It appears that our fairly typical early fall weather pattern will continue for the foreseeable future. The jet stream wave pattern is quite “rhythmic” and progressive, with alternating ridges and troughs moving across the northern tier of the U.S. and southern Canada. As a result, our pattern of alternating surface high pressure systems, interrupted by cold frontal passages will continue. As a trough in the jet stream wave pattern moves in, as it is now, a cold front at the leading edge ushers in cooler air from Canada (north of the jet stream). Surface high pressure then builds in behind the cold front, along the western side of the trough, as it progresses eastward. This brings relatively clear skies (descending air) and cool temperatures. Once the trough moves to our east and a ridge begins to build in, milder air within the ridge (south of the jet stream) surges northward, driven at the surface by southwesterly winds around the western side of the surface high as it moves to our east (clockwise flow). Another trough follows the ridge and a cold front at its leading edge once again lifts the mild, moister air in place. This results in a period of rain or showers (cold front passage) followed by cool, dry surface high pressure…..and this pattern continues…..

A seasonably cool surface high pressure system is presently building slowly in behind the cold/cool front which passed through Berkshire County this morning. As a result, skies should clear this evening and we can look forward to a clear, cool night tonight followed by a mostly sunny and seasonably cool day on Wednesday, with weather Wednesday night similar to tonight (clear and cool). There will likely be some patchy fog after midnight tonight as residual moisture from today’s rain condenses in the low levels of the atmosphere as temperatures drop into the low 40s by morning.

On Thursday, a weak cool front will pass through Berkshire County from the north around midday. However, the air in advance of the front will be quite dry so we are likely to only see some clouds from this frontal passage, with only a slight chance of a sprinkle or light shower. Even stronger, and slightly cooler high pressure will build in behind the front and it should be mostly sunny and even cooler on Friday, with temperatures struggling to get out of the 50s in most locations, despite the sunshine. In fact, frost is a strong possibility Thursday night as winds die out as the surface high pressure system crests over the region. The high pressure system will then drift to our east on Saturday and a ridge will begin to build in. Southwesterly winds around the high will drive daytime temperatures to near 70 on Saturday and, likely, into the 70s on Sunday before the next cold front passes with showers Sunday night or Monday.

Tuesday Night

Becoming mostly clear and cool with light winds. Patchy fog likely developing after midnight.

Low temperatures generally in the low 40s, with some upper 30s possible over the elevated terrain.

Light northwesterly winds.

Wednesday

Mostly sunny and seasonably cool.

High temperatures in the mid 60s in Pittsfield, Adams, North Adams, Willliamstown, Lee and Stockbridge; mid to upper 60s in Great Barrington and Sheffield; and near 60 to low 60s over the elevated terrain.

Light westerly winds.

Wednesday Night

Mostly clear, calm and cool.

Low temperatures, again, generally in the low 40s.

Calm winds.

Thursday

A mixture of clouds and sun with seasonably cool temperatures. The day should start out and end mostly sunny, with a period of greater cloudiness from the late morning into the early to mid afternoon, with only a slight chance of a light shower or sprinkle, as a cold front passes.

High temperatures in the mid 60s in Pittsfield, Lee and Stockbridge; low to mid 60s in Adams, North Adams and Williamstown; mid to upper 60s in Great Barrington and Sheffield and; near 60 to low 60s over the elevated terrain.

Northwesterly winds at 5-10 mph in the morning, increasing to 10-15 mph during the afternoon.

Thursday Night and Friday

Clear, calm and cold overnight, with possible frost. Low temperatures in the mid 30s, with some spots in the low 30s. Mostly sunny and quite cool on Friday. High temperatures averaging in the upper 50s.