This Week's Drought Summary (9/13)
A stalled cold front draped across the southern Plains, middle Mississippi and Ohio Valleys, and mid-Atlantic, plus ample Gulf moisture from Tropical Storm Gordon, was the focal point for moderate to heavy showers and thunderstorms. Widespread amounts of 2-4 inches, with locally 6-12 inches, were common in the southern and central Great Plains, along the Gulf Coast (Gordon), in the lower and middle Mississippi, Tennessee, and Ohio Valleys, western Great Lakes region, and the mid-Atlantic. Similar to last week, additional improvements were made in the Midwest, but this week, major modifications (improvements) were also done in the southern Plains (especially Texas) and lower Mississippi Valley. Elsewhere, little or no precipitation fell across the western third of the Nation, although some light showers finally dampened western Washington. In addition, the northern Plains, parts of the Southeast (Georgia and Carolinas), and extreme northern New England saw little or no rain. Temperatures averaged below-normal across the middle third of the Nation and in New England, and above-normal in most of the West, Southeast, and mid-Atlantic.
After the first 6 days of the period saw little or no precipitation across most of New England and degradation looked probable, Day7 brought widespread light to moderate rains (1-2.5 inches) to the drought and abnormally dry areas of upstate New York, Vermont, New Hampshire, and coastal Maine. Decent rains (1-2 inches) also fell on far western New York (near Buffalo) during Day6. As a result, short-term deficits were reduced enough to allow for some 1-category improvements in western New York, the southern sections of the drought area in upstate NY, Vermont, and New Hampshire, and along coastal Maine, with the latter area’s recently introduced D2 improved to D1, and the former D1 area shrunk. USGS 1- and 7-day averaged stream flows responded to the rains, rising into the near normal category except for far northern locations that received lower amounts and stayed in the 10th (much below) to 24th (below normal) percentile. In contrast, little or no rain was reported in far eastern Maine (Aroostook county) where 30- and 60-day precipitation was below normal along with low 7-day averaged stream flows, thus the two separate D0 areas in eastern Maine were merged. The D0 in eastern Massachusetts remained unchanged as enough rain fell (~1 inch) to prevent deterioration, but not enough for D0 removal with shortages remaining at 30-, 60-, and 90-days.
Except for localized heavy rains from Tropical Storm Gordon (which made landfall near the MS-AL border), much of the Southeast saw little or no precipitation through Day6. On Day7 however, scattered showers and thunderstorms (0.5-2 inches) developed over parts of the central Carolinas and eastern Georgia, dropping enough rain to keep conditions status-quo in the Carolinas (still 2-5 inch deficits at 60-days), but erasing shortages in eastern Georgia, thus D0 was removed there. In east-central Georgia, however, little or no rain fell, and D0 was slightly expanded to depict the growing 60-day departures of 1-3 inches. Farther west, Gordon dropped a narrow band of 4-12 inches of rain from the extreme western Florida Panhandle northwestward into east-central Mississippi and west-central Alabama, erasing D0 in the latter two areas, and in Baldwin County in southern Alabama. Just to the east, however, scattered, lighter precipitation occurred, producing 60-day deficits of 2-5 inches that resulted in some D0 expansion and new D1 areas in northern Alabama, northern Mississippi, and southwestern Tennessee.
Moist inflow from the western Gulf and a stalled cold front across the south-central Plains triggered widespread, numerous showers and thunderstorms across much of Texas into central Oklahoma, along with bands of heavy showers across most of Louisiana. The remnants of Gordon also dropped spotty but heavy amounts of rain on parts of Arkansas, Mississippi, and western Tennessee (see Southeast). About the only area in the South that missed out on decent rains were eastern Oklahoma, western Arkansas, and the northern Texas and western Oklahoma Panhandles. With widespread weekly totals of 4-8 inches and locally more, much of southern, central, western, and southeastern Texas saw 1-2 category improvements as most station SPIs were normal or wet at 2-, 3-, and 6-months, and even 12-month SPIs were improved. Average 1-, 7-, and 14-day USGS stream flows responded into the 76th (above normal) or 90th (much above) percentiles where the improvements occurred. In Oklahoma, 1.5-6 inches of rain allowed for a 1-cat improvement in parts of southwestern, south-central, and northeastern Oklahoma. In Louisiana, scattered showers and thunderstorms (1.5-4 inches) continued this week as a wetter pattern has gradually improved conditions, although a few dry pockets of D2-D3 remained in the northwest. In Arkansas, rain across the southern, northern, and eastern portions of the state eroded some of the previous week’s D0 and D1. 1-cat improvements were also made in parts of northwestern Mississippi and western Tennessee as Gordon rains dropped 2-5 inches.
