Following Hurricane Irma’s arrival in Florida on September 10 and subsequent demise across the Southeast, generally dry weather dominated the country for a few days. However, the first two significant autumn storms of the season arrived across the northern Plains and Northwest, starting on September 14. Eventually, precipitation fell as far south as the Intermountain West and eastward into the upper Midwest. Several areas of the country, however, remained mostly dry and continued to see mounting short-term rainfall deficits. As a result, portions of the central and southern Plains, as well as the mid-South and lower Midwest, experienced general increases in the coverage of dryness and drought. In mid-September, there was an abrupt weather-pattern change that not only provided the northern Plains and Northwest with much-needed precipitation, but also brought a warming trend to the eastern half of the nation and notably cooler weather to the West.
Mostly dry weather prevailed as slowly weakening Hurricane/Tropical Storm Jose churned the western Atlantic Ocean. Late in the drought-monitoring period, heavy surf, gusty winds, and rain showers associated with Jose grazed the middle and northern Atlantic Coast. Many areas of the Northeast are experiencing short-term dryness, although impacts are minimal due to reduced water demands associated with seasonal change. At this time, moderate drought (D1) is confined to eastern Maine. According to the U.S. Department of Agriculture, topsoil moisture on September 17 was rated 48% very short to short in Vermont, along with 33% in Maine and 25% in Massachusetts.
Pockets of Southeastern dryness (D0) remained limited to Virginia and North Carolina, as the remnants of Hurricane Irma lingered across the region during roughly the first half of the drought-monitoring period. Due to short-term rainfall deficits, the area of dryness in central North Carolina was extended northeastward into southern Virginia. The U.S. Department of Agriculture noted that Virginia’s topsoil moisture was rated 24% very short to short on September 17, up 8 percentage points from the previous week.
Pockets of significant rain spread into the Midwest, but were mostly concentrated across western areas from Minnesota to Missouri. The rain resulted in local trimming of dryness and drought. However, several other areas of the Midwest saw further drought expansion. Specifically, moderate drought (D1) was introduced or expanded in areas such as the Minnesota-Iowa-Wisconsin triple point; southeastern Wisconsin, south-central Michigan; west-central Indiana; central Illinois; and east-central Missouri. In southern Iowa, a small area of severe to extreme drought (D2 to D3) expanded slightly. According to the U.S. Department of Agriculture, topsoil moisture in Illinois rated very short to short increased from 51 to 75% during the 2-week period ending September 17. Also on the 17th, topsoil moisture was rated at least one-half very short to short in Michigan (61%), Missouri (52%), Iowa (51%), and Indiana (50%). On the same date, nearly half of the pastures in Iowa (47%) and Illinois (46%) were rated in very poor to poor condition. Almost one-third (31%) of Michigan’s pastures were rated very poor to poor.
The High Plains region had a mix of improvement and deterioration. Improvement in the drought depiction was most prominent in the western Dakotas, where heavy rain fell, while deteriorating conditions affected parts of Kansas and environs. In fact, some additional severe drought (D2) was introduced in Kansas, where topsoil moisture rated very short to short (by the U.S. Department of Agriculture) jumped from 38 to 58% during the 2 weeks ending September 17. In contrast, North Dakota’s topsoil moisture was rated 44% very short to short on the 17th, down 18 percentage points from the previous week. However, rangeland and pastures in the Dakotas were slow to recover—typical following a hard-hitting drought—with 58% rated very poor to poor in both states on September 17. Also on the 17th, South Dakota led the nation—among major production states—in very poor to poor ratings for sorghum (29%), corn (28%), and soybeans (19%).
Short-term dryness brought modest expansion of dryness (D0) and moderate to severe drought (D1 to D2) to various areas. Topsoil moisture rated very short to short increased at least 10 percentage points each of the last 2 weeks to reach 57% by September 17 in Texas, 47% in Oklahoma, and 43% in Arkansas, according to the U.S. Department of Agriculture. Winter wheat planting is underway in the region, and the crop will soon need rain to support proper autumn establishment. In Texas, 14% of the intended winter wheat acreage had been planted by September 17; Oklahoma had planted 11%.
