Area Forecast Discussion
Issued by NWS Boston, MA

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897
FXUS61 KBOX 160230
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
930 PM EST Sun Dec 15 2019

.SYNOPSIS...
High pressure will build into the region late tonight and into
Monday. A storm system expected to pass near the waters of
Southern New England will bring a round of wintry weather Monday
night into Tuesday with accumulations of snow and ice to parts
of interior Massachusetts, Connecticut into northern Rhode
Island. A major Arctic blast moves in Wednesday night,
accompanied by strong wind gusts, possible snow squalls, and
very low wind chills. Dry and cold weather Thursday into
Saturday. Low chances for snow showers Sunday.

&&

.NEAR TERM /THROUGH MONDAY/...

930 PM update...

Overnight...

A secondary pulse of 40 to 50 mph wind gusts moved through the
region earlier this evening, but it has exited the coast.
Nonetheless, steep lapse rates will continue to result in some
30 to 40 mph wind gusts for another few hours. Winds will
continue to diminish though especially after midnight as a ridge
of high pressure begins to build in from the southwest. Low
temps should be mainly in the 20s by daybreak, so quite a bit
colder than the past two.

Monday...

Ridging then settles over the region on Monday, with lightening wind
speeds and thermal advection transitioning from neutral to weak warm
advection late. We start the day mostly sunny, but sun becomes
increasingly filtered from SW to NE by afternoon and by early
evening toward mostly cloudy/OVC south coast northward to the Mass
Pike and partly to mostly cloudy for northern MA. How warm
temperatures can get before the clouds come in is pretty critical,
though most guidance shows 2-m wet bulb temperatures by sunset that
are in the low to mid 20s and near 30 towards the Cape. Kept highs
in the upper 20s to mid 30s with light W winds.

&&

.SHORT TERM /MONDAY NIGHT THROUGH TUESDAY/...
** Accumulation of snow and ice to impact travel Tuesday **

We continue to monitor developments regarding another round of
accumulating wintry weather for the very late Monday night into
Tuesday timeframe. While most areas are expected to begin as snow,
warmer air aloft should transition initial period of snow over to a
wintry mix across a good part of the interior. While confidence
continues to increase as it pertains to projected and envisioned
impact, timing the changeovers between p-types and on the
northward extent of precip remain the biggest sources of
uncertainty on the snow and ice accumulation forecasts.

Today`s guidance has trended a bit slower on onset by a few hours
and also slightly further south; thermal structure is slightly
colder as well in most of the guidance. Given the slower tendency,
this may allow enough time for temperatures to cool further with
good confidence that snow should be falling as initial p-type at
onset.

Surface wave will be moving from the northern VA Blue Ridge to
coastal SE NJ by 12z Tuesday. Clouds should be on the increase
through the first part of Monday night, though given antecedent dry
low-levels, initial precip towards central CT, coastal RI, into
coastal SE Mass doesn`t appear to start until early overnight. As
precipitation lifts northward and eventually overspreads to the Mass
Pike by the northern end of the warm nose by the pre-dawn hrs,
warmer air aloft will be transitioning the southern end of the
precip shield over to sleet and eventually freezing rain across much
of northern CT into northern/central RI by daybreak. North of the
Pike towards I-95 and Route 2, an uptick in snow intensity toward
moderate and even briefly heavy at times as southerly flow increases
with the so-called front-end thump (GFS shows a related ribbon of
850-700 mb frontogenetic forcing that builds northward during the 09-
15z timeframe from Springfield-Worcester northward into southern
VT/NH that could spur some steadier snowbands). Even though models
have slowed down the timing by a few hours, we`re still looking at
the Tuesday morning commute becoming adversely impacted by
accumulations of snow/sleet and freezing rain.

Even as precipitation is expected to continue on Tuesday, there
remains uncertainty in precipitation type due to varied handling of
thermal structure, and this is especially the case for after the AM
commute. ECMWF remains the coldest of all the guidance and would
keep accumulating snow or snow/sleet going across most areas N/W of
I-95. It`s also cold enough for mainly snow even towards the North
Shore into the Boston area. Most of the other guidance keeps mainly
snow or snow/sleet into the Berks and along/north of Route 2, with
max warm nose temps of +1 to +3C supporting sleet/freezing rain
from the Mass Pike south into northern CT/northern RI. Tough
call toward the North Shore and into the Boston area; did show
rain/snow mix by afternoon, but if the surface low keeps
shifting south and forces winds to be ageostrophically more from
N/NNW versus E/ENE, it could help to draw colder/freezing
temperatures in for a longer period of time than a more onshore
component would provide. While most of the interior shakes out
to be continued "wintry", and SE Mass and the coast will have
transitioned over to rain, colder air on the backside of the
system should result in a southeastward changeover to snow
during the late day hrs (PM commute into early Tues night)
areawide, though amounts are less than during the first part of
the day as the system pulls away into the Canadian Maritimes.

