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Climate Prediction Center


October - December 2013


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Latest Seasonal Assessment - Widespread, heavy monsoonal rains fell across the Southwest during the past few weeks, leading to significant improvements in regional drought conditions. With the summer monsoon season now winding down, persistence of remaining drought is anticipated over this area. In the Northwest, autumn (especially November and December) typically heralds a return to the rainy season. Areas of drought Improvement and Removal are expected over far northern and western portions of the region, with Persistence anticipated elsewhere. Across most of the Rockies and Great Plains, drought conditions are likely to persist. Short-term exceptions may include southwestern and far southern Texas, where heavy rain is predicted within the next 2 weeks or so. Some of this rain may originate from an area of disturbed weather currently located just west of the Yucatan Peninsula. The potential for late season tropical systems to affect the lower Mississippi Valley offsets climatological drying, resulting in a middle-ground forecast of Improvement. Drought Persistence is anticipated across the Midwest, which has experienced flash drought conditions for the second summer in a row. However, this year's flash drought started significantly later in the growing season, helping to mitigate crop losses. Drought improvement and removal are predicted across most of the central and southeastern interior of Alaska, while drought persistence and development are favored for the Hawaiian Islands, especially the Big Island.

Discussion for the Seasonal Drought Outlook

Tools used in the U.S. Drought Outlook (USDO) included the official updated CPC temperature and precipitation outlooks for October 2013, the long lead forecast for October through December 2013, various short- and medium-range forecasts and models such as the 5-day and 7-day precipitation totals from the Weather Prediction Center, 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 four-month Palmer drought termination and amelioration probabilities, climatology, and initial conditions.

A very active monsoon has brought much needed rain to the desert Southwest during the past few weeks. The Advanced Hydrologic Prediction Service (AHPS) graphics for the past 30-days depict rainfall amounts in excess of 5 inches across much of New Mexico, northwestern and central Arizona, north-central Colorado, and southern Utah. Some locations embedded within these areas received between 8-10 (or more) inches of rain in the past 30-days, as confirmed by the raingauge-based CPC Unified Precipitation Analysis (UPA). The forecast for the next 7-days calls for a significant reduction in precipitation amounts across the Southwest (less than a half-inch). The CPC 6-10 day and 8-14 day outlooks favor near- to below-median rainfall across most of the Southwest (with the exception of near- to above-median rainfall favored over far northern portions of Nevada, Utah, and Colorado. The CPC 30-day precipitation outlook for October 2013 shows Equal Chances (EC) over most of the Southwest, with a slight tilt in the odds for below-median precipitation favored in northeastern Colorado. The October-December 2013 seasonal outlook shows no substantial tilt in the odds towards any one tercile, hence a forecast of EC. With monsoon season winding down, and barring the influx of late season tropical moisture from the eastern Pacific, drought persistence is favored across the region during OND 2013.
Forecast confidence is moderate to high across the Southwest.

In the Northwest and northern Rockies, the October precipitation outlook indicates enhanced odds of above-median precipitation over Washington and Oregon, and EC elsewhere. The OND outlook favors above-median precipitation from eastern Washington to eastern Montana. Typically during the autumn, the mid-level westerlies strengthen and shift southward, helping to steer Pacific storm systems across this region. Historically (going back to 1931), Idaho receives 30-35 percent of its annual precipitation during the OND season, while southwestern Montana receives between 15-30 percent of its annual precipitation during OND. During the same period, southwestern Oregon and northern California typically receive 35-40 percent of their annual precipitation. Drought removal is expected in north-central Idaho and southwestern Oregon, while improvement is favored over northern coastal California and southwestern Montana. Elsewhere, drought conditions are expected to persist.
Forecast confidence is moderate across the Northwest and northern Rockies.

For the northern half of the Great Plains (North Dakota through Nebraska), drier conditions have returned during the past month, especially across the lower Plains. According to AHPS and the CPC UPA, 1-3 inch deficits have accumulated during this period over eastern portions of both South Dakota and Nebraska. Most other areas have received near- to above-median precipitation during the past 30-days. Surpluses of 2-4 inches (locally a bit higher) were observed in northwestern South Dakota and adjacent sections of southwestern North Dakota. The official 30-day outlook for October 2013 calls for slightly elevated odds of below-median precipitation across Nebraska and southern South Dakota, with EC anticipated elsewhere. There is no discernible tilt in the odds towards any one category during the OND 2013 season, so EC is deemed the best bet. Persistence of drought is therefore indicated on the U.S. Seasonal Drought Outlook.
Forecast confidence for the northern half of the Great Plains is low to moderate.

Across the southern half of the Great Plains (Kansas southward through Texas), OND typically signals the onset of the dry season. During this period, percent of annual precipitation ranges from 10-15 percent over the High Plains to 20-30 percent over the Lower Plains. For both October and OND, the forecast for most areas is EC, with the exception of elevated odds of below-median precipitation over northwestern Kansas during October. The Drought Outlook depicts Persistence of drought over most of this region, with the exceptions of southwestern, far southern, and far eastern Texas, where Improvement is indicated. This forecast is heavily based on heavy rain that is anticipated during the first few weeks of the Outlook period. Tropical moisture may reach as far north as southern Texas in association with the area of disturbed weather currently near the Yucatan Peninsula. Some moisture may also get into this region in association with the enhanced phase of the MJO. Beyond these first few weeks, mostly dry conditions are favored.
Forecast confidence for the southern half of the Great Plains is moderate.

Within the past 2 months, the Midwest experienced a return to flash drought conditions, similar in some ways to the flash drought of last summer, but later in the growing season. An extended period of excessive heat also contributed to the rapid onset of drought conditions. North American Land Data Assimilation System (NLDAS) ensemble mean soil moisture anomalies (as of September 14th) indicate widespread deficits of 2-5 inches (within the top 1-meter of soil) from the north-central Mississippi Valley eastward across Indiana and western Ohio. The Constructed Analog on Soil Moisture (CAS) tool predicts soil moisture deficits of 1.5-2.5 inches by the end of October, and also by the end of December. This area typically receives 10-30 percent of its annual precipitation during the OND season, with the higher percentages farther east. With a precipitation forecast of EC anticipated for both the October and OND time periods, it was decided to keep the region in drought persistence, despite the expectation of relatively wet conditions in the next 2 weeks.
Forecast confidence for the Midwest is low to moderate.

The lower Mississippi Valley typically experiences a drier pattern during the autumn season. This is offset to some degree by the fact that this area is still vulnerable to tropical systems originating in the Caribbean or Gulf of Mexico basins. With the CPC 30-day and 90-day outlooks both calling for EC, it is thought that drought Improvement would be the most appropriate prediction.
Forecast confidence for the lower Mississippi Valley is low to moderate.

In Alaska, continuing drought Improvement (and in some cases, removal) appears to be most likely for much of the central Interior, with the exception of Persistence across the southeastern Interior. CPC's October and OND 2013 outlooks call for EC across the Alaska drought areas.
Forecast confidence for Alaska is low to moderate.

Drought persistence and development is forecast for much of the Hawaiian Islands, especially the Big Island, as supported by the OND seasonal precipitation outlook.
Forecast confidence for the Big Island of Hawaii is moderate, but low for the remaining Islands.

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Page Author: Climate Prediction Center Internet Team
Page last modified: September 19, 2013
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