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000
FXUS61 KBOX 191923
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
223 PM EST Mon Nov 19 2018

.SYNOPSIS...

*/ Near- and short-term forecast updates only...

Developing storm center tonight through Tuesday night will bring
accumulating snows mainly along and N of the MA-Turnpike, impacting
the Tuesday AM commute for some. Otherwise an all rain or rain/snow
mix. Turning cooler and drier behind the storm, breezy, as an Arctic
front sweeps across the region late Wednesday followed by near record
cold and bitterly cold wind chills Wed night through Thanksgiving
Day. Dry weather with less wind and moderating temperatures Friday
and Saturday. However wintry weather may return to Southern New
England Saturday night into Sunday.

&&

.NEAR TERM /UNTIL MIDNIGHT TONIGHT/...

230 pm update...

Break in the action. Between the waves, quiet conditions prevailing.
But precipitation moving back in during the evening hours, becoming
widespread towards midnight. Overall light to start, and we`ve got
some warmer air to overcome out ahead. Anticipating a start to
snow for the high terrain whereas as lower elevations around the
Berkshires and Worcester Hills it`ll start off as a rain / snow
mix prior to the lower column cooling and snow prevailing. N winds
will be driving through this forecast period, no longer light and
variable, and likely wet-bulbing will prevail. Most of the impacts
along and N of the MA-pike, can`t rule out some travel hazard
conditions developing especially in the way of reduced visibility.

&&

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

Tonight through Tuesday...

Strengthening storm system across New England bringing accumulating
around 3-inch snows mostly N of the MA Turnpike, especially the high
terrain. Tuesday AM travel impacts with reduced visibility and snow
on area roadways for those areas noted.

Synoptically. Initial low sweeps New England with maturation further
downstream. Mainly open wave, not so much closed H925-85, tracking
over interior New England around the MA / VT / NH border, low level
convergent focus of the anti-cyclonic warm-moist conveyor belt out
ahead of a digging, sharpening mid-level trof axis and accompanying
vortmax energy. Initiated low to mid level thermal packing as well
as deformation / frontogenesis beneath mid-level ascent and initial
diffluence aloft, majority low-level forcing up thru the dendritic
growth zone, H6-7, so not impressively deep. Omega values through
the column weak, especially through the dendritic growth zone which
happens to exhibit some fairly steep lapse rates. Overall, a decent
front-end thump leading out ahead of the surface to H85 low. The
storm becomes better organized through Tuesday as it exits into the
Gulf of Maine having swept across the Cape Cod Canal around 1 pm.

Challenges. Thermal fields / thickness tell the story. Right on the
freezing cusp with deeper Arctic air absence N, hardly a N/NE high
over Canada. While N isallobaric flow, it`ll only drain S surface
temperatures ranging around 30 with dewpoints a tad lower such that
wet-bulbing can only do so much. Low-level column nearly isothermal
up through H8, warmer air intruding as the H925-85 flow flutters
along the MA / NH / VT border, possibly some drier air working in
through the dendritic growth zone, while fluttering around freezing
as the event evolves into the daylight hours, dealing with forecast
headaches concerning precipitation types and snowfall accumulations
as well as snowfall intensity. Speaking of, as model forecast mass
fields look pretty similar, small-scale focusing forcing mechanisms
vary as discerned via model forecast plume diagrams. Considerable
spread in respect to liquid-equivalent precipitation and snowfall,
the latter ranging from sub-advisory to warning levels in particular
locations. Relates back to snowfall intensity, some low probability
of 1"/hr noted in high-res guidance, and thermal field alignment as
to where the rain / snow cut-off line will be at any one particular
time. Forcing majority is, however, below the dendritic growth zone
with this synoptic setup. Of last note, model forecasts assume that
everything which falls accumulates on the ground which we know is
not always the case. So it`s definitely not easy but we`re going to
do our best as we always do.

Decisions. Despite uncertainties and challenges noted, a consensus
weighted forecast does not seem like a bad idea. Gave greater weight
towards the HREF blended within. With probabilities noted, and the
19.12z forecast suite slightly jogging S, focus is around the MA /
NH / VT border. Coating N of the MA-pike, with 1-3" lower elevations
while 3-6" higher, 500 feet and above especially. Potentially near
warning-level snows up around the N MA towns of Ashburnham, Ashby,
and Townsend. Keep with present N/W MA WINTER WEATHER ADVISORIES.
The storm center sweeping NE across the Cape Cod Canal important
with respect to blustery N/NE winds out ahead.

