Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 241950

Area Forecast Discussion
National Weather Service Taunton MA
250 PM EST Tue Jan 24 2017

A powerful coastal storm east south of Long Island will move
northeast to Nantucket this evening then into the Maritimes
Wednesday. Showers with locally brief heavy rain lingers through
tonight with areas of freezing rain confined to the high terrain
of the Worcester Hills and the Berkshires of Massachusetts. High
pressure builds in behind the departing storm and brings dry mild
weather Wednesday and Thursday. A cold front crosses New England
Thursday followed by cooling temperatures and blustery winds
through the weekend.



230 pm update...

*** Areas of freezing rain likely lingers into tonight across the
    high terrain especially at elevations above 1000 ft ***

Winter Weather Advisory...

Temperatures continue to warm aloft as closed low east of NJ
continues to advect Trof of Warm Air Aloft (TROWAL) into the
region. This will result in predominantly rain this afternoon and
tonight. Although as column cools later tonight there is a chance
of some sleet mixing in at times with the rain.

As low pressure tracks south of New England surface winds will
remain from the NNE. This will keep the shallow cold air over
interior MA in place thru today and likely into tonight. Thus
freezing rain is expected especially at elevations above 1000 ft
across the Worcester hills and Berkshires. Already received a
report of 3/16" of ice accretion at 1130 am from Berlin MA and
0.25 inch of ice in North Adams MA. Therefore expect some icy
roads this afternoon and tonight across the high terrain of MA. As
precip and ice accretion continue there will be a low risk of down
branches and isolated power outages across this region into
tonight. Forecasting additional ice accretion of up to a tenth of
an inch. This is supported by the 00z NCAR ensembles, 12z Euro/NAM
and 09z SREF.


Despite subtropical plume of moisture northeast of the region into
ME and the Maritimes...closed low east of NJ is circulating a new
plume of moisture off the Atlantic which is beginning to form a
comma-head over southern New England. This radar will continue to
fill in with additional showers. Precip shield is fragmented due
to some drier air aloft from the dry slot offshore advecting into
the region. However this dry air aloft is serving to steepen the
mid level lapse rates and enhancing updraft potential. Thus low
risk for isolated thunder this afternoon into tonight. But more
importantly these steepening mid level lapse rates will support
heavier showers with brief downpours. A model blend of global and
Hi Res guid supports widespread qpf of 0.15 to 0.35 thru tonight
with local amounts up to a 0.50 where heavier embedded showers


Core of low level jet is now along ME coast into the maritimes,
however 40-45 kt low level NE jet continues to wrap cyclonically
across the region as low pres remains south of New England. These
winds will slowly weaken this afternoon and tonight...remaining
below wind advisory criteria. However these gusty NE winds will
continue the raw/damp chilly feel thru tonight.



Big Picture...

West Atlantic ridge in place midweek will shift out to sea. Pacific
high pressure builds east into the Western USA, kicking an upper
trough east to the Eastern USA over the weekend. Ridge west/trough
east pattern continues early next week.

Shortwave embedded in the upper trough will move northeast with the
trough, crossing New England during Thursday. Upper jet associated
with this shortwave sweeps east and crosses New England later
Wednesday night and Thursday, then remains well to  our south over
the weekend.

Contour heights at 500 mb will be above average during midweek, then
below average Friday through Monday. This means mild weather midweek
followed by colder weather over the weekend.



Low pressure moves off through the Maritimes Wednesday morning and
carries off any leftover clouds and showers. Atlantic high pressure
builds over Southern New England with dry weather. Mixed layer
reaches to at least 950 mb, possibly a little higher to 925 mb.
Temps in this layer would support max sfc temps of 45 to 50.

Not much sky cover in the evening, but the trend during the night
will be for increasing clouds as the upper jet approaches and
enhances upper venting. Previous model runs had weakened the jet as
it moved overhead, but this 00Z suite maintains the jet strength
while the moisture in the column increases. Expect a few showers
from late night into Thursday morning.

