Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 160015
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
715 PM EST Wed Feb 15 2017

.SYNOPSIS...

Rapidly intensifying low pressure will be lifting into the
Canadian Maritimes Thursday morning. This will result in rain
changing to accumulating snow across eastern Massachusetts
overnight into early Thursday morning. Otherwise, dry but
blustery and cold weather continues for the remainder of the
work week. Dry weather continues this weekend, but with a
significant moderation in temperatures especially by Sunday
when much of the region may see highs exceeding 50.

&&

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

***Accumulating snow likely late tonight into early Thursday
morning across eastern MA especially on the north shore***

7 PM Update...

Triple point low pres noted across Nantucket Sound as seen on
KBOX 88D radar loop, matching up with the wind directions across
SE Mass, Cape Cod and the islands. Also noting a trough cutting
eastward across W Mass into N Central CT on 21Z surface front
analysis. Areas of showers, with some spotty higher
reflectivities seen on KBOX 88D radar loop since 23Z.

Line of showers steadily moved E out of the lower Hudson valley
moved steadily SE across W CT from 2245Z-23Z. Local media
reported thunder across SW Hartford county about 2310Z. Sure
enough, noting several clouds lightning pulses and a positive
C/G strike across W Hartford county near Bristol at the same
time. Another smaller spot of lightning pulses and a cloud flash
at 2325Z near Plainville. This area of showers is moving SE
into S central CT. Something else to keep an eye on.

Another line of light to locally heavy showers moving across E
CT/W RI and heading E though no lightning activity noted with
this line. This line may break apart as it approaches the main
area of precip along the E coast of Mass, while the southern
extent of the line may actually become entrained into the
circulation of the low as it shifts N.

Starting to see the precip across Plymouth county pivoting to a
more northerly direction. Big question as to whether the trough
may start to invert further S than previously forecast. Will
monitor this aspect very closely.

Colder air working into central and western areas as winds shift
to N-NW though remaining light, while wind directions remain
E-SE across SE coastal Mass. Noted mixed precip on 00Z Ob from
KORH with the temp down to 34 degrees. Lots of changes moving in
over the next few hours.

Have updated near term forecast to bring conditions current.
Kept similar trends with respect to inverted trough development
and movement overnight, for now.

Previous Discussion...

The main focus for precipitation tonight will become focused
across eastern MA and particularly northeast MA. This a response
to vigorous shortwave energy to our west coupled with a rapidly
intensifying ocean storm that will be lifting into the Canadian
Maritimes. This will setup the inverted trough which are
notoriously tough to forecast.

Ptype/Uncertainty...

As mentioned above the scattered rain and high terrain snow
showers across the interior should come to an end this evening.
Most of tonight will be dry in these locations with the focus
shifting to eastern MA. The guidance is in general agreement in
showing a period of good low level convergence with the inverted
trough overnight...particularly in northeast MA. This coupled
with a period of strong omega in the snowgrowth region/deep
moisture and total totals between 55 and 60 will result in an
ideal setup for moderate to heavy snow.

However, there are a couple of limiting factors that need to be
take into consideration. The first is that the boundary layer
will likely be too warm for snow into the first half of the
evening. Based on model soundings...rain will probably change
to snow between 9 pm and midnight across most of eastern MA. The
second is that the inverted trough looks to be progressive, not
stationary over our area for an extended period of time.
Therefore, the duration of the moderate to heavy snow will
probably be limited to a few hours. The last and most important
factor is that the models often perform quite poorly in
depicting the axis of heavy snow associated with inverted
troughs. This can result in a significant snowfall bust on the
high or low side for specific locations. In this case, the
biggest uncertainty is does the axis of heavy snow drop for
enough south to affect our region.

Snow Accumulations/Headlines/Timing...

Based on the above reasoning have opted to go with a Winter
Weather Advisory for 3 to 6 inches of snow across Essex county.
Have also issued a special weather statement for the possibility
of an inch or two of snow further south across the rest of
eastern MA. While the heaviest snow will probably be over by mid
morning on Thursday... roads may be snow covered and slippery
for the morning rush hour. As mentioned will have to closely
monitor trends this evening given the uncertainty.

Winds...

Winds will shift to the northwest and increase overnight behind
the rapidly intensifying low pressure system. Should see
northwest wind gusts of 25 to 40 mph develop across the coastal
plain. Will be close to wind advisory criteria across the
Cape/Nantucket but kept things just below for now.

&&

.SHORT TERM /6 AM THURSDAY MORNING THROUGH 6 PM THURSDAY/...
Thursday...

While the steadiest snow should be over by mid morning on Thursday
across eastern MA...enough low level moisture/steep lapse rates coupled
with diurnal heating may allow hit and miss scattered snow
showers to develop into mid afternoon. In fact...may see the
activity develop further back across the interior as a result of
the diurnal heating depending on how much moisture is left. It
will also be windy with high temps in the 30s.

&&

.LONG TERM /THURSDAY NIGHT THROUGH WEDNESDAY/...
Highlights...

* Generally dry conditions through most of this period
* Blustery and cold Friday
* Moderating temperatures Saturday, and unseasonably mild on Sunday
* Above normal temperatures continue into early next week

Details...

Thursday night...

Leftover NW winds at the surface and aloft will continue. Skies
will become mostly clear, though some clouds may linger across
the E slopes of the Berkshires and outer Cape Cod, along with a
few leftover snow showers Thu evening. Winds will gust up to
25-30 kt mainly along the coast Thu evening, then should
diminish as the pressure gradient from storm system over the
Maritimes decreases.

