574
FXUS61 KBOX 191530
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
1030 AM EST Sun Jan 19 2025

.SYNOPSIS...
Low pressure will bring a quick hitting snowstorm to most of the
region later today and especially tonight...which may begin as a
brief period of rain. An arctic airmass will bring frigid
conditions and wind chills on the order of 5 to 15 below zero at
times Monday night through Thursday morning. Temperatures
moderate a bit in the Thursday through Saturday time frame, but
they will remain below normal. Dry weather will generally
prevail, but a bit of light snow/flurries may clip the southeast
New England coast late Tuesday night into Wednesday morning.
Another wave of low pressure will bring the low risk for a
period of snow late Thursday night into Friday if the system
tracks close enough to the coast.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Key Messages...

* Patchy black ice early this morning

Dense Fog Advisory was allowed to expire at 9 AM. Still dealing
with patchy black ice on untreated surfaces.

Incorporated some of the timing from the last few runs of the
HRRR into the forecast this morning. Still thinking much of
southern New England remains dry through 1 PM, although clouds
will be increasing. Still anticipating precipitation to begin
2-5 PM Worcester County west in MA and northern CT, and 4-7 PM
farther east. This is slightly faster than the previous
forecast timing, and accounts for radar trends across PA and NY
state.

Minor tweaks to temperatures, but will need to monitor those
closely. These will be critical for determining precipitation
types this afternoon into this evening, and ultimately impact
snowfall accumulations as well. As of this writing, still
thinking areas along and south of the I-395 corridor in MA and
CT, as well as inside I-495, start as mainly rain then
transition to all snow by evening.

Previous Discussion...

The first half of Sunday will be quiet with yesterday`s
system/rain exited to the northeast. Early this morning, the
main concerns will be black ice and areas of fog. A combo of
recent rainfall and temperatures at or just above freezing by
sunrise will support areas of black ice, especially on bridges
and overpasses. If traveling, take caution. Surface observations
and nighttime microphysics satellite imagery indicate areas of
fog across parts of the CT River Valley, Cape Cod, and northeast
MA. Some locations have reported 2 miles to as low as 1/4 mile.
A Dense Advisory remains in effect through 9 AM this morning,
particularly for the areas mentioned above.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM MONDAY/...
Key Messages...

* Plowable snow late Sunday and through Sunday night.

* A Winter Storm Warning in effect for along and northwest of
  Boston-Providence line. Winter Weather Advisory for areas
  southeast of I-95

* Potential for bands of localized higher snow rates/amounts
  setting up tonight. Low confidence on where they set up.


Temperatures rise into the upper 30s and low 40s today with
breaks of sun developing mid- morning. This improvement will be
short-lived as clouds increase by early afternoon ahead of the
incoming storm. This afternoon, our snowmaker of a storm will
track offshore of the mid-atlantic coast as a surface low ahead
of a broad upper trough. This surface low is expected to track
up the east coast Sunday night into Monday. The track of this
storm has come into better agreement among model and ensemble
guidance. However, there are still subtle differences that are
enough to create differing solutions in snowfall amounts.

Ingredients and features...

Ingredients are there to support a system capable of bringing
snow Sunday PM through Monday. Southern New England sits in an
optimal position for large scale ascent with respect to the
upper level jet. This will be coupled with ample moisture with
precipitable water values around 180-200% of normal. Despite
small differences in strength/track of the low, there is good
agreement among guidance for frontogenetical forcing extending
across portions of southern New England. Features to watch as
they will provide additional forcing/lift to help generate
higher rates within snow bands. HREF in particular indicates
these bands across north and central MA and another positioned
across SE MA along the coastal front. HREF means show potential
for 1-2" rates within these bands Sunday night(7pm- 1am). All
supported by the localized forcing and cold air advection for
higher ratios.

Storm track...

Deterministic and ensemble guidance has come into better and
consistent agreement on the track of the system. Still these
subtle differences in track and strength will have impacts on
expected totals and other details such as precipitation type at
the onset of the event. Some models have consistently showing
higher totals than others (e.g. GFS) while other models have
leaned lighter(ECMWF). There has also been some run to run
inconsistencies even this close to the event, particularly with
respect to banded snowfall amounts. Track will also influence
precipitation type with a more inland track meaning rain or
rain/snow for longer before switching to snow across areas
southeast of I-95. A blend of solutions support a start as rain
or a rain/snow mix for those areas. This seems realistic as high
resolution guidance shows the coastal front with ocean modified
air pushing into those areas Sunday afternoon while it`s colder
to the north and west. The kicker will be how this coastal
front continues to position tonight as CAA works in from the
north as the low tracks across. As a result, areas generally
south and east of I-95 will hold a larger degree of uncertainty
in amounts.

