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Summer in September - A Record Pace?

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Feel like deja vu? The start of September 2014 was similar to this year's. It certainly feels like the seasons have been shifted a month over the last couple of years...with winter starting a month late, summer starting a month late, and fall starting a month late! Likely a coincidence and not indicative of a permanent shift.

Daylight may be fading (by about 3 minutes a day) and that's inevitable, but the summer temps are doing their best to hang on this September. After about 24 hours of an 'autumn preview' we're back on the upswing this week. By the end we may even close in on another daily record high after setting two of them last week. And when all is said and done, the persistent warmth may be enough to do something that's only happened once since weather records began in Boston. Before we get to that, let's look at the setup.

This whole week will be dominated by high pressure and a large area of subsidence, which is a science geek way of saying 'sinking air.' Sinking air helps to keep clouds at a minimum, and so we'll be bathed in sunlight all week long with barely a cloud to be found the entire work-week. It's a good thing we picked up some much-needed rainfall over the past several days, because there isn't a drop of it in the forecast. Keep those garden hoses handy. We temporarily fought back against the annual precipitation deficit, but it will begin to creep back up over the next couple of weeks. In general, September is known as the most peaceful weather month of the year. The wildcard of this reputation is the tropics, and there's no threat there in the near-term. In the absence of tropical impact, it's expected to be a dry stretch.

2015 Pattern(1)

At the same time jet stream energy will be staying to our north, keeping the cooler air bottled up in Canada and also pushing the storm track to the north. So each day will get a little bit warmer, and all of them will be well above average this week. What exactly is 'average'? Well that figure is a middle ground between swings to either side, which are more 'typical' than the average mark itself. But an average high is in the low 70s this time of year in Boston, with lows in the mid/upper 50s. Monday was exactly that, but we'll trade in the +8-16º range for the rest of this week.

Friday is the one day where a daily record high may be threatened. In Boston the record is 90F (set in 1955 and tied in other years ) and at Bradley Airport it's 89F (set in 1906). I think we'll probably end up falling a couple degrees short, but it's close enough to keep an eye on. The deciding factor will be whether or not a decent southwesterly wind will be generated. If that happens and we mix down some warm air aloft (+17C at 850mb) as well as keep the sea breeze at bay, we'll have a chance.

2015 Dewpoint Chart(2)

The good news is that as all this is going on, humidity won't become much of an issue. Dew points will generally stay in the 50s on Tuesday and Wednesday, and hover near 60F for Thursday and Friday. That means nights will cool off nicely into the 50s to around 60 degrees and air conditioners won't have to work too much. Plus, cooling sea breezes will develop on Wednesday and Thursday to help keep things comfortable in eastern Massachusetts.

The end of the warmth should arrive on Sunday as a weak cold front pushes through. Right now our rain chances don't look too promising with this front, although I wouldn't rule out a shower or two. The main change will be a wind shift and a change of air mass, dropping us back into the 70s on Sunday and much cooler by Monday as high pressure builds in from southeastern Canada.

Will it end up being a record-setting month? Looking back over the last 140+ years, only 1 year has featured and average September temperature in the 70s. That year was 1983, which if you remember was also the last time we had a September heat wave before the one we notched last week! It was a wild weather month - six days topped 90F, one of them a whopping 99F. But there were also seven days that stayed in the 60s. Lots of swings and when all was said and done the average monthly temperature was 70.6F.

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To date (through Monday) the average monthly temp is sitting at 72.6F in Boston. With the current forecast going forward, I'm thinking the average temp for the stretch of Tuesday through Sunday will be close to 71-72F. So overall, the figure we're at now shouldn't change much at all through the weekend. That gets us to September 21st with about 2 degrees of wiggle room for the warmest September ever recorded in the city and 3-4 degrees of wiggle room for a Top 5 finish. Long story short - it will be close and that a Top 5 warmest September is a very strong possibility at the very least. Get outdoors and enjoy it...because we all know it will get cold soon enough and last long enough!

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