Accumulations were in general little changed from prior forecasts; I
did increase snow totals across northern MA given the FGen signal in
the GFS during the late-overnight/early Tuesday period, while also
reducing the amount of freezing rain in CT/northern-central RI by a
few hundredths (e.g. towards more sleet). We are still calling for 2-
5" for areas north of the Mass Pike and N/W of I-95 (with locally up
to a half-foot pending steadier bands, best chance in the eastern
Berks). Something in the 2-3" range with increasing freezing rain
towards a tenth near the Mass Pike to near two tenths to the
Hartford-northern Providence area. Looking at a slushy couple inches
or less toward the E/SE Mass, central/southern RI and towards the
Cape.

At the moment, this has the look of a solid Advisory event with
combo of snow/icing for a large part of the interior (thinking
mainly N/W of I-95). Greatest impacts during the Tuesday AM commute
into the Tuesday morning period; with a second round of travel
difficulties possible into the PM commute, though snow amounts here
should be more manageable. After collaboration with NWS Albany,
and with it being a mainly Tuesday (late Period 3/Period 4)
event, will hold on issuing Advisories with this package to
allow another set of model data to be re- evaluated to try to
better firm up thermal structure and p-type changeovers.

Lows Monday night into the 20s with slow rises by daybreak; highs on
Tuesday mainly in the upper 20s to near freezing N/W of I-95, lower
30s towards the North Shore into Providence and upper 30s towards
the Cape.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...
Highlights...

* Scattered snow showers late Wed afternoon into early Wed night
  with some snow squalls possible.

* Major Arctic blast comes in Wed night, accompanied by strong gusty
  winds and Wind Chill Indices from -10 to -20 degrees in central
  and western sections.

Details...

Tuesday night... Lingering light snow and rain comes to an end from
west to east by around midnight, except on outer Cape Cod, where a
mix of rain and snow could linger overnight. Lows in the 20s, except
30 to 35 Cape Cod and the Islands.

Wednesday... Skies will be partly cloudy with some widely scattered
snow showers across the higher terrain and possibly across Cape Cod
and the Islands. A major Arctic cold front will approach western
areas late in the afternoon.

Wednesday night... A major Arctic cold front will blast across the
region before midnight. it may be accompanied by a band of snow
squalls. BUFKIT soundings and modeled winds aloft show a 3 to 6 hour
window of very strong wind gusts... perhaps 40 to 50+ mph...after
the front passes. Gales likely over the coastal waters. 850 mb
temperatures will dive to about -23C across the region. Surface
temperatures will plummet to 0 to 10 above, except 10 to 15 in RI
and southeast MA. Wind Chill Indices will drop to 0 to -8 in eastern
MA, RI, and northern CT...and to -10 to -20 in central and western
MA, where Wind Chill Advisories may be needed.

Thursday through Friday...

Expecting dry conditions with the frigid air mass in place and high
pressure advancing eastward from the Ohio Valley. Highs Thursday
will remain in the teens in western areas and only reach the lower
to mid 20s in northern CT, RI, and southeast MA. It will still be
windy, especially over the coastal waters. Wind Chill Indices will
range from 5 below zero to 10 above zero all day. Bundle up!

It should be noted that the ECMWF is more pronounced with the upper
low than other models and correspondingly spins up a surface low
over the waters well east of southern New England. ECMWF ensembles
are farther east than the operational run. Will need to keep an eye
on this in case the trend is for any snow showers to back into
eastern MA as a result of that potential system.

Another super cold night is expected Thursday night. Lows again in
the single digits and teens, except could get a degree or two below
zero in Franklin County, MA and could be in the lower 20s over the
Islands. At least winds will be significantly diminished Thursday
night.

Some moderation in temperatures is expected Friday, with highs in
the 20s, with lower 30s Cape Cod and the Islands.

