By AccuWeather senior meteorologist
Updated Jul. 14, 2021 10:50 AM EDT
As the atmosphere remains stuffed with tropical moisture, AccuWeather meteorologists are warning that more incidents of life-threatening flash flooding can occur in parts of the Northeast.
As has been the case over the past several days, and most recently in the northern suburbs of Philadelphia late Monday afternoon, rainfall of up to several inches per hour can occur through Wednesday since the air is so moist. Similar flooding events have recently occurred in the area from Binghamton to Albany, New York, Sunday, and part of the New York City metro area last week.
Weather systems tend to move very slowly during July and very warm and humid conditions are a given during the summertime. However, this combination can cause big trouble when weak systems join up or even stand alone --- and the current setup at hand is not done causing trouble.
This image was captured during midday Tuesday, July 13, 2021, and shows a push of tropical air, strewn with clouds, showers and thunderstorms over much of the eastern half of the United States. (CIRA at Colorado State/GOES-East)
"Due to the extremely humid conditions and weak steering winds, any shower or thunderstorm has the potential to produce localized flooding downpours through at least Wednesday," AccuWeather Senior Storm Warning Meteorologist Brian Wimer said.
Tuesday evening, heavy storms sent torrents of rainfall through the Northeast, even producing small hailstones around central Pennsylvania.
NOAA's Storm Prediction Center noted dozens of wind reports in the Northeast Tuesday evening, but one of the strongest occurred in Yates County, New York. Before 9 p.m. local time, a wind gust of 78 mph was reported in Penn Yan, New York, which sits at the northern end of the east branch of Keuka Lake.
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Additional channels of thunderstorms were evident on radar from the Tennessee and Ohio valleys to the Great Lakes region Tuesday.
But the Northeast and Midwest won't be the only areas to see heavy storms. The risk of flooding downpours will continue through midweek in parts of the Southeast. Into Tuesday night, a non-tropical storm pushed onshore along the southern Atlantic coast with the risk of pockets of torrential rain in Florida, Georgia and South Carolina.
On Wednesday and Wednesday night, thunderstorms capable of producing flooding downpours and gusty winds will likely continue over the central Appalachians. However, storms are also forecast to push eastward toward the mid-Atlantic coast and into New England.
So while Philadelphia, Baltimore and Washington, D.C., could get hit each day through Wednesday, the best chance for flooding downpours from New York City to Boston is at midweek.
Flooding downpours have the potential to hit areas that have dodged high water thus far and could make a repeat performance in areas that have been slammed by torrential rain in recent days.
In this pattern, flooding of streets and highways can happen almost immediately. Shallow puddles can turn into raging torrents in a matter of a few minutes, which can stall vehicles and lead to major travel disruptions. Underpasses can rapidly fill with water as rain pours down. Motorists may want to avoid flood-prone stretches of highways, especially during the afternoon and evening hours.
Even in areas that avoid flooding, there is the likelihood of sudden downpours, lightning strikes and strong wind gusts. Where the soil is saturated, trees may topple more easily when compared to dry ground. A few incidents of hail and an isolated tornado cannot be ruled out as well.
It may take until the early or middle part of next week before the steamy part of the air eases up. So while there may be a lull in thunderstorm activity in some areas by Thursday, more downpours are inevitable late this week to the start of next week.
What's causing the intense and frequent flooding downpours?
In the case of this week, the circulation around high pressure near the Atlantic coast has been pumping moisture northward, while at the same time weak disturbances in the jet stream are drifting along into that steamy air.
This high pressure area has been strong enough to limit or prevent rainfall through Monday in many of the beaches from southern New Jersey to North Carolina.
But, instead of the high pushing well inland and limiting storms to the Appalachians on west, its smaller size allowed storms to encroach on parts of the Interstate 95 corridor Monday.
The small disturbances in the jet stream represent pockets of cool air over the middle and upper layers of the atmosphere. Since the effects of the sun are so strong in July, warm air that gathers near the ground then rises up through the cooler air and forms towering clouds capable of producing torrential tropical downpours.
In a pattern as moist as this, it may be possible to physically watch the cumulus clouds rapidly grow during the afternoon hours. If the air was not as moist and/or the air upstairs was not so cool, this effect would be greatly reduced or might not occur at all.
What is making matters worse in the Eastern states is that the ground is moist over a broad area and saturated in some cases. July tends to bring a more widespread dry soil condition due to high evaporation rates. However, in recent days, and weeks in some cases, frequent showers and thunderstorms have limited the evaporation process.
This image shows rainfall for the seven-day period ending on Tuesday morning, July 13, 2021.
Despite the cool air aloft, the storms will do little to cool down the steamy conditions.
"Because the air is so moist even high up in the atmosphere, it may feel just as sticky after the downpours as opposed to beforehand," AccuWeather Senior Meteorologist Tom Kines noted.
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