Heavy Snow in Massachusetts, December 5, 2024

Area Forecast Discussion
National Weather Service Boston/Norton MA
1246 AM EST Thu Dec 5 2024

.SYNOPSIS...
A quick hitting system will bring accumulating snow near and
northwest of I-495 tonight into Thursday morning especially in
the high terrain with mainly rain on the I-95 corridor. A strong
cold front will likely bring a few snow squalls Thursday and
also result in strong winds developing. Gusty conditions and
below normal temperatures Friday into Saturday with a low
chance of an isolated snow shower or snow squall. A weak clipper
system passes through northern New England early Sunday and
brings a light snow shower to areas mainly north of Route 2 in
northern Massachusetts. Then the first-half of next week
features mild temperatures and a more active weather pattern.

&&

.NEAR TERM /THROUGH TODAY/...

Clouds have moved in across southern New England ahead of an
approaching Alberta Clipper system. Out ahead of this system
southerly flow form a modest LLJ continues to advect "warmer"
air into the region. A couple of pesky light rain and snow
showers have been around but, the bulk of the moisture remains
out to the west. Widespread rain and snow will hold off until
the mid to late evening hours which we will discuss below.

&&

.SHORT TERM /TONIGHT THROUGH 6 PM FRIDAY/...
Key messages...

* Accumulating Snow tonight into Thu AM near and NW of I-495
* Greatest impacts in the Berks/Worcester Hills & Hills of NE CT
* Ptype mainly rain near and especially southeast of I-95
* A few snow squalls late Thu AM/Thu afternoon with poor vsbys
* Becoming Windy & Turning Colder Thu into Thu night

Details...

1) Ptype & Expected Snow Accumulations...

The Alberta Clipper will be tracking to our north across Quebec
tonight and into northern New England Thu. As this happens...it will
induce a strong SSW low level jet of 45 to 55 knots tonight. This
will increase the forcing for ascent and allow widespread rain &
snow to overspread the region from west to east in the 7 pm to 11 pm
time frame this evening.

The biggest forecast challenge revolves around Ptype as well as snow
accumulations & impacts. Given the fact that the surface low will be
tracking north of our region...southwest flow will warm temps aloft
and in the boundary layer to result in mainly rain near and
southeast of the I-95 corridor. Ptype could end as a bit of snow or
mix in this region Thu morning...but think any impacts will be
northwest of I-95. The main concern for accumulating and plowable
snowfall near and especially northwest of I-495. Thermal
profiles/soundings indicate that temps should be cold enough in this
region for mainly snow or rain quickly changing to snow. Given
marginal boundary layer temps...greater impacts/snow accumulations
will be in the higher terrain. Strong low level southwest flow will
also result in some mesoscale enhancement along the western side of
the Berks, Worcester Hills and northeast CT Hills. This can be seen
in a lot of the high resolution model guidance QPF fields. So while
the snow amounts should be elevation dependent to some
degree...expect to see the higher snow accumulations on the
western/upslope side vs the eastern side at the same elevations.

We continued the Winter Weather Advisory for western and central MA
and also include Tolland county to account for their higher
elevation. We generally expect 2-6" of snow to fall within the
Winter Weather Advisory with the greater of those amounts in the
high terrain...particularly on the western upslope side.
However...some travel impacts are expected in the lower elevations
and this may coincide with the Thu AM commute even just northwest of
I-95. Thermal profiles very marginal in this region though...so a
lot will be dependent on intensity which will be more of a nowcast
situation.

In a nutshell...motorists planning travel overnight into Thu morning
should be prepared for slippery travel northwest of I-495 and
especially in the high terrain. Confidence is high in a plowable
snow above 500 feet in elevation...but more uncertain and rate
dependent on the lower elevations.

2) A Few Heavy Snow Squalls later Thu morning and afternoon...

The other big concern for Thursday will be the potential for a few
heavy snow squalls later Thu morning and afternoon behind the
widespread activity. Several parameters are quite impressive with a
potent shortwave/cold front...steep 1000-700 mb lapse rates along
with some MUCape. This coupled with left over low level moisture
should result in a few heavy snow squalls developing later Thu
morning into the afternoon. The main concern is across the
interior...where roads could quickly become snow covered with strong
wind gusts and very poor visibility. We certainly may need to issue
some snow squall warnings. While this activity may impact the I-95
corridor...warm boundary layer may result in possibly a mixture of
rain and snow showers but limited impact on roadways.

