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Coldest Air Of The Season On Deck

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Sometimes it seems like Head of the Charles Weekend is one you can set your watch to for a pattern change. Maybe it's just a way of New England weather adding some adversity to the event. Perhaps it's just mid-October being mid-October. But here we are again and HOCR weekend will feature a sharp change from the mild autumn we've been enjoying. It's not even out of the question that New England could see some of the first snowflakes of the season! Too soon? Probably for most. But here are the details on a chilly blast from Canada headed our way.

Most of this week features a gradual cool-down back to seasonal norms. After 70s on Monday and Tuesday, we dip into the mid/upper 60s for Wednesday. Low 60s on tap for Thursday, and then near 60 for Friday. Just a couple degrees of change each day. Although once we hit Friday, we start a series of fronts that will bring progressively colder air into the region through the weekend. Friday's will bring some rain showers along with it as well, and then it's time to break out the autumn vests and perhaps even the (eek) winter jackets.

saturday

ECMWF showing hints of low pressure and a lingering trough providing wet weather on Saturday. Something to watch as we head through the week. Source: Weatherbell

There are hints of a wave of low pressure trying to form near the coast on Saturday, which may at the very least dangle a trough across New England to start the weekend. I'm leaning toward this more pessimistic forecast right now, with areas of chilly showers/rain developing. Best case scenario is that this doesn't materialize and any precipitation stays out to sea, but it's on our radar for now. Colder air aloft should cap our high temps in the low 50s, with possible a few mid 50s. That in itself is not terribly unusual for this time of year - an average high in Boston by Saturday is 61F.

On Saturday night enough cold air is in place to straddle the fence between raindrops and snowflakes. Air temps will be above freezing (unless we somehow clear out, in which case it's a moot point) but they should cool close to 40F. The air aloft is plenty cold (about -7C at 850mb), and so if there's moisture around we're not too far away from allowing some wet snowflakes to splat on the pavement. I think this is a low probability for now and even if it happens it would have no impact except for the novelty of it all.

Across northern New England there is a much better chance of seeing some flakes, especially for the higher terrain. An upslope flow should squeeze moisture out of the air mass, and with a couple of passing disturbances in the atmosphere I certainly think we could see some mountain snow. It wouldn't accumulate to much, but again it's just a sign of the seasonal changes that are upon us! The west-facing slopes of Vermont would stand the highest chance, as well as far northern Maine. Get ready for a barrage of ski areas tweeting out their freshies. If we're lucky we could get some frosted trees mixed with the foliage - which always makes for some incredible photos.

snowfall

Nice graphic courtesy the National Weather Service in Taunton, MA

But would snow in mid-October be unusual? Not particularly. It's happened before, and will certainly happen again at some point. There was even a widespread Northeast snow event on October 10th, 1979. Just a few years ago, in 2009, Boston recorded its earliest measurable snow on record (on the 18th). And of course there's the infamous 'Snowtober' event that pasted New England with widespread heavy snow on October 29th-30th, 2011. So flakes in October are never off the table. Fortunately we're not looking at any kind of similar situation this year.

snowtober

Epic snow that produced a heap of trouble in late October of 2011.

On the topic of 'not unusual' - how about this cold shot? While we can still see 90s in the area this time of year, we can also have some pretty impressive cold. In fact, if you haven't snagged a frost in your yard yet, it's late to arrive. There's a chance on Saturday night if this storm doesn't materialize, but the best chance is Sunday night. By Monday morning, it's October 19th. And if you look at the climatology (below) the majority of the area averages its first frost before that date. The exception is Cape Cod & the Islands which usually take a little longer thanks to the moderating influence of the ocean. So it's not like we're seeing a dramatic end to our growing season - it's actually been quite lengthy.

frosty

In terms of records, I currently don't think any are in jeopardy. The coldest max temp for Sunday is 45F (1898) in Boston and 36F (1989) in Worcester. It would be tough to match or beat these marks. How about overnight lows? The Monday morning records are 28F (1922) in Boston and 24F (1974) in Worcester. Again, it would be difficult to get that cold with the expected air mass. In fact, Boston has not had a single overnight temp in the 20s during the month of October since 1976. It only has happened 4 times since measurements moved to Logan Airport in 1936. In short, it's really tough to dip into the 20s this early in the season at a location on the water and one impacted by the Urban Heat Island effect of the city. Can't rule it out, but it's unlikely.

anomaly

First big blast of cold air impacting the eastern U.S. on Sunday. Temps should run a solid 10-20F below average. Source: Weatherbell

Even without records, the 24-hour period of Sunday into Monday looks to feature the core of the chilly air. Most locations should fail to escape the 40s during the day Sunday, plus a gusty northwest wind that will make it feel more like late November than mid-October. Sunday night should bring our best chance for a widespread frost to date, and perhaps even a hard freeze for much of the area. The only region that may avoid it is Cape Cod/Martha's Vineyard/Nantucket. After that cold start we'll stay in the 40s to low 50s on Monday, with another cold (subfreezing for some) night to follow. But this is a 3-day cold shot with strong warm air advection kicking back in by Tuesday. Temps will sneak back up all week, and so this is more of an early signal of changing seasons than it is a permanent shift in the move toward winter. So bundle up, enjoy a fire in the fireplace, stick some chili in the crock pot, and enjoy.

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