Drought Monitor: Rain, cold front moves east across Great Plains

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Weather Summary: The prior week featured a couple of storm systems that produced significant rains across the Pacific Northwest, Upper and Middle Mississippi River Valley, and Ohio Valley.   Early in the week, precipitation spread eastward along a warm front that extended from the Northern Great Plains to the Northeast.  South of the warm front, some tropical moisture was able to stream northward across the southeast.  Later in the week, precipitation was focused along a cold front that moved from the Great Plains to the east coast by Tuesday.

The Northeast and mid-Atlantic:  Significant rains fell across many portions of the northeast, with some parts of Pennsylvania receiving nearly 3.0 inches of rain (0.5 – 1.5 inches was more common across New Jersey and New York).  Much of the rainfall occurred west of the Appalachians, missing the driest areas along the coast.  Where the rains did encroach on the areas depicted in moderate (D1) or severe (D2) drought, the rains were not enough to bring the 30-day totals back to near normal.  As a result, the drought depiction remained nearly unchanged, except for some improvement where the rainfall totals were higher (1.0-3.6 inches) across Pennsylvania, west of the Susquehanna River.  Rains continued to fall across the region near the data cutoff.

Across Maryland, dry conditions continued, with the moderate drought conditions being expanded westward to near the triple point of Virginia, Maryland, and West Virginia.

The Southeast and Tennessee Valley:  A plume of tropical moisture moved northward across the Gulf of Mexico and brought heavy rains to portions of Alabama, Florida, Louisiana and western Georgia.  The rains prompted some trimming of each drought level across southwestern Alabama and extreme western Florida.  Extreme drought was removed from Okaloosa and Santa Rosa Counties in Florida, and along the I-65 corridor, north of Mobile.  As a result of isolated convective rains (1.0 - 2.4 inches), reductions in the coverage of drought conditions were also pursued across west-central Georgia and eastern Alabama.

Across northeastern Florida, the rains missed the areas already under severe or extreme drought, so D4 (exceptional drought) was expanded to cover Saint Johns county.  Additional expansion of D3 (extreme drought) was included over Flagler County.  Standardized Precipitation Index (SPI) values for the past 3 months indicate moderate drought, but SPI values keyed to longer periods of record (6, 9, and 12 months) all indicate extreme or exceptional drought across this region.

Most of the rains with the cold front later in the week fell west of the Appalachians, resulting in the trimming away of small areas of D0 (abnormal dryness) across Tennessee.  What rains did make it across the southern Appalachians; to the Carolinas were enough to stem the tide of drought.  No changes were pursued across North Carolina or South Carolina.

The Ohio Valley and Midwest:  Moderate drought conditions expanded over eastern Missouri and western Kentucky, where severe drought was introduced across Caldwell and Hopkins counties.  Drought indicators, such as SPI values for 3 months and Percent of normal Precipitation for 90-days or less, indicate much drier conditions than indicators based on longer time periods.  Percent of normal precipitation at 30 and 60 days were used to shape the current depiction, with SPI being used to determine the intensity.

Due to recent rains (0.75 – 5.0 inches) and some moistening of soils, improvements to the drought conditions were pursued across Indiana and Illinois.  A 1-category improvement was made across this region.   Land Data Assimilation System (LDAS) data from NASA continues to depict surface and root zone soil moisture as below normal, but showed increased wetness compared to last week.

The Northern Plains: Improvements were also indicated from the Northern Great Lakes region to the northern Great Plains.  Recent rains (1.0 – 5.0 inches) prompted 1 and locally 2-category improvements over Iowa and southern Minnesota.  Across Iowa, precipitation since July 15, 2011 (a period of unusually dry weather for NW Iowa) still shows substantial deficits (only 64 percent of normal except for a few scattered counties).   However, stream flows have rebounded a great deal and soil moisture has also increased substantially with recent rains.

Across Nebraska, long-term deficits in precipitation and soil moisture are still prevalent.  To maintain current conditions, which means receiving rainfall at climatological levels, areas of Nebraska would need to receive nearly 1 inch per week  Anything less, combined with increasing agricultural needs (0.20 inches per day for corn fields), would lead to a drying of the soils.

Across the Dakotas and much of Minnesota and Wisconsin, further reductions in drought coverage were depicted.  Heavy rain came to eastern South Dakota (top national CoCoRaHS report of 6.22 inches as of Sunday morning).  This fell right on the D1 region in eastern South Dakota.  Across Wisconsin and the UP of Michigan, SPI values out to 9 months, and precipitation departures from normal over the past 30 and 60 days, were used to redefine the drought depiction.  Moderate drought is now impacting a smaller area.

Central and Southern Plains:  Reductions in the coverage of exceptional (D4) and extreme (D3) drought were made across western Texas and southeastern New Mexico.  Rains this week were not exceptional, but a recent wet pattern has helped to alleviate some of the dryness across that region.  Heavy rains across central Texas, occurring right up to the data-cutoff, prompted improvements there, but poor groundwater storage and slowly responding reservoir levels continue to mitigate the recovery, so the only modest reductions in coverage were indicated.

Across southwestern Kansas, River Forecast Center (RFC) precipitation data indicated small pockets of significant rains (0.5 - 1.5 inches), so small areas of D0 and D1 were removed.

The West: Across Utah, moderate and severe drought conditions were expanded across the western portions of the state.  Data from the United States Department of Agriculture, Natural Resources Conservation Service (NRCS) continued to indicate low snow-water equivalent (SWE) values across most of the state.  The conditions are impacting inflows into the major reservoirs across Utah and Colorado.

Recent wetness across the Pacific Northwest and Northern Rockies (30 and 60-day periods) resulted in the removal of some D0 from Oregon, Washington, and eastern Montana.  The rains have missed much of central and western Montana, Idaho, and Nevada as the moisture is squeezed out over the higher terrain.  As a result, some areas of abnormal dryness were added to Idaho, Wyoming, and central Montana.  The drought impact lines were also redrawn over Nevada and Utah to indicate the lengthening of this dry period beyond 6 months.

Hawaii, Alaska and Puerto Rico:  No changes were made to the depictions of drought across Alaska, Hawaii, or Puerto Rico.  Rainfall totals exceeded 2.0 inches at many reporting stations across Puerto Rico.  Windward showers helped to keep drought confined on Hawaii.  Southern portions of Alaska remained wet, but interior portions are starting to become dry, so this area will be closely monitored.

Looking Ahead:  National Weather Service forecasts indicate an active southern storm track during the next 5 days with soaking rains (locally near 4 inches) forecast across Texas and the western Gulf Coast states.  Remnants of cold front are also anticipated to bring another period of wet weather to the northeast.  Beyond this upcoming week, forecasts from the Climate Prediction Center indicate higher than normal chances for wet conditions across the Southeast and Central Rockies.  Dry conditions are expected to continue from Arizona to Oregon, and across the Great Lakes, with the odds for warm conditions across the northern tier of the contiguous 48 states.  Forecasts for Alaska indicate a wet pattern for the southern portions of the state.

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

Dryness Categories
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
S ... Short-Term, typically <6 months (e.g. agricultural, grasslands)
L ... Long-Term, typically >6 months (e.g. hydrology, ecology)


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