Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 040254
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
954 PM EST Sat Dec 3 2016

.SYNOPSIS...
High pressure builds across the area allowing for clear
conditions and light winds Sunday. A period of light accumulating
snow is possible Monday morning into early afternoon before ending
as light rain. High pressure returns Monday night with mainly dry
conditions into Tuesday. Low pressure off the mid Atlantic coast
may bring some mixed light rain and/or snow into southern New
England Tuesday night into Wednesday night, though timing and
track remain uncertain. A polar front crosses the region Thursday
with rain showers possibly changing to snow showers. Very windy
and cold conditions Friday and next Saturday with wind chill
indices in the single digits and teens at night.

&&

.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...

10 PM update...

Chilly evening across the region with dry but cold northwest
flow.  850 temps currently down to about -9C across the area (per
latest SPC mesoanalysis) with -12C 850 mb air upstream along the
Canadian/northern New England border. Thus cold night ahead.
Winds on the verge of decoupling in the valleys and typical
sheltered locations.

Cold air streaming across the relatively warm ocean waters is
yielding ocean effect rain/snow bands east of Cape Ann and Cape
Cod. As low level winds back to the NNW overnight and into early
Sunday morning these bands may clip the outer cape per Hi Res
guidance.

Previous forecast captures these details nicely so no major
changes with this update.

Previous discussion...

Gradual pressure rises as high pressure builds in from the W as
low pressure exits E. Slight dampening of the winds but remaining
blustery out of the NW with gusts up to 30 mph. Scattered to
broken cloud decks continue as mid-level energy continues to
rotate through the overall cyclonic flow. Plenty of moisture and
steep lapse rates below H8-7.

Tonight...

A quiet, cold night of weather. Scattered to broken cloud decks are
forecast to continue across the N/W high terrain of S New England
and over the Outer Cape given N/NW flow relaxing through the night
as high pressure builds in from the W. Given the breezy conditions
somewhat cautious of the coldest guidance. Should be clear over S/E
interior S New England and should winds become light, especially
within the sheltered valleys, then it should drop off. So with a
combination of cold air advection and/or radiational cooling, most
locations will see lows in the 20s. Add in the winds, going to feel
like the teens for several locations by Sunday morning.

&&

.SHORT TERM /6 AM SUNDAY MORNING THROUGH MONDAY NIGHT/...

Sunday...

Pleasant but cool day on tap. Surface high pressure building into
the region as heights rise aloft, winds should dampen beneath the
anti-cyclonic flow. Clouds dissipating giving all of S New England
mostly clear conditions. Recent dousing rains along with a h925
airmass aloft around -4 to -6C, hesitant to go with the warmest of
guidance but believe we will come close. Thinking highs around the
upper 30s with warmest locations around the low 40s across the lower
CT Valley and interior S/E portions of S New England. Light NW wind.

Sunday Night into Monday...

 * Snow and impacts anticipated for the Monday morning commute.
 * Takeaways: reduced visibilities at times, slippery conditions.
 * Light accumulations forecast, coating up to 1 inch.
 * Highest snow accumulations possible above an inch over the east
   slopes of the Berkshires.

Mid to upper level feature flattening while becoming sheared by the
strong W flow aloft. Overall weakening as synoptics stretch, there
is still a measure of lift of a continental tropical airmass over-
running, especially along the 285-295K theta surfaces (H9-7), across
a diffuse warm frontal boundary beneath a measure of mid to upper
level forcing, enough to generate some low to mid level fronto-
genesis, squeezing the available moisture out of the atmosphere as a
mix or rain or snow. Went closely with the EC with this forecast.

One thing for certain is the environment ahead of inclement weather.
Prior to clouds on the increase W to E, conditions will be clear as
winds will be light allowing 2m temperatures to drop into the 20s.
Much of the area except along the immediate coast will be below
freezing before onset of precipitation towards early Monday morning.
Also will be fairly dry dewpoints as well.

But as to the availability of moisture aloft, especially within the
snow growth region (-12 to -18C), there is some uncertainty. Dry air
will be eroding the continental tropical airmass especially within
the stronger W flow. Anticipating top-down drying with mechanical
mixing given the faster flow aloft. Aside, will lean towards the
presence of ice (thinking its availability down to -8 to -10C).
Will keep any and all wintry precipitation as snow, no freezing rain.

