Localized flash flood risk to continue in eastern US
By
Alex Sosnowski, AccuWeather senior meteorologist
Updated Jul 24, 2023 2:57 PM EDT
Torrential rain left many motorists stranded in flash flood waters in Bucks County, Pennsylvania, on July 11.
As tropical air continues to flourish, AccuWeather meteorologists warn that more torrential downpours will be on the prowl that can bring new and repeat incidents of flash flooding through midweek and beyond.
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 setup at hand is not done causing trouble.
"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.
As has been the case over the past several days, and most recently in the northern suburbs of Philadelphia late Monday afternoon, the area fro Binghamton to Albany, New York, on Sunday, and part of the New York City metro area last week, rainfall up to several inches per hour can occur through Wednesday since the air is so moist.
"A weak, back-door cool front may play a role in limiting intense downpours from the northeastern corner of Pennsylvania and northern New Jersey to parts of New England into Tuesday evening," AccuWeather Senior Meteorologist Joe Lundberg said.
Low clouds and slightly cooler air in the immediate wake of this feature may be enough in preventing big storms and big downpours.
"A better bet for strong thunderstorms with torrential downpours seems to be from the Susquehanna Valley on west where the leading edge of that front provides some extra lift for towering clouds into Tuesday night," Lundberg added.
People in or traveling through the metro areas of Harrisburg, Lancaster, Reading, Williamsport and Scranton, Pennsylvania, as well as Hagerstown, Maryland; Martinsburg, West Virginia; and Elmira, New York into Tuesday night, could be in the zone for the localized torrents of rain.
Those on the road, working a construction project or have outdoor plans should keep an eye on the weather and stay alert for changing conditions.
CLICK HERE FOR THE FREE ACCUWEATHER APP
Additional channels of thunderstorms were evident on radar from the Tennessee and Ohio valleys to the Great Lakes region on Tuesday. These bands of focused and repeating downpours will likely lead to incidents of flash flooding over the Midwest into Tuesday night.
And it's not just the Northeast and Midwest at risk. Even in parts of the Southeast, the risk of flooding downpours will continue through midweek. Into Tuesday night, a non-tropical storm will push 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, the focus of thunderstorms capable of producing flooding downpours and gusty winds will likely continue over the central Appalachians, but is 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 be almost immediate. 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.
Why so many downpours? Why are they so intense? Why so much flooding?
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 is not quite as large as originally portrayed from last week, but it 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 on 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. But in recent days, and weeks in some cases, there has been a prolific amount of showers and thunderstorms so that the rainfall has overcome the evaporation process.
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.
Keep checking back on AccuWeather.com and stay tuned to the AccuWeather Network on DirecTV, Frontier, Spectrum, FuboTV, Philo, and Verizon Fios.
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News / Severe Weather
Localized flash flood risk to continue in eastern US
By Alex Sosnowski, AccuWeather senior meteorologist
Updated Jul 24, 2023 2:57 PM EDT
Torrential rain left many motorists stranded in flash flood waters in Bucks County, Pennsylvania, on July 11.
As tropical air continues to flourish, AccuWeather meteorologists warn that more torrential downpours will be on the prowl that can bring new and repeat incidents of flash flooding through midweek and beyond.
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 setup at hand is not done causing trouble.
"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.
As has been the case over the past several days, and most recently in the northern suburbs of Philadelphia late Monday afternoon, the area fro Binghamton to Albany, New York, on Sunday, and part of the New York City metro area last week, rainfall up to several inches per hour can occur through Wednesday since the air is so moist.
"A weak, back-door cool front may play a role in limiting intense downpours from the northeastern corner of Pennsylvania and northern New Jersey to parts of New England into Tuesday evening," AccuWeather Senior Meteorologist Joe Lundberg said.
Low clouds and slightly cooler air in the immediate wake of this feature may be enough in preventing big storms and big downpours.
"A better bet for strong thunderstorms with torrential downpours seems to be from the Susquehanna Valley on west where the leading edge of that front provides some extra lift for towering clouds into Tuesday night," Lundberg added.
People in or traveling through the metro areas of Harrisburg, Lancaster, Reading, Williamsport and Scranton, Pennsylvania, as well as Hagerstown, Maryland; Martinsburg, West Virginia; and Elmira, New York into Tuesday night, could be in the zone for the localized torrents of rain.
Those on the road, working a construction project or have outdoor plans should keep an eye on the weather and stay alert for changing conditions.
CLICK HERE FOR THE FREE ACCUWEATHER APP
Additional channels of thunderstorms were evident on radar from the Tennessee and Ohio valleys to the Great Lakes region on Tuesday. These bands of focused and repeating downpours will likely lead to incidents of flash flooding over the Midwest into Tuesday night.
And it's not just the Northeast and Midwest at risk. Even in parts of the Southeast, the risk of flooding downpours will continue through midweek. Into Tuesday night, a non-tropical storm will push 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, the focus of thunderstorms capable of producing flooding downpours and gusty winds will likely continue over the central Appalachians, but is 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 be almost immediate. 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.
Why so many downpours? Why are they so intense? Why so much flooding?
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 is not quite as large as originally portrayed from last week, but it 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 on 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. But in recent days, and weeks in some cases, there has been a prolific amount of showers and thunderstorms so that the rainfall has overcome the evaporation process.
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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Keep checking back on AccuWeather.com and stay tuned to the AccuWeather Network on DirecTV, Frontier, Spectrum, FuboTV, Philo, and Verizon Fios.
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