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FXUS61 KBOX 280004
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
704 PM EST Wed Jan 27 2021

.SYNOPSIS...
Northerly onshore flow will produce intermittent periods of
light ocean-effect rain/snow showers through tomorrow afternoon.
Most of this activity will take place over the Cape and Islands
with more isolated activity along the east coast of MA. A surge
of cold arctic air will bring the coldest temperatures of the
year to southern New England by Friday morning with sub-zero
wind chill temperatures likely. Cold will continue through
Saturday with moderating conditions Sunday. There is the
potential for a winter storm to affect the area Monday and
Tuesday.

&&

.NEAR TERM /UNTIL 7 AM THURSDAY MORNING/...
700 pm update...

radar showing light returns along and just east of the coast
from Portland ME down to Cape Cod. Web cams and obs have
indicated a little bit of light snow and rain associated with
the radar signatures. Hi-res models show the development of a
weak surface low overnight just offshore of Cape Ann. In
association with that, a more consolidated area of light
precipitation is expected to develop. So it appears to be a weak
mesocale feature combining with instability over the "warmer"
ocean waters to generate ocean enhanced precipitation. Models
suggest this feature will be most organized late tonight thru
mid-morning Thursday. A little discussion here in the office
that we might see some 1" or so accumulations over the far
eastern tip of Cape Ann by daybreak, per 12z HREF guidance and
latest HRRR output. At this point I'm not going to make any
substantial changes to the forecast which has 30-40% chance of
snow showers across much of eastern MA down to the Cape and
Islands for later tonight into Thursday. The forecast already
has accumulations of 1-2" by tomorrow afternoon for parts of
Cape Ann, and generally a dusting to 1" elsewhere.



430 PM Update...

Light ocean-effect rain/show showers continue over the Cape Cod
and The Islands. Still expecting intermittent periods of
scattered ocean-effect snow/rain showers through this evening
and overnight mostly along the south shore/south
coast/Cape/Islands. Low temps overnight fall into the low-mid
20s.

&&

.SHORT TERM /7 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/...
* Ocean-effect showers continue through Thursday afternoon.
* Skies gradually clear from west-east mid-late afternoon
Thursday
* Cold arctic air brings single digit temperatures and sub-zero
wind chills Friday morning.

Northerly flow associated with an offshore low-pressure system
will continue to produce light ocean-effect snow/rain showers
along the east coast of MA and Cape Cod through tomorrow
afternoon. As this system slowly moves offshore we'll begin to
see clearing over southern New England late tomorrow afternoon.

As the offshore low moves further east over the Atlantic waters,
strong CAA at 850 hPa will usher an Arctic air mass into
southern New England Thursday night. This will strengthen
the pressure gradient over the northeast and produce gusty
northwesterly winds that will allow for wind chill temperatures
well below 0. A wind chill advisory may be needed as some of the
western zones could see wind chill temperatures as low as -15
F. At this time however, most locations are forecast to be above
the -15F advisory threshold.

&&

.LONG TERM /FRIDAY THROUGH WEDNESDAY/...
Friday Through Wednesday

Pretty active weather pattern through the period. 12z guidance in
fairly decent agreement given the overall evolution of the upper air
pattern this weekend and into the middle of next week. Given that
relative agreement, relied heavily on the NBM (Nat'l Blend of
Models) as a starting point, with some adjustments here and there.
In general looking at a shot of pretty cold weather for Friday and
Saturday as the flow turns directly out of the north. Cold upper
trough lifts out Sunday, however a potent longwave trough develops
across the eastern third of the country to start next week. Model
guidance showing increasing potential of a fairly strong winter
storm to impact the region. Let's break it down day by day below:

Friday: For most of southern New England it's going to be a
downright cold and windy day. With -18 to -22C air at 925mb moving
over the region, surface highs are going to be held generally in the
teens. A few lucky areas near the coast and Cape and Islands will
top out in the lower 20s. This is a good 15 to 20 degrees below
normal. The cold air will promote excellent mixing to about 3000 ft.
This will tap into 30-35kt winds and bring them down to the surface.
Just what we need when it's cold...a lot of wind. Expect wind chills
below zero much of the day. Wanted to also mention that we expect
freezing spray to be an issue over the coastal waters. See the
marine section for more details.

Probably the more meteorologically interesting feature that we will
need to keep a close eye on is an inverted surface trough, extending
outward from low pressure well to our east, that will swing from
north to south just to the east of Cape Cod during the day. It's now
getting into better "view" of the hi-res models, but since the
global models have been showing the formation of this feature for
days, that's a signal of it's intensity. NAM3km shows some very
strong warm air advection and frontogenesis signatures at 850mb
and to a slightly lesser extent at 700mb. At the same time, the
deep and cold 500mb low center (w/temps about -38C) will move
just off the coast. With all the cold air aloft and relatively
warm ocean temperatures, this will also result in a very
unstable low level lapse rate. Snow squall index values are
pegged on the high end just off the coast. Expect that we will
see a band of heavy snow showers and squalls just offshore with
that surface trough and move fairly rapidly south during the
afternoon. Big question is exactly how close it comes to the
Cape and southeast MA. Think the bulk will be just east of the
Outer Cape, but close enough that a period of moderate snow
showers will occur. Looks to be primarily 16-21z timeframe that
the event occurs. How strong the convective snow showers will
get is a bit uncertain, but think a quick 1-2" in less than an
hour is possible. Did bump PoPs up across the Cape. If the
trough comes a little further west, 4-6" totals are not out of
the question from P-Town down to Chatham. Stay tuned!