Timing and Amounts...

There is sufficient confidence on the timing of onset for this
event with precipitation beginning in the 3-5pm timeframe for
northern CT and western MA and 4-6pm across central, east,
southeast MA and RI. Again, rain or a rain/snow mix is looking
likely in areas at onset, more likely across RI, east and SE MA
before flipping to snow this evening. Snow continues overnight
with the most likely period for banded, localized heavier
snowfall falling in the 7pm-1am timeframe. Snow decreases from
west to east with snow diminishing by 7 am Monday.

This is generally looking like a high end advisory to low end
warning event for areas north and west of I-95 with 6-8"
forecast. If banding does materialize, can`t rule out the low
chance at localized amounts up to 10 inches (per HREF ens max).
Confidence is lower on the where these bands set up tonight.
These areas are under a Winter Storm Warning. For southeast of
I-95, there is greater uncertainty in how quickly it changes to
snow which will impact snow amounts. High resolution guidance
has hinted at the potential for a frontogenetical band
positioning in this area Sunday night and if cold enough could
enhance snow amounts with localized amounts near warning
criteria. Given the large amount of uncertainty, we put southern
RI and MA (southeast of I-95) in a Winter Weather Advisory for
3-6".

Impacts...

This storm will bring the usual impacts brought in with a
winter storm such as reduced visibilities and snow covered roads
making for hazardous in areas Sunday night. Snow will likely be
done by 7am for those who are participating in the AM commute.
Strong cold air pushes into the region, starting the streak of
frigid temperatures this week.

&&

.LONG TERM /MONDAY THROUGH SATURDAY/...
Key Points...

* Arctic Outbreak Mon PM through Thu AM
-Wind Chills 5 to 15 below zero and perhaps as low as -20 in spots
-Actual Lows in the single digits with some below zero
-Actual highs Tue & Wed mainly between 15 and 20

* Temps moderate a bit Thu/Fri/Sat but will remain below normal

* A period of light snow or flurries may clip southeast New England
  esp the Cape/Islands late Tue night-Wed AM...Dependent on track

* Low risk for a period of snow late Thu night into Fri

Details...

Monday through Thursday morning...

All the model guidance continues to agree on an arctic outbreak
in the Mon through Thu am time frame. 850 MB temperatures will
drop to around -22C/-23C as arctic air pours into the region
from the northwest. Highs on Mon should reach the lower to
middle 20s for most locations with gusty NW winds. By Tue and
Wed, expect highs to mainly be in the 15 to 20 degree range.
Overnight low temps will be in the single digits with some below
zero readings at times...particularly Tue night when winds
decouple. Wind Chills will drop to between 5 and 15 below zero
and perhaps as low as -20 in spots. Cold Weather Advisories will
be needed.

The vast majority of this time will feature dry weather. We may
see a few brief spot snow showers Mon PM as the arctic air
rushes into the region. We may also see a period of light snow
or flurries clip the southeast New England coast late Tue night
into Wed am from a distant wave of low pressure. At this
point...even if it happens does not look to be a big impact.
Nonetheless...some very minor accumulations would be possible
especially if winds can turn more NNW which would result in some
ocean effect/enhancement across the Cape and Nantucket.

Thursday afternoon into Saturday...

The core of the coldest air begins to shift northeast and away
from our region. It still will be rather cold though with
highs mainly in the 20s Thu and perhaps 30 to 35 by Fri and Sat.

Also, we will be watching a low pressure system that will bring
snow/ice right down to portions of the Gulf Coast early this
week! This wave of low pressure will then track northeast,
likely passing southeast of the Benchmark. The majority of the
GEFS/EPS/CMC ensembles indicate this storm will miss us or just
bring us a glancing blow. That being said there are a few
ensemble members that are further northwest and would bring a
better shot at snow to the region...especially the coastal
plain. We will likely need a few more days to have a better idea
on this risk.

Monday...High Confidence.

Mainly VFR outside a few brief spot snow showers that will be
possible. NW wind gusts of 20 to 30 knots.

KBOS TAF...Moderate Confidence in TAF. Heaviest of the snow
roughly between 00z and 05z. 1" per hour snowfall rates possible
at times.

KBDL TAF...Moderate Confidence in TAF. Heaviest of the snow
roughly between 22z and 03z. 1" per hour snowfall rates possible
at times.

IFR/MVFR this evening, becoming VFR overnight. VFR most of
Sunday. Light snow and MVFR CIGS move in between 21-23z.

Outlook /Monday through Thursday/...

Martin Luther King Jr Day: VFR. Windy with local gusts up to
30 kt. Slight chance SHSN.

Monday Night through Tuesday: VFR. Breezy.