For the third consecutive week, large portions of the Midwest were inundated with heavy rainfall (2-8 inches), although this week the greatest rains fell along the Ohio River Valley after missing out the previous 2 weeks. Due to these rains, a 1-cat improvement was made in parts of the Missouri Bootheel, western Kentucky, southern Illinois, southwestern Indiana, and northern Ohio. Moderate to heavy rains also fell on portions of the middle Mississippi and lower Missouri Valleys, although it was more scattered in nature, producing bands of improvement where the greatest totals (2-6 inches) fell. A band of moderate to heavy rain (2-6 inches) also fell from southeastern Minnesota northeastward into the UP of Michigan, erasing a few small D0 areas in central Wisconsin and Michigan’s Luce and western Chippewa Counties. Lighter totals (0.5-2 inches) were reported in lower Michigan, but with last week’s rainfall, 30- and 60-day deficiencies were vanquished, leaving only smaller 90-day deficits. Thus, some 1-cat improvements were made to the D0 and D1 areas where the 1- and 2-month shortages were erased. The rains have steadily improved growing conditions as the Aug. 12 USDA/NASS report of pasture conditions for IA and MO were 28 and 76% poor or very poor; by Sep. 9, those values had dropped to 17 and 44%, respectively. In contrast, D0 was expanded in northern Minnesota (Cass and Hubbard Counties) in areas that missed out on the decent rains the past 30 days and USGS stream flows dropped below into the 10-24th percentile, while D1 was added to Lake of the Woods and western Koochiching Counties with 90-days deficits of 2-4 inches and very low stream flows.
Generally little or no precipitation fell across the western High Plains while light rains (0.5-1 inch) fell on parts of North Dakota, southeastern South Dakota, eastern Nebraska, most of Kansas, and central Colorado. Scattered locations in extreme southeastern SD, northeastern NE, and eastern KS received more (1.5-3 inches, locally to 7 inches in southeastern KS), but most of this rain fell on non-drought areas. An exception to this was north-central and southeastern Kansas where a 1-cat improvement was made, but unfortunately missed most of the core D2-D4 area in northeastern Kansas (again). Kansas pastures and ranges have also improved, with a NASS/USDA Aug. 12 value of 35% poor or very poor versus a Sep. 9 value of 20%. Subnormal weekly temperatures (anomalies of -2 to -6 degF) in the central Plains also eased evaporative demands. In contrast, drier and warmer weather in the Dakotas have steadily lowered moisture and growing conditions. Under half of normal 30- and 60-day precipitation has fallen on parts of North and South Dakota, and field reports are indicating declining farming, ranching (no forages to graze), and hydrologic (shallow wells and small ponds drying up) conditions. The NASS/USDA poor and very poor pasture conditions for ND and SD have slowly climbed from 7 and 10%, respectively, on July 29, to both at 25% on Sep. 9. Accordingly, D0-D2 was expanded into the driest areas, and a D3 was added in McHenry County, ND, where the field reports and drought tools lined up.
