8 to 14 Day Forecast: August 28 to September 3 Looks Close to Normal for Many…But What About the Tropics?


We're at the time of year where a complete 8 to 14 day forecast, offered in my weather broadcasts every Monday night on NECN, must include an overview of how the pattern will impact any tropical threats to the United States.

The basic pattern across the contiguous U.S. features a continuation of the general configuration of the Westerlies that we've seen over the last several weeks - a somewhat zonal flow over the Northern Tier of the United States that wavers in response to shortwaves associated with stronger vorticies in Canada.  This allows for multiple southward penetration of Canadian cool air, and when combined with anticyclogenesis (development of high pressure) over the Central and Eastern United States, affords near-normal temperatures for a large segment of the nation, except near and under the large Western U.S. ridge, where both hot and dry conditions continue.


Below normal precipitation, of course, continues beneath the ridge, while an average weakness between the Lower 48 ridge, and the large dome of high pressure over the Atlantic, keep the Southeast US open to Gulf of Mexico moisture.  Of course, this also means the Southeast US - and the Eastern United States - are open to potential tropical cyclones should they be in the vicinity when these weaknesses in the ridge open wide enough.  As for the last days of August into the first days of September, it appears the keys will be: 1) strength of the upper level low digging into the Southeast US during the middle of the period, and 2) position and strength of Westerlies in the Northern Tier.


The Southeast US upper low will surely feed above normal precipitation when coupled with Gulf of Mexico moisture, but also would afford a southerly flow for a time Tuesday to Wednesday, which would open the East Coast to a tropical cyclone.  If this upper low doesn't dig quite so deep, this leaves Florida and the Gulf more vulnerable.  At this early juncture, I expect the low to dig deep enough to open the Southeast US Tuesday 8/28 to Thursday 8/30, but that may be an incorrect expectation.  The Westerlies, on the other hand, seem unlikely to relent - that is, the upper low in the Southeast probably will not be strong enough to grab and reconfigure the polar jet stream, which will still flow quickly and rather zonally across the Northeast, meaning even a northward turning cyclone by the Southeast, should that occur, would likely recurve thereafter, without running the remainder of the Eastern Seaboard.  With such a small window - Tuesday, Wednesday and perhaps Thursday - this essentially means that any tropical cyclone (as of this writing, I expect we'll see both Atlantic tropical waves develop soon) has a greater chance of wandering into the Gulf, unless it hits this window at the same time.  As such, it's entirely possible we see the first disturbance head into Florida and/or the Gulf, and the second more open to the northward turn.

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