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Climate Prediction Center


July - September 2012


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Latest Seasonal Assessment - Dryness and moderate drought have been increasing both in extent and intensity across much of the Corn Belt region, the middle and lower Mississippi Valley, and much of the Great Plains. Drought is likely to either develop, persist or expand across these areas. Scattered relief may come in the form of cold front passages, or organized thunderstorm clusters (MCS’s), but for the most part, summers are usually a fairly dry time of year for the central part of the nation. For the northern tier states, such as Montana, North Dakota, Minnesota, and upper Michigan, chances are better for getting frontal passages since these areas reside close to the average position of the polar jet stream during the summer. In the Southeast, drought improvement is expected across Florida and coastal portions of Georgia and South Carolina, due to the greater likelihood of a tropical cyclone affecting these areas, and also from sea-breeze driven thunderstorm activity. Across the Southwest, at least some improvement is anticipated across much of Arizona and New Mexico, with the seasonal monsoon coming into play. At this time, it is uncertain as to how widespread or intense this year’s monsoon is likely to be. Finally, drought persistence is the best bet across the remaining portions of the Western U.S., given that summertime is usually their dry season.

Discussion for the Seasonal Drought Outlook

Tools used in the U.S. Drought Outlook (USDO) included the official CPC temperature and precipitation outlooks for July 2012 and the long lead forecast for July through September 2012, various medium- and short-range forecasts and models such as the 6-10 day and 8-14 day forecasts, the most recent 384-hour GFS total precipitation amounts, the soil moisture tools based on the Constructed Analog on Soil (CAS) moisture, the Climate Forecast System (CFS, versions 1 and 2), the four-month Palmer drought termination and amelioration probabilities, climatology, and initial conditions.

Light to moderate rain (less than 2 inches) fell across much of the Southeast during the past 7-days, along with very localized areas of heavy rain (2-3 inches) across parts of Florida. AHPS Departure from Normal Precipitation (DNP) for the last 30 days reveals significant positive departures (ranging from 2-8 inches) across northern portions of Florida, far southern Alabama, and coastal portions of both Georgia and South Carolina. Much of this surplus precipitation is attributed to the passage of former Tropical Storm Beryl several weeks ago. In the ensuing 5 days, most of the Southeast is expected to remain on the dry side, with the striking exception of the Florida peninsula, where up to 10 inches of rain may fall near Fort Myers due to a large batch of tropical moisture. In recent days, dynamical models have generally done a good job in forecasting this significant event. The extended-range forecasts (6-10 day and 8-14 day) anticipate elevated odds of above-median rainfall across Florida, and below-median rainfall for most of the remainder of the Southeast. Though CPC’s monthly outlook for July calls for Equal Chances (EC) of below-, near-, and above-median precipitation across the Southeast, the seasonal outlook for July-September favors wetter-than-median conditions over a large portion of the region.
Forecast confidence for the Southeast is moderate.

During the past 2 weeks, light to moderate rainfall and below-normal temperatures over the Northeast and mid-Atlantic have helped to maintain a predominantly drought-free region. However, the precipitation has largely missed the area that needs it most – central and southern Delaware, and adjacent parts of Maryland’s Eastern Shore. This core drought area shows up clearly in the DNP’s out through the past 180 days. In the short-term (next 5 days) about a half-inch of rainfall is expected for the Delmarva region, but there are no clear indications at this time for significant mitigation of the core drought area at least for the duration of the summer.
Forecast confidence for the Northeast and Mid-Atlantic regions is moderate.

Much of the Corn Belt continues to experience increasing dryness, with DNP’s over the past 30 days on the order of 2 to 4 inches across central and eastern Iowa and much of Illinois and Indiana. Current NLDAS soil moisture anomalies (Ensemble Mean LSM, total column) reveal widespread moisture deficits on the order of 4 to 6 inches. Current root-zone (top 1-meter) soil moisture deficits range from 2 to 5 inches across this region. Though western and central Iowa are forecast to receive between 1.5 and 2.0 inches of rain in the next 5 days, the 6-10 day and 8-14 day forecasts call for enhanced chances of below-median rainfall. Temperatures are predicted to be close to normal during this same period, with an upper ridge axis and above normal temperatures just west of the area. CPC’s official predicted 500-mb height pattern for the extended range has the southern edge of the westerlies in the vicinity of the Corn Belt. If correct, it suggests at least the potential for organized thunderstorm complexes (MCS’s) to bring relief to parts of the area. The CPC July 2012 outlook calls for elevated chances of above-normal temperatures and below-median precipitation. CPC’s 90-day outlook for July-September indicates increased odds of above- normal temperatures, but there is no clear precipitation signal for the region, so EC is favored.
Forecast confidence for the Corn Belt is moderate.

