Posted at 3:00 pm
Summary and Analysis:
Because classes at BCC were cancelled today I have time for a bit longer post than usual so, if you are not in the mood for a little lecture then feel free to skip down to the forecast…..
Berkshire County is being impacted by a very strange “noreaster” for this time of year. This resulted in a prolonged period of sleet (ice pellets) for most of the county last night with 1 – 2″ of accumulation. There was some snow mixed in along the higher ridges in the northeastern portions of the county, but for the most part, there was no snow from a low pressure system that, in a normal winter pattern, would have given us all snow, on the order of 6 – 12″ or so. Is this strange scenario made more likely by the changes in the atmospheric circulation pattern produced by global warming? Probably…..
What strange pattern am I talking about? Well, what we call the polar front, which separates very cold arctic air from milder subtropical air, and along which the main jet stream tends to form, is displaced well to the north from its usual location, into north-central Canada. In fact, temperatures are in the 30s along the southern portion of Hudson Bay and as “warm” as the single digits in northern Canada. For perspective, temperatures in northern Canada are usually in the 20s or 30s “below” zero in January. The polar front usually meanders along the northern tier of the U.S. this time of year. This displacement of the polar front explains the unusually mild weather we have seen recently. As a result of this displacement, a second front/jet stream has developed separating cool/mild air over the northern and central U.S. from warm subtropical air to the south. This is sometimes referred to as a “subtropical” jet.
Our midlatitude storm systems, identified as surface low pressure, develop along these fronts, underneath the jet stream. In particular, they develop along the eastern margin of the troughs in the wave pattern the develops along these fronts. The north-south (meridional) temperature gradient along these fronts determines how strong the jet stream will be and, since the storms derive their energy from the energy of their jet stream, the stronger the jet stream, the stronger the potential storm.
Normally, the polar front has the steepest temperature gradient and thus the stronger jet stream. Presently, the opposite is the case and the “noreaster” which developed along the Mid-Atlantic coast yesterday formed along the eastern edge of a trough in the subtropical jet. Just a note about the term “noreaster”…. It is the name given to a strong low pressure system along the East Coast of the U.S. It was given this name by sailors and fisherman along the New England coast back in the 1800s because, since winds circulate counterclockwise around low pressure (in the NH), they knew that if winds were freshening out of the northeast there was a big winter storm moving up the coast.
In any case, since all of the really cold air is displaced well to the north, temperatures at the surface and up to about 10,000 feet have been above freezing for the past week or so. If the “noreaster” had formed along the polar front (as is usually the case) and taken the track it is taking, to our south and east, the air over us would have been more than cold enough for us to get a snowstorm. With this present, unusual scenario, with all of the above freezing air above us, this should have been a plain old rainstorm. However, surface high pressure system developed well to our north, in northeastern Canada, along the polar front over the weekend. Clockwise circulation around this surface system brought air that was just cold enough (below freezing) from the northeast so that we developed a layer of below freezing air up to about 5000 feet with above freezing air above that (where the precipitation was forming). So rain fell through a thick layer of cold air and froze into ice pellets (sleet).
Berkshire County short-term
So, how do things progress from here. The storm will move northeast across Cape Cod and into the Gulf of Maine overnight. A band of precipitation is wrapping around the backside of the low and will move across Berkshire County this evening. The county should see a mixture of sleet and rain in the lower elevations, with sleet and freezing rain over the elevated terrain where the temperature has remained below freezing. The precipitation should be light this afternoon but will pick up in intensity this evening. By around 10 pm, enough cold air will have wrapped around the system on northwesterly winds to change the back edge of the precipitation to snow. Snow should around midnight or shortly thereafter. Most likely accumulations are a dusting to 1″ with a few spots over the higher terrain seeing as much as 2″. Wednesday will be a tranquil day, with a mixture of clouds and sun and temperatures up into the 30s, as the low moves away into the Canadian Maritimes.
Berkshire County long-term
As the low moves away on Wednesday, a trough in the polar front wave pattern will deepen and move eastward from the western U.S. The eastern edge of the trough will move into our region on Thursday as a cold front. The frontal passage will generate, first some rain showers Wednesday night into Thursday morning, changing to snow showers Thursday afternoon. The trough will then broaden and stall over the central and eastern U.S. for several days. As a result, we will see a return to average, to slightly below average temperatures into next week. There will be some intermittent lake-effect and lake-enhanced snow flurries and snow showers brought into the county on west-northwest winds from Thursday night into the weekend. Because the trough is broad, and the eastern edge will be well off the coast, any storms that develop look to be too far off the coast to give us significant snow for awhile.
Berkshire County Forecast:
Tuesday Night
A mixture of sleet and rain during the evening in the lower elevations, sleet and freezing rain over the elevated terrain. Up to 0.5″ of sleet and 0.25″ of freezing rain over the elevated terrain. The precipitation will likely change to snow by around 9 or 10 pm and then end around midnight. Snow accumulations a dusting to 1″ with as much as 2″ in a few spots over the highest terrain. Probability of precipitation near 100%. Low temperatures in the upper 20s to near 30, mid to upper 20s over the elevated terrain. Northwesterly winds at 10 – 15 mph.
Wednesday
A mixture of clouds and sun. High temperatures in the mid 30s in the lower elevations of central and northern Berkshire, upper 30s in South County and low to mid 30s over the elevated terrain. West-northwest winds at 10 – 15 mph early, diminishing to 5 – 10 mph during the afternoon.
Wednesday Night
Partly to mostly cloudy. There is a chance of rain showers after midnight. Low temperatures near 30 during the evening and then rising through the 30s overnight. Light southwesterly winds in the evening, becoming southerly after midnight.
Thursday
Mostly cloudy with rain showers possible through early afternoon changing to snow showers late. It will be mild, with high temperatures rising into the upper 30s to low 40s by early afternoon. It will turn blustery and colder late, with temperatures falling through the 30s. Southwesterly winds at 5 – 10 mph becoming west-northwesterly at 10 – 15 mph with higher gusts late.