Northeast heat to end with severe storms, possible flooding

July 12, 2024

A disruptive episode of strong to severe thunderstorms, including the risk of flooding and a few tornadoes, is expected across the Northeast on Thursday. Damaging winds will accompany the storms, which look to peak in intensity during the evening commute along one of the busiest highway corridors of the United States.

The elevated risk zone for severe storms includes Washington, Baltimore, Philadelphia, New York and Boston.

The National Weather Service issued a severe thunderstorm watch for the zone from northeast Pennsylvania to southeast Maine until 8 p.m., including Scranton, New York, Hartford, Boston and Portland.

“This is a severe weather setting which is uncommonly experienced in Southern New England, and the potential for brief tornadoes and straight-line wind damage is a credible risk in environments like this one,” the Weather Service office in Boston wrote.

In addition, a flood watch is up for most of Connecticut, Rhode Island, Massachusetts, Vermont, New Hampshire and southern Maine. Thunderstorms may train, or pass repeatedly, over the same areas, dumping heavy rain on an area that has received more than its fair share of water in recent weeks. That could revive flood concerns, particularly in urban areas.

Farther south, into the Mid-Atlantic, the Weather Service cautioned that severe storms could also develop, but may not be as widespread as in New England. A severe thunderstorm watch was also issued until 11 p.m. for the zone from Northern Virginia through east central Pennsylvania and much of New Jersey, including Washington, Baltimore, Philadelphia and Allentown, Pa.

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Despite the tempestuous forecast, the storms do nothing to quell the heat. Much of the eastern half of the stretch between Washington and Boston is under heat advisories, with highs forecast to peak in the upper 90s to near 90 south to north. A few areas within this zone, including New York City, are under excessive-heat warnings. Here, temperatures in the mid-90s or higher will overlap with oppressive humidity to make it feel like at least 105 degrees.

The same storm system that will trigger storms in the Northeast brought severe gusts to the Great Lakes and Ohio Valley on Wednesday. Detroit’s Metropolitan Airport measured gusts to 67 mph, and a 70 mph gust was clocked in Stockton, Ill. Three people were injured by falling trees in Presque Isle State Park in Erie, Pa.

Thursday’s severe weather risk

Areas affected: A level 2 out of 5 risk of severe weather has been drawn by the National Weather Service Storm Prediction Center. It includes parts of the Mid-Atlantic and New York City Tri-State area, as well as New England away from the immediate South Coast of Massachusetts, the Cape and the Islands. Within the risk zone are D.C.; Baltimore; Philadelphia; New York City; Scranton, Pa.; Hartford, Conn.; Providence, R.I.; Boston; Manchester, N.H.; and Portland, Maine.
Timing: Showers and storms in the western portions of the Northeast will increase in coverage and intensity as they approach the lee of the Appalachians by 3 or 4 p.m. By then, they will be strong to severe, and should affect the Interstate 95 corridor between 4 and 7 p.m.; thereafter, they will gradually fizzle while shifting toward the coast.

Effects: With a swift jet stream moving overhead, storms are likely to mix momentum to the surface in the form of strong to locally damaging straight-line winds. A few gusts over 70 mph are possible. Hail, perhaps nickel to quarter size, may accompany the strongest storms. A few isolated tornadoes are possible as well somewhere in the area, particularly in the Connecticut River or Blackstone Valleys, as well as north-central Massachusetts, southeast Vermont and southern New Hampshire.

Discussion

A decaying MCS, or mesoscale convective system, is exiting the Great Lakes. It was surfing a pulse of strong west-southwesterly winds at the mid-levels of the atmosphere.

That jet stream energy is associated with a subtle pocket of high-altitude cool air, low pressure and spin aloft. Because it is a comparatively weak trigger, there will not be a meaningful drop in temperatures behind the system.

That said, storms will not need a strong trigger to become severe, given the ample heat and moisture present.

The greatest risk of a tornado will be near the Massachusetts-Vermont and Massachusetts-New Hampshire borders. That is where a warm front will be present. Warm fronts tend to feature a little extra twist, which storms can gobble up and use to rotate more profusely. Moreover, that is also where a couple of rogue supercells, or more discrete, isolated rotating storms, are expected to form.

Heavy rain and flood potential

Storms also may produce heavy rainfall, particularly considering how moisture-loaded the atmosphere is, especially in northern New England. Precipitable water, a measure of how much moisture is present in a column of atmosphere, is around two inches, which is much above normal. As rain falls, the moisture-loaded air will gradually be replenished by south-southwesterly surface winds. That means storms could wring all that moisture out of the air, much like squeezing a sponge, and then ingest more moisture and do it again.

Across northern Vermont and New Hampshire and western and central Maine, at least 1.5 to 2 inches of rain is expected.

In southern New England, weather models project somewhat less rain, or around a half-inch to an inch. It is important to note that, given the scattered nature of the storms, some places could receive twice or triple that, and some will largely miss out. That is why a few pockets of flooding are possible.

New England has been waterlogged as of late, and with saturated soils, the ground will not be able to handle much additional water. That will bolster the risk of flooding.

Hartford’s Bradley International Airport has experienced its wettest July on record, with 12.43 inches. The records date to 1949.

Most of New England is running 4 to 8 inches above average for the past 30 days. That is why any additional rain could be problematic.

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