It's hard to believe how quickly we've flown through September, but this week's 8 to 14 day forecast takes us into the start of October, already. This time of the year a lot is changing - including losing about 3 minutes per day of daylight, and a precipitous fall in average temperature, with the normal high temperature for Boston, MA, down to 67 by October 1.
To further highlight the change, the jet stream winds aloft that steer our storms and act like a thermostat for the atmosphere, separating cold to the north from warm to the south, will take a big dip over the Eastern United States for the final week of September. This pattern suggests the door will be open for cool, Canadian air to drop southward, penetrating all the way into the Southeastern United States, and putting most of New England very near normal or perhaps even just slightly below, particularly with the likelihood for some chilly overnight lows to keep average daily temperatures down.
The dip, or trough, in the jet stream will be so significant, that deep moisture capable of producing appreciable precipitation is likely to be shunted offshore of the Eastern Seaboard - at least after a chance of Sunday rain in the Northeast on September 23 (just prior to this forecast period) - meaning near-normal precipitation owing to the potential for energetic upper level disturbances touching off occasional rain showers, though there is certainly a chance for below normal precipitation in this cool, dry air.