By AccuWeather senior meteorologist
Published Aug. 12, 2020 12:51 PM
Some areas of the East Coast that were recently pummeled by Isaias' flooding rainfall and winds will face more weather hazards into this weekend. Portions of the Southeast, central Appalachians and mid-Atlantic could face daily downpours and, in some cases, multiple downpours each day that could raise the risk of flash flooding.
Flash flood watches were issued by the National Weather Service (NWS) Wednesday along a corridor of the mid-Atlantic from Virginia to New York.
Flash flooding in northern Virginia caused a 50 foot by 100 foot sinkhole to open up in Manassas Park and led to water rescues early Wednesday morning, according to The Washington Post. One car was swept by floodwaters into a creek, and two other cars were “in danger of going into the sinkhole,” Capt. Frank Winston of the Manassas Park Police told The Washington Post.
Fairfax County Police also reported that roads were closed due to high water early Wednesday.
Flash flood watches (shown in green) were in effect from Virginia to New York on Wednesday as gusty storms loomed for parts of the region. (AccuWeather)
As a push of dry air sags southeastward from the Great Lakes region and encompasses areas from central Pennsylvania to Maine into Wednesday night, more showers and thunderstorms will erupt from Virginia to part of southeastern New England.
Along this boundary, the air will be very moist and could fuel torrential rainfall. Since the ground is saturated along much of this same swath that includes many of the major metro areas from Washington, D.C., to Philadelphia and New York City, there is an enhanced risk of flash urban and small stream flooding into Wednesday night. Motorists should expect substantial delays.
Storms may unleash too much rainfall in a short duration, but the wet weather will also usher in some heat relief. Places like Philadelphia and Washington, D.C., that have been enduring days of sweltering 90-degree Fahrenheit heat will see high temperatures trimmed back by about 5 to 10 degrees after the midweek storms. Thermometers have been teetering near the 90-degree mark in New York City, which will also see a similar trend.
The push of dry air is forecast to sputter and eventually stall from part of the Ohio Valley to the mid-Atlantic coast during Thursday and Friday.
After some drying takes place in southeastern New England and the New York City area for a time on Thursday, some shower and thunderstorm activity can pulse northeastward once again for a time Thursday night, before retreating southward on Friday.
"There will be a very weak storm system that moves along the frontal boundary on Thursday and Thursday night," AccuWeather Senior Meteorologist Brett Anderson said. "That little storm will pull moisture northeastward a bit then allow it to dip southward on Friday, but not by much."
Farther south, much of the Southeast and the lower part of the mid-Atlantic will be in the zone where there can be a shower or thunderstorm at any time through this weekend with the bulk of the activity during the afternoon and evening hours.
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Where downpours repeat or a storm stalls over a location, the risk of flash flooding will increase. Some moisture could not only reach areas that have become dry in recent weeks, but also other areas that have been picking up precipitation on a daily basis.
Thanks in large part to Fay and Isaias, some locations of the mid-Atlantic have received twice their normal rainfall over the past six weeks.
In Philadelphia two single days during the period have accounted for most of the rainfall. Fay unleashed 4.15 inches of rain on July 10, and Isaias dumped 4.16 inches of rain on Aug. 4. Total rainfall from July 1 through Aug. 11 was 11.17 inches, compared to a normal rainfall of 5.66 inches.
Meanwhile, in Atlanta, which dodged both tropical systems, rainfall since July 1 has only been about 45 percent of the average of 6.64 inches through Aug. 11. Boston also managed to avoid direct rainfall from both tropical systems with less than 2 inches of rain to show for the same approximate six-week period.
Even though some areas that have been dry recently will pick up some needed rainfall, not everywhere will get drenched by the upcoming pattern. Some of the neediest areas may be bypassed by what would be beneficial moisture. Abnormally dry to moderate drought conditions are affecting areas from central Pennsylvania up through much of New England, according to the U.S. Drought Monitor.
"The same pattern that will keep the Southeast unsettled, will also allow a wedge of dry air to set up from part of upstate New York and northern Pennsylvania to New England this weekend," AccuWeather Meteorologist Dean DeVore said.
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"However, the pattern is rather complex with some areas of the central Appalachians and upper mid-Atlantic region on the fence between moisture to the south and dry air to the north," DeVore explained.
As the weekend progresses, moisture from the Southern states will percolate northward and moisture from the Plains will advance eastward. The leading edge of this shower and thunderstorm activity will overspread areas west of the Appalachians later Saturday and Saturday night. It is possible that downpours return to dry areas of Pennsylvania and New York state that will miss out on rainfall during the second half of this week.
Vast areas of the eastern United States were free of clouds and rain on Wednesday morning, Aug. 12, 2020. However, satellite images may look much different this weekend, especially over the southeastern quarter of the nation as a wet pattern evolves. (NOAA / GOES-East)
"This is the type of pattern, especially in areas from Maryland and West Virginia on south, where there will be flooding downpours, but nearly impossible to say where the downpours will be from one day to the next and which areas will end up with the most rain over a five- to seven-day period," AccuWeather Senior Meteorologist Dan Kottlowski said.
Through this weekend, the areas most likely to be wet most often will be in the Southeastern quarter of the nation, and areas most likely to be dry most often will be from eastern upstate New York to central and northern New England.
During early next week, wet and unsettled conditions with showers and thunderstorms are likely to sweep across much of the Northeast, including areas that manage to stay dry much of the time through Sunday.
Keep checking back on AccuWeather.com and stay tuned to the AccuWeather Network on DirecTV, Frontier and Verizon Fios.
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