Across the lower/middle Mississippi Valley and the lower Tennessee and Ohio Valleys DNP’s spanning the most recent 2 weeks depict 1 to 2 inch deficits over northern Arkansas, Tennessee, most of the lower Ohio Valley, and northwestern Missouri. Over the past 60 days, AHPS DNP’s show widespread deficits across most of the region, most notably over western Arkansas and northern Louisiana where precipitation deficits range from 8 to 12 inches. Prospects for significant drought relief at all time scales through the JAS 2012 seasonal outlook appear rather limited at this time.
Forecast confidence for the lower/middle Mississippi Valley and the lower Tennessee and Ohio Valleys is moderate.

Over a large portion of the upper Mississippi Valley and northern Great Plains, ample rain has fallen over the course of the past 2 weeks. One to two inch surpluses are common across northeastern Montana, western and central portions of the Dakotas, and much of Minnesota. Even greater surpluses (4 to 8 inches) are evident in southeastern Minnesota. Occasional fronts and associated thunderstorm activity have helped keep this general area relatively moist. During the summer months, the polar jet stream tends to remain near the Canadian border, so this region (as well as Wisconsin and upper Michigan) can expect continued cool front passages, in addition to organized thunderstorm clusters (MCS’s). However, dryness is increasing just south of this region across southeastern South Dakota, Nebraska and Iowa.
Forecast confidence for the upper Mississippi Valley/Northern Plains is moderate.

Areas of heavy rainfall (2 inches or greater) were observed during the last 7-days over much of southeastern Nebraska, central Kansas, central Oklahoma, and far eastern portions of Texas. During the last 30 days, rainfall deficits of 3 to 5 inches accumulated over south-central and southeastern portions of Texas, southeastern Oklahoma and eastern Kansas, with lesser (2 to 3 inch) deficits noted across central Nebraska. Though some areas are expected to receive light to moderate precipitation in the upcoming 5-day period, the CPC extended-range forecasts show a tilt in the odds towards warm, dry conditions under an upper ridge of high pressure. CPC’s 30-day and 90-day outlooks favor a tilt in the odds for above-normal temperatures for both July and July-September. The precipitation outlooks for these same time periods is less certain, with Equal Chances predicted.
Forecast confidence for the central/southern Plains is moderate.

In the last 30 days across the Southwest, 1 to 2 inch precipitation deficits have mounted in central and southeastern New Mexico, western-, southern- and northeastern Colorado, and central sections of Utah. In contrast, 1 to 3 inch surpluses (very localized amounts of 4 to 5 inches) have been reported in an area just east of the Colorado foothills, between Denver and Limon, and up toward Fort Collins. This same area coincides well with the greatest concentration of severe weather reports. Late spring/early summer is the height of the damaging hail season in this region. Rainfall surpluses of comparable magnitudes also accumulated over northeast parts of New Mexico. Typical onset for the Southwest Summer Monsoon is in early July, with peak rainfall often occurring during August. Once the Monsoon becomes established, thunderstorms have the potential to bring local drought relief, but as is often the case, there is considerable uncertainty in the monsoon’s intensity and extent forecast.
Forecast confidence for the Southwest is low.

Widespread moderate to severe drought covers much of the remainder of the Southwestern U.S. While the Southwestern Monsoon can bring moisture throughout the Four Corners States, the summer is climatologically dry across the Great Basin, California, and the Northwest. CPC’s monthly and seasonal outlooks favor enhanced odds of below-median precipitation over the Northwest, and above-median precipitation over parts of Arizona and New Mexico.
Forecast confidence for the remainder of the West is high.

The summer months are climatologically dry in Hawaii. Therefore, little improvement in existing drought areas is expected through September.
Forecast confidence for Hawaii is high.

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Page Author: Climate Prediction Center Internet Team
Page last modified: June 21, 2012
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