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674
FXUS61 KBOX 171556
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
1056 AM EST Fri Jan 17 2025

.SYNOPSIS...
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Plenty of sunshine with temperatures moderating to seasonable
levels this afternoon. An approaching cold front will result
in a southerly flow of even milder air for Saturday with a
period of rain showers Saturday afternoon into early evening.
Dry weather should prevail for the first part of Sunday behind
the cold front. A wave of low pressure will ride up and along
the front and likely bring an accumulating/plowable snowfall
later Sunday and especially Sunday night. This will be followed
by an arctic outbreak later Monday into Wednesday with Wind
Chills likely dropping to between 10 and 15 below zero.
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&&

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

* Warmer with high temperatures around normal

Not much to change with this update. Did bring the forecast
back in line with observed trends. Sunny across most of southern
New England, with some ocean-effect clouds lingering across
portions of the Cape and islands.

Previous Discussion...

Mid-level ridging and weak high pressure will support a dry day
across southern New England. Light NW winds this morning will
transition to SW in the afternoon as the surface high pushes
offshore. Temperatures aloft warm a few degrees from yesterday
supporting more seasonable high temperatures in the mid 30s to
near 40. Most areas will experience minimal cloud cover with the
exception of the Cape and islands where there will be lingering
clouds through early afternoon. Can`t rule out a stray snow
shower or flurry across the outer Cape today, but otherwise it
will be dry today.
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&&

.SHORT TERM /6 PM THIS EVENING THROUGH SATURDAY NIGHT/...
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Key Messages:

* Above normal temperatures and breezy conditions Saturday.

* Rain showers developing in the afternoon, becoming more
  widespread by the evening.

Tonight:

A weak mid-level shortwave approaches from the west which will
bring our next chance of precipitation Saturday. Winds from the
SW become more enhanced overnight as the pressure gradient
strengthens. Clouds increase ahead of a weak warm front. With
the cooler airmass exited and increased cloud cover, tonight
will trend warmer than the previous couple of nights. Low
temperatures range in the low to mid 20s across the interior and
mid to upper 20s closer to the coastline.

Saturday:

A weak shortwave trough and cold front approaches the region
Saturday. Moisture gradually increases into the afternoon with
precipitable water values around 150-200% of normal by early
evening. This wave will bring a broad area of ascent supporting
our next chance of precipitation. Showers begin to develop in
the afternoon from west to east, becoming more widespread mid-
afternoon to early evening.

There will will be a good amount of WAA advection ahead of this
system with 850mb temperatures warming 5-8C from Friday
bringing highs into the 40s, potentially 50 in spots. This will
put us around 6-12 degrees warmer than normal. That means this
will fall as rain with the exception of the Berkshires which
will see light snow. It will also be breezy Saturday with a low
level jet/enhanced gradient positioned across the region. No
impactful winds are expected, but gusts 20-25 mph are possible.
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&&

.LONG TERM /SUNDAY THROUGH THURSDAY/...
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Key Messages...

* Rain showers depart the region early Sat evening
* Plowable snowfall likely late Sun/Sun night ending by Mon AM
* Arctic Outbreak Mon/Tue/Wed with Wind Chills -10 to -15 below zero
* Actual Lows Mon night/Tue night single digits with some below zero
* Actual Highs Tue & Wed only in the middle teens to the lower 20s

Details...

Saturday night...

The associated shortwave/cold front will be pushing east of the
region Sat evening taking the deeper moisture/forcing with it.
So lingering showers will be exiting the region from west to
east during the first part of Sat evening. Otherwise, a drier
northwest flow of air will be working into the region. We
expect partial clearing...but the airmass initially behind this
front is not that cold. Low temps Sat night should only bottom
out in the upper 20s to the middle 30s.

Sunday and Sunday night...

The first part of Sunday should feature dry weather and probably
will see some partial sunshine in the morning. Given the airmass
behind the initial front is not that cold, high temps should
top off in the upper 30s to the middle 40s.

The main concern with this forecast revolves around the
increasing confidence in an accumulating/plowable snow event
later Sun and especially Sun night across the region. That being
said, there is still considerable spread amongst the guidance
and this forecast is not set in stone. The biggest question is
how far northwest an anafrontal wave tracks to the coast. The
00z NAM is the eastern outlier with a shutout across southern
New England as a weak wave of low pressure tracks well southeast
of the Benchmark. Meanwhile...the 00z RGEM continues to be the
western outlier with a significantly stronger low pressure
system that tracks much further northwest across southeast MA.
This solution is so far west that Ptype would mainly be rain
across the coastal plain with most of the snow across the
interior.

