NATIONAL WEATHER SUMMARY

SEPTEMBER 2017

3-9: Mostly dry weather dominated the nation in the days leading up to Hurricane Irma’s September 10 arrival across Florida. However, ongoing late-season heat in the West contrasted with chilly conditions across most of the eastern half of the nation.
Weekly temperatures ranged from as much as 10F below normal in the Tennessee Valley and environs to more than 10F above normal in a few Northwestern locations. Cool weather in the Midwest continued to slow the pace of corn and soybean maturation. In
addition, scattered frost affected the upper Midwest, while clouds and scattered showers plagued the Great Lakes region and parts of the northern Plains. Western heat remained extreme in early September and eased only slightly as the week progressed. On
September 3, Colorado Springs, CO, tied a monthly record with a high of 95F. The only other time Colorado Springs attained 95F in autumn was on September 17, 1936. September records were also tied or broken on the 3rd in Idaho locations such as
Pocatello (98F) and Stanley (93F). Pocatello had also achieved 98F on September 5, 1976, but Stanley had never been above the 90-degree mark after August—having peaked at 89F on September 4, 1988. On September 4, daily-record highs soared to exactly
100F in locations such as Burns, OR; Elko, NV; Reno, NV; and Winslow, AZ. Winnemucca, NV, logged four consecutive triple-digit maxima from September 2-5, including a daily-record high of 103F on September 3. Later, steamy weather in advance of
Hurricane Irma’s approach resulted in daily-record highs in Florida locations such as Miami (94F on September 7) and Key West (93F on September 8). Farther north, however, cool weather gripped much of the central and eastern U.S. Daily-record lows
dipped to 36F in Imperial, NE (on September 6), and Flint, MI (on September 9 and 10). Chadron, NE, reported 32F on September 6—not a record for the date. Farther east, record setting lows for September 7 fell to 43F in St. Joseph, MO, and
Crossville, TN. By September 8, daily-record lows slid to 45F in Lynchburg, VA, and London, KY. However, hot weather quickly returned to the northern High Plains, where Havre, MT, registered a daily-record high of 94F on September 9.

10-16: Hurricane Irma tore across the Florida Keys on the morning of September 10, followed by a final landfall on Marco Island, FL, during the afternoon. Irma was packing sustained winds near 130 mph upon reaching Cudjoe Key at 9:10 am EDT, marking the
first time that two Category 4 hurricanes achieved a U.S. landfall in the same year. (Hurricane Harvey had crossed the middle Texas coast near Rockport with winds near 130 mph on August 25.) When Irma made landfall on Marco Island at 3:35 pm EDT,
sustained winds had dropped to 115 mph— a Category 3 hurricane. Cool conditions in the South and lower Midwest contrasted with late-season warmth across the North and West. Weekly temperatures averaged at least 5F below normal in much of the Southeast,
while readings averaged as much as 10F above normal across the upper Midwest. The upper Midwestern warmth was especially beneficial for late-developing corn and soybeans. Late in the week, an abrupt pattern change brought sharply cooler air to the
Northwest (and warmer weather to the Southeast), as well as drought-easing precipitation to northern sections of the Rockies and Plains. The cool, wet weather aided wildfire containment efforts and boosted topsoil moisture for newly planted winter wheat.
Cool air settled across the south-central U.S. early in the week and gradually spread eastward. Cape Girardeau, MO, notched a daily record low of 46F on September 11. In Texas, Corpus Christi tallied a trio of daily-record lows (64, 61, and 63F) from
September 12-14. High temperatures only reached 59F in Crossville, TN, on September 11 and 13. In contrast, heat lingered in the West, where Santa Rosa, CA, registered a daily-record high of 100°F on September 10. Heat covered much of the western
half of the country by September 12, when highs soared to daily-record values of 110°F in Phoenix, AZ, and 100°F in Valentine, NE. On September 14, additional triple-digit, daily-record highs included 104F in Midland, TX, and 100F in Kansas locations
such as Dodge City and Hill City. Late in the week, however, markedly cool air arrived across the Northwest. In Montana, high temperatures on September 15 failed to top the 40-degree mark in Butte (38°F) and Helena (40F). In Oregon, record-setting
lows for September 16 dipped to 23°F in Burns and Baker City.

