The first month of meteorological fall sees average high temperatures drop about 10° from the start of the month to the end. Signals for this September aren't really showing anything notable, which may lead to a rather mundane month for the most part. With that said, full summer heat may be very limited and just about done for the season except for a couple rogue days.
Pawtuckaway State Park in Nottingham, New Hampshire in September 2020
TEMPERATURES
September will open with a cold front slicing through New England, putting a quick end to building heat and humidity going into early next week. The first week of September will feature a ridge building in the west with a trough digging into the Great Lakes and northeast. Surface high pressure will also build during this time, reinforcing a general northwest flow.
500mb height anomaly showing the ridge in the west with a trough/cold pool over the northeast:
This will start September off with near to slightly below average temperatures. This setup will lead to chilly mornings with afternoons dominated by the low to mid 70s through midweek next week. Heading later into next week, the cold pool aloft seen above will likely begin to shift east as the ridge in the west flattens out, at least temporarily. This should allow a general warm-up for New England at the end of next week. Overall, summer warmth will remain out of New England to start the month, with much of it bottled up in the west.
Heading toward the middle of the month, a slight eastward progression of the large-scale setup is expected. That is to say the ridge over the west may begin to slide eastward, but remain to the west of New England. Generally speaking, a ridge-in-the-west-trough-in-the-east pattern may remain, continuing the theme of near to slightly below average temperatures as we head into week two. The trough could be centered to the south of New England, with the chance for below average temperatures higher in southern New England.
This general setup can be seen by looking toward the NAO index and PNA index. The NAO index can indicate troughing or ridging over the east while the PNA index does the same for the west. Heading toward the middle of September, the NAO index is generally forecast to be in a negative phase while the PNA index heads for a positive range. As always, it’s important to remember that these indices are not the only, tell-all factor, it’s just one piece of the puzzle used to determine potential weather.
This general setup is also supported by major global models, which generally show below average heights and troughing over the east with a new ridge and above heights building across the west once again.
500mb height anomaly showing below average heights continuing across the east in the middle of September:
Heading toward the end of September, signals tend to show the continued gradual evolution of the large-scale pattern described above. This would favor more ridging across the eastern United States as the eastward progression continues. This could lead to a warmer end to September relative to average. Average temperatures drop around 10° from September 1st to the 30th.
Overall, we're predicting a mundane month when it comes to temperatures with plenty of days around seasonal averages. We're thinking temperatures will likely skew below average more often than above average. This may change heading toward the end of the month, but, as always, small-scale factors that can't be predicted this far out will come into play by that time.
PRECIPITATION
An expansive surface high pressure will start off September. This will get the month started very dry and calm. With general troughing and below average heights through the first couple weeks of September, however, this will likely change heading toward week two of the month. Next weekend may see a system move through New England, bringing more widespread precipitation back to the region.
Overall, there aren't many strong signals for above or below average precipitation. When this is the case, it usually points to precipitation ending up around average. This is supported by the Climate Prediction Center's September precipitation outlook, which shows New England with equal chances for precipitation to end up above or below average.
The big wild card with precipitation in September is the fact that hurricane activity peaks in September. Whether New England sees a direct impact from a tropical cyclone or remnants of what once was move through the region, this can push what had been a below average or near average precipitation month to being above average. Only time will tell how this factor will play out.
LOOKING BACK
August panned out mostly as expected. We predicted a cool down in the middle of the month would be flanked by a warmer start and end. A hot start to the month was followed by generally cooler times in the middle. It started to warm up near the end once again before temperatures came back down over the past couple days. The cooler times were a bit more persistent and further below average than we anticipated, but those are the small scale factors coming into play for you.
Daily observed temperatures for Boston (1st image) and Burlington (2nd image). The bars show the observed temperature while the lines show seasonal averages:
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