Last Wednesday started as a typical fall day in the Northeast, with crisp morning lows and highs set to crest in the 60s. A chance of thunderstorms with isolated gusty winds was in the forecast.

By Wednesday night, nearly 400,000 customers across the Northeast would be without power, with a damaging derecho felling thousands of trees and claiming two lives. The fatalities, in Clifton Park, N.Y., and Great Barrington, Mass., occurred when trees collapsed on vehicles.

A derecho is a fast-moving complex of thunderstorms with severe wind gusts that travels an extended distance that can span multiple states. The National Weather Service says a line of thunderstorms must persist for at least 240 miles to be classified as a derecho; the American Meteorological Society lists that threshold at 400 miles.

The National Weather Service confirmed Monday that last week’s storm complex satisfied its derecho criteria.

The most recent previous derecho to affect New England occurred on May 15, 2018, when a spattering of tornadoes swept across western Connecticut. Just this past June, a violent derecho tore across eastern Pennsylvania, claiming three lives.

2020′s most devastating derecho struck Iowa in August, where winds up to 140 mph caused hurricane-like destruction in Cedar Rapids and nearby communities, with damage estimated at $7.5 billion.

A derecho develops

Strong wind gusts associated with a developing line of thunderstorms were noted over Lake Erie around lunchtime last Wednesday. Damage from the storms was observed as soon as they arrived in Oswego County, N.Y., where wind gusts topped 60 mph.

Wind gusts began to become more scattered to widespread as the storms moved east around 2 or 3 p.m. A brief EF0 tornado touched down in Canajoharie, in Montgomery County, N.Y., destroying a barn in its fleeting 30-second display of power. The narrow vortex was estimated to have produced winds up to 85 mph.

A trio of microbursts, which are intense, localized downbursts of winds, were confirmed in Root, Pittstown and Johnsonville, N.Y., southeast of Saratoga Springs, with wind speeds pegged at 80, 90 and 100 mph respectively. The most significant affected a swath on the eastern side of the Tomhannock Reservoir, about 15 miles northeast of Albany, where numerous hardwood trees were snapped.

The National Weather Service in Albany noted that the intensity of the event was among the greatest in 2020, on par with a thunderstorm outbreak May 15.

Strong storms race east

Storms raced east into Massachusetts and northwest Connecticut at forward speeds between 50 and 70 mph.

Atop Mount Tom, 15 miles north of Springfield, Mass., winds were clocked at 88 mph. The mountain hosts numerous television and radio transmitting towers because of its elevation.

A 74 mph wind gust was measured in Westfield, Mass., while a 68 mph wind gust was reported from the Blue Hill Observatory in Milton, just south of Boston. At the coast, winds gusted to 82 mph in Quincy.

Heavy damage was reported in Hanover and Hingham, Mass., south of Quincy.

The storms produced winds estimated between 60 and 70 mph in southeastern Massachusetts near the Cape Cod Canal; they weakened before heading over the Cape.

Nearly a quarter-million customers in the Bay State found themselves in the dark Wednesday, with a similar scenario unfolding for 150,000 customers in eastern New York. More than 45,000 customers in Massachusetts alone remained without power Thursday night.

A tricky forecast

On Wednesday morning, the Storm Prediction Center had placed most of central and eastern New York and southeast New England in a level 1 out of 5 “marginal risk” for severe weather.

A threat of isolated damaging winds was mentioned but only at the lowest-level risk category. Autumn severe weather events are notoriously difficult to predict in the Northeast, because instability — or the amount of “juice” in the atmosphere that helps air to rise — is frequently too meager to support severe storms.

In this case, that lackluster instability, caused by cool temperatures, was overcome by powerful wind dynamics at the mid-levels of the atmosphere, where jet stream winds over 100 mph screamed east. Extremely dry air was also found at the mid-levels.

“I think a big part of it was we had really strong winds aloft,” said Hayden Frank, a meteorologist at the National Weather Service in Norton, Mass. “You kind of had a synoptic setup you’d typically see during the latter half of fall, so you had very strong winds just off the ground, even though the instability was marginal.”

He also cited as contributing factors very steep low- and mid-level lapse rates — the rapidity with which temperature decreases with height. If the air cools off quickly above the ground, it’s easier for strong updrafts to develop. The atmosphere can also more efficiently transfer momentum to the surface, which includes mixing down strong wind gusts.

“The low-level lapse rates were very useful in transporting those strong winds down to the ground,” Frank said.

An approaching cold front served as a triggering mechanism to set the whole process in motion.

Only once the storms formed were forecasters able to issue timely downwind warnings.

A derecho in an unusually dry environment

Temperatures at the time of Wednesday’s derecho were in the lower to mid-60s across most of Southern New England, and dew points — a measure of how much moisture is in the air — were in the upper 40s to near 50 degrees. Those are both very low values to ordinarily sustain severe thunderstorms.

It’s not unheard of, though. On Nov. 30, 1989, an evening derecho rolled through Pennsylvania into the Tri-State area and Mid-Atlantic in an environment with dew points in the 30s.

Bill Bunting, chief of forecast operations at the Storm Prediction Center, said that Wednesday’s storm complex could be considered a “hybrid low dew point derecho.”

Low dew point derechos form when very dry air near the surface makes it possible for thunderstorms to exhale strong wind gusts at ground level. But getting thunderstorms to develop in such dry environments is difficult, making low dew point derechos comparatively rare.

Bunting said that Wednesday’s derecho formed in a rather dry environment and exhibited some of the characteristics of both a low dew point and a regular derecho; he considers the episode a hybrid event.

The storms weren’t all bad, though. As they departed, the setting sun combined with an impressive display of pouch-like mammatus clouds produced a brilliant display of pastel hues that rivaled sunsets behind storms on the Great Plains.