The upper jet drives a shortwave trough and surface cold front
through our area. Current suite suggests timing during the afternoon
but this may shift a little as the event gets closer. Colder air
moves in behind the front, and the cold advection should increase
mixing depth to 850 mb. Winds in this layer reach 25 knots, possibly
30 knots...which is higher than model guidance. We will adjust
forecast values higher. Timing of the cold front will determine the
high temperature.  If the cold air arrives mid afternoon then max
temps may again be in the mid 40s to lower 50s.

Friday through Monday...

Broad cyclonic flow with cold pool clouds and a cooling trend on
temps aloft. Temps aloft trend from around -8C Friday to -11C or
-12C Sunday and Monday, so max sfc temps trend from either side of
40F Friday to the 30s early next week.  Another shortwave moves
through the flow over the weekend and may increase the sky cover as
it passes, but current forecast data shows moisture too limited for


Forecaster Confidence Levels...

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

20z update...

Thru tonight...high confidence.

Not much change with mainly IFR conditions in showers and fog with
marginal IFR/MVFR CT river valley. Main concern late this
afternoon thru tonight is freezing rain for the high terrain of MA
including KORH. Up to a tenth of an inch of ice accretion is
possible. NNE winds up to 35 kt late this afternoon slowly slacken
and become NNW overnight.

Wednesday...high confidence.

Any MVFR cigs over Cape Cod and island at sunrise quickly move
offshore followed by VFR. Elsewhere VFR prevail with modest WNW
winds, dry weather and good vsby. Low risk of leftover showers
Cape Cod and Nantucket at sunrise.

Wednesday night...high confidence.

VFR to start then lowering to VFR-MVFR toward sunrise Thu. Low
risk of IFR-MVFR across the high terrain of MA/CT and possibly
northwest RI toward daybreak. Light WNW winds become SSW toward
morning. Chance of showers toward daybreak as well.

KBOS TAF...High confidence in TAF.

KBDL TAF...High confidence in TAF.

Outlook /Wednesday through Saturday/...Moderate confidence.


VFR Wednesday. Ceilings and vsbys trend to MVFR after midnight in
rain and snow as a cold front approaches from the west. Areas of IFR
are likely Thursday morning. Conditions trend back to VFR after the
cold front moves through Thursday afternoon. West winds gust to 20
knots Wednesday and 25 to 30 knots Thursday afternoon.


VFR. West winds gust to 25 knots each day.


Forecaster Confidence Levels...

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

*** Winds slacken tonight but very rough seas continue ***

245 pm update...

Thru tonight...

Low pres south of Long Island at 3 pm tracks NE to Nantucket this
evening and then across the Gulf of ME tonight...then south of
Nova Scotia Wed. NNE winds 20 to 30 kt with a few gusts up to 35
kt late today will slacken slowly and become NNW overnight. Vsby
lowering to 1 mile or less in showers and fog..but improving
after midnight and especially toward daybreak.

Outlook /Wednesday through Saturday/...Moderate confidence.

NW winds diminish Wednesday and back from the WSW. Lingering gusts
to 25 knots.  A cold front sweeps across the waters Thursday,
followed by colder air and gusty west winds 25 to 30 knots.  West
winds gusting to 25 knots continue through Saturday.

Seas will diminish through the period, starting 5 to 10 feet on the
exposed waters Wednesday and diminishing Wednesday night. Seas will
rebuild reaching 5 to 10 feet Thursday in the west winds behind the
cold front, then slowly diminish with 5 to 7 foot seas lingering on
the outer waters through Saturday.

A Small Craft Advisory will be needed for some of all of the waters
through this time period.


MA...Winter Weather Advisory until 4 PM EST this afternoon for
MARINE...Small Craft Advisory until 10 AM EST Wednesday for ANZ231>237.
     Small Craft Advisory until 5 PM EST Wednesday for ANZ254-255.
     Small Craft Advisory until 4 PM EST Wednesday for ANZ256.


NEAR TERM...Nocera
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