3) Strong Winds...

Lastly...the other concern will be for strong winds. The first
concern is tonight across the Cape and Islands. As the strong SSW
low level jet develops this evening into the overnight hours...it
still will be colder than the comparable SST. This will allow for
good mixing and Bufkit soundings indicate the potential for southwest
wind gusts of 40-50 mph across the Cape and Islands. The area around
Buzzards Bay is particularly vulnerable to strong winds in this
setup. Wind Advisory posted tonight for this region.

Across the rest of the region...the main concern for strong winds
will be later Thu morning into Thu evening in the strong cold
advection pattern behind the cold front. A westerly 45 to 55 knot
low level jet will develop at 850 mb and strong cold advection will
allow for excellent mixing. We are thinking this should generate
wind gusts on the order of 35-50 mph. We opted to go with a Wind
Advisory for western/central MA and northern CT. Good mixing on
westerly flow in the cold advection favors high terrain and areas
down wind. These winds may also be enhanced by any snow squalls etc.
Later shifts may need to consider extending

4) Turning Colder Later Thu into Thu night...

Strong cold advection will result in temps beginning to drop later
Thu and especially Thu night. Temperatures should fall into the 20s
Thu night under strong cold advection and will probably see some
upper teens in the high terrain by daybreak. The strong winds will
result in wind chills dropping into the single digits and teens by
early Fri morning.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
Key Messages:

* Below normal temperatures and gusty wind on Friday and Saturday.

* Low confidence in a snow shower/squall late Friday night into
  early Saturday.

* Weak clipper system passes to the north early Sunday with a snow
  shower or two possible for far northern Massachusetts.

* First-half of next week features mild and unsettled conditions.

Friday and Saturday: The low pressure system deepens as it moves
toward the mouth of the Saint Lawrence River, while a surface high
pressure builds across the south/central CONUS. Pressure gradient
builds and leads to gusty west to northwest flow on Friday and
Saturday, gusting between 25 to 35 MPH, below advisory criteria. CAA
ushers an Arctic airmass, with 850mb temperatures crashing, between
-12C and -18C on Friday and then -15C and -20C on Saturday. No
surprise that temperatures remain well below normal with highs in
the 30s and nighttime lows in the teens to low 20s. This period is
largely dry, the west to northwest flow may pick up moisture from
the relatively warmer Great Lakes and cause an isolated snow shower,
similar to what was experienced Tuesday evening. FWIW, the CIPS Snow
Squall Parameter highlight moderate values for squalls late Friday
night or early Saturday morning. At this time have low confidence in
occurrence, but if were to occur may lead to travel impacts, will
have to wait and see what the higher resolution models show once it
begins to cover this period. At this point, did not introduce any
POPs to the forecast as confidence remains low, though could see
those added to the forecast once we are able to get the NAM3KM and
HRRR (models like this) to cover this period in question. Stay
tuned!

Sunday: The one wrinkle in the forecast during the second-half of
the weekend comes from a weak clipper system with limited moisture
passing to our north early to mid-morning. Bulk of the moisture will
be over Vermont and New Hampshire and the greatest forcing is
displaced a bit further north in Quebec. Still, cannot rule out a
snow shower or two for northern Massachusetts, areas along or north
of the Rt. 2 corridor. Temperatures moderate Sunday afternoon to the
upper 30s north and mid 40s south.

Monday to Wednesday: First-half of next week features a warm-up as
the surface high moves over Bermuda and brings deep southwest flow
to our region. Highs return to the low and middle 50s with nighttime
lows in the 30s and low 40s. Additionally, it is a more active
pattern with the next chance for rain late Monday through the
overnight and possibly again Wednesday, but confidence remains low
with the timing given the time frame.

&&

.AVIATION /06Z THURSDAY THROUGH MONDAY/...
Forecast Confidence Levels:

Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.

06z TAF Update...

This morning...Moderate Confidence.

Widespread low end MVFR- IFR conditions with periods of LIFR
conditions across the interior through 15-18z. Across the
interior terminals precip type will be SN.

