Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 192147
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
447 PM EST Wed Jan 19 2022

.SYNOPSIS...
A cold front will cross the region late tonight followed by a period
of snow or rain quickly changing to snow Thursday morning, mainly
near and south of the Mass Pike. This will coincide with a portion
of the Thursday morning commute with 1 to 3 inches of snow
accumulation expected. Very cold Thursday night with subzero wind
chills in interior Southern New England. Other than some ocean
effect snow showers across the outer Cape, high pressure supports a
cold but dry Friday. An ocean storm will likely remain well to the
south on Saturday with minimal impact to southern New England.
Below normal temperatures continue into the middle of next week.

&&

.NEAR TERM /UNTIL MIDNIGHT TONIGHT/...

The peak of the wind is expected through late afternoon when
the low level jet strengthens over SE New Eng before exiting.
This is when a few gusts to 45 mph are possible over Cape
Cod/Islands and SE coastal MA. Otherwise, diminishing wind this
evening with dry conditions through midnight.

&&

.SHORT TERM /MIDNIGHT TONIGHT THROUGH THURSDAY NIGHT/...

Late Tonight into Thursday...

* Light snow accumulations expected late tonight into Thu, mainly
  near and south of HFD-ORH-BOS line

* Some impact to the Thu morning commute with rain changing to
  light to occasionally moderate snow

Fairly robust northern stream shortwave moves through the Gt Lakes
tonight with the attendant cold front moving south across SNE 06-
12z. The mid level flow amplifies enough for an anafrontal wave
developing on the boundary Thu morning. Deep moisture plume ahead of
the shortwave moves into SNE late tonight into Thu with the column
saturating which will allow a swath of precip to develop north of
the front 3-6 am. Initially, the column is warm enough for precip to
begin as rain across RI and eastern MA with mostly snow to start in
the interior. But there should be a fairly quick transition from
rain to snow from 5-7 am in RI and SE MA as colder air moves in from
the north. This transition will take a bit longer for the
Cape/Islands with the changeover occurring between 8-10 am.

The northern extent of the precip shield remains somewhat uncertain
as there will be drier air working in from the N. The greatest risk
for a period of steady snow will be south of the Pike, especially
across eastern CT, RI and SE MA. This is where the axis of greatest
moisture is located along with tightening low level thermal gradient
which may enhance snowfall a bit. Further to the N across northern
MA, drier air may win the battle with just a bit of light snow for a
few hours with little to no accum. Snow growth is favorable north of
the immediate south coast with a brief period of up to 15 units of
omega in the snow growth region Thu morning so a few hours of
moderate snow is possible. But not looking at more than 0.5-0.75"/hr
rates in the heavier snow.

QPF averages one tenth to one third of an inch from the Pike south
to the coast. Some of this QPF will be wasted on rain and initial
melting of snow across RI and SE MA, so current thinking is a
general 1-2 inches accum south of BDL-ORH-BOS but can`t rule
isolated 3 inch amounts, especially over the hills in NE CT and
western RI where mostly snow is expected with less melting as temps
will be a few degrees colder. There will be some impact to the
morning commute with reduced vsbys and possibly some slippery travel
but think most roads in the coastal plain will be wet with much of
the accum on the grassy surfaces. Temps here will be above freezing
when the snow begins and only cool to around 32F in the snow with
lack of heavy snowfall rates. There will be a greater impact in the
hilly terrain of NE CT, northern and western RI and adjacent MA
where slightly colder temps may lead to snow covered roads.

Snow will taper off and end during early afternoon from west to east
for areas south of the Pike followed by colder air filtering into
the region. Increasing sunshine is expected in northern and western
MA during the afternoon with partial clearing clearing working its
way to the south mid/late afternoon.

Thursday night...

Another surge of arctic air moves into SNE. Low temps by Sat morning
will bottom out in the single numbers with teens along the immediate
coast. Some sub zero readings are possible in NW MA. There will be a
modest north wind with wind chills zero to -10 across much of SNE
with a few locations in the higher elevation close to -15. Generally
mostly clear to partly cloudy skies expected across the region, but
the cold air over the ocean will result in decent instability with
300-400 J/kg of ocean induced Cape developing. With N trajectory,
expect scattered ocean effect snow showers developing over the outer
Cape late evening and overnight. Localized accums of a coating to an
inch are possible.

&&

.LONG TERM /FRIDAY THROUGH WEDNESDAY/...
Highlights...

* Chilly but mostly dry Friday.

* It appears increasingly likely that a coastal system looks to
  pass south of our area this weekend. Nonetheless, this can
  still change given we are still three days out.

* Could potentially turn unsettled first half of next week but
  confidence is very low in any specific outcome. Higher
  confidence in below normal temps continuing, however.

Details...

Friday into Friday Night:

Cold and dry for the most part on Friday. The only exception would
be for the Cape and Nantucket, there could be a few lingering ocean
effect snow flurries. Otherwise, a 1040 mb surface high over the
Saint Lawrence River Valley brings us a quiet weather day with
afternoon temperatures in the teens across the interior and 20s
coastal plains. For reference, average highs this time of the year
are in the mid to upper 30s. With northerly winds 10 to 15 mph,
apparent temperatures will range between -5 to +15F.

For Friday night, clouds will be on the increase after midnight
and especially east of western MA and CT. Though sub-zero lows
are possible given more clearing across portions of western MA,
lows should generally bottom out in the single digits to low
teens, except for upper teens to low 20s across Cape Cod and the
Islands.