So altogether, looking at a window of impacts around early to mid
morning Monday, roughly 4 am to 10 pm. Moistening low levels with
time, ran a wetbulb procedure closer to temperature with this event
given how light it is, but keep it below freezing longer along with
the presence of snow. A consensus of forecast solutions along with
ensemble probabilistics, at least a coating for much of interior S
New England with highest amounts of a tenth or two inch over W New
England with up to half an inch over the N/W high terrain.

May be too bullish on amounts as latest forecast has on average a
half an inch for much of S New England. But aside, snow is expected
and the main takeaway is that travel will become hazardous given the
timing of the event during the Monday morning commute and should
roads become snow covered. At a minimum snow will result in reductions
in visibility.

Towards midday conditions should quickly improve with winds becoming
breezy out of the W and conditions clearing out. Temperatures remain
stable during the morning given cloud cover and W winds. More than
likely by midday into afternoon with clearing we`ll see temperatures
bounce to highs around the upper 30s, low 40s.

Monday night...

Turning quiet and once again cold. High pressure building back into
the region parent with rising heights aloft. Winds should become
light out of the N as conditions become mostly clear. Especially for
those areas with any snow cover, should see radiational cooling
proceed resulting in lows well down into the 20s, perhaps teens for
the high terrain. Leaning with the coldest MOS guidance.

&&

.LONG TERM /TUESDAY THROUGH SATURDAY/...

Highlights...

* Very active weather pattern
* Mixed precipitation possible late Tuesday night and Wednesday
* Very strong polar front blasts through on Thursday
* Windy and very cold into next weekend with strong gales over
  the waters

Tuesday... High confidence.

High pressure over eastern Quebec and
northern New England will provide a mostly sunny day. However
there will be cool northeast to east winds and increasing high and
mid cloudiness from the southwest during the afternoon. Highs only
upper 30s to lower 40s.

Tuesday night through Wednesday night...Moderate confidence.

The upper level closed low from the southwest states will have
opened up and what is left aloft will be moving across the Mid-
Atlantic states then passing eastward...to the south of New
England. Meanwhile a strong upper low will be moving across
western Ontario. The net effect will be to cause a splitting of
the flow over us. Low pressure at the surface off the VA/NC coast
will cause cloudiness and some light precipitation to spread into
mainly western and southern portions of southern New England
Tuesday night. The precipitation could be a mix of snow and rain
in northern CT and central/northeast MA and mainly light snow in
northwest MA...with rain elsewhere.

On Wednesday...GEFS and ECMWF ensembles all show the main low
moving straight east and passing well to our south. However...with
light moist easterly flow over the area we should remain cloudy
and there could still be some light rain southeast and a light
rain/snow mix northwest. Not expecting any more than an inch of
snow in the northwesternmost areas. Highs upper 30s northwest to
mid 40s southeast. The light precipitation may continue into
Wednesday night with the best chances of light snow still in the
slopes of the Berkshires especially in northwest MA. Some light
accumulations still possible there Wednesday night.

Thursday and Thursday night...Moderate confidence.

Still some big timing differences in models regarding the
progression of a very sharp upper level trough and arctic cold
frontal passage. The GFS and its ensembles are roughly 12-24
hours quicker than the ECMWF and its members. We have taken a
blend.

Southerly winds ahead of the front will cause temperatures to rise
to the upper 30s northwest and upper 40s southeast Thursday.
Showers are likely ahead of the front. They will mainly be rain
showers, but if the front enters northwest MA before evening, the
rain showers could change over the snow showers there late in the
day.

The arctic front should pass through Thursday night, accompanied
by a chance of snow showers across most of the region, except rain
in southern RI, Cape Cod, and the Islands. Lows 25 to 30 except
mid to upper 30s Cape Cod and Islands.

Friday and Saturday... Moderate confidence.

Ensemble means show low pressure bombing out over southeastern
Canada...anywhere from Labrador to Nova Scotia. A very strong
pressure gradient over New England will cause northwest winds to
gust to 25-35 mph inland and to 35-45 knots over the coastal
waters, with perhaps some 50 knot gusts over the waters. Depending
on timing, the max winds could be Friday afternoon or Friday
night. Highs Friday will struggle to rise through the 30s except
lower 40s Cape Cod and Islands. Models are indicating that lake
effect snow showers from the eastern Great Lakes may actually make
it over the Berkshires and into portions of western and central
Massachusetts. Have gone with 20-30 percent PoPs for those areas.

