Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 110904
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
404 AM EST Mon Dec 11 2017

.SYNOPSIS...
Mainly dry today after early showers or snow showers. A mixed
wintry precipitation event begins early tomorrow morning and
continues into tomorrow evening, with many areas changing to all
rain. A shot of arctic air follows Wednesday and Thursday with
a period of strong winds and bitterly cold wind chills. Another
low pressure system may bring more snow to the region sometime
in the Thursday night to Friday night timeframe, if it tracks
close enough to the coast. Somewhat milder weather will arrive
by the end of the upcoming weekend.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
WV imagery suggests upper lvl wave is sliding through PA/NY at
this time, and should push across New England during the mid
morning hours. Dwpts and soundings suggest a slight increase in
area moisture this morning, which has led to light echos on
radar falling out of a mid-upper deck of SC. POPs associated
with the frontal passage will be focused mainly across SE MA/RI
and offshore, where moisture peaks and there is an every so
slight increase in lower lvl instability which would enhance
precip potential. Elsewhere, have highlighted a risk for
flurries or sprinkles based on sfc temps until the wave fully
clears the region. Any accums of either liquid or snow would be
light.

After the passage, SC clears as a weak ridge of high pres moves
through the area. In spite of an increase in lower lvl mixing,
the lingering snowpack will keep highs mainly in the mid and
upper 30s. Raw 2m temps were best in this case, so leaned most
heavily on these numbers.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/...

*** Mixed Wintry Precip Event Late Tonight into Tomorrow ***

Setup...
Dual, unphased shortwaves will generate double barreled low
pres situation late Mon night into Tue. The leading wave is
currently sliding from Saskatchewan into the N Plains, while the
secondary wave slides out of Manitoba into N Ontario. The fact
that these remain unphased, allows the initial parent low pres,
developing the great lakes to transfer energy to a secondary
warm frontal low pres which will slide across central New
England or along Gulf of Maine. In either case, developing 40+
kt H92 LLJ will push fully into S New England Tue morning,
allowing a nose of warm air between 975-900hPa. This allows the
+2C isotherm to reach near the NH/MA border, using an average of
available guidance, with slightly cooler temps along the MA/VT
border. Given this, precip onset within overrunning ahead of the
warm front should start as mainly SN, but then gradually give
way to liquid precipitation from SE to NW, with a low risk for
mainly SN especially extreme NW MA and into the Berkshires.
Thermal profiles are tricky, with more on that below. Latest
trends are also slower as the unphased wave takes on a negative
tilt and low dwpt depressions take time to overcome tonight,
suggesting overall lower QPF than previous runs.

Thermal profiles...
While soundings mainly H85 and above remain below 0C, the nose
of warm air associated with the LLJ falls mainly in the lower
100hPa. Mean temps in this layer rise above 0C to about route
+2C between 12Z and 15Z Tue. Given the dwpt depressions, wet-
bulbing would suggest light SN at precip onset, Sfc temps are
likely to warm slower, thanks to a lingering snowpack. Areas
closest to coastlines could start as a frozen mix.. Not a
classic damming signature, but low lvl mass fields do show some
weak inverted ridging and mesoscale guidance does have light
drainage flow particularly in the CT valley and Worcester Hills.
In any case, this is a typical scenario where guidance warms
the sfc/2m temps too quickly. The transition to all liquid
should generally occur no later than mid day, except in the
higher terrain of NW MA, which could remain cool enough for
SN/frozen precip.

QPF/Snowfall/Ice...
As mentioned above, thanks to a dry column initially overnight,
timing of precip onset have slowed and the resulting overall
QPF is lower, totaling around 0.30-0.50 Given this, there may
be less SN at onset with some areas starting as RA/FZRA. The
warm influence also favors lower ratio snows, therefore, have
lowered expected snowfall through. The FZRA/SN thinking with
this update is backed up by latest WPC Day 2 data. Ice accums
would be light as the warm air should translate to the sfc
mainly a tenth or so at best, as sfc temps are not expected to
be marginal, and not in the 20s, in which ice accumulation is
more efficient. Snowfall totals are now lighter, with the best
chance for advisory lvl snowfall in the high terrain of NW MA,
where the low lvl warm air will be slowest to arrive, if it even
ever shifts above 0C. 3-6 inches is the latest forecast for NW
MA but with another set of guidance still to come, these are
still likely to change a bit.

Timing/impact...
Precip, likely mainly in the form of light snow, gradually
transitioning to a mix of FZRA and RA begins mainly after 1AM
from W to E during the early morning hours Tue, with widespread
precipitation likely between 7AM and 1PM, and continuing to
change form wintry precip to then gradually ending from S-N
mainly after 3PM through 7PM. The Tue AM commute has the highest
risk for impact, as SN or even a wintry mix could impact
portions of the commute as the warm air build in aloft.
Untreated sfc could become slippery thanks to a light SN accums
and/or light icing. By the afternoon commute, precip still
occurring, but mainly rain outside of the higher terrain of MA
particularly.

With all these factors, will hoist Winter Weather Advisories
for portions of western and central Massachusetts, where
confidence is highest in snowfall totals in the realm of
advisory levels, and the best chance for cold air damming
supporting some ice accumulations. These may need to be expanded
based on new data today.

&&

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

* Low risk for a few brief snow squalls late Tue night/Wed
* Arctic air Wed into Thu w/strong winds and bitter cold wind chills
* Period of snow possible sometime in the Thu night-Fri night period
* Milder weather by the end of the weekend

Details...