Impacts. Expect hazardous travel over N/W MA especially the higher
terrain of Worcester Hills and Berkshires. Route 2, western-half of
MA I-90, N MA I-95, I-190 out of Worcester, and the NW beltway of
495 all have the risk of accumulating snows on roadways. Reduced
visibility as well especially within any moderate to heavy snow as
near 1"/hr snowfall rates are possible. Thinking a very low risk of
any mixed wintry precip-types mainly in the form of freezing rain.

Tuesday night...

Turning colder and drier. The storm continuing to deepen into the
Gulf of ME, blustery NW winds follow. Some lingering snow showers
possible, especially for the high terrain. Otherwise the main storm
is the driving cold along with breezy conditions that`ll make it
feel 5 to 10 degrees colder than the forecast nighttime lows around
the low to mid 20s, possibly upper teens for the high terrain.

&&

.LONG TERM /WEDNESDAY THROUGH SUNDAY/...

4 AM update...

Highlights...

* Arctic front late Wed with snow squalls possible
* Near record cold & bitterly cold wind chills Wed night/Thanksgiving
* Moderating temps and dry Friday into Saturday
* Wintry weather possible Sat night into Sunday

Wednesday...

Arctic front moves thru the region 18z-00z. Models differ on track
of arctic frontal wave. The NAM is farthest to the south across the
NH/MA border, while the remainder of the guidance is across northern
New England. The NAM solution would yield more frontal convergence
into northern MA resulting in a greater risk of snow squalls. While
reminder of guidance keeps greater risk across northern New England.
Still almost 72 hrs out so will have to see how later model runs
trend. NAM solution supports steep low level 0-2 km theta-E lapse
rates and plenty of low level moisture for instability to be
realized. This combined with shallow robust convergence would
support potential for a few strong snow squalls. Given very busy
holiday travel Wed will have to watch this closely. Regarding high
temps, 925 mb temps peak around -4C ahead of the arctic front, so
could see highs 35-40 before the frontal passage.

Wednesday night and Thanksgiving...

Near record cold and accompanied by bitterly cold wind chills as
arctic airmass overspreads the region. By 12z Thu model guid has 850
temps down to -20C to -22C across the region! These values are -3
standard deviations colder than climo. 1000-500 thicknesses are
impressive as well, with 498 dam contour traversing northern MA!
Given the magnitude of arctic airmass we followed by the coldest
guid which yields mins just before sunrise Thu of 5 to 15 followed
by temps only rebounding into the upper teens and low 20s
Thanksgiving Day. In addition strong northwest winds will provide
bitterly cold wind chills.  For outer Cape Cod, arctic air streaming
in on NNW winds will resulting in possible ocean effect snow showers
with the greatest probability from PVC to CQX given wind trajectory.

Friday...

Cold anomalous trough moves offshore with arctic 1037 mb high
settling over the region. Very cold morning with lows in the single
digits and teens! Temp moderate into the upper 20s and lower 30s.
Thus not as cold as Thu and especially less wind. However still much
colder than normal as normal highs should be around 50! Nevertheless
much more tolerable than Thu.

Next Weekend...

Dry weather possibly lingers into much of Sat. However active
weather pattern with next trough approaching the region Sat night
into Sunday. Its a southern stream system so abundant moisture is
available. Given cold air from Thu/Fri retreats looks warm enough
for rain however still 6-7 days away. Nonetheless heavy rain
potential that would impact heavy travel day Sunday.

&&

.AVIATION /19Z MONDAY THROUGH SATURDAY/...

Forecaster Confidence Levels...

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

Short Term /through Tuesday Night/...High Confidence.

Rest of today...
Mix MVFR to low-end VFR. Will hold IFR for the high terrain and
closer to the MA / NH / VT border with respect to CIGs and VSBYs.
Light winds overall. Pockets -FZRA for high terrain terminals
through 16z, otherwise -RA lingering through early afternoon
then a break before the next storm into this evening.

Tonight and Tuesday...

A mix of MVFR-IFR to start except SE New England terminals,
conditions lowering with onset RA/SN. SN for N/W MA terminals
especially over the high terrain. RA/SN mix around the MA-pike
while rain elsewhere or transitioning to rain. VSBYs more than
likely impacted by SN. Can`t rule out IFR-LIFR conditions. N
winds prevailing, stronger NE along the E MA coastline as a
storm center passes over the Cape Cod Canal around 18z. Gusts up
around 25 kts possible for E coastline terminals.

Tuesday night...

