During September 15-21, an upper-air low slowly meandered across the Delta and Southeast, drawing moist tropical Gulf and Atlantic air northward and westward. As a result, widespread, numerous, slow-moving clusters of showers and thunderstorms inundated many portions of the Delta and Southeast, including upsloping airflow that greatly enhanced rainfall over parts of the southern Appalachians. Many locations from eastern Oklahoma and southern Missouri eastward into the western Carolinas and Georgia recorded well over 4 inches of rain, with locally up to 18.43 inches measured from a CoCoRaHS observer in Lilburn (north-central Georgia) during September 18-22. Georgia’s State Climatologist Dr. David Stooksbury reported that many locations in north Georgia experienced a 100-year rain event Sunday into Monday. This means that a 24-hour rain exceeding 8 inches has only a 1 in 100 chance (1 percent) in any year of occurring in central Georgia. Needless to say, severe, widespread flooding occurred across much of northern Georgia, with localized flooding elsewhere across the South. Amazingly, it was just about 2 years ago that much of this area was in the grips of exceptional (D4) drought. Elsewhere, light to moderate rain also fell on the southern Plains (including Texas again), Four Corner region, Pacific Northwest, northern Plains, western Corn Belt, and upper Great Lakes region.

In contrast, high pressure over the Southwest brought seasonably dry but abnormal warmth to California and the Great Basin, although a resurgent monsoon flow triggered scattered showers and thunderstorms in eastern Arizona, most of New Mexico and Colorado, eastern Utah, and southern Wyoming. Mostly dry weather also affected most of the northern and central High Plains, upper Midwest, and Northeast. Temperatures averaged much above-normal (more than 10 degrees F) from Montana to Minnesota, while the Northeast and south-central Plains experienced unseasonably cool weather.

Atlantic Coast States and Eastern Ohio Valley: A very dry September continued for much of the eastern Corn Belt, central sections of the Appalachians, Piedmont, and North Carolina, and eastern South Carolina. Many locations in this area have measured less than 0.5 inches of rain this month (and during the past 30 days), and month-to-date deficits have accumulated between 2 and 4 inches. Likewise, most USGS 1-, 7-, 14-, and 28-day averaged stream flows have also dropped to below (10-24 percentile) or much-below (less than 10 percentile) normal values in the same area. Negative departures were also common at 60- and 90-days. Accordingly, D0(A) was expanded to incorporate these growing short-term deficits, while D1(A) was added to last week’s D0 areas where the lowest 30-day percent of normal precipitation occurred (central Appalachians and east-central Corn Belt). In addition, a dry September has also affected the eastern Great Lakes region and interior New England, but thanks to a very wet summer, surpluses are still common at 60-days and longer periods, so no designation was placed there yet.

Southern Plains and Gulf Coast Region: After last week’s deluge and drought improvement across much of Texas, it appeared as though a return to dry weather and status-quo would be the story for this week. A swath of showers and thunderstorms, however, swept through the eastern half of the state on day 7 (24-hour period ending 7am CDT Tuesday, Sep. 22), dropping more beneficial rains (1.5 to 2.5 inches) from Del Rio northeastward to Texarkana. Light to moderate rains also fell earlier in the period on extreme southern Texas (up to 2.5 inches). Unfortunately, the rains missed north-central and south-central Texas. But with the Texas A&M SPI blends updated through 7am CDT September 22, a reassessment was made for Texas, including: A 1-category trimming of the northern and eastern drought borders (D0-D2), some improvement in the lower Rio Grande Valley, erasing some of the northern D0 portion in central Louisiana and southern Mississippi (1 to 3 inches of rain), and a redefining of the core D4 area, shifting it farther west into Duval, Live Oak, and Bee counties, and southward out of Brazoria and Matagorda counties. The small D0-D1 area in north-central Texas was slightly expanded with dry weather, while little or no rain failed to alleviate the small D0 in northern Oklahoma.