Saturday and Sunday... The high pressure area moves offshore
Saturday. Significant model disagreement on whether weak low
pressure develops over the region by Sunday.  For now, have
increasing cloudiness Saturday night and Sunday with a chance of
snow showers...possibly rain showers near the coast...by Sunday.
This is low confidence at this point. Still cold, with highs in the
30s both days and lows mainly in the 20s.

&&

.AVIATION /03Z MONDAY THROUGH FRIDAY/...
Forecaster Confidence Levels...

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

Short Term /through Tuesday/...

0025z TAF Update:

Through 00z Monday...High confidence.

Tonight...High confidence.

VFR conditions. W/WNW winds still gusty around 30-35 kt early,
though steady decrease in wind gusts to 20-22 kts overnight as
pressure gradient slackens.

Monday...High confidence.
VFR with lightening/easing winds as ridge builds in. Expecting
a gradual lowering/thickening of cloudiness late day.

Monday Night...Moderate confidence.

VFR to deteriorate to MVFR-IFR after 05z with snow and wintry
mix. Snow may fall at moderate-intensity or even briefly heavy
for a time by daybreak N of ORH-BAF-BED. Should see transition
from snow to wintry mix (or snow to rain towards the coast)
though only moderate confidence on timing p-type changeovers.
Winds become NE 6-10 kts.

Tuesday...Moderate confidence.

Mainly IFR to MVFR. Snow to wintry mix in the interior with cold
rain towards SE Mass, Providence into Cape Cod will eventually
transition back to light snow by Tuesday evening. Best chance of
slickened runways N/W of I-95 where wintry weather is the most
prolonged. N/NE winds become N-NW 5-10 kts.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Tuesday Night through Friday/...

Wednesday Night through Thursday: VFR. Strong winds with gusts
up to 40 kt.

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

Friday: VFR. Breezy.

Friday Night: VFR.

Saturday: Mainly VFR, with local MVFR possible.

&&

.MARINE...
Short Term /through Tuesday/...

Gale warnings continue on most waters with rough seas. Decreasing
trends to winds and seas over the next 24 hrs, with existing gale
warnings eventually to be replaced with SCAs. E/NE winds then
increase Monday night into Tuesday as a warm front approaches the
southern coast.

Tonight: W/NW winds at gales thru 3 or 4 AM across the outermost
waters, then decreasing 20 to 30 kts. Seas decreasing from 8-12
ft to 6-10 ft, highest offshore. Good visibility.

Monday: W/NW winds 10-15 kts with gusts 20-25 kts (highest
offshore/eastern) decrease to 5-10 kts. Seas 6-8 ft, decrease below
5 ft by midday to 2-4 ft levels by early Monday evening. Good
visibility.

Monday Night: Light winds early become E/NE around 10 kts with gusts
to 20 kts southern waters. Seas around 2-4 ft. Wintry mix
toward the southern waters after midnight.

Tuesday: Probable period of SCAs. E/ENE winds 10-20 kts with
gusts near 25 kts eastern offshore waters. Seas build to 5-8 ft
on southern/southeastern waters. Wintry mix to rain with
visibility 1-3 miles.

Outlook /Tuesday Night through Friday/...

Wednesday Night: Moderate risk for gale force winds with gusts
up to 40 kt. Rough seas up to 13 ft. Slight chance of snow
showers. Visibility 1 to 3 nm.

Thursday: Moderate risk for gale force winds with gusts up to
40 kt. Rough seas up to 13 ft.

Thursday Night: Moderate risk for Small Craft Advisory winds
with gusts up to 30 kt. Areas of rough seas.

Friday: Winds less than 25 kt. Local rough seas.

Friday Night: Winds less than 25 kt. Seas locally approaching
5 ft.

Saturday: Winds less than 25 kt.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Gale Warning until 4 AM EST Monday for ANZ232>235-237-250-
     254>256.
     Gale Warning until 1 AM EST Monday for ANZ230.
     Gale Warning until 5 AM EST Monday for ANZ231-251.
     Small Craft Advisory until 3 AM EST Monday for ANZ236.

&&

$$
SYNOPSIS...Loconto/GAF
NEAR TERM...Frank/Loconto
SHORT TERM...Loconto
LONG TERM...GAF
AVIATION...Loconto/GAF
MARINE...Frank/Loconto/GAF



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