32º In Boston? A Rare October Feat

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'Tis the season for earlier sunsets, chilly mornings, and icy scenes starting to creep back in across New England. The winter hounds will rejoice (I know there are at least a few of them that read this blog), and many will start to curl up into the fetal position and/or book flights to Florida. But the seasons are as inevitable as death and taxes, and there's no stopping the turning of nature's clock. This weekend will certainly serve as a reminder of that. Any WBZ viewer or reader of our weather blogs knows I'm a context junkie and a stat geek to boot, so here's a look at how the cold blast on the way stacks up.

For starters, let's talk a little Urban Heat Island effect. Some of you have probably noticed that Boston is typically the warmest spot on the map most nights. The urban heat island effect helps that cause. The collection of paved surfaces, asphalt, building density, and general humanity helps to absorb the sun's radiation during the day. At night, these surfaces continue to radiate out that warmth. So while the quiet countryside, or even MetroWest towns, quickly cool off at night - the city does not. It always lags, and so it often does not reach the chill of the suburbs. For instance, many towns have come close to or even dropped below 32F this fall (some several times). The low, protected, valley locations tend to do this first. But in Boston, the coldest temp so far this season has been 48F. Not even close. The reporting station is also right on the water at Logan Airport. The ocean has a moderating effect on air, and also helps to keep a breeze up at night. So in short, Boston is one of the last places you're going to see hit the freezing mark in a given fall.

It may surprise you to hear that the city has never recorded a freezing temperature in the month of September at all. Not even when the station was located inside the city and away from the water. In over 130 years, no September frost in downtown. We can take that a little further and say that freezing temps in October are relatively rare, especially in the past 30 years. Part of that is more urban development, the other is a general warming of the Earth over that time.

How rare? Well Boston has averaged just about 1 night touching or diving below freezing in October per decade since the 1980s. We haven't picked one up yet in the 2010s, and the last time it happened was in 2002. Looking at the above graph, you can see why those 'New Ice Age Coming' headlines were in the news in the 1970s! Still waiting on that glacier in the backyard.

Some more 'fun' tidbits:

• Since records moved to Logan in 1936, there have only been 3 occurrences of a freezing temp earlier than 10/19 (Monday)
• The last subfreezing reading in October was Halloween morning of 1988
• Boston hasn't touched the 20s in October since 1976 (on the 27th)
• Boston has only reached the 20s in October on 5 days since records began
• The coldest all-time October temperature in Boston is 25F (reached in both 1870 and 1936)

NAM forecast temperature anomalies at 850mb on Sunday. You can see the core of the cold right on top of New England, running 15C below average for this time of year at that level (a few thousand feet up). Source: WSI Energycast

So let's take a peek at the forecast. It's looking awfully cold Sunday into Monday, when an anomalous air mass will push into the region. Pretty much all of New England is looking at a killing freeze, except for perhaps the southern coast and Cape Cod/The Islands. It's been a good run, and a freeze this time of year is not unusual at all. It's time to say goodbye to those gardens that treated us well in the warm months. If you have mums, the plants can survive the cold but the blooms get frostbite and turn brown/purple. So you may need to bring them in or cover them up Saturday night, Sunday night, and Monday night if you want to get some more life out of them (pattern looks warmer after this cold shot, so would be worth the effort).

How cold exactly are we talking? It's several days out, but this is what I'm thinking we'll all wake up to Monday morning. Oh joy. As if Mondays weren't rough enough (especially after watching the Patriots late Sunday night), this is going to be a tough one to get out of bed. Most areas outside the city should dip well down into the 20s. I wouldn't be completely shocked if a town or two could reach the 10s, although that seems unlikely. It's certainly cold enough to kill off all annual plant life and to warrant a flick of the furnace switch. We'll be back down in the 20s/low 30s Monday night as well.

As you can see, I'm shooting for a temp of 32F in the city. It will come down to a couple of factors. 1) Do we completely clear out and 2) Will the atmosphere decouple? Decoupling essentially means that winds go light/calm, very cold air gathers near the surface, and this pool of cold gets cut off from the atmosphere above it. We see this on calm, radiational cooling nights. There's still a hint of a pressure gradient on Sunday night, so the winds may not completely calm down in time. Plus, there are several disturbances rotating through the Northeast trough all throughout the weekend. One more pushes through Sunday, and there may be a few clouds that come with it. But if the winds go calm and the skies clear out, lows 20-25F in the suburbs look extremely likely and certainly we could hit the freezing mark in Boston.

The record low in Boston Monday morning is 28F set in 1922, and the record low in Worcester is 24F set in 1974. They're probably both safe, but we won't be too far off. In any case get ready for a frosty, bundled up start to the week. But fear not cold haters - this pattern won't stick around long. By Tuesday afternoon we're back up near 60, and closing in on 70 for Wednesday and Thursday. This is a coming attraction and not a feature presentation....yet.