September 2013

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Latest Monthly Assessment - The September Drought Outlook is based on initial conditions, short and medium range forecasts, the CPC monthly outlooks, and climatology. During August 2013, drought rapidly developed across parts of the Midwest and eastern Dakotas. According to the U.S. Drought Monitor on August 27, severe drought exists across southern Iowa and northern Missouri with an increase in moderate drought coverage across other areas of the Midwest. Persistence and additional development are forecast for the eastern Dakotas and Midwest. Persistence is also forecast for the Great Plains, lower Mississippi Valley, and Texas although forecast confidence decreases across the high Plains and south Texas. A continued active monsoon is expected to result in either improvement or removal of drought across parts of the Southwest. A dry climatology favors persistence along the West Coast and Hawaii. Drought removal is forecast for the southern area of moderate drought across interior Alaska.

Tools used in the monthly U.S. Drought Outlook (MDO) included the official CPC temperature and precipitation outlooks for September 2013, various short- and medium-range forecasts and models such as the 6-10 day and 8-14 day forecasts, the NAEFS precipitation outlooks, the soil moisture tools based on the Constructed Analog on Soil (CAS) moisture, dynamical models (CFSv2, NMME, and IMME), the 384-hour total precipitation forecasts from several runs of the GFS, the 7-day quantitative precipitation forecast from the NCEP Weather Prediction Center (WPC), climatology, and initial conditions. ENSO Neutral conditions are expected to continue through the fall.

A late summer flash drought occurred recently across parts of the Midwest where 30-day rainfall deficits are 2 to 5 inches across southern Indiana, Illinois, Iowa, northern Missouri, Minnesota, and southern Wisconsin. Following below normal temperatures during early to mid-August, maximum temperatures in the 90s during the final week of August intensified drought conditions, stressing corn and soybeans, and degraded pasture conditions. As of August 29, Champaign and Decatur, Illinois have recorded less than 0.1 inches of rainfall during August. Many locations across the Midwest are expected to have one of their driest Augusts on record. On August 28 and 29, moderate to locally heavy rain mostly fell to the north of these new drought areas. Since the first half of September is expected to remain relatively dry, persistence and additional development are forecast for the Midwest. Short-term drought expansion farther east across the Ohio Valley is possible, but the 1-3 Day WPC precipitation outlook indicates moderate rainfall and the CPC monthly outlook indicates enhanced odds for above median precipitation across most of Indiana and Ohio.
Forecast confidence for the Midwest is high.

Similar to the Midwest flash drought, the eastern Dakotas have experienced a rapid development of drought during the past month. Since no precipitation tools offer a wet signal during September, persistence is forecast for the eastern Dakotas.
Forecast confidence is high for the eastern Dakotas.

Tropical Storm Ivo tracked near the Baja California Peninsula from August 23-25 and resulted in a surge of low-level moisture north from the Gulf of California to the desert Southwest. Heavy rain triggered flash flooding across western Arizona and southern Nevada. Tropical Storm Juliette developed on August 28 and also tracked north near the Baja California Peninsula, contributing to a persistence of deep monsoon moisture across the Southwest. Locally heavy rain is expected to continue into the beginning of September across Arizona, southwest Colorado, western New Mexico, and southern Utah. The 6-10/8-14 Day outlooks favor above median precipitation across these areas. Although the monsoon begins to wane during September, improvement or drought removal (moderate/D1 areas currently designated in the U.S. Drought Monitor), is forecast for the Southwest.
Forecast confidence is high for drought improvement or removal across Arizona, southwest Colorado, western/northern New Mexico, and southern Utah.

Although the 6-10/8-14 Day precipitation outlooks tilt the odds towards above median precipitation, persistence is expected for the long-term drought areas of the Great Basin and northern half of the northern Rockies through the end of September. Unseasonably warm temperatures during late August and early September could intensify drought conditions across parts of this region
Forecast confidence is moderate for the Great Basin and northern half of the Rockies.

A sharp gradient in drought conditions exists across the central and southern Great Plains. According to the U.S. Drought Monitor on August 27, extreme (D3) to exceptional (D4) drought exists across the central and southern high Plains with eastern Kansas and northeast Oklahoma drought-free. Precipitation tools for early September, including the WPC 1-5 day precipitation forecast and CPC’s 6-10 Day precipitation outlooks favor below median precipitation across the central and southern Great Plains. Therefore, persistence is expected for the central and southern Great Plains. However, forecast confidence decreases across the central and southern high Plains where the 384-hour total precipitation forecasts from several past runs of the GFS indicate the potential for moderate to heavy rainfall across this region.
Forecast confidence is moderate for the central and southern Great Plains.

Climatology supports improvement across south Texas, but precipitation tools including the NAEFS indicate a dry signal during early September. Since the CPC monthly outlook calls for equal chances of below, near, or above median precipitation for south Texas, the expected dry start to September favors persistence.
Forecast confidence is low for south Texas.

Persistence is forecast for ongoing drought areas of the lower Mississippi Valley where the 6-10/8-14 Day Outlooks favor dryness and the CFSv2 indicates near to below median precipitation in this region. In addition, September is a relatively dry time of year for southern Arkansas and northern Louisiana.
Forecast confidence is high for the lower Mississippi Valley.

The wet season typically begins later in the fall season along the West Coast with September remaining a relatively dry time of year. Climatology favors persistence along the West Coast.
Forecast confidence is high for the West Coast.

Removal of the southwest part of the ongoing drought area across Alaska is based on seasonably cooler temperatures and increased chances for above median precipitation during the first half of September. For the remainder of the Alaska drought area, forecasted lighter precipitation amounts favor persistence.

Climatology favors persistence across Hawaii. Development is not forecast during the 1-month time period covered by this outlook.
Forecast confidence in Hawaii is moderate.