The pumpkin patch might become a tad toasty this week. The start of October typically signals the arrival of crisp, sweet, comfortable air ideal for sweaters and for most seasonal outdoor activities.  That won’t be the case this year. Thanks to an expanding subtropical ridge, summer’s heat will rebound northward across much of the eastern two-thirds of the US. Perhaps a trip to the beach would be more appropriate to kick-off this October.

The subtropical ridge was already dominating the weather across most of the country. However, its high was centered just south enough over the Southeast US for the jet stream to settle over the northern third of the country, which brought typical fall weather to the northern US for several days.  Now the ridge is expanding, pushing summer-like air northward. Why would a subtropical ridge intensify in autumn?

Rosa’s remnants are pummeling the southwestern US with up to a half-year’s worth of rain as of Monday afternoon. Meanwhile, in California, Nevada and Utah, an unseasonably strong upper-level trough of low-pressure is prompting the development of scattered downpours, which will likely persist into Wednesday. All of this rain represents a wide area over which condensation of water vapor into liquid droplets is occurring. The latent heat released when  all of this water vapor condenses into rain is contributing to the development of subtropical, warm-core high pressure. In the upper Midwest, a shortwave will spark strong to severe thunderstorms this week, which will release even more latent heat. With all of this latent heat being released over such a large area, it’s no wonder that the subtropical ridge is growing.




The heat will slowly spread northward throughout the week, becoming warmer each day through Wednesday in the Midwest and Thursday in the South, the Mid-Atlantic and Southern New England.  Areas along and to the north of I-90 in New England, New York, and the Upper Midwest will miss out on summer’s return.

The mercury will be just shy of 80°F as far north as Newark, NJ and Indianapolis, IN Monday. Temperatures in the mid to upper 70s will dominate just to the north for cities like Chicago, Akon, and Hartford. Tuesday, the 80°F  boundary will race northward through the plains, pushing just north of Pierre, SD. A cold front will stall the advance of the 80°F  boundary in the Great Lakes and northern Mid-Adlantic, but by then temperatures near or exceeding 90°F will have begun to expand into the Southern Plains.

Wednesday will be the peak of the warmth in the Midwest.  The region containing pockets of 90°F  will balloon as far north as the I-80 Corridor near Lincoln, NE. Meanwhile, the 80°F boundary will have surpassed just north of New York City, Pittsburgh, Cleveland, and Chicago. Fortunately for the Midwest, a cold front will sweep across the region Wednesday night, delivering the return of autumn weather north and west of the Ohio Valley.




Thursday will feature the most intense heat in the South and the Mid-Atlantic. The mercury will exceed 85°F  in Philadelphia, Washington, Louisville, and St. Louis. The mercury may even flirt with 90°F  to the south in cities like Richmond, Raleigh, Atlanta and Birmingham.  In southern New England, temperatures my flirt with 80°F  as far north as Hartford.

Thursday night, a cold front will erase the heat in the Northeast and Ohio Valley, but only temporarily. Summer will loiter in the South and the southern Mid-Atlantic. Thus, the heat may return as early as this weekend. We’ll be sure to provide details on the next bout of heat if another resurgence of summer continues to look likely.



Author

Josh is a lifelong nature and weather enthusiast as well as the Head Meteorologist at WeatherOptics. He began regularly forecasting for New Jersey, Long Island and New York City in 2014 on social media, contributing to community pages such as SBU Weather. He holds degrees in Physics and in Atmospheric and Oceanic Sciences from Stony Brook University, from which he graduated in 2018. In the Fall of 2018 Josh will start graduate school for his M.S. in Marine and Atmospheric Sciences at Stony Brook, continuing his research on approaches to non-convective wind gust forecasting.

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