Upper Midwest: Similar to the eastern Ohio Valley and Atlantic Coast States, September continued to be quite dry across the upper Midwest (less than 0.5 inches), especially in northern Wisconsin, UP of Michigan, northwestern lower Michigan, north-central and south-central Minnesota, and northern Iowa. The drought has been exacerbated by unseasonable warmth (weekly temperatures averaging 6 to 14 degrees F above normal, highs in the 80s) and long-term subnormal precipitation, with some areas (e.g., northern Wisconsin and southern Minnesota) receiving less than 70 percent of normal precipitation during the past year (deficits of 8 to 12 inches), and about 80 percent the past 2 years (deficits of 10 to 16 inches). Several USGS 1-, 7-, 14-, and 28-day averaged stream flows were at or near record low (lower 2 percentile) levels in east-central Minnesota, northern Wisconsin, and southern UP of Michigan. As a result, D2 was increased and D3 introduced to this 3-state region, while D0 and D1 was expanded into the rest of Minnesota, northern Iowa, UP of Michigan, and northwest lower Michigan. The news, however, was not all bad as the dry and warm weather accelerated late-season crop progress that was far behind schedule due to the cool and occasionally wet spring and summer months. This eased fears of an early frost that could have reduced the quality and quantity of crop yields. For example, Minnesota’s corn dented went from 56 percent of the crop to 76 percent in a week, while soybeans dropping leaves went from 16 percent to 49 percent (according to USDA/NASS). Additionally, spring grain harvesting rapidly progressed (MN: 68 percent to 86 percent spring wheat harvested).

Northern and Central Plains: Hit and miss showers and thunderstorms developed over parts of the northern and central Great Plains while the High Plains mostly saw little or no rain. Fortunately, light to moderate rains (0.4 to 1.5 inches) fell on portions of south-central and eastern North Dakota, providing enough moisture to eliminate the D1(A) in east-central North Dakota, and trimming away the abnormal dryness in Emmons county (south-central ND). Elsewhere, the rains (0.1 to 0.4 inches) were not enough to remove D0 in the remainder of the Dakotas and southeastern Nebraska. And similar to the upper Midwest, the mostly dry and warm weather spurred late-season crop development (which was way behind schedule), harvesting of spring grains, and the planting of winter wheat.

The West: Seasonal dryness and slightly above-normal readings prevailed in California and the Great Basin as the autumn wildfire season fast approached. Since September is normally dry, conditions were quite typical for this time of the year, thus status-quo was maintained. As of Sep. 22, the NIFC reported that there were 7 large active wildfires, all in the Far West, including the long-burning Station fire in California’s Angeles National Forest. At 94 percent containment, Station has blackened nearly 161,000 acres. Farther to the north, a weak Pacific system dropped light to moderate rains (0.1 to 0.5 inches, locally to 1.6 inches near Quillayute, WA) on the Pacific Northwest, halting any further deterioration but not great enough to make any improvements. In contrast, a second week of a resurgent monsoon brought widespread showers and thunderstorms to the Four-Corner states and southern Wyoming. One to two inches of rain fell on western two-thirds of New Mexico and Colorado, east-central Arizona, northeastern Utah, and south-central Wyoming. Accordingly, short-term D1 was erased from central New Mexico, as was D0(A) in western parts of the state. East-central Arizona was improved by 1-category, while northwestern Colorado, north of the Colorado River, received enough rain to remove D0 there. In southwest Colorado, however, flows on the Animas and Los Pinos Rivers above the Vallecito Reservoir remain below normal, and the Lemon Reservoir (near Durango) is currently 25 percent full, or 48 percent of the normal storage, placing this area into D1.

Hawaii and Alaska: Light to moderate daily showers (generally 0.1 to 0.5 inches; 1 to 1.5 inches at a few sites) from Thursday through Sunday were common on the windward sides of Kauai, Oahu, Maui, and the Big Island, but little or no rain fell on most leeward portions. With no drought designation on the windward sides, and the summer and early fall months typically dry on the leeward portions, status-quo was kept.

In Alaska, stormy weather brought plentiful precipitation (2 to 10 inches) along the southern and southeastern coast, but also into east-central sections. One-half to 2 inches of rain fell from Nenana eastward to Chicken as 7-day averaged USGS stream flows rose into the upper tenth percentile in response to the heavy rains; therefore the northern extent of the D0 was trimmed back.

Looking Ahead: During the next 5 days (September 24-28), widespread rains from a slow-moving storm system are forecast for the eastern half of the Nation, with the largest totals (more than 1.5 inches) expected in Iowa, extreme southern Texas, the Tennessee Valley, and southern Appalachians. In drought areas, moderate rains (an inch) may fall on parts of the upper Midwest, lower Delta, mid-Atlantic and Carolinas. Dry and warm weather is forecast for the West, although temperatures should start to moderate by the weekend.

The CPC 6-10 day forecast (September 29-October 3) calls for above-normal precipitation in the Northwest, Plains, and Delta. Drier weather should return to Arizona and New Mexico, while the eastern third of the Nation should record subnormal precipitation. Unseasonable warmth is forecast for the southern half of the Plains and western Corn Belt, while subnormal temperatures occur in the Northeast and West. Wet weather is probable for the southeastern quarter of Alaska.

Author: David Miskus, JAWF/CPC/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
A ... Agricultural
H ... Hydrological

Drought Monitor: Dry In The Corn Belt & Midwest, Rain In Eastern Texas