First Call For Weekend Snows
Major Accumulations "Blocked" From Region
Latter November Stratospheric Warming Impacts On The Forecast
The time has come for an Eastern Mass Weather mea culpa of sorts as it pertains to the stratospheric warming this past November. The winter outlook published during the early portion of November referenced the early December 2000 analog in postulating that this warming would not result in a reversal of the 850 zonal winds across the arctic. It was also (incorrectly) asserted on December 16th that this event did in fact barely fall short of the reversal criteria.
This was an honest mistake, as it has been brought to my attention by the very meticulous Larry of the Americanwx.com forum, that there was barely a marginal reversal on November 28th.
This is in more in line with the December 4th, 1981 warming that was cited as a potential alternative scenario that would in fact yield a full reversal. Ultimately, while the warming episode in-and-of-itself was relatively well forecast using the 1981 and 2000 analogs, the ramifications of it were certainly understated in that high latitude blocking was not expected to materialize prior to the new year, following the mid-month relaxation. However, it is now clear that this will be incorrect given that NAO blocking has rapidly burgeoned into existence on modeling in the period centered around the holiday week.
This is taking place approximately 30 days after the 11/28 reversal and consistent with the time lag of the development of high latitude blocking following a reversal per research, and this will have major ramifications on the potential weekend storm.
Synoptic Evolution Yields North Atlantic "Traffic Jam"
The emergence of the north Atlantic block this Christmas, on the eastern side of the NAO domain in response to the previously referenced stratospheric warming late last month, is what sets in atmospheric "traffic jam" of sorts.
Normally, high latitude blocking helps to facilitate snow storms for the area, but in this instance the opposite is actually the case. Northern stream energy dumping into the Maritimes pools underneath the developing block once off of the coast and amplifies. At the same time, the energy for the weekend potential ejects out of the Pacific trough careens east-north-east over the top of the central US ridge and on a collision course with the Great Lakes by Boxing Day.
The lead wave has developed into a deep, closed 500mb low near the 50/50 position by this time and is largely impeded from exiting to the NE by the nascent block to the southeast of Greenland. This results in yet another zone of confluent flow over New England since this system can not "get out of the way", so to speak, which shears and redirects the weekend system to the southeast as it approaches the region after descending from the peak of the ridge.
This should relegate any significant snowfall to the southwestern quadrant of southern New England, primarily impacting portions of the state of Connecticut.
Expected Storm Evolution
Snowfall should begin to overspread the region from southwest to northeast late in the evening on Friday night.
Reaching the Boston area during the predawn hours on Saturday morning.
However, as this point it will begin to encounter resistance from the block and associated confluence to the northeast, thus flurries will struggle to reach far northeastern Massachusetts by late morning.
Snowfall should begin too taper region wide by late afternoon and early evening.
FIRST CALL:
Final Call will be Thursday night or Friday.