Friday Night: COLD! Still breezy. Lots of 0-10 degree lows, sub-zero
in far western areas. Winds still gusting 10-15mph, so wind chills
in most areas will be 0 to 10 below.

Saturday: Sunny and still cold, but not as bad as Friday. Highs
generally in the 20s area wide.

Sunday: Still below normal temperatures, but moderating. Primary
weather feature will be increasing clouds.

Monday/Tuesday: Longer range global model guidance suggesting a
significant winter storm to impact the region. 12z runs of ECMWF,
CMC and GFS all show a similar patter with a slow moving low
pressure center developing off the mid-Atlantic coast and trekking
northeastward. Typical nor'easter track. GFS and Canadian brings it
just outside the 40n/70w "benchmark", while the ECMWF is just
inside. Obviously the details hinge on quite a few items...exact
track, speed, intensity etc. But given the support by the ensembles
as well, it appears as the region will be dealing with winter
weather to start the week. Stuck with the NBM blend which suggests
a rain-snow line somewhere across southeast MA, with the heaviest
snows just northwest of that line. Deterministic models suggest QPF
of 1-2" of liquid are possible, especially across SE MA. Certainly
plausible given the slow motion of the storm and the deep trough
tapping moisture from the Gulf of Mexico and Bahamas. How much is
rain vs snow, way too early to tell...but winter fans will likely
embrace the snowfall opportunity and perhaps already salivating at
the potential. As always, later forecasts will continue to fine tune
details. We are still 5-6 days away -- a lot can change.

&&

.AVIATION /00Z THURSDAY THROUGH MONDAY/...
Forecaster Confidence Levels.

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

00Z Update

Overall moderate confidence in the forecast.

Currently conditions ranging from VFR (primarily western areas)
to MVFR across eastern areas, though pushing IFR across the
Cape. Forecast concerns revolve around the development of snow
showers along the east coast of MA. There is uncertainty into
how organized the precipitation will be, though latest guidance
suggests a period of light snow across northeast MA developing
after 08z or so and lasting into mid-morning. This would
primarily impact BOS, with lesser impacts at nearby airports
such as BED and OWD. Additional scattered mainly snow showers
(though perhaps mixed with rain across the Cape) will continue
into the early afternoon for areas south of BOS, into the Cape
and Islands. A mix of MVFR and IFR conditions will occur within
the areas of precipitation.

Across western areas, VFR conditions are expected to predominate
overnight and especially all day Thursday.

A cold front will be moving through the region later in the day.
As this happens, surface winds will start to gust above 20
knots, and clearing conditions will develop from west to east.

KBOS TAF...Moderate confidence. Uncertainty in the visibility
reductions due to expected light snow. Potential for less than
2sm visibilities between 11-15z, but confidence is too low at
this point to add to the forecast.

KBDL TAF...High confidence in TAF.


Outlook /Friday through Monday/...

Thursday Night: VFR. Windy with gusts up to 35 kt.

Friday: VFR. Strong winds with local gusts up to 40 kt. Slight
chance SHSN.

Friday Night: VFR. Windy with gusts up to 35 kt.

Saturday: VFR. Breezy.

Saturday Night through Sunday: VFR.

Sunday Night: Mainly VFR, with local IFR possible. Breezy.
Chance SHSN.

Monday: Mainly IFR, with areas MVFR possible. Breezy. Chance
SN, chance RA.

&&

.MARINE...
* Gale Watches issued for all waters.

Intermittent periods of isolated to scattered rain showers will
continue over the eastern waters overnight tonight. Winds will
continue to prevail out of the north/northwest tomorrow.

By tomorrow night, a strengthening pressure gradient will result
in gusty northwest winds over the coastal waters. Currently have
a Gale Watch in effect that may be upgraded to a Gale Warning in
the next forecast package. Additionally, a surge of cold arctic
air in from the north will work in tandem with the gusty winds
to create light to moderate freezing spray conditions. Currently
have a freezing spray advisory in effect from Midnight Thursday
through about 3PM Friday.

Outlook /Friday through Monday/...

Thursday Night: Moderate risk for gale force winds with gusts
up to 40 kt. Rough seas up to 11 ft. Freezing spray, slight
chance of snow showers. Areas of visibility 1 to 3 nm.

Friday: Moderate risk for gale force winds with gusts up to
40 kt. Rough seas up to 16 ft. Freezing spray, chance of snow
showers. Areas of visibility 1 to 3 nm.

Friday Night: Low risk for gale force winds with gusts up to
40 kt. Rough seas up to 16 ft. Freezing spray, slight chance of
snow showers. Areas of visibility 1 to 3 nm.

Saturday: Winds less than 25 kt. Areas of rough seas. Freezing
spray. Local visibility 1 to 3 nm.

Saturday Night: Winds less than 25 kt. Local rough seas.

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

Sunday Night: Winds less than 25 kt. Areas of seas approaching
5 ft. Chance of snow showers. Areas of visibility 1 to 3 nm.

Monday: Moderate risk for Small Craft Advisory winds with areas
of gusts up to 30 kt. Rough seas up to 12 ft. Rain likely,
chance of snow. Areas of visibility 1 to 3 nm.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Gale Watch from late Thursday night through Friday evening for
ANZ230>237-250-251-254>256.
Freezing Spray Advisory from 1 AM to 3 PM EST Friday for
ANZ231>233-235-237-250-251-254>256.

&&

$$
SYNOPSIS...RM/Nash
NEAR TERM...RM/Nash
SHORT TERM...RM
LONG TERM...Nash
AVIATION...RM/Nash
MARINE...RM
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