Given the considerable differences between the NAM/RGEM, there
remains plenty of uncertainty on the exact outcome. While no
solutions can be ruled out at this point...we are favoring the
GEFS/EPS Ensembles along with the UKMET that support a middle
ground with the wave of low pressure tracking near the
Benchmark. This solution would bring a period of
accumulating/plowable snowfall which may start as a brief period
of rain. A very preliminary snowfall forecast indicates that
2-6" of snow is possible across a good portion of the region,
which is supported by the latest NBM model. That is the most
likely solution based on the current data...but given that there
still is a large spread in the models this is not set in stone.
There still is a low risk for little if any snowfall if the
storm track trends east like the NAM or a far western solution
like the RGEM...which would support mainly rain on the coastal
plain. There also would be a low probability for a swath of over
6"+ of snow with a stronger low pressure system tracking just
inside the Benchmark. But right now odds favoring more on the
order of a 2-6" type of snow for most locations...but probably
less towards the Cape/Islands where Ptype issues are more
likely.

Monday through Wednesday...

The good news is that the steady snow will pretty much be over
by daybreak Mon. Otherwise, the main story will be an arctic
outbreak and likely the coldest weather since early February
2023! The arctic airmass begins to work into the region Mon on
blustery NW winds. Highs on Mon probably mainly in the lower to
middle 20s with the bitterly cold air works into the region late
Mon/Mon night and continuing into Wed. Overnight low temps will
be in the single digits Mon night and Tue night with some below
zero readings in the outlying locations...especially Tue night
when winds start to diminish. High temps Tue and Wed will only
be in the middle teens to the lower 20s. Blustery Northwest
winds will probably drop Wind Chills into the -10 to -15 below
zero. Cold Weather Advisories will likely be needed.

Thursday...

The core of the coldest air will begin to depart, but thinking
high temps will still be quite cold in the 20s to near 30.
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&&

.AVIATION /16Z FRIDAY THROUGH TUESDAY/...
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Forecast Confidence Levels:

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

Today...High Confidence.

Lingering MVFR conditions towards the Cape/Islands should
gradually improve to VFR this afternoon. VFR elsewhere. NW
winds 5-10 knots shift to SW after 20Z.

Tonight...High Confidence.

VFR. Light/Calm winds early this evening become S later tonight
at speeds of 10 knots or less.

Saturday...Moderate Confidence.

VFR conditions through mid morning or so gradually deteriorate
to MVFR thresholds in the afternoon and early evening.
Localized IFR conditions possible by early evening. Rain
showers will also overspread the region during the afternoon
hours from west to east. A few scattered showers are possible
after 15z. S winds of 10 to 15 knots with gusts around 25
knots.

KBOS TAF...High Confidence in TAF.

KBDL TAF...High Confidence in TAF.

Outlook /Sunday through Tuesday/...

Saturday Night: Mainly VFR, with local IFR possible. Breezy.
Slight chance SHRA, patchy BR.

Sunday: Mainly VFR, with local MVFR possible. Breezy. Chance
SN.

Sunday Night: Mainly IFR, with local MVFR possible. Breezy. SN
likely.

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

.MARINE...
Forecaster Confidence Levels:

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

High confidence.

Generally light winds (15 kt and less) and seas are expected
through Friday night. A system will bring a round of rain
Saturday afternoon and evening. Seas approach 5 ft for the
outer waters by early afternoon Saturday with winds 25-30 kts.
This will be short-lived with winds/seas decreasing overnight
into Sunday.

Outlook /Sunday through Tuesday/...

Saturday Night: Winds less than 25 kt. Areas of seas
approaching 5 ft. Chance of rain showers, patchy fog.

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

Sunday Night: Moderate risk for Small Craft Advisory winds with
gusts up to 25 kt. Seas up to 5 ft. Rain, chance of snow.
Visibility 1 to 3 nm.

Martin Luther King Jr Day through Monday Night: Moderate risk
for Small Craft Advisory winds with gusts up to 25 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,
slight chance of snow showers.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...None.

&&

$$

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