By Terry Eliasen, Meteorologist, WBZ-TV Exec. Weather Producer


BOSTON (CBS) – Never has a first frost of the season been so welcomed. Typically the first frost is not cause for celebration, as it signals the end of a growing season and, in many ways, the official death of summer. This year, however, with the Eastern Equine Encephalitis (EEE) mosquito threat being at an all-time high, folks have literally be calling, emailing and begging for that first frosty night.

Well, I have some good news – Friday night just might be the night!

First, a refresher on what exactly it means to get a frost versus a freeze.

A frost advisory is issued (like tonight) when temperatures are expected to fall anywhere from about 32 to 36 degrees. Essentially, temperature cools to the dewpoint overnight and a frosty layer of dew is formed on plants, grass, etc. This does not assure the killing of all plants and mosquitoes, in fact it is unlikely that tonight will completely do the job. For that you really need several successive frosty nights or more particularly, a freeze.

A freeze warning is issued when temperatures are expected to fall to 32 degrees or lower. When this occurs, it almost guarantees the killing-off of most plans and mosquitos, especially if it is expected to last for several hours. That is NOT in the cards Friday night, nor is it forecast anytime in our immediate future.

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So, look at Friday night as the first step in ridding ourselves of the mosquitos, but unlikely to be the kill-all event that many have been waiting for. Currently we are not forecasting any such “hard freeze” in the next 7 days.

The biggest wildcard Friday night is the wind.

To really get temperatures to bottom out, you need a night with very little wind. Temperatures on clear and calm nights will plummet due to “radiational cooling,” essentially the warm air at the surface radiates back up into the atmosphere.  If the winds stay busy, the air stays mixed and radiational cooling is slowed or halted. The winds Friday during the day will be quite gusty, only slowly decreasing overnight. Current forecasts are for the winds to calm to about 4-to-8 mph after midnight, which likely would be just enough to kick the cooling process into gear. However, if the winds don’t drop off quite as quickly, temperatures will likely hover just above frost levels.

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This first frost is coming just about on schedule. Most of the suburbs outside of Route 128 and certainly outside of Interstate 495 typically receive their first frost between October 1-10. Boston, areas closer to the coastline and also Cape Cod will not frost over Friday night, and typically they need to wait until late October or early November.

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Boston didn’t get its first 32 degree reading last year until November 14th, a week or so later than the average.

Follow Terry on Twitter @TerryWBZ.

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