Summary: A surface high pressure system will build in over the region following this mornings cold front passage. This high pressure system will become centered directly over berkshire county from tonight into Thursday. This will result in bright sunny days and clear, cold nights for the next few days (typical autumn weather?). Since high pressure will be directly over us, there will be no pressure gradient (typical for the center of surface high pressure) so winds will become calm overnight tonight and Tuesday night. This will result in ideal radiational cooling conditions both nights with clear skies, dry air and calm winds. This will permit the earth’s surface to emit infrared radiation into space without clouds to absorb it and re-radiate it back to earth (no insulating blanket) or wind to mix any heat which results from absorption of the radiation by greenhouse gases in the atmosphere above back down to the surface. Also, low levels of water vapor in the air (water vapor is the most powerful greenhouse gas) will limit absorption of the infrared radiation being emitted by earth’s surface. Bottom line: near surface temperatures will drop rapidly and substantially overnight tonight and Tuesday resulting in the threat for the first frost of the season in many locations. In fact, the National Weather Service has issued a Frost Advisory for overnight tonight. Temperatures should reach the mid 30s in most locations so any frost should be relatively light. However, the frost could be heavier, with temperatures in the low 30s in the usual favored valleys, particularly low spots over the elevated terrain (this occurs since cold air is heavier than warm air and tends to slide down into the valleys and low spots). I think that a light northerly breeze will continue in most locations for a good part of the night tonight and that this breeze will limit frost to only the valley spots that are most protected from the wind. Still it would probably be a good idea to cover or bring in any vegetation that is most frost sensitive. I think it will be calm all night Tuesday night so frost is probably more likely tomorrow night. I will update this tomorrow afternoon.

The continental polar (cP) airmass that is moving in will moderate somewhat, as the axis of the trough in the jetstream which brought it in begins to lift out, so temperatures will begin to warm as the week progresses. In fact, we will approach 70 on Wednesday and temperatures should move into the 70s by Thursday and Friday. However, although more clouds will work their way in later in the week, the surface high pressure system should continue to dominate our weather and we should remain dry until the next jetstream wave dips over us and brings the next cold front through Saturday afternoon or night.

Monday Night

Clearing skies early this evening. Becoming clear and cold with light winds overnight. There is the threat of the first frost of the season in the entire county, but frost is most likely in the wind protected valleys, particularly over the higher terrain.

Any frost should be relatively light in most locations, with low temperatures in the mid 30s county wide with some low 30s in the coldest valleys.

North-northwesterly winds at 5-10 mph becoming light (< 5 mph) and shifting to north-northeast after midnight.

Tuesday

Sunny and cool.

High temperatures near 60 in Pittsfield, Adams, North Adams and Williamstown, low 60s in Great Barrington and Sheffield and mid to upper 50s over the elevated terrain and hilltowns depending on elevation.

Light north-northeasterly winds in the morning becoming light and variable in the afternoon.

Tuesday Night

Clear, calm and cold. There is, once again, the risk of frost.

Low temperatures in the mid 30s countywide with some low 30s in the colder valleys.

Calm winds.

Wednesday

Sunny and a bit milder.

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

Light westerly winds in the morning, becoming light southwest in the afternoon.