Montana and environs finally received much-needed precipitation, starting on September 14. Prior to the storminess, Cut Bank, Montana, had reported 88 days (June 18 – September 13) with precipitation totaling 0.01 inch or less, breaking the station’s record of 74 days set from October 22, 1908 – January 3, 1909. Cut Bank’s streak ended with a 0.47-inch total on September 14-15. Elsewhere in Montana, daily-record totals for September 15 included 1.22 inches in Billings; 1.10 inches in Great Falls; 1.02 inches in Helena; and 0.65 inch in Butte. Helena had just completed its own record-setting streak—61 consecutive days (July 10 – September 8) without measurable precipitation. Helena’s previous warm-season record for days without accumulating precipitation had been 38 days, from September 1 – October 8, 1880. And, Helena’s previous longest spell without measurable precipitation—60 days—had occurred in the dead of winter from December 15, 1986 – February 12, 1987. Farther west, the season’s first significant precipitation arrived in the Northwest a few days after Montana’s event. Spokane, Washington, did not receive measurable rain from June 29 – September 16, a record-setting span of 80 days (previously, 75 days in 1917), but netted 0.84 inch from September 17-19. More substantial precipitation fell in western sections of Washington and Oregon, curbing the wildfire threat and aiding containment efforts. Through September 19, year-to-date U.S. wildfires had consumed 8.53 million acres of vegetation. In recent years, only 2015 (8.85 million acres) and 2012 (8.61 million acres) had a higher burned acreage on that date.
The rain and high-elevation snow brought some improvements in the drought depiction. Nevertheless, areas that received the most significant precipitation—including some of Montana’s extreme to exceptional drought (D3 to D4) areas—experienced up to one category of improvement. Since drought relief occurred so late in the growing season, and because the drought has been historic in nature, improvement in rangeland and pasture conditions may not be realized until spring. Rangeland and pastures rated very poor to poor on September 17 included 77% in Montana and 65% in Washington. However, topsoil moisture rated very short to short by the U.S. Department of Agriculture improved in Montana from 99 to 65% during the week ending September 17, while Idaho improved from 68 to 44%. The improvements in topsoil moisture should benefit recently planted winter wheat; Washington led the nation with 43% of its wheat planted by September 17.
Hawaii continued to experience warmer- and drier-than-normal weather. Lihue, Kauai, posted daily-record highs of 89°F on September 12, 15, 17, and 18. Although Hawaii’s drought depiction was largely unchanged, there was some further deterioration (mainly, expansion of D3, or extreme drought) on the lower leeward slopes of the Kohala Mountains on the Big Island due to declining pasture conditions. Meanwhile, Alaska has experienced mild, showery weather in recent weeks and became free of dryness when D0 was removed from a portion of the Copper River basin. Finally, Puerto Rico was battered and drenched by powerful Hurricane Maria on September 20. Maria’s direct hit occurred just 2 weeks after Hurricane Irma grazed the island.
During the next couple of days, a storm system and its attendant cold front will push eastward toward a ridge of high pressure parked over the eastern U.S. Initially, the front will make little progress, resulting in an axis of heavy rain stretching from the upper Midwest to the southern High Plains. Five-day rainfall totals could reach 2 to 4 inches or more along that axis, while isolated 1- to 3-inch amounts can be expected from the Pacific Northwest to the northern Rockies. Early next week, a warming trend will commence in the Far West, while cool conditions will shift eastward across the Plains. Late-season warmth and general dryness will continue, however, in the East.
The NWS 6- to 10-day outlook for September 26 – 30 calls for the likelihood of below-normal temperatures across large sections of the Rockies and Plains, while warmer-than-normal weather will prevail in the Pacific Coast States and across the eastern one-third of the U.S. Meanwhile, below-normal rainfall in the Southeast and Northwest should contrast with wetter-than-normal conditions across New England, the upper Great Lakes region, and southern portions of the Rockies and Plains.