It’s not quite astronomical fall yet, but parts of New England this weekend are bracing for some of their coldest temperatures this month. So cold that a freeze warning and frost advisories have been issued across interior sections of New Hampshire, Maine, and Vermont from overnight Saturday into Sunday morning.
Overnight lows will take a dive to subfreezing temperatures, plunging as low as 27 degrees, in some areas as daytime heating escapes under clear night skies and as cooler air settles in. It might be wise tonight to make sure you take in your plants.
A strong area of high pressure from northern Canada will push into New England early Friday afternoon, bringing very dry, cold air. That will help cool the atmosphere quickly overnight and into the start of the weekend. The freeze warning and frost advisory are in effect until 8 a.m., Sunday.
Folks up north will see Sunday morning temperatures in the low 30s right before sunrise, depending on the elevation of where you live. Frost will appear on lawns and likely on car windshields to start the day, but despite the cooler air mass working into the region, the sun angle is high enough this time of year that the frost will melt quickly and should be gone by mid-morning.
No frost advisories are in effect for Massachusetts and the rest of Southern New England, although they will also see the coldest lows so far this season — in the 40s; the 50s on the Cape.
What’s the difference?
A freeze warning is issued when temperatures are forecast to fall below 32 degrees for a long period of time (several hours) over a widespread area during growing season. Plants left outdoors may die. A hard freeze is when temperatures are below 28 degrees.
A frost advisory, on the other hand, means temperatures are 33 to 36 degrees with clear skies and light winds over a widespread area that would promote the development of frost. Plants left outdoors may be damaged. A freeze watch is issued for potentially significant, widespread freezing temperatures within the next 24 to 36 hours.
When does first frost normally arrive?
The first frost of the season, also called “first freeze,” is marked by the date when overnight lows dip to freezing (32 degrees) or below. For the Boston area, the average date for the first frost is Nov. 5, according to National Weather Service records dating back to 1872.
In New England, the historical timeframe for the first frost falls within the first two weeks of October over interior portions of New Hampshire, Vermont, and Maine. But this fall, a few higher elevations are seeing the first frost before September comes to a close, given that the first two weeks of the month have been below average for most of the region.
Coastal Maine and southwest through the interior portions of Central and Southern New England will reach freezing by the end of October.
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Ken Mahan can be reached at ken.mahan@globe.com. Follow him on Instagram @kenmahantheweatherman.