It will remain uncomfortably warm and humid overnight tonight (one of the warmest and muggiest nights of the summer) and during much of the day Saturday. However, a fairly strong cold front, at the leading edge of a trough in the jet stream wave pattern, will pass through the county, from northwest to southeast, between 5 and 7 pm on Saturday. This will result in a marked drop in temperature and humidity as a continental polar surface high pressure system from the upper midwest replaces the warm and humid air mass, resulting from the maritime tropical “Bermuda High”, which is presently in control of our weather.

The cold frontal passage will generate fairly widespread thunderstorms during the afternoon and evening on Saturday, as the cooler air behind the front lifts the warm, moist, unstable air over the region. The air will become even more unstable as we will likely get enough sunshine through midday to drive surface temperatures to, and probably, exceeding 80 degrees. The Storm Prediction Center has issued a forecast of a “slight” chance of “severe” thunderstorms (15% probability within 25 miles of any point). However, the risk is primarily for damaging “straight-line” wind gusts. It does not appear that there will be enough wind shear to generate significant supercell (rotating) thunderstorms that could spawn tornadoes. Some of the high-resolution models are generating a “prefrontal” band of thunderstorms which would move through the county from northwest to southeast during the early afternoon. I think this is a likely scenario. A second band of thunderstorms, accompanying the frontal passage, will likely pass through the county between 5 and 7 pm. I believe most places in the county will experience a thunderstorm at some point on Saturday, possibly in the early afternoon, but more likely during the late afternoon to evening period as the front passes.

Temperatures will plummet from the 80s to the 60s as the thunderstorms move through, then into the 50s during the evening. Temperatures will continue to drop overnight Saturday, falling to near 50 by Sunday morning. It will be mostly sunny both Sunday and Monday as surface high pressure re-establishes control of our weather. Temperatures will return to seasonal levels, with highs in the low 70s on average and it will be much drier, with dewpoints in the upper 40s, compared with the low 70s of today. This high pressure system should maintain control of our weather into mid-week as it drifts slowly off the coast. Another, much more amplified, trough in the jet stream wave pattern will likely develop over central portions of the continent during the week next week and propagate through our region late in the week and through next weekend. It looks like this trough will contain much cooler air from Canada and could give us our first taste of Autumn, with temperatures struggling to get out of the 50s (?) next weekend.

Friday Night

Mostly clear. Unseasonably warm and humid.

Low temperatures generally in the upper 60s to near 70 with some low 70s in South County.

South-southwesterly winds at 5-10 mph this evening, becoming light after midnight.

Saturday

Mostly sunny to partly cloudy into early afternoon. Very warm and humid. Thunderstorms likely in the afternoon into early evening, with a band of thunderstorms possible in the early afternoon, and a second band more likely between 5 and 7 pm. Gusty winds and heavy downpours will likely accompany some of these storms. Probability of precipitation 80%.

High temperatures in the low 80s in Adams, North Adams, Williamstown and over the elevated terrain; low to mid 80s in Pittsfield, Lee and Stockbridge; and mid 80s in Great Barrington and Sheffield.

South-southwesterly winds at 5-10 mph in the morning, becoming southwesterly and increasing to 10-15 mph in the afternoon, and then shifting to west-northwesterly between 5 and 7 pm. Strong gusts likely in thunderstorms.

Saturday Night

A few lingering showers early, then clearing skies and much cooler and drier.

Temperatures falling through the 60s and into the 50s during the evening. Low temperatures near 50 to low 50s by morning.

Northwesterly winds at 5-10 mph, becoming light after midnight.

Sunday

Mostly sunny, with seasonable temperatures and low humidity.

High temperatures in the low 70s in Pittsfield, Adams, North Adams, Williamstown, Lee and Stockbridge; low to mid 70s in Great Barrington and Sheffield and; upper 60s to near 70 over the elevated terrain.

Northwesterly winds at 5-10 mph.

Sunday Night and Monday

Mostly clear overnight and mostly sunny on Monday. Seasonable temperatures and low humidity, with overnight lows averaging in the mid to upper 40s with highs in the low 70s.