17-23: Across the U.S. mainland, relatively quiet weather prevailed for much of the week. However, in a reversal from early September, warm air overspread the South, East, and Midwest, while a late season heat wave abruptly ended across the West.
Weekly temperatures averaged at least 10F across portions of the central and southern Plains and much of the Midwest, pushing developmentally delayed summer crops—including corn and soybeans— toward maturity. Starting on September 20, widespread
readings above 90F covered much of the Corn Belt. For a few days, unsettled showery weather stretched from the Pacific Northwest to the northern Rockies, boosting soil moisture for recently planted winter grains and further aiding wildfire containment
efforts. Toward week’s end, a band of heavy rain developed from the southern High Plains into the upper Midwest. Much of the remainder of the country, particularly the South and East, experienced a long stretch of warm, dry weather. A sudden surge of
warmth across the central and eastern U.S. reversed an earlier regime. By September 19, daily-record highs of 100°F were reported in locations such as Larned and Russell, KS, and Wichita Falls, TX. Other triple-digit, daily-record highs included 102°
F (on September 20) in San Angelo, TX; 101F (on September 21) in Dodge City, KS; and 100°F (on September 21) in McCook, NE. On September 20, a week-long heat wave commenced across the Midwest. From September 20-25, Chicago, IL, noted six consecutive
daily-record highs (92, 94, 94, 95, 93, and 92F). Chicago’s previous latest occurrence of at least four consecutive highs of 92°F or greater had been September 16-19, 1955, and the latest observance of at least three consecutive highs of 94°F or
greater had been September 13-15, 1927 and 1939. Similarly, La Crosse, WI, registered daily-record highs (95, 94, and 93F) from September 22-24. That represented La Crosse’s latest trio of 90-degree readings on record (previously, September 16-18,
1955). Among hundreds of other daily-record highs were readings of 97°F (on September 20) in St. Louis, MO; 96°F (on September 20 and 22) in Springfield, IL; and 95°F (from September 22-24) in Traverse City, MI. In contrast, September 22 featured
daily-record lows in California locations such as Montague (29°F) and Stockton (46F). In Oregon, daily-record lows included 19F (on September 23) in Burns and 25F (on September 22) in Klamath Falls. On September 23-24, Prescott, AZ, posted consecutive
daily-record lows of 38 and 34F, respectively.

24-30: Dry weather dominated the East and Far West. However, the early- to mid-week period featured record-setting warmth across the Midwest and Northeast, while cool weather covered the West. Later, warmth began to return to the Far West, while
seasonably cooler air overspread the eastern half of the U.S. Weekly temperatures averaged at least 10F above normal in several locations from the lower Great Lakes region into New England, but ranged from 5 to 10F below normal across the Intermountain
West. Elsewhere, light showers were mostly limited to the Pacific Northwest, Intermountain West, and Northeast. A few heavier showers dotted Florida. East of a line from northeastern Texas to Lower Michigan. Unprecedented, late-season warmth continued
for several days across the Midwest and Northeast. Chicago, IL, reported a streak of at least 7 consecutive days (September 20-26) with highs of 92F or greater for the first time since August 25 – September 3, 1953. It was also Chicago’s latest
heat wave (highs of 90F or greater) on record lasting at least 7 days, previously established from August 24 – September 3, 1953, and August 25 – September 3, 1973. In Michigan, six consecutive daily-record highs were set or tied from September 21-
26 in locations such as Saginaw and Flint. Saginaw’s high temperatures during the heat wave ranged from 90 to 95F, while Flint’s readings ranged from 91 to 94F. In Northeastern locations such as Scranton, PA, and Burlington, VT, 4-day heat waves
lasting from September 24-27 were the latest on record. In Scranton, for example, the previous latest occurrence of a heat wave (at least 3 days at 90F or higher) had occurred from September 14-16, 1915. Burlington’s previous latest heat wave had
been September 8-10, 2002. Burlington also tied a 1945 record with 4 days of 90-degree heat in September. Also, 3-day heat waves in New York locations such as Albany (from September 24-26) and Syracuse (from September 25-27) broke long-standing records.
Albany’s previous stretch with at least three 90-degree readings had been observed from September 21-23, 1895, while Syracuse’s latest heat wave had been September 10-14, 1931. Finally, records for the latest 90-degree reading were established in
several Northeastern communities, including Burlington, VT (90F on September 27; previously, 92F on September 16, 1939), and Glens Falls, NY (90F on September 25; previously, 90F on September 13, 1952). Later, in advance of a cold front, record-breaking
heat made a brief push into the Southeast. On September 27-28, Fayetteville, NC, collected consecutive daily record highs (94 and 95F, respectively). Pensacola, FL, tallied a trio of daily-record highs (94, 95, and 95F) from September 27-29. Augusta,
GA, reported 15 consecutive highs of 90°F or greater from September 15-29, including a daily-record high of 97F on the 28th. Meanwhile in Arizona, the week opened on September 24-25 with consecutive daily-record lows in locations such as Flagstaff (25
and 24F) and Winslow (36 and 31F). Other Southwestern daily-record lows for September 25 included 32F in Grand Junction, CO, and 41F in Douglas, AZ.

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