It’s more than two weeks into meteorological fall and less than five days from the start of astronomical fall, yet it still feels like the middle of summer throughout much of the eastern half of the country. After a brutally hot summer for much of the Eastern US, millions of Americans have been looking forward to the crispness of autumn. Instead, the heat index will continue to climb into the middle 90s as far north as the Central Plains this week. There will be a few intermittent cool shots but summer won’t give up without a fight. The question remains; where is fall?
Canada has been knocking for the past few weeks but the knocks have gone unanswered. The loss of daylight and solar radiance has already begun cooling the arctic tundra. In autumn, arctic air masses have intensified and penetrated further south. But as this cool weather spreads southward across North America, it continues to encounter a wall of heat in the northern tier of the US. The strong temperature gradients have intensified the jet stream, which has been unable to drape southward without retaliation from tropical high pressure centered over the southern US. Only the Pacific Northwest, the northern Midwest, and northern New England have enjoyed prolonged tastes of fall, owing to their proximity to the jet stream. The cool air will make some headway over the next week, but summer is still expected to dominate.
The temperature will climb into the low 90s as far north as Sioux City, IA Monday. The 90 degree heat will extend as far west as the Rocky Mountains and as far east as the Mississippi River. The undead remains of Florence will draw northerly winds to moderate the temperatures further east. Low to middle 80s are expected eastward to the Ohio Valley. Ahead of Florence’s remains, temperatures will climb into the low 80s along the Northeast’s I-95 corridor northward into southern Ontario and Québec. With so much moisture drawn from the Gulf of Mexico and the Atlantic Ocean, dew points will surge into the low 70s such that even where the temperature does not climb above 90, the humidity will likely make it feel as if it were.
Though heat may dominate, fall is mustering its strength to fight back. An amplified trough of low pressure will begin to wash ashore the West Coast early Tuesday, pushing eastward against the subtropical ridge responsible for the heat. The trough will draw moisture from the Pacific and generate monsoon rains in the Desert Southwest Wednesday and Thursday. These rains will spread as far north as the Rocky Mountains. Meanwhile, the trough will drag a cold front southward against the heat in the Plains and northern Midwest, spawning several rounds of thunderstorms.
The latent heat release from the thunderstorms and the pressing of the cold front against the ridge will invigorate the jet stream to the west and to the north of the heat dome. All that momentum will push the ridge south and eastward through the end of the week, leaving crisp, fall air in its wake. Unfortunately, the heat won’t be eliminated. Rather, it will simply shift eastward.
Tuesday will likely be another 90 degree day for the central Plains. The only relief will be north of I-80, where fall will have already left a taste of 70 degree weather. Elsewhere, it will continue to be hot. Just east of the Rocky Mountains in Colorado, southern Nebraska, and western Kansas, temperatures will soar into the upper 90s to near 100 degrees due to air warming as it rushes down the Rockies. The only sign of fall will be the spatial distribution of the heat. Incredibly, temperatures will only reach the lower 70s less than 100 mi north of this region in central Nebraska. Meanwhile, upper 80s and lower 90s will again reach as far east as the Mississippi River. Topeka, Tulsa, Kansas City, St. Louis, and Springfield will all crack the 90 degree mark. Dew points in the low 70s will make temperatures feel even hotter. As Florence’s remains move out of the Ohio River Valley, lower and middle 80s will hit the Appalachians.
Slowly, the ridge will continue to shift east. The heat will moderate west of the Missouri River Wednesday. This will likely be the last 90 degree day until at least the middle of next week for the Plains. East of the Missouri River, the heat will slightly intensify. Pockets of 90 degree heat could reach as far east as the Ohio River Valley. Northerly winds behind Florence’s remnant low will keep temperatures in the 60s and 70s in the Northeast. Dew points will be in the 50s and low 60s, giving the Northeast a quick taste of fall before summer returns late week.
The heat will crest Thursday in the Midwest ahead of a cold front drawn south and eastward by the upper-level trough. This will likely be the last hot day until mid-week east of the Mississippi River. Before the cold frontal passage, 90 degree heat could reach as far north as Chicago. Clouds from convection will likely keep temperatures just shy of 90 between the Rockies and the Missouri River.
The cold front will race through the Central US Thursday and Thursday night, sparking widespread thunderstorms, some of which could become severe. This will suppress the ridge and its heat southeastward, drawing cool, crisp air just in time for the weekend in most of the Central US. Temperatures will be stuck in the upper 60s to upper 70s until at least Monday before summer puts up another fight.
While the Central US enjoys relief Friday, the shift of the ridge will mark the beginning of a muggy weekend in the Northeast. Lower and middle 80s will return to the Mid-Atlantic and southern New England east of the Appalachian mountains Friday and Saturday, before a cold front returns cooler weather just in time for the start of fall Sunday.
Unfortunately, the weekend relief will not be long-lasting. A subtropical ridge will redevelop early next week. But model guidance suggests that the season’s most intense trough yet could approach just behind it, with very cool weather late next week. It is still uncertain whether such a system will bring more intense relief, so keep checking back as we learn more.