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Area Forecast Discussion

Forecast Discussion for BOX NWS Office
000
FXUS61 KBOX 220146
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
946 PM EDT Thu Mar 21 2019

.SYNOPSIS...
Intensifying low pressure will bring a period of heavy rain
overnight, which will taper off to scattered showers Friday morning
and afternoon.  Scattered showers should change to a period of snow
Friday night with some accumulations expected in the high terrain
and it will become windy across the entire region. The strong
winds will continue into Saturday, but it will be dry. It turns
milder Sunday and Monday, then a return to colder than normal
conditions is expected Tuesday into mid next week.

&&

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

10 PM Update...

*** Rain Arrives After 11 pm From South to North ***

Potent storm system taking shape this evening over DE/NJ and
coastal waters with negative tilt closed mid level low inducing
cyclogenesis. Satellite imagery indicating an abundance of
lightning within the warm conveyor belt off DE/NJ coast with a
firehose of moisture off the Atlantic as PWATs increasing above
an inch, +2 standard deviations from climo. Dry air over
MA/RI/CT eventually erodes and gives way to rain overspreading
the region mainly after 11 pm. Initial rain band approaching
the south coast may erode as it encounters dry air over
CT/RI/MA, so expecting steady rains to arrive after 11 pm from
south to north.

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

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

Things will change during the late evening and overnight hours
as an intensifying low pressure system approaches the southeast
New England coast. Although the storm system is quite
progressive, a 925 mb easterly jet 3 to 4 standard deviations
above normal along with Pwats of 1+ above normal will result in
a period of heavy rain. The bulk of the steady/heavy rain will
arrive after 10 or 11 pm and be over by 5 or 6 am. Most
locations should receive rainfall amounts on the order of 0.50
to 1", but some locally higher totals are possible.

While rain is on tap for the vast majority of the region, it is
possible that areas of the highest terrain in northwest MA see
a period of wet snow with a slush inch or two of accumulation.
The NAM indicates that it will be too warm for this to occur,
but other guidance indicates that strong forcing/dynamic cooling
may allow for ptype to be in the form of wet snow for a period
of time. If this were to occur, it would be confined to mainly
elevations over 1500 feet in far northwest MA.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/...

* Accumulating snow expected Friday night in the high terrain
* Windy later Friday and especially Friday night in all locales

Friday...

While the steady/heavy rain will be over by early Friday morning,
scattered showers are expected at times through the afternoon along
with rather cloudy skies. This a result of cyclonic flow around the
mid level closed low.  High temperatures should be in the 40s, so
ptype will mainly be in the form of rain showers.  However, it is
possible that it becomes marginally cold enough for some wet snow in
the high terrain along the east slopes of the Berks especially by
late in the day.

Friday night...

A second piece of vigorous shortwave energy will drop south of New
England Friday night.  The result will be an increase in areal
coverage/intensity of the precipitation.  Thermal profiles will be
cooling...so any rain will quickly flip to snow in the high terrain
and eventually even the lower elevations. In addition...strong winds
will develop as low pressure lifts into eastern Maine. Some
locations may be near wind advisory criteria, but given this is a
3rd and 4th period marginal event, opted to hold off on any wind
headlines at this point.

With that said though, the westerly flow coupled with deep
moisture/forcing will result in some enhanced upslope snowfall in
the Berkshires.  A few inches of snow seem likely in northwest MA,
but the highest terrain may see a swath of 4 to 8" above 1500 feet.
Will have to watch this region for a few power outages given the
snow coupled with gusty winds.  We may also see an area of 1 to 3
inches of snow across the higher terrain of the Worcester/CT Hills,
especially in the western Hills where upslope flow will result in
some enhancement. Elsewhere in the lower elevations...a dusting
to 1 inch of snow is certainly possible, but we can not rule
out the low risk of some location seeing 2 inches. Overall
though the main impacts appear to be in the high terrain and
especially in the Berkshires. Given that we have issued a Winter
Weather Advisory for western Franklin/western Hampshire county.
Snow amounts may vary quite a bit in this region too, highly
dependent on elevation.

&&

.LONG TERM /SATURDAY THROUGH THURSDAY/...

Highlights...

 * Windy Saturday, then milder Sunday.
 * Cold front brings rain showers Monday, possibly starting as snow
   showers inland.
 * Colder weather follows through midweek.

Closed low exits New England this weekend as it heads into
Maritimes. Colder air aloft coupled with steep lapse rates will
promote diurnal clouds and gusty NW winds of 35-45 mph Saturday,
before skies clear and winds diminish Saturday night.

Milder weather arrives Sunday as upper flow becomes more zonal and
high pressure off Carolinas results in SW flow across region.
Blended guidance yields highs well into 50s away from colder ocean
influence (South Coast, Cape, and Islands) but with plenty of sun
it`s possible we reach lower 60s in some interior locations.

Ensembles in good agreement on timing of cold front through SNE
Monday which should reach South Coast around 18z before slowing down
a bit, as weak wave of low pressure may form along it as mid level
short wave approaches. Most of forcing and deeper moisture should be
located along and south of boundary, so while most of rainfall
(light) will be focused south of Mass Pike, we can`t rule out some
showers farther north as well. Should they occur it`s possible to
see some wet snow at start across NW MA where temperature profiles
support snow at least initially. Not looking for any real
accumulation at this point however.

