Thunderstorms are going to light up the sky over parts of the Northeast on Tuesday evening, and some of them will probably be severe.

The Storm Prediction Center issued a “moderate” risk for severe thunderstorms from the Poconos into parts of southern New England, which is actually pretty high. It’s the second-highest on the center’s five-point scale, and it means widespread severe thunderstorms are likely. They will be long-lived and intense.

If you work in the Interstate 95 corridor from New York to Boston, and have the ability to head out early, that might be a good idea. A line of thunderstorms — or a squall line — was already tracking across Pennsylvania and Upstate New York early Tuesday afternoon. Forecast models suggest that it will arrive in the I-95 corridor around 5 p.m. in New York City and Boston.

To the south, there is a heavy rain and flooding risk in the Washington and Baltimore region Tuesday evening, but damaging winds are somewhat less likely there.

Early Tuesday afternoon, the National Weather Service issued several tornado and severe thunderstorm watches that covered more than 36 million people between Pennsylvania and southern Vermont. While tornadoes are possible, especially in storms that pop up ahead of the squall line, the biggest threat from this line of storms is destructive wind.

Between Pennsylvania and western New England, the Storm Prediction Center forecasters said that “scattered severe wind gusts appear likely which may yield a derecho, and thus damaging wind coverage probabilities have been increased,” meaning there’s even more likelihood that this line of storms will be destructive.

Derechos are large line of thunderstorms that bows out, arrowhead-like, “as if a fist of wind punches through the line,” according to our severe-weather expert Jeff Halverson.


“Instead of updrafts, these storm systems are dominated — propelled forward, in fact — by a powerful, spreading, surging downdraft,” Halverson writes. “In a thunderstorm, the downdraft is the chilly blast one feels just before the onslaught of heavy rain. But in a derecho, the windy blasts from dozens of extreme storm downdrafts (called downbursts) coalesce into a single, massive wind surge that races forward at speeds of up to 60 to 70 mph.”

In 2012, a particularly destructive derecho tracked from Chicago to Washington in less than 12 hours. Three million homes lost power and 18 people were killed. The storm did around $2.9 billion in damage.

Although a storm as strong as the 2012 derecho is not likely on Tuesday, the same types of impacts will apply in affected areas of the Northeast – well north of where the 2012 derecho struck. Trees and branches will come down in strong wind, which will lead to thousands of power outages. Because of the timing, the evening commute will be tangled and potentially dangerous.

By 8 or 9 p.m., the line of thunderstorms will probably be past the I-95 corridor. It will reach the cool marine air of the Atlantic Ocean and will weaken.