Potentially damaging frosts and freezes moving into Northeast midweek

Air from northern Canada will drop southward into the Upper Midwest and northeastern United States from the middle to late parts of the week. This will not only set off snow in parts of the northern tier but will also bring the risk of a damaging frost and freeze to many areas, AccuWeather meteorologists warn.

The batch of air, encapsulated by a dome of high pressure, will affect the area from Wednesday to Friday.

The first of two Canadian high-pressure areas delivered temperatures as low as the frigid low 20s F in northern Michigan and northern New York. On Monday morning, temperatures in the frosty mid- to upper 20s were common from interior southern Michigan to northern Ohio, central Pennsylvania, central and southern New York state and central and northern New England.

The cold air and threat of frosts and freezes have sent orchard and vineyard operators scrambling to raise temperatures by a few degrees by using wind machines and bonfires in their fields. Regarding vineyards, the non-native varieties of grapes are most susceptible to damage.


Temperatures have been much higher than the historical average since the start of the winter. This has forced bud break, blossoming and leaf-out of many forms of vegetation ranging from flowering trees and shrubs to deciduous trees to fruit trees, vines and bushes. Anytime a heavy frost or hard freeze occurs after bud-break and leaf-out occurs, there is the risk of damage.

Garden centers, agricultural interests or anyone who has put out tender plants in areas where a frost or freeze can occur from Wednesday night to Friday morning will be at risk.

"Widespread low temperatures in the 20s to the low 30s are in store from much of the Great Lakes region [away from the immediate shores of the lakes] to the central Appalachians, the interior mid-Atlantic and much of New England on Thursday morning," AccuWeather Senior Meteorologist Matt Benz said.

The shores around large, deep lakes tend to be warmer than the surrounding landscape on a clear, calm and chilly night because of the relatively warm water.

Also, the urban "heat island" effect may prevent frost in some large cities but only to a certain point. There is the risk of frost reaching some coastal areas of the Northeast on Thursday and Friday mornings, including in Boston and Philadelphia, as well as New York City and Washington, D.C. Frost is a concern around Chicago, Detroit, Cleveland and Pittsburgh -- at least for Thursday morning.

"Temperatures on Thursday night may be a few degrees higher than Wednesday night, especially from the central Great Lakes on west," Benz said, adding, "However, it is still likely to get cold enough for a frost or freeze from parts of the central Appalachians to much of New England on Friday morning."

As a special note, forecast low temperatures are intended for a location 4-6 feet off the ground, where official temperature readings are taken. During a clear, calm night, the temperatures can dip 5-10 degrees lower at ground level compared to 4-6 feet above the ground. In conditions such as those anticipated in the region from Wednesday night and Thursday night, it may be necessary to subtract at least 5 degrees from the forecast low temperature for a more accurate reading at ground level.

This is why taking measures to guard against a frost or freeze is advised when forecast low temperatures are above the freezing mark -- 32 degrees Fahrenheit.

As the leading edge of the cold air pushes in, temperatures may dip low enough as moisture lingers for a mixture of rain and snow over the mountains of the northern tier of the Northeast from Wednesday to Wednesday night, Benz explained.

"Up to a few inches of snow may fall from parts of the Green and White Mountains to northern Maine," Benz said.

Farther to the south, the same cold front responsible for the snow over the northern tier set off showers and locally heavy thunderstorms over parts of the Midwest into Tuesday night. Thunderstorms brought hail and gusty winds across portions of Wisconsin and Michigan.

On Wednesday, the same cold front may set off thunderstorms in parts of the mid-Atlantic.

After the high-pressure area cycles through late this week, upcoming pushes of Canadian air are likely to be less potent and not quite as cold. While it is a bit premature to say there will not be any more risk of frosts or freezes moving forward in May, it is likely they will not be as intense nor as widespread as that of this past weekend or what lies ahead from Wednesday to Friday.

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