Beyond Monday, broad upper trough remains in place for much of week
which will result in below average temperatures, at least through
Wednesday. High pressure will dominate with dry weather before
heading offshore later next week, so warmup appears more likely by
then.

&&

.AVIATION /02Z FRIDAY THROUGH TUESDAY/...
Forecaster Confidence Levels...

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

Short Term /through Friday Night/...

03z update ...

Steady rains overspread MA/RI/CT after 11 pm with periods of
heavy rain and IFR/LIFR overnight. Previous discussion below.

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

Tonight...Moderate to high confidence.
VFR conditions early this evening deteriorate to low end MVFR-
IFR around or after 04Z and through the overnight hours. Spotty
showers will slowly transition to steady rain working into the
region, which will be heavy at times around or after 05Z. A
period of easterly wind gusts to 30 knots are expected for a
time overnight on the Cape/Islands with the low level jet. LLWS
of 40 to 50 knots will also be a concern overnight for all
locations.

Friday...Moderate to high confidence.
The steady rain should come to an end from S-N Friday morning,
but scattered showers will remain possible through the
afternoon. IFR conditions should improve to mainly MVFR levels
in most locations by mid afternoon. Winds shift to the west by
afternoon with gusts to 30 knots expected to develop.

Friday night...Moderate to high confidence.
Scattered showers will become more numerous and quickly flip to
snow in the high terrain during the evening with a few inches
of accumulation expected. In the lower elevations, scattered
showers should change to a period of snow showers but
accumulations should generally be a dusting to an inch with
mainly wet runways. MVFR to IFR conditions anticipated along
with westerly wind gusts of 30 to 40 knots.

KBOS terminal...Moderate to high confidence in TAF.

KBDL terminal...Moderate to high confidence in TAF.

Outlook /Saturday through Tuesday/...

Saturday: Mainly VFR, with local MVFR possible. Strong winds
with gusts up to 40 kt.

Saturday Night: VFR. Windy with local gusts up to 30 kt.

Sunday: VFR. Breezy.

Sunday Night: VFR. Slight chance SHRA, slight chance FZRA.

Monday: VFR. Chance SHRA.

Monday Night: VFR. Breezy. Slight chance SHRA, slight chance
SHSN.

Tuesday: Mainly VFR, with local MVFR possible. Breezy. Slight
chance SHSN.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Friday Night/...

Tonight...High confidence.
Intensifying easterly low level jet will bring a period of
easterly wind gusts of 25 to 35 knots from south to north. Seas
will build to between 5 and 9 feet across the open waters by
daybreak Friday with the long easterly fetch. Rain overspreads
the region and will be heavy at times overnight, reducing vsbys
for mariners at times.

Friday and Friday night...High confidence.
Winds will slacken for a time Friday morning as low pressure
tracks across the region. However, as the strong low lifts to
the northeast of our area the pressure gradient will increase.
Westerly winds will increase to between 35 and 45 knots later
Friday and especially Friday night. Gale Warnings are posted for
all coastal waters.

Outlook /Saturday through Tuesday/...

Saturday: Moderate risk for gale force winds with gusts up to
45 kt. Rough seas up to 15 ft.

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

Sunday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Local rough seas.

Sunday Night: Winds less than 25 kt. Areas of seas approaching
5 ft.

Monday: Winds less than 25 kt. Areas of seas approaching 5 ft.
Chance of rain showers.

Monday Night: Winds less than 25 kt. Areas of rough seas.
Slight chance of rain showers.

Tuesday: Winds less than 25 kt. Areas of rough seas. Slight
chance of rain showers, slight chance of snow showers.

&&

.TIDES/COASTAL FLOODING...
The strongest winds/seas will arrive after the overnight high
tide. Therefore, worst case scenario appears to be some very
minor splash over during the 1 AM high tide.

The next high tide occurs early Friday afternoon, but by that
time winds will have diminished. Nonetheless, enough pressure
falls, lingering seas and relatively high astro tide will
likely result in pockets of splash over and minor coastal
flooding in the typically most vulnerable areas along the
eastern MA coast. We hoisted the Coastal Flood Advisory for the
eastern MA coast with the greatest risk along the north shore,
where winds will be slowest to slacken. Nonetheless, not
expecting any property damage or significant impacts anywhere
in our region.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...Coastal Flood Advisory from 11 AM to 3 PM EDT Friday for
     MAZ007-015-016-019.
     Winter Weather Advisory until 8 AM EDT Saturday for MAZ002-008.
RI...None.
MARINE...Gale Warning from 2 PM Friday to 4 PM EDT Saturday for
     ANZ232>235-237-254>256.
     Gale Warning from midnight Friday night to 4 PM EDT Saturday
     for ANZ230-236.
     Gale Warning from 8 PM Friday to 4 PM EDT Saturday for ANZ231-
     250-251.

&&

$$
SYNOPSIS...Frank/JWD
NEAR TERM...Frank/Nocera/EVT
SHORT TERM...Frank
LONG TERM...JWD
AVIATION...Frank/Nocera/EVT/JWD
MARINE...Frank/JWD
TIDES/COASTAL FLOODING...

NWS BOX Office Area Forecast Discussion

 



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