With clearing skies Friday night...am forecasting temperatures to
drop to below model consensus guidance levels...into the upper
teens in the Worcester hills and slopes of the Berkshires with 20
to 25 elsewhere and upper 20s to near 30 Cape and Islands.
Winds will still be 20-30 mph and this causes some single digit
wind chill index values in northwestern Massachusetts Friday night.

High pressure builds into the Mid-Atlantic states on Saturday, but
despite sunshine it will still be blustery and cold across
southern New England. Highs will only reach the upper 20s to mid
30s. Wind chill indices Saturday night could dip to near zero in
northwest Massachusetts.

&&

.AVIATION /03Z SUNDAY THROUGH THURSDAY/...
Forecaster Confidence Levels...

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

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

10 pm update...

No major changes from 00z TAFs. Earlier discussion below.

=============================================================

Tonight...
VFR. NW winds diminishing but initially blustery. Gusts up around
25 kts dampening. Cigs dissipating.

Sunday...
VFR. Mostly clear with light NW winds.

Sunday Night into Monday...
Mostly -SN with coastal -RA for the Monday morning push. Will see
conditions deteriorate around 9-15z to IFR with -SN as VSBY will
most likely be impacted. Cigs mainly MVFR, IFR for the high terrain.
W winds around 5 to 10 kts. Improving VFR beginning around 18z Monday.
Snow accumulations possible, at least a coating, for all interior
terminals.

Monday Night...
VFR. SKC. Light NW winds.

KBOS TAF...Gusty NW winds continuing around 25 to 30 kts.
Diminishing through the overnight period.

KBDL TAF...Gusty NW winds continuing around 25 to 30 kts.
Diminishing through the overnight period.

Outlook /Tuesday through Thursday/...

Tuesday...High confidence. VFR.

Tuesday night through Wednesday night... Low to moderate confidence.
MVFR to local IFR ceilings in light rain RI and southeast MA...
a mix of rain and snow northern CT, central and northeast MA, and
light snow northwest MA.

Thursday... Low to moderate confidence. Mainly MVFR ceilings in
areas of light rain showers ahead of an approaching arctic cold
front. Depending on timing, rain showers could change to snow
showers with local IFR conditions in western MA late in the
afternoon.

&&

.MARINE...
Forecaster Confidence Levels.

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

Short Term /through Monday Night/...

Through Sunday...

High pressure building into the waters will allow winds and seas
to relax. Initially, NW gusts up to 30 kts expected, dropping out
overnight into Sunday morning. Wave action dampens through Sunday.
This will allow small craft advisories to conclude. Ocean effect
rain showers likely to be an issue throughout, especially over the
outer waters.

Sunday night into Monday night...

Weak disturbance will sweep W to E across the waters during the
early to mid Monday morning period. Winds will overall be light
and out of the W. Expect showers, perhaps some slight reductions
in visibility. Winds increase late going into Tuesday morning out
of the NW, could see some gusts up to 20 kts. Waves remaining
below 5 feet throughout.

Outlook /Tuesday through Thursday/...Moderate confidence.

Tuesday through Wednesday night...Winds and seas remain below
small craft criteria for most of the waters. NE to E winds may
gust to 15 to 20 knots at times. Seas may build to near 5 ft
over the southern outer waters by Wednesday night.

Thursday...Small craft advisories will likely be needed. Ahead of
an approaching Arctic front, winds will shift to the southeast to
south and may gust up to 25 to 30 kt by late Thursday afternoon.
Seas will be building to 5 to 7 ft over the outer waters.

Attention mariners... Although it is just beyond this current
forecast...you should be aware that northwest to west strong gales
with some potential for storm force gusts are expected to develop
Friday into early next weekend. Stay tuned to the forecast.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 10 AM EST Sunday for ANZ230>237-251-
     256.
     Small Craft Advisory until 4 PM EST Sunday for ANZ250-254-255.

&&

$$
SYNOPSIS...Sipprell/GAF
NEAR TERM...Nocera/Sipprell/GAF
SHORT TERM...Sipprell
LONG TERM...GAF
AVIATION...Sipprell/GAF
MARINE...Sipprell/GAF



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