Ptype will mainly be snow northwest of I-495 with generally
rain southeast of that region overnight into Thu morning. Ptype
could mix with or change to snow towards the I-95 corridor for
a brief time before the bulk of the precipitation ends Thu
morning. Snow accumulations of 2-6" are expected across much of
the interior with the higher end amounts confined to the higher
elevations. Lower amounts as you go east from Worcester Hills.
Little if any snow accumulations on the I-95 corridor with
precipitation mainly in the form of rain.

SSW winds increasing this morning with gusts up to 35 kts by
12z. Cape/Islands trend higher with gusts up to 40 kts.

Thursday...Moderate confidence.

MVFR/IFR changing to MVFR/VFR. Widespread RA/SN ending 15-18z
from west to east but scattered snow squalls will be possible in
the afternoon briefly dropping visibilities with a quick
coating of snow. Winds turn from SW to W/NW increasing in the
late morning and afternoon, gusting 25-40 kts.

Thursday night...High confidence.

VFR. Gusty NW winds 25 to 35 kts.

Friday...High confidence.

VFR. NW winds gusting 20-25 kts.

KBOS Terminal...Moderate confidence in TAF.

This morning: Rain may turn to snow this morning but
accumulation isn't expected. Main batch of precip exits 13-16z
with a few showers possible behind it through the afternoon.
Will see some improvements in ceilings this morning with periods
of VFR possible in between showers. Chance for brief RA/SN
showers/squalls in the afternoon. Timing/chances lower
confidence due to hit or miss nature of showers.

KBDL Terminal...Moderate confidence in TAF.

Today: Main batch of precipitation exiting by 13-15z. Brief
improvements in ceilings toward MVFR in the AM in between
showers, even brief VFR possible. Chance for brief
showers/squalls in the afternoon with brief vis reductions and
increased precip rates. Strong SW to W winds with gusts up to 40
kts.

Outlook /Friday Night through Monday/...

Friday Night through Saturday: VFR. Breezy.

Saturday Night: VFR.

Sunday through Sunday Night: VFR. Breezy.

Monday: Mainly VFR, with local IFR possible. Breezy. RA likely.

&&

.MARINE...
Forecaster Confidence Levels:

Low - less than 30 percent.
Medium - 30 to 60 percent.
High - greater than 60 percent.

* Gale Warnings tonight through Thu night with 45 knot gusts

Tonight through Thursday night...

Alberta Clipper tracking to our north will induce a strong southwest
LLJ tonight. This will allow SSW wind gusts of 30 to 45 knots to
develop later this evening and especially during the overnight
hours. The strongest of those winds tonight will be across the
southern waters.

This system will result in a strong cold frontal passage across the
region on Thu. Strong cold advection will result in westerly wind
gusts of 35 to 45 knots developing Thu and continuing Thu night.
Gale Warnings posted all waters though this time and seas will be
quite high and rough.

Outlook /Friday Night through Monday/...

Friday: Low risk for gale force winds with gusts up to 35 kt.
Rough seas up to 11 ft.

Friday Night: Moderate risk for Small Craft Advisory winds with
gusts up to 25 kt. Seas up to 5 ft. Slight chance of snow.

Saturday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Seas up to 5 ft.

Saturday Night: Winds less than 25 kt. Areas of seas
approaching 5 ft.

Sunday through Sunday Night: Winds less than 25 kt. Local rough
seas.

Monday: Winds less than 25 kt. Areas of seas approaching 5 ft.
Chance of rain, patchy fog. Local visibility 1 to 3 nm.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Wind Advisory from 10 AM this morning to 10 PM EST this
     evening for CTZ002>004.
     Winter Weather Advisory until 10 AM EST this morning for
     CTZ003.
MA...Wind Advisory from 10 AM this morning to 10 PM EST this
     evening for MAZ002>004-008>012-026.
     Winter Weather Advisory until 10 AM EST this morning for
     MAZ002>004-008>012-026.
     Wind Advisory until 10 PM EST this evening for MAZ022>024.
RI...Wind Advisory until 10 PM EST this evening for RIZ008.
MARINE...Gale Warning from 7 AM this morning to 7 AM EST Friday for
     ANZ230.
     Gale Warning until 7 AM EST Friday for ANZ231>237-250-251-
     254>256.

&&

SYNOPSIS...Frank/Dooley
NEAR TERM...Frank
SHORT TERM...Frank
LONG TERM...Dooley
AVIATION...BW/Mensch
MARINE...Frank/Dooley