Tuesday Night through Wednesday: VFR. Breezy. Slight chance
SHSN.

Wednesday Night through Thursday: VFR. Breezy.

&&

.AVIATION /15Z SUNDAY THROUGH THURSDAY/...
Forecast Confidence Levels:

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

Today and tonight...Moderate Confidence.

Mainly VFR conditions into this afternoon. Precipitation spreads
across region from west to east between 20z and 23z with conditions
lowering to MVFR/IFR.

Ptype will mainly be snow NW of I-95 in MA and RI, but a period
of rain may occur near and especially southeast of I-95. Ptype
should transition to mainly snow northwest of the Cape Cod Canal
by 00z/01z or so. Heavier snow rates will likely occur 00z
through 04z/05z when IFR/LIFR conditions are expected. Snowfall
rates of 1-2" per hour will be possible at times. Total snow
accumulations of 3-6"/4-8" will be common for much of the region
northwest of the Cape Cod Canal. Ptype probably changes to all
snow before ending even across the Cape. The last of the steady
snow should exit the coast by 09z/10z Mon. Conditions will
improve to VFR by daybreak Monday.

Winds should become N at 5 to 10 kts this afternoon. Winds
become more NW later tonight with gusts of 20-25 kts developing
and perhaps up to 30 kts at ORH and Cape terminals briefly.

Monday...High Confidence.

Mainly VFR outside a few brief spot snow showers that will be
possible. NW wind gusts of 20 to 30 knots.

KBOS TAF...Moderate Confidence in TAF. Heaviest of the snow
roughly between 00z and 05z. 1" per hour snowfall rates possible
at times.

KBDL TAF...Moderate Confidence in TAF. Heaviest of the snow
roughly between 22z and 03z. 1" per hour snowfall rates possible
at times.

IFR/MVFR this evening, becoming VFR overnight. VFR most of
Sunday. Light snow and MVFR CIGS move in between 21-23z.

Outlook /Monday through Thursday/...

Martin Luther King Jr Day through Tuesday: VFR. Breezy.

Tuesday Night: VFR. Slight chance SN.

Wednesday: VFR. Breezy. Slight chance SN.

Wednesday Night through Thursday: VFR. Breezy.

&&

.MARINE...
Forecaster Confidence Levels:

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

High confidence...

Sunday winds are light most of the day, increasing to 15-20 kts
Sunday night with gusts to 30 kts as a low pressure passes from
southwest to northeast. Gusts up to 35 kts possible for the
northern outer waters tonight through early Monday resulting in
a Gale Warning. Otherwise, expect SCAs generally from 00z
tonight through 12z Tuesday. Seas 4-9 ft.

Outlook /Monday through Thursday/...

Martin Luther King Jr Day: Moderate risk for Small Craft
Advisory winds with gusts up to 30 kt. Rough seas up to 9 ft.
Freezing spray, slight chance of snow showers.

Monday Night: Low risk for Small Craft Advisory winds with
gusts up to 30 kt. Areas of rough seas. Freezing spray, slight
chance of snow showers.

Tuesday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Areas of seas approaching 5 ft. Freezing spray.

Tuesday Night: Winds less than 25 kt. Seas locally approaching
5 ft. Freezing spray, chance of snow showers.

Wednesday: Low risk for Small Craft Advisory winds with gusts
up to 20 kt. Seas locally approaching 5 ft. Freezing spray,
chance of snow showers.

Wednesday Night: Winds less than 25 kt. Seas locally
approaching 5 ft. Freezing spray.

Thursday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Freezing spray.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Winter Storm Warning until 7 AM EST Monday for CTZ002>004.
MA...Winter Storm Warning until 7 AM EST Monday for MAZ002>016-026.
     Winter Weather Advisory until 7 AM EST Monday for MAZ017>021.
RI...Winter Storm Warning until 7 AM EST Monday for RIZ001-002.
     Winter Weather Advisory until 7 AM EST Monday for RIZ003>007.
MARINE...Small Craft Advisory from 1 AM to 7 PM EST Monday for ANZ230-
     236.
     Small Craft Advisory from 10 PM this evening to 7 PM EST
     Monday for ANZ231>235-237.
     Gale Warning from 7 PM this evening to 7 AM EST Monday for
     ANZ250.
     Small Craft Advisory from 7 PM this evening to 7 AM EST
     Tuesday for ANZ251.
     Small Craft Advisory from 7 PM this evening to 3 PM EST
     Tuesday for ANZ254>256.

&&

$$

SYNOPSIS...Frank/Mensch
NEAR TERM...Belk/Mensch
SHORT TERM...Mensch
LONG TERM...Frank
AVIATION...Belk/Frank/Mensch
MARINE...Frank/Mensch