Warmer weather returned to the Northwest after two consecutive weeks of near or subnormal temperatures. Deteriorations in this week’s dry areas were limited to an expansion of D1 into central Washington’s Chelan and Kittitas Counties (2, 3, and 4-month SPEIs -1 to -1.5); a slight increase in D3 in Oregon’s northern Harney and Malheur Counties (soil moisture models, EDDI > 6 weeks, Vegetation Health, VegDRI, low stream flows); reassessment of northern California (D0-D2 shifted eastward into Siskiyou and Modoc Counties); D2 expanded into southern Idaho (Oneida County) with similar conditions in bordering northern Utah (Box Elder County); D3 expanded in southeastern CA due to the weak to non-existent summer monsoon rains. In contrast, improvements were made in central Arizona (courtesy of the Flagstaff WFO) that noted the past 8 weeks have seen well above-normal monsoon rains (in some areas 40-70% of average ANNUAL rainfall) in central Yavapai County and the Mogollon Rim region; and in eastern New Mexico that received significant rains (but less than neighboring west Texas) – with some improvements to Colfax and Mora Counties [D3 to D2], San Miguel and Guadalupe Counties [D2 to D1], DeBaca and Chavez Counties [D3 to D2], and Roosevelt County [D1 to D0]. Conversely, some D2 was added into eastern Otero County, NM, and northeastern Hudspeth County, TX, after a reassessment of conditions. The rest of the West was left untouched as the Pacific Northwest awaits the start of its wet (Fall) season.
Alaska, Hawaii, and Puerto Rico
Based upon the full set of August rainfall and FSA input, adjustments were made to the Big Island and Maui depictions. In the Big Island, the districts of Kau and South Kona improved from D1 to D0, and sufficient rainfall occurred on the slopes of the North Kona and South Kohala districts for a D2 to D1 change; however, lower elevations received much less rain, and were left unchanged. In Maui, the area from Maalaea to Kihei measured surplus monthly amounts and improved from D2 to D1, but the lower leeward slopes of the West Maui Mountains received far less rain and remained at D2. The rest of the islands were status-quo.
In Puerto Rico, heavy showers (2-8 inches) fell across the northwestern interior quarter of the island, with lesser totals (1-2.5 inches) measured in the northeast and southeast. Little or no rain fell along most of the immediate southern and northern coasts, and in the east-central interior. Based upon the past 7-, 60- and 180-day departures, the D0 area was removed in the north-central region, and expanded slightly into southeastern sections.
In the southeastern Alaskan Panhandle, the areas of D0 and D1 were left unchanged as light to moderate weekly totals (1-3 inches) were not large enough to remove longer-term deficits (where the SPI for climate division 12 was in the moderately dry or worse category for most periods from 2-36 months), but not small enough for deterioration. This area is moving into its wet season, which says how much precipitation this part of Alaska typically gets. Some locations have WYTD (since Oct. 1, 2017) deficits of more than 30 inches, and stream flows are below normal as many salmon return this time of year.
For the ensuing 5 days (September 13-17), the focus will be on Hurricane Florence’s landfall and where it tracks thereafter. As of Wednesday afternoon, the most likely scenario is landfall near the NC-SC border, with the hurricane slowing down and dropping catastrophic amounts of rain (over 20 inches near landfall), with possible devastating floods in parts of the Southeast and mid-Atlantic. The 5-day QPF targets the Carolinas and Virginia with 4-10 inches, with heavy rains moving northward into the Northeast during Days 6-7. Elsewhere, a tropical disturbance is expected to move into the western Gulf from the Caribbean, dumping more heavy rain (2-6 inches) on the southern half of Texas. Pacific systems traversing along the US-Canada border may drop light to moderate amounts from Washington to Minnesota. Little or no precipitation is expected elsewhere. Temperatures should average below-normal in the Far West, northern Rockies, and Texas, and above-normal from the Southwest northeastward into New England.
For the CPC 6-10 day extended range outlook (Sep. 18-22), the odds favor above-normal precipitation in the northern half of the Plains, in the eastern quarter of the country (from Florence), and northwestern Alaska. Chances are good for subnormal precipitation in most of the West, south-central Plains, lower and middle Mississippi Valleys, and southern Alaska. Above-normal temperatures are likely in the southern and eastern sections of the U.S. and most of Alaska, with subnormal readings limited to the Northwest and northern Rockies and Plains.