September marks the start of meteorological fall (September 1-November 30). From September 1st to September 30th, average temperatures drop 10-12 degrees across New England. Boston drops from 78° to 68°. New England is looking at a warm start to the month, how long will it last?
TEMPERATURES
September will be off to a very summery start. The stretch of weather coming up to kick off the month will be the most prolonged summer-like weather New England has seen this season. This heat will be spreading eastward from the Midwest into New England as yet another heat dome sets up across the central United States.
These domes of intense heat over the Midwest have been common this summer. This has led to a very persistently hot summer to our west. For the most part, New England has missed out on this heat thanks to blocking patterns. That doesn't look to be the case for the start of September as a ridge in the jet stream, along with high pressure, will allow the heat into our neck of the woods for the first full week of the month.
The upper Midwest has really baked this summer, and Minneapolis is once again looking at air temperatures over 100° this weekend into Labor Day. The heat will moderate as it spreads eastward into New England, so it won't be getting quite that hot, however, widespread mid 80s to low 90s will very likely occur through much of next week (September 3-9). This is a good 10-15 degrees above average.
An onshore flow will keep the coastlines cooler, likely being held to the mid 70s to mid 80s. Boston has yet to see a heat wave this summer, and this onshore flow will make it extremely difficult for them to see one next week. Heat waves (which are at least 3 consecutive days at 90+°) will be possible across the interior of southern New England and the Champlain Valley of Vermont.
Heading into week two of the month (September 10-16), there are signals that keep the warmth around, although not to the extent of week one. It does appear that the ridge will weaken or break down and a cold front will cross the region late next week (September 7-8). The question becomes does the ridge rebuild in week two? Right now, it looks like it just might. Another question will be exactly how it sets up for mid-September, will New England end up on the warm side, or more on the fence between the warm and cool air masses, which was the case in August.
The NAO Index is looking to climb to its highest positive phase of the summer for week one, which is a good indication of the above average weather expected (positive NAO favors above average weather temps, negative NAO is the opposite). The index does look to take a dive back toward neutral territory heading into the middle of the month. This would, overall, support an end to the summer-like heat, but above average temperatures would still be in the cards. By mid-September average highs are in the 60s to mid 70s in New England.
Heading into late-September, signals are pointing toward a cool down (relative to average). This is mainly thanks to the typical cool-air intrusions from Canada and increased (and stronger) cold fronts crossing the region.
Overall, western areas of New England will have a higher chance to be above average for the month than eastern areas. The entire month will likely end up above average, although this will likely end up being the case because of how warm it will be right at the beginning of the month, not necessarily because it remains warm the entire month. We're predicting a warm first half or the month with a cool down come late September.
RAIN
New England is getting a much earned break from all the rain to start to the month as high pressure dominates the region. The breakdown of the ridge toward the end of next week (September 7-8) will support a return to more unsettled weather as we head into the middle of the month. A system, and its cold front, is looking to bring this dry stretch to an end next Thursday or Friday (September 7-8).
A big wild card in the unsettled weather is the remnants of Idalia. Models have struggled with figuring out where this storm will go since it left Florida. At one point, some models had the storm do a loop and hit Florida again (which clearly did not, and is not, going to happen).
A couple days ago, models had the storm heading straight for Cape Cod. Models are now shying away from that as well. The question is will it come close enough to be a rain maker or not, and how will it interact with other systems moving through. All of this uncertainty with Idalia points toward the system getting cut off from the main flow.
September is always a difficult month to figure out mainly because this is when Atlantic hurricane activity peaks. These large, powerful storms have a big influence on both temperature and rainfall outlooks, and the storms can't be predicted a month, or weeks in advance.
LOOKING BACK
Our August outlook panned out almost as expected. We predicted a very average month, with upper 70s and low 80s dominating. That was the case as the month did not feature a single big warm up or significant cool down. We did expect a warm up in late-August. This did not end up happening. Although, looking at how early-September is shaping up, the warm up is coming, just a bit later than expected.
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