With the NW flow in place, will see H85 temps will fall to -10C
to -13C, lowest across the interior higher terrain. Expect lows
in the mid to upper teens across N central and W Mass, ranging
to lower-mid 20s along the immediate coast.

Friday and Saturday...

High pressure ridge builds across the eastern seaboard Friday,
then will steadily shift E Fri night into Sat. As the H5 vortex
across the Maritimes finally lifts toward Greenland Sat, W-NW
winds will relax with the ridge`s arrival. Mid level winds back
to W as well.

Another H5 short wave will slide SE out of central Canada by
later Sat, so some mid and high level clouds will increase from
the NW during the afternoon across western areas. Temps will
return close to seasonal levels, with highs in the mid 30s to
lower 40s.

Saturday night-Sunday...

As a cutoff low moves eastward across the mid Mississippi
valley, the other system over southern Canada remains N of the
region. This system will push a warm front into northern New
England, with only some clouds working across the region Sat
night into early Sunday. The main moisture from both systems
will remain outside the region, so expect little if any precip.
There may be enough moisture for some patchy fog to develop
inland late Sat night into Sun morning.

W-SW early Sunday will bring milder air to the region, so
expect temps to reach the mid 40s to lower 50s, up to 10 degrees
above seasonal normals under mostly sunny skies.

Winds will shift back to NW late Sunday into Sunday night as
both systems push off the coast.

Monday through Wednesday...

Quite a bit of solution spread amongst the model suite during
this timeframe, mainly due to continued somewhat amplified but
progressive split mid level flow across the lower 48. Looks like
a fast W-NW flow continues across the northern stream, while a
cutoff low moves along the southern stream down to the Gulf of
Mexico. So, have rather low confidence with this portion of the
forecast.

At this point, may see another warm front approach around late
Mon night or Tue as low pressure moves across Quebec. May see a
chance for rain and/or snow showers, but timing is in question.
May see another weak system cross during Wed, but again timing
and track are in question. Have carried slight chance POPs for
this system.

&&

.AVIATION /00Z THURSDAY THROUGH MONDAY/...
Forecaster Confidence Levels...

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

Short Range /through Thursday/...

Tonight...Moderate confidence. Areas of low end VFR to MVFR
conditions dominates, with rain changing to snow across eastern
MA between through 05z. This should result in periods of IFR to
LIFR conditions in snow. Focus for snow will be across
northeast MA but may affect the rest of eastern MA for a time
late tonight into early Thu AM.

Thursday...Moderate confidence. MVFR CIGS look to hang around
much of the day. There will also be scattered snow
showers...especially across eastern MA during morning hours.
This should result in localized lower conditions. Northwest wind
gusts of 20 to 30 knots expected.

KBOS TAF...Moderate confidence in TAF. Best chance for a period
of accumulating snow will be between 04z and 12z.

KBDL TAF...Moderate confidence in TAF.

Outlook /Thursday Night through Monday/...

Thursday night...High confidence. A few leftover -SHSN possible
across the E slopes of the Berkshires Thu evening. May also see
leftover MVFR CIGS across the E slopes of the Berkshires and along
the outer Cape. Should improve by around 05Z. Otherwise, VFR
conditions.

Friday and Saturday...High confidence. Mainly VFR conditions.

Saturday night-Sunday...Moderate to high confidence. May see patchy
MVFR-IFR CIGS/VSBYS develop Sat night across the CT valley and E
slopes of the Berkshires ahead of warm front. MVFR-IFR CIGS may
linger Sunday and early Sunday night along the E slopes of the
Berkshires. Elsewhere...mainly VFR.

Monday...High confidence. Mainly VFR conditions.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Range /through tonight/...

Tonight and Thursday...Moderate to high confidence. E-SE winds
shift to NW and increase to between 30 and 40 knots later this
evening and overnight. Winds will diminish a tad on Thursday,
but strong cold advection will result in gusts between 25 and
35 knots. Gale warnings posted for all waters except small craft
headlines for Boston Harbor/Narr Bay.

Outlook /Thursday Night through Monday/...

Thursday night...High confidence. Expect NW winds gusting to 30-35
kt Thu evening, then will diminish. Gale warnings will likely be
lowered to small crafts over the open waters. Light freezing spray
is likely along the near shore waters.  Seas up to 6-9 feet on the
outer waters.

Friday-Friday night...High confidence. W-NW winds will continue to
gust up to 25-30 kt so small crafts will likely continue. Light
freezing spray will likely continue through Fri morning, then should
end as winds diminish. Seas 6-9 ft early, then will slowly subside
but remain at or above 5 ft through Fri night on the outer waters.

Saturday through Sunday...High confidence. W-SW winds gusting up to
20 kt. Seas below 5 ft.

Monday...Moderate confidence. NW winds may approach small craft
criteria across the open waters.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...Winter Weather Advisory until 10 AM EST Thursday for MAZ006-
     007.
RI...None.
MARINE...Gale Warning from 10 PM this evening to 10 AM EST Thursday for
     ANZ231>235-237-251.
     Small Craft Advisory from 10 PM this evening to 7 PM EST
     Thursday for ANZ230-236.
     Gale Warning from 10 PM this evening to 7 PM EST Thursday for
     ANZ250-254>256.

&&

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



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