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Summary: A highly amplified, negatively-tilted jet stream trough over the western U.S. is generating a deep surface low pressure system along its eastern margin in the Midwest today. This system has a well developed classic midlatitude cyclone structure, with a warm front extending out well to the east of the surface low and a strong cold front extending well to the south of the low. As the cold front plows into the unstable air in the warm sector (between the two fronts) it will generate a squall line (at the cold front) and a few pre-frontal bands of thunderstorms. The air in the warm sector is unstable and there is enough vertical wind shear so that some supercell thunderstorms will likely form late this afternoon over the lower Mississippi Valley and then the Tennessee and Ohio Valleys overnight as the cold front plows eastward. The Storm Prediction Center has forecast a moderate probability of severe thunderstorms in parts of the Ohio and Tennessee Valleys, mainly for strong winds and hail, although there is a 5% probability of tornadoes within 25 miles of any point within this region. It is only late February, but we are already heading into tornado season (March-June, with a peak in May). There is no concern for severe thunderstorms here since, by the time the cold front reaches us late in the day on Friday, it will be moving into much more stable air.

The center of the low is located over the upper Mississippi River near the Iowa, Missouri Illinois borders. As a result, heavy snow is falling to the northwest of this low, in Iowa and Minnesota. As the low moves northeastward, through the western Great Lakes tonight, northern and western Wisconsin will also receive heavy snow. As the low moves through the Great Lakes and into Canada, it will pull its warm front over us tonight and, eventually, drag its cold front through here late in the day tomorrow.

We will see the typical progression of precipitation types associated with a warm frontal passage this time of year overnight. Precipitation will likely be moderate to heavy at times from around 7 pm until well after midnight tonight with liquid total precipitation equivalent of 0.25″-0.50″ from southern into central Berkshire and 0.50″-0.75″ from central into northern parts of the county. The precipitation will likely start as snow but then changeover fairly quickly to sleet (ice pellets), by 9 pm or so. The changeover will be a little sooner to the south and a little later to the north. It may snow hard at the outset, particularly in North County, with only a dusting in South County, a dusting to 1″ in central Berkshire with 1-2″ possible in North County. The higher elevations in North County may see as much as 2-3″. The sleet will then change fairly rapidly to rain and freezing rain, by 9 or 10 pm in South County and 11 pm or midnight in North County. There should only be a coating of sleet on top of whatever snow falls.

Beyond this changeover the forecast gets tricky. As the precipitation changes to liquid form, temperatures will be at or just above freezing in the lower elevations but near 30 over the elevated terrain. Temperatures will rise very gradually after midnight so, even if the rain freezes initially in the lower elevations, there should not be much ice accumulation there as temperatures that are not over 32 at the outset will rise above this level shortly after rain starts falling. Over the elevated terrain, freezing rain may persist all night, with temperatures not rising above freezing until the steady rain ends about 4 am. Therefore, in the elevated terrain of central to northern Berkshire as much as 0.25″ of ice could accumulate on some surfaces. Hopefully, this will not be enough to create any power outages. In any case, travel may be treacherous everywhere this evening for a time, with the risk greater and for a longer period in North County. After midnight conditions should improve in the lower elevations but may remain slipper over the elevated terrain for a time.

Steady rain and freezing rain should end by 4 am with only spotty drizzle or freezing drizzle persisting through the morning hours. Any freezing drizzle over the elevated terrain should change to plain drizzle by daybreak or shortly thereafter as temperatures rise above freezing. We will be in the warm sector for a good part of the day but cold air is likely to be trapped near the ground so temperatures will likely have trouble rising out of the 30s in most locations. The approaching cold front will generate a band of steady rain that will move through from mid afternoon until evening. The rain may mix with a little wet snow for a brief period before it ends early Friday evening.

Once this mess clears Friday evening, it looks like we are in for a fairly nice weekend as surface high pressure builds in to give us mostly sunny skies and mild temperatures on Saturday, although it will be breezy. A weak, secondary cold front will rotate through Saturday night with nothing more than the chance of a few mixed snow and rain showers. Sunday should be a little cooler, but high temperatures will still be in the 30s, with partly cloudy skies.

Don’t get too used to the milder temperatures because it looks like a deep trough in the jet stream will settle over us next week and allow a cold arctic air mass for this late in the winter to drift south over us from Canada by late in the week and into the weekend.

Thursday Night

Snow developing by 7 or 8 pm, changing rapidly to sleet and then rain in South County. In central and northern Berkshire the snow and sleet will likely persist a little longer, with snow changing to sleet by 9 or 10 pm and then rain or freezing rain by 11 pm. In the lower elevations, temperatures will be near or just above freezing at the onset of the rain but then will rise gradually so if there is any freezing rain it should change to plain rain by midnight or just after. Freezing rain could persist in some spots in the elevated terrain. Likely snowfall totals a dusting in South County, a dusting to 1″ in central Berkshire and 1-2″ in North County. Ice accumulation should be minor in the lower elevations but could reach 0.10″- 0.25″ in a few locations in the elevated terrain. Any rain/freezing rain should taper to drizzle or freezing drizzle by 4 am or so. Probability of precipitation near 100%. Liquid equivalent totals 0.25-0.50″ South County to central Berkshire and 0.50-0.75″ in North County.

Low temperatures this evening in the low 30s, near 30 over the elevated terrain. Temperatures rising a few degrees after midnight, reaching the low to mid 30s by morning.

Light east-southeasterly winds this evening, increasing to 5-10 mph by morning.

Friday

Overcast with spotty light rain and drizzle until mid-afternoon. A steadier period of rain and showers developing mid-afternoon and continuing until early evening. Probability of precipitation 90%. Likely rainfall totals 0.20-0.35″.

High temperatures in the upper 30s in Pittsfield; near 40 in Lee, Stockbridge, Adams, North Adams and Williamstown; low 40s in Great Barrington and Sheffield and; mid 30s over the elevated terrain.

East-southeasterly winds at 5-10 mph, shifting to westerly late.

Friday Night

Some lingering clouds early, then becoming mostly clear.

Low temperatures in the mid 20s.

West-southwesterly winds at 5-10 mph.

Saturday

Mostly sunny, breezy and mild. Clouds may increase late in North County with just the slight chance of a rain shower there toward evening.

High temperatures in the mid 40s in Pittsfield, Lee, Stockbridge, Adams, North Adams and Williamstown; mid to upper 40s in Great Barrington and Sheffield and; low 40s over the elevated terrain.

Southwesterly winds at 10-15 mph.

Saturday Night and Sunday

Variable clouds with the chance of a rain or snow shower, mainly before midnight. Turning colder after midnight with low temperatures in the low 20s. Sunday will likely be partly cloudy and cooler, with high temperatures in the 30s.