Summary: A fairly deep surface low pressure center located over eastern Kentucky is presently generating heavy snow over the upper Ohio Valley. The surface low is beginning to redevelop along the Mid-Atlantic coast. This is a common occurrence with eastern U.S. winter storms as the most upper-level divergence and upward motion in the atmosphere shifts to the coast, where the greatest temperature contrast is located. The surface low should track to the Delmarva Peninsula by midnight and just south of Long Island by Thursday morning. It will then track over Cape Cod and into the Gulf of Maine during the day Thursday.
The air over the Northeast is cold enough at all levels for the precipitation to begin as snow everywhere. However, the track of the surface low will be right along the coastline and the counterclockwise circulation around the low will bring much milder air from over the ocean (surface water temperatures still in the 40s) northward and westward where it will override the cold air near the surface. This will definitely cause a fairly rapid changeover to ice and eventually rain in areas to our south and east where snow accumulations will not be particularly great.
The precipitation will start as snow in Berkshire County from south to north, at around 5 pm in South County and near 7 pm in North County. Berkshire County will lie right along a transition zone between snow and ice late tonight and tomorrow morning. The farther south and east within the county one is located, the more likely that the snow will change to sleet and, possibly, freezing rain after midnight tonight. The precipitation should stay snow in North County and, there, over a foot of snow is likely to fall.
In South County, a changeover to sleet and, possibly, freezing rain is likely after midnight. However, even there 4-8″ of snow will likely fall before the changeover. An additional 0.50″ to 1″ of sleet and 0.25″ of ice (freezing rain) could fall during the morning hours of Thursday.
Central portions of Berkshire County provide more of a forecast challenge. If there is a changeover, it looks like 6-12″ of snow will likely fall before any changeover to sleet well after midnight. Another 0.50″ of sleet could fall during the early morning hours, with an additional thin coating of ice. This is presently the “party line” (all public forecast outlets are forecasting a changeover). However, my feeling at this point is that it will remain all snow in much of Central Berkshire which would result in an accumulation of over one foot there as well.
The intensity of precipitation will be the heaviest overnight and will begin to taper off during the morning Thursday. It will also become quite windy overnight tonight as the pressure gradient intensifies as the low approaches the county. Winds will peak out of the east-northeast at 20-30 mph with gusts to 40-50 mph after midnight. Gusts to 50-60 mph could occur along east-facing elevated terrain. Winds will begin to weaken toward morning and during the day Thursday. Precipitation will change back to all snow during the afternoon and evening on Thursday as the surface low pulls east and northwest winds bring colder air back in at all levels. We could receive an additional 1-3″ in “backlash” snow showers during this time as the storm moves out.
I expect snowfall totals for the entire storm to be 12-16″ in North County and much of Central Berkshire. Accumulations could be on the order of 16-20″ in some of the favored east-facing slopes along the elevated terrain (e.g. Savoy). Snowfall totals in South County should range from 4-8″ in most locations but up to 10″ if the changeover occurs late.
It will become partly cloudy and seasonably cold on Friday. A weak storm will move by to our south on Saturday. Right now it looks like only 1-3″ of snow from that system.
I will try to update this forecast later today as more information becomes available…..