By AccuWeather senior meteorologist
Updated Aug. 31, 2021 9:48 AM EDT
Ida is expected to continue to wreak havoc days after making landfall and more than a thousand miles to the northeast of the point where it crashed onto United States soil. After the system unleashes a deluge across parts of the Tennessee and Ohio valleys, a tremendous amount of rain will pour down from portions of the central Appalachians to the mid-Atlantic and New England, with the potential for life-threatening and damaging flooding.
The system will continue tracking to the northeast across the central and eastern United States, losing wind intensity as it takes that path. Ida was a tropical depression, with 30-mph sustained winds, over northern Mississippi on Tuesday morning, but it will likely be a tropical rainstorm by the time it reaches the Northeast during the middle of the week.
Even after Ida moves off the New England coast later Thursday, residents across the Northeast may still feel the storm's wrath in the days that follow.
"There is going to be some interaction with Ida and an approaching dip in the jet stream in the northeastern United States," AccuWeather Chief On-Air Meteorologist Bernie Rayno said, adding that this could cause the air pressure at the center of Ida to drop. The intensification could lead to even heavier rainfall across parts of the area.
That interaction will unleash tremendous rainfall and will even compensate for a swift, steady forward movement of the storm.
"Many areas along the path of Ida from the central Appalachians as well as parts of the mid-Atlantic and southern New England are likely to have rounds of rain over a 12- to 18-hour period, but intense rainfall can last six to eight hours," AccuWeather Senior Meteorologist Dan Pydynowski said.
"During that six- to eight-hour period, rainfall rates of 1-2 inches per hour can occur, which is enough to trigger flash urban and small stream flooding even where dry or average soil moisture is present," Pydynowski explained.
A band of 4-8 inches of rain is forecast from northern West Virginia and southwestern Pennsylvania to northern New Jersey, the Hudson Valley of New York state and portions of southern and central New England. Farther to the southeast, 2 to 4 inches of rain is anticipated. And in both areas, pockets of heavier rain can fall with the risk of double-digit rainfall in a few cases. An AccuWeather Local StormMax™ of 12 inches of rain is most likely to occur from near the tri-state area of northeastern West Virginia, northern Maryland and south-central Pennsylvania to part of southeastern Pennsylvania.
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Many locales across the Northeast have already had a wet end to the summer. Rainfall during August has been above average to excessive in the same areas of the Northeast that Ida is forecast to impact. One and a half to three times the normal monthly rainfall has fallen in the corridor that will experience Ida's torrential rain.
In some areas, the excessive rainfall was due to the impacts of Tropical Rainstorm Fred and/or Tropical Storm Henri in prior weeks, but in other areas, an almost daily dose of pop-up showers and thunderstorms has drenched and saturated the landscape.
The wet August is evident in some of the major population centers. New York City's Central Park has received a staggering 10.32 inches of rain as of Aug. 30, compared to an average of 4.30 inches. Boston has picked up 7.00 inches of rain this month, compared to an average of 3.13 inches through Aug. 30. When compared to normal, Washington, D.C., has been even wetter with a whopping 288% of the city’s normal rainfall as of Monday.
The action of pop-up drenching thunderstorms will continue in some locations, even ahead of Ida's arrival. These pre-Ida downpours can lead to incidents of flash flooding and travel disruptions.
The combination of saturated ground, hilly terrain and extensive paved surfaces will lead to significant runoff from Tuesday to Thursday as Ida arrives and rolls through and interacts with the non-tropical system.
This runoff will first hit streets, highways, poor drainage areas and small streams.
Later, significant rises on larger streams and the major rivers seem likely in Ida's path. Some of the rivers that have the potential to experience a significant water rise include the lower portion of the Susquehanna, both the main stem and the west branch; Delaware; Lehigh; Schuylkill, Juniata; Conemaugh, Passaic; Raritan; Potomac; Monongahela; Youghiogheny and Kanawha.
In portions of the central Appalachians and the mid-Atlantic, there are some similarities to the current forecast and events that unfolded in the late summer of 2011, when back-to-back tropical systems, Hurricane Irene and Tropical Storm Lee, triggered torrential rainfall and major flooding, AccuWeather Senior Weather Editor and meteorologist Jesse Ferrell said.
"In 2011, after Hurricane Irene and Tropical Lee drenched Pennsylvania, the flooding on the Susquehanna River was the highest since Hurricane Agnes in 1972 in Harrisburg, Pennsylvania," Ferrell stated.
With more than a foot of rain from Tropical Storm Lee alone, part of downtown Harrisburg and the town of Hershey, Pennsylvania, were inundated.
"Damage in Dauphin County, Pennsylvania, alone was more than $150 million," Ferrell added.
AccuWeather meteorologists are expecting a sharp edge to the rain in this case. In 2011, torrential rain from Lee spread across much of northern New York state and part of northern New England. In the situation with Ida this week, little rain is forecast in comparison.
The National Weather Service river gauge forecast, which may not yet account for all of the predicted rainfall from Ida, shows the Conemaugh River at Seward, Pennsylvania, and the Juniata River near Williamsburg, Pennsylvania, both nearing major flooding stage Wednesday night. At Williamsburg, that's higher than the river rose in 2011. At Seward, that reading would be the highest since 1977.
This image shows the predicted water level of the Conemaugh River at Seward, Pennsylvania, based on National Weather Service hydrological information. The predicted level is subject to change based on additional rainfall data. (NWS)
People living along unprotected areas of small streams and rivers will need to keep a watchful eye and heed advisories from officials as they are issued. Removal of debris along small streams and city streets ahead of the heavy rainfall might reduce the risk of flooding.
AccuWeather forecasters urge people to avoid camping along small streams and to avoid attempting to drive through flooded roadways. It could be a life-saving move to find alternative routes and avoid areas prone to flooding at times of heavy rain, such as underpasses and low-lying roads along small streams. In hilly terrain, there is also the potential for road washouts, mudslides and rock slides.
As of Tuesday morning, flash flood watches had been posted along a 1,200-mile-long swath from the Florida Panhandle to southern New England.
And to make matters worse, flooding is not the only concern from Ida in the Northeastern states.
"There is the risk of a severe weather threat that includes tornadoes in part of the mid-Atlantic region from Wednesday to Wednesday night," Rayno warned.
A sweep of cool air in the upper part of the atmosphere will plunge into the zone of moisture from Ida, and that cooler air aloft is a key ingredient for severe thunderstorms to erupt. Since winds may be blowing from different directions at different altitudes, it could set the stage for severe thunderstorms and isolated tornadoes due to added spin in the atmosphere.
This risk may extend from southeastern Virginia, the Carolinas and southeastern Maryland on Wednesday to perhaps as far to the north as northern Virginia, northeastern Maryland, Delaware, southern New Jersey and southeastern Pennsylvania late Wednesday and Wednesday night.
An added danger will be the potential for tornadoes to be concealed by rainfall or under the cover of night's darkness. AccuWeather forecasters emphasize the importance of having the means to receive any severe thunderstorm and tornado warnings. Pockets of damaging wind gusts may occur even where tornadoes do not touch down.
AccuWeather will continue to provide updates on Ida as this dangerous tropical system continues to move through the U.S.
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