 on pervious maps), lack of reported impacts from drought, and reported impacts of too much mud.  Across northwestern Texas, rainfall ranging from 0.5 to 1.7 inches prompted the removal of much of the area of abnormal dryness.  Two- and six-month weighted SPI values from the Office of the State Climatologist of Texas supported the retention of D0 from Howard County to Stonewall County.

The Upper Mid-West:  No changes were made in this region.  The Lake Effect Snows are falling on frozen ground so soil moisture is not increasing but water demand during the winter is minimal.

Hawaii and Alaska: In Hawaii, another mostly dry week, except for rains across Kauai (1.1 to 2.9 inches) and Oahu (0.4 to 4.2 inches).  Reports from Maui indicated rains near 0.5 inch while reports on the Big Island showed less than a quarter inch.  The continued lack of rainfall this week prompted an additional area of extreme drought across the southern growing areas of the Big Island.

In Alaska, little or no precipitation fell across much of the state.  Reports from near the southern shore, adjacent islands, and along the panhandle indicate precipitation amounts ranged from 1.4 to 5.1 inches.  The frozen ground and dormant vegetation provide minimal demand for ground water, so the depiction was maintained from last week.

Author: Matthew Rosencrans, NOAA/NWS/NCEP/Climate Prediction Center

D0 ... Abnormally Dry ... used for areas showing dryness but not yet in drought, or for areas recovering from drought.

Drought Intensity Categories
D1 ... Moderate Drought
D2 ... Severe Drought
D3 ... Extreme Drought
D4 ... Exceptional Drought

Drought or Dryness Types
A ... Agricultural
H ... Hydrological

Updated February 3, 2010