This is a fantastic AFD. Gives you the Drag “chills” feel reading it
Per collaboration with surrounding NWS offices, Winter Storm Watches
are now hoisted for western and central MA into northern CT. This is
mainly due to the potential for a prolonged period of freezing rain
and sleet. Though sensitivity to low-level thermal structure and
precipitation type changes results in some uncertainty in forecast
ice and snow/sleet accumulations, model solutions are coming around
to the idea of potentially significant icing with potential for tree
damage, power outages and transportation impacts in the Watch
area.
First part of this expected precip event looks to affect the region
starting Sunday evening. Warm-frontal precipitation is expected to
overspread from central and southern NY into Sunday night. Though
surface temperatures initially support rain as a predominant weather
type, northeast ageostrophic flow related to an in-situ high
pressure ridge/wedge over ME/NH will draw increasingly colder/sub-
freezing low-level air southwestward mainly across areas north and
west of Interstate 495. This process will facilitate a changeover
towards sleet and freezing rain in these areas, with warm-nose
temperatures between +1 to +4C following the ECMWF/NAM guidance.
Surface ridge over northern New England remains essentially in place
through most of the first part of the event, and this will maintain
and lock in sub-freezing surface temperatures across several hours
in these interior areas. Precipitation appears most focused with
this first portion of the event during the pre-dawn into Monday
afternoon hours with impact potentially to both commutes. In this
period, of some concern is NAM-based guidance indicating a corridor
of elevated instability (up to 150 J/kg) on the nose of a SSW low-
level jet of 40-50 kts, which may help to enhance freezing rain and
sleet accumulation rates. Temperatures across most of the South
Coast and near/SE of I-95 should remain above-freezing supporting
predominantly rain.
Later Monday night, surface winds become east to east-southeast as
we await the second part of this event, taking the form of a
developing coastal cyclone that looks to progress from the east
coast of NJ towards SE MA by 12Z Tuesday. While the GFS remains on
the eastern/offshore side of the guidance envelope regarding this
system, ECMWF ensemble membership is rather tightly clustered either
near the MA E coast or inland. A closer-inland track appears
preferred and follows the ECMWF/GEM solutions. System looks to pull into
coastal Maine by the evening of New Years Eve, with winds becoming
westerly and cooler air filters in on west winds.
The Winter Storm Watch was hoisted in areas where significant
freezing rain accretions are possible, with amounts up to a half-
inch forecast. It sometimes can be difficult to reach Warning
criteria purely from freezing rain accumulations alone. Small
changes to low-level thermal structure will make significant
differences in observed sleet/freezing rain accumulations, and
pending subsequent QPF and accumulation trends, some of those
areas currently in the Watch may end up becoming converted to
Advisories on that basis. The potential exists for significant
icing in portions of the Watch, however, and this is especially
the case for parts of the hills in Worcester and Tolland/Windham
Counties and into the eastern slopes of the Berkshires given
northeast/east upslope flow. Indeed, trending on the lower end
of that range by aftn.ndividual NAM/ECMWF/GFS model guidance
freezing rain accumulations are quite substantial, and while
likely overdone, in some cases are over three- quarters of an
inch. Further expansion of the Watch could be needed in
subsequent updates.
