By AccuWeather senior meteorologist
Unusually cool conditions brought an early autumn preview across the Midwest and Northeast this week, but a return to more typical sultry August weather is forecast in the coming days. AccuWeather meteorologists predict that several major cities will see highs throttle up once again late this week into next week -- with more opportunities for the mercury to hit 90 degrees Fahrenheit.
A big southward dip in the jet stream is currently to blame (or thank) for the conditions that were more typical of mid-September during the first half of this week. The jet stream is forecast to retreat during the latter part of this week, but not before allowing drenching rain to spread northward into parts of New England.
"That storm with its tropical moisture connection will unload a general 1-3 inches of rain and can produce localized flash flooding from eastern Long Island, New York, to southeastern Maine from Wednesday night to Thursday," AccuWeather Senior Meteorologist Brett Anderson.
As the jet stream retreats, humidity and temperature levels will gradually increase over the balance of this week.
Typical early August high temperatures range from the upper 70s in the northern Great Lakes, northern New England and the highest elevations of the central Appalachians, to near 90 around the Chesapeake Bay and the lower Ohio Valley. Low temperatures tend to range from the mid-50s in the northern tier and in the mountains to near 70 over the Ohio Valley and the mid-Atlantic Interstate 95 corridor during the first part of August.
From late this week to this weekend, high temperatures will trend upward from the 70s and low 80s to widespread highs in the 80s with a few 90-degree highs in some of the major cities in the Midwest and mid-Atlantic.
The upcoming pattern will allow some locations to add to their number of 90-degree days. Many of the major cities, such as Chicago, New York City, Philadelphia, Detroit, Boston, Pittsburgh, Cincinnati, St. Louis and Washington, D.C., have the potential to add a number of 90-degree days to their tally for the season. Many cities are on track to finish near average for the year in terms of hot days.
D.C. has recorded 30 90-degree days, just six shy of the seasonal average. Similarly, Philadelphia has experienced 20 days at 90 F or higher, compared to its annual tally of 27. The Big Apple is five days shy of its normal yearly total of 17 days of 90-degree heat.
Burlington, Vermont, has already exceeded its average of seven days at or above 90 F with eight such days recorded thus far. However, Cincinnati is lagging behind with eight compared to a seasonal average near 20 days. Pittsburgh has only had two 90-degree days so far this summer, compared to an average of 10 days per year.
During the weekend, another southward dip in the jet stream is forecast to pivot from the Midwest to the Northeast and may put the brakes on the warming trend, Anderson said.
The brief jet stream plunge is likely to be much less pronounced, and it will have virtually no cool air associated with it. However, it may bring an uptick in shower and thunderstorm activity, according to Anderson.
At the very least, spotty thunderstorms are likely to erupt in eastern parts of the Great Lakes and Ohio Valley during Saturday afternoon and evening, followed by areas along the mid-Atlantic coast and in New England Sunday.
"During much of next week, temperatures are likely to inch a bit higher so that highs ranging from the mid-80s in the northern tier and over the mountains to the upper 80s and low 90s in many of the major Midwest, mid-Atlantic cities will be common," Anderson said.
With this in mind, there can still be some subtle day-to-day fluctuation in temperature thanks to sea and lake breezes in some areas, or spotty, cooling thunderstorms from the prior day.
During August, a decline in daylight hours tends to limit the amount of heating just enough so that morning fog episodes can become more common and thunderstorm activity can have a more lingering, negative impact on peak temperatures.
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