Tuesday night through Thursday..

An arctic cold front will move across the region Tue night/Wed
accompanied with impressive shortwave energy.  A few brief passing
snow showers are likely to impact the region with the arctic front.
The bigger question is do we see a few localized intense snow
squalls. The shortwave energy is pretty dynamic with some steep 1000
to 700 mb lapse rates.  However, moisture is the wild card and it is
usually a limiting factor for this activity making it over the
Berks.  We do think there is at least a low risk for a couple.

The other issue will be a period of strong winds.  Bufkit indicates
very deep mixing with steep lapse rates and an impressive 850 mb
westerly jet. This will bring the potential for westerly wind gusts
of 40 to 50 mph Wed into Wed night and wind headlines may eventually
be requited.

Lastly...very cold air will be ushered into the region with highs
Wed and Thu mainly in the 20s to around 30.  Lows Wed night/early
Thu am should be well down into the teens with some single digit
readings possible across western MA.  This should result in wind
chill values between 5 above and 5 below zero.

Thursday night into Friday night..

This portion of the forecast remains uncertain.  Another piece of
shortwave energy swings down from the northwest and allows for an
areas of low pressure to develop over the ocean.  The ensembles
differ considerably as to where this low develops.  Some solutions
depicted are fairly close to our region, while others are further
south and east.  This will determine if we see a period of snow
sometime in the Thursday night to Friday night time range or
precipitation remains out over the ocean. Will just have to wait
this one out and see how models trend over the next few days.

Saturday and Sunday...

The upper trough lifts out of our region allowing more west to
southwest flow aloft.  If guidance stays the course most of the
weekend will feature mainly dry weather with moderating temperatures
by Sunday.

&&

.AVIATION /09Z 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/...

Through today...High confidence.
Mainly VFR except brief MVFR CIGS mainly across Cape/Island
terminals this morning. Occasional very light SHSN/SHRA through
mid day.  W winds gradually shift to the SW.

Tonight...Moderate confidence.
VFR to start, will give way to MVFR then IFR early AM Tue as
CIGS lower and light SN/FZRA moves in mainly after 06Z from W-E.
SN accums would be light. Winds shift around to the E-NE.

Tomorrow...Moderate confidence.
Mixed MVFR and occasional IFR conditions in a mix of SN/FZRA
giving way to all RA through sunrise into mid day Tue. Risk for
NW MA to remain SN long enough for accums to exceed reach 3-6
inches with generally less further SE. Low risk also for a
period of FZRA before the change to RA. Conditions improve after
20Z from S-N.

KBOS Terminal...High confidence in TAF, lower categories don`t
move in until after 06Z tonight.

KBDL Terminal...High confidence in TAF.

Outlook /Tuesday Night through Friday/...

Tuesday Night: MVFR/IFR conditions possible. Windy with local
gusts to 30 kt. Chance SHSN, slight chance SHRA.

Wednesday: MVFR/IFR conditions possible. Strong winds with
gusts to 45 kt. Chance SHSN.

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

Thursday: VFR. Windy with areas gusts to 30 kt.

Thursday Night: Mainly VFR, with areas IFR possible. Breezy.
Chance SN, slight chance SHSN.

Friday: MVFR/IFR conditions possible. Breezy. Chance SN.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Tuesday/...

Today and Tonight...High confidence.
Winds will continue to diminish through the day today, and seas
will also recede. Current Small Craft Advisories will be able to
be dropped through the day, with mainly quiet boating weather
thereafter.

Tomorrow...Moderate confidence.
Winds gradually shift around to the E. Gusts 25-30 kt expected
on the S waters with 35-40 kt possible on the E waters. Gale
Watches will be issued, with the need for small craft advisories
where Gales are not observed. Otherwise, rainy conditions
expected on the waters.

Outlook /Tuesday Night through Friday/...

Tuesday Night: Moderate risk for Small Craft Advisory winds
with gusts up to 30 kt. Rough seas up to 12 ft. Chance of rain
showers, chance of snow showers. Local visibility 1 to 3 nm.

Wednesday: Moderate risk for gale force winds with gusts up to
45 kt. Rough seas up to 15 ft. Chance of snow showers.
Visibility 1 to 3 nm.

Wednesday Night: Moderate risk for gale force winds with gusts
up to 40 kt. Rough seas up to 16 ft. Local visibility 1 to
3 nm.

Thursday: Moderate risk for Small Craft Advisory winds with
gusts up to 30 kt. Rough seas up to 12 ft.

Thursday Night: Winds less than 25 kt. Areas of seas
approaching 5 ft. Chance of snow, slight chance of snow
showers. Visibility 1 to 3 nm.

Friday: Winds less than 25 kt. Areas of seas approaching 5 ft.
Chance of snow, chance of rain. Visibility 1 to 3 nm.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...Winter Weather Advisory from 4 AM to 4 PM EST Tuesday for
     MAZ002>004-008>012-026.
RI...None.
MARINE...Small Craft Advisory until 1 PM EST this afternoon for
     ANZ231>234-250.
     Small Craft Advisory until 6 PM EST this evening for ANZ235-
     237.
     Gale Watch from Tuesday morning through Tuesday evening for
     ANZ250-251-254.
     Small Craft Advisory until midnight EST tonight for ANZ254>256.

&&

$$
SYNOPSIS...Frank/Doody
NEAR TERM...Doody
SHORT TERM...Doody
LONG TERM...Frank
AVIATION...Frank/Doody
MARINE...Frank/Doody


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