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Gardens Beware: Coldest Air Of The Season Ahead

Some areas have already seen frost. This was taken Monday morning in Grafton County, NH. Photo via @CapeAnnWeather on Twitter

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Remember that stretch of heat (in fact of the warmest all year) that kicked off September? Nope, me neither. Amazing how quickly we flipped the switch straight to fall. Eight of the past nine days have all featured below average temperatures after that scorching start. Mornings have been downright chilly, and afternoons have been pleasant, if not with a tinge of chill in the air. Lovers of fall are soaking it in, while others have already begun the annual pre-winter freak out thinking about the real deal to come.

After a couple of nice days ahead, our attention will turn to the Thursday night through Saturday morning time frame. A reinforcing cold front will head through the area Thursday (again bringing no rainfall - a growing trend around here) and behind it is the coldest air of the season. Temps at 850mb, a few thousand feet up, will be at the freezing mark. That, combined with dew points falling into a winter-like 20-32º range locally, will help temperatures plummet. We've got a pretty ideal radiational cooling situation setting up for Friday morning, although it may not be perfect.

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Forecast 850mb temps Friday morning along with MSLP. Strong high pressure cresting over New England with the coldest temps likely in northern New Hampshire & Maine. Source: WeatherBell

Here's the situation by late Thursday night. We've got this blast of chilly air, clear skies, and high pressure building in. Most of the models are indicating that this strong, cold-season style high will rest more toward northern New England and New York State than right over us in southern New England. That means the most calm and perfect radiational cooling conditions will be across NY/VT/NH/ME. A bit of a breeze should keep us from reaching down as far as those northern locales. Looking at it now, I think there's a high likelihood that much of northern New England & NYS will see a killing hard freeze by Friday morning. That's the end of the growing season, an end to the mosquitoes, and some ragweed relief too.

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Forecast GFS 2m temperatures Friday morning. That's a lot of chill to end the week! Source: WeatherBell

For us in the 'BZ viewing area - frost is definitely in the cards. Models notoriously underestimate overnight lows during radiational cooling setups, and I'm always altering them downward on our graphics in anticipation. There's moderate to high confidence that many towns NW of Boston will see a frost Friday morning. The highest probability is in southern NH and northern MA, with the lowest potential for frost being in the immediate Boston area, right along the coast, and far SE Mass.

Now there's a caveat to that - there are a few spots that are usually excellent radiators at night. Communities around Taunton, Norwood, down to Carver and throughout the bogs tend to be much chillier than surrounding towns. The weather station on Martha's Vineyard is another great radiator and cool spot. So I wouldn't rule out some patchy frost well to the SE corner of the state. All dependent on whether the ENE wind stays alive for a good chunk of the night.

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Current thoughts on the frost forecast...stay tuned for any changes as we get closer.

If you've got a garden still doing well or have done some fall decorating, plan on covering up your goods. It's a good idea to cover pumpkins and mums. Even though they're hardy, the flowers on mums can be frostbitten and will turn brown/black. A pumpkin will start to rot faster if it gets freezer burn from Mother Nature, so don't forget about them either! You may want to just throw in the towel on your annuals...there's not a whole lot of warmth left ahead to sustain them unless you're hardcore.

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Average first frost dates across the state. Of course, there can be quite a bit of variability from year to year.

Feeling like this is all a bit too soon and grumbling about the early onset of an icy scene? You have a legit gripe. If this frost verifies for a good chunk of the area, it would be well ahead of schedule by a week or two...perhaps even by three weeks in some towns. So certainly something to monitor to see if any variables change in the next 48 hours. One has to wonder - will any ski areas test their snow guns?

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Wider view of average first frost dates

With highs only reaching the upper 50s on Friday, we'll be right back at the cold stuff Friday night into Saturday morning. It may end up just a feel degrees 'warmer' as some milder air arrives aloft and a light southerly flow kicks in. But I think many towns will be back into the 30s, with perhaps a few 20s to the far north. It may be a second round of frost for quite a few towns and one of those mornings where you notice the cold feeling of the tile on your feet when you get out of bed! Have that hot coffee ready to go to kick off the weekend - but fear not. Warmer air will quickly move in - with afternoon highs close to 70 on Saturday and 80 on Sunday! You gotta love the ride.

Find Eric Fisher on Twitter and Facebook

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