Saturday into Saturday Night:

We continue to monitor details regarding a potential coastal or
ocean storm that appears to take shape over the Carolinas early
Saturday morning. While some uncertainty remains, and some new
snowfall is still possible across southeastern and eastern portions
of Southern New England, there is a tendency towards a less robust
outcome.

It does look like we only get a glancing blow from the coastal storm
with it tracking well south of the 70W/40N benchmark. As a reminder,
a storm tracking close to the benchmark increases the potential for
accumulating snowfall along the Boston-Providence corridor. So a
reasonable scenario at this time is for some minor snowfall
across southeastern New England, with a shut out further north
and west. This does make sense given the antecedent strong and
very dry high pressure over northern New England. PWATs are
indeed unimpressive, with values between 0.5 and 2.5th
percentile on Saturday and only up to 10th percentile on Sunday
according to NAEFS situational awareness table. Overall, this
favors a suppressed solution with the bulk of the precipitation
passing to our south, or a Mid Atlantic seaboard special.
Nonetheless, snow lovers should not be too downbeat at this time
given we are still 72 hours out and have to see how the Thursday
system evolves.

Sunday into Wednesday:

Above average confidence in below normal temperatures in this
forecast period. Initially dominant northern stream with a
prominent longwave trough developing across most of north-
central into eastern North America, with anomalous upper ridge
over the West Coast or just offshore. This should support
periodic, reinforcing shots of colder air supporting below
average temperatures. Again for reference, normal highs are in
the mid to upper 30s.

As for meaningful weather, there is very little agreement
amongst model guidance. The CPC 6 to 10 day precipitation
outlook shows an equal chance of below and above normal precip
for our area, which reflects the mixed teleconnection signals
of a neutral PNA and positive AO/NAO pattern. There is potential
for bouts of unsettled weather, but whether that could help eat
into our snowfall deficit for the season (currently all four
climate sites are running 6 to 10 inches below normal) remains
to be seen. So largely stuck to blended model guidance for this
time period.

&&

.AVIATION /22Z WEDNESDAY THROUGH MONDAY/...
Low - less than 30 percent.
Medium - 30 to 60 percent.
High - greater than 60 percent.

18z Update:

Through 00z...High confidence.

VFR. SW wind gusts 25 to 35 knots with a few gusts up to 40
knots along the far southeast New England coast.

Tonight...High confidence.

VFR this evening with diminishing wind, then MVFR/IFR
conditions developing 08-12z mainly along and south of BDL-ORH-
BOS in brief rain changing to snow.

Thursday...High confidence.

IFR in snow south of the Pike with rain changing to snow
Cape/Islands. Snow ends during early to mid afternoon across
RI/SE MA followed by improving conditions. Accums of 1-3
inches possible south of the Pike. Conditions improving to VFR
north of the Pike as any light snow ends early.

Thursday night...High confidence.

Mainly VFR, but areas of MVFR cigs developing late evening and
overnight over outer Cape and ACK with scattered ocean effect
snow showers.

KBOS Terminal...High confidence in TAF. Greatest risk for a
period of accumulating snow will be 11-16z. A coating to 2
inches possible.

KBDL Terminal...High confidence in TAF. A period of light snow
may bring a coating to an inch 09-14z.

Outlook /Friday through Monday/...

Friday through Friday Night: VFR. Breezy.

Saturday: Mainly VFR, with areas IFR possible. Breezy. Slight
chance SN.

Saturday Night: VFR.

Sunday: Mainly VFR, with local IFR possible.

&&

.MARINE...
Forecaster Confidence Levels.

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

Tonight...High confidence. Gale force SW winds will linger into this
evening before diminishing overnight.

Thu into Thu night...High confidence. Winds shift to NW with gusts
to 20 kt Thu increasing to 25 kt Thu night. Vsbys reduced in rain
changing to snow Thu with improving conditions by late Thu. Areas of
freezing spray developing Thu night with scattered snow showers over
waters adjacent and east of Cape Cod.

Outlook /Friday through Monday/...

Friday: Winds less than 25 kt. Areas of rough seas. Chance of
freezing spray, slight chance of snow.

Friday Night: Low risk for Small Craft Advisory winds with
gusts up to 20 kt. Areas of rough seas. Chance of freezing
spray, slight chance of snow. Local visibility 1 to 3 nm.

Saturday: Low risk for Small Craft Advisory winds with gusts up
to 20 kt. Rough seas up to 8 ft. Chance of snow. Visibility
1 to 3 nm.

Saturday Night: Winds less than 25 kt. Areas of rough seas.
Slight chance of snow. Areas of visibility 1 to 3 nm.

Sunday: Winds less than 25 kt. Areas of rough seas. Visibility
1 to 3 nm.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...Wind Advisory until 6 PM EST this evening for MAZ019-021>024.
RI...Wind Advisory until 6 PM EST this evening for RIZ008.
MARINE...Gale Warning until 10 PM EST this evening for ANZ231>235-237-
     250-251-254>256.
     Gale Warning until 7 PM EST this evening for ANZ230-236.

&&

$$
SYNOPSIS...KJC/Chai
NEAR TERM...KJC/Chai
SHORT TERM...KJC
LONG TERM...Chai
AVIATION...KJC/Chai
MARINE...KJC/Chai


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