Winds shifting NW as conditions improve towards VFR, the 18z
storm center lifting out into the Gulf of ME. Gusts up to around
25 kts possible especially over the high terrain and along
coastline terminals.

KBOS Terminal...
A break but CIGs only lifting 2-3 kft with a lull in outcomes from
earlier -RA. Then looking at lowering conditions evening and over-
night with SNRA mix through early morning before changing over to
all RA. MVFR-IFR CIGs and VSBYs with increasing N/NE winds through
the day shifting NW into evening and overnight. Gusts potentially
as high as 25 kts with NE winds around 18z before turning NW and
remaining blustery.

KBDL Terminal...
Keep MVFR prevailing. Then lower conditions moving back in towards
evening continuing through overnight. RASN mix right through the
Tuesday morning push, becoming all rain towards midday. Mostly IFR,
improving MVFR to VFR late Tuesday into Tuesday evening. N winds
throughout shifting NW late. Somewhat breezy, especially late with
potential gusts up to 20 kts.

Outlook /Wednesday through Friday/...

Wednesday: Mainly VFR, with local IFR possible. Windy with
gusts to 30 kt. Slight chance SHSN.

Wednesday Night through Thanksgiving Day: VFR. Windy with gusts
to 35 kt.

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

Friday: VFR.

&&

.MARINE...

Forecaster Confidence Levels...

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

Short Term /through Tuesday Night/...High confidence.

Storm center passing over the Cape Cod Canal around 18z Tuesday.
Will see increasing E/NE winds out ahead especially over the E
waters, S/SW for the S waters. Behind the storm center, winds
turning NW, remaining blustery. Throughout, gusts up around 25
kts possible. Can`t rule out near gale force gusts. SMALL CRAFT
HEADLINES in effect for the later-half of Tuesday going into
Tuesday night as seas build up around 5 to 7 feet on the outer
waters.

Outlook /Wednesday through Friday/...

Wednesday: Low risk for Small Craft Advisory winds with gusts
up to 30 kt. Seas up to 5 ft. Slight chance of rain showers.

Wednesday Night: Low risk for gale force winds with gusts up to
35 kt. Areas of rough seas. Slight chance of snow showers.
Areas of visibility 1 to 3 nm.

Thanksgiving Day: Low risk for gale force winds with gusts up
to 35 kt. Areas of rough seas. Slight chance of snow showers.
Visibility 1 to 3 nm.

Thursday Night: Moderate risk for Small Craft Advisory winds
with gusts up to 30 kt. Local rough seas. Local visibility 1 to
3 nm.

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

&&

.CLIMATE...

With the potential of the coldest air of the season moving into
the region around Thanksgiving, here are the record temperatures
for November 22nd and the holiday of Thanksgiving.

November 22nd
Location / Record Low / Record Minimum High

Boston..........9 (1879) / 24 (1880)
Hartford.......14 (1969) / 27 (1978)
Providence.....16 (1987) / 30 (2008)
Worcester......11 (1987) / 24 (2008)

Thanksgiving
Location / Record Low / Record Minimum High

Boston.........11 (Nov 27, 1873) / 24 (Nov 28, 1901)
Hartford.......12 (Nov 28, 2002) / 27 (Nov 23, 1989)
Providence.....14 (Nov 23, 1972) / 30 (Nov 28, 1996)
Worcester.......9 (Nov 23, 1989) / 22 (Nov 23, 1989)

&&

.BOX WATCHES/WARNINGS/ADVISORIES...

CT...None.
MA...Winter Weather Advisory from 10 PM this evening to 5 PM EST
     Tuesday for MAZ002>004-008-009-026.
RI...None.
MARINE...Small Craft Advisory from 4 PM Tuesday to 4 AM EST Wednesday
     for ANZ232>234.
     Small Craft Advisory from 1 PM Tuesday to 1 AM EST Wednesday
     for ANZ231-251.
     Small Craft Advisory from 4 PM Tuesday to 6 AM EST Wednesday
     for ANZ235-237.
     Small Craft Advisory from 10 AM Tuesday to 6 AM EST Wednesday
     for ANZ250.
     Small Craft Advisory from 1 PM Tuesday to 6 AM EST Wednesday
     for ANZ254>256.

&&

$$

SYNOPSIS...Nocera/Sipprell
NEAR TERM...Nocera/Sipprell
SHORT TERM...Sipprell
LONG TERM...Nocera
AVIATION...Nocera/Sipprell
MARINE...Nocera/Sipprell
CLIMATE...Sipprell