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000
FXUS61 KBOX 100917
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
417 AM EST Tue Dec 10 2019

.SYNOPSIS...

Unseasonably mild temperatures continue today with another round of
showers developing later this morning and especially this afternoon.
A strong cold front crosses the region this evening, followed by
much colder temperature.  Showers will change to a period of
accumulating snow later tonight into Wednesday morning, especially
south of the Massachusetts turnpike where a significant impact may
occur to the morning commute.  Snow will end by Wednesday afternoon
as large high pressure will bring cold and dry conditions Thursday
and Friday. Another system will impact the region Saturday with
potentially periods of heavy rain, possibly beginning as freezing
rain over northwest Massachusetts. A trend toward drier and cooler
weather is scheduled for Sunday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...

The core of the strong southerly LLJ and deeper moisture has shifted
offshore with the departing shortwave.  This has brought an end to
the bulk of the showers very early this morning.  While a few spot
showers will remain possible through daybreak, dry weather will
dominate but it will remain unseasonably mild with temps in the 50s
to near 60 across parts of the coastal plain.

Showers will begin to increase in areal coverage again later this
morning and early afternoon across portions of western MA/CT as the
forcing for ascent increases ahead of an approaching cold front.
These showers should arrive in eastern MA/RI by mid to late
afternoon. Despite an abundance of clouds today temps are already
very mild. Given southwest flow and 850T near +8C expect highs to
reach well into the 50s across the interior and lower 60s across
much of the coastal plain.  It is possible the record high of 64 in
Boston could be challenged today.  It will remain breezy today,
given modest LLJ so expect southwest wind gusts of 20 to 30 mph
strongest of which will be across eastern New England.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM WEDNESDAY/...

* Accumulating snow expected late tonight into Wed AM for much of
  the region, but a sharp cutoff is possible in northern MA

* Significant impact to Wed AM Rush hour possible, especially across
  parts of northern CT, Rhode Island and Southeast MA

Tonight and Wednesday...

Quite the challenging forecast tonight into Wednesday morning. This
revolves around snow amounts and a potentially significant impact to
the Wed morning commute.  Greatest concern is across parts of CT,
RI/SE MA with a possible sharp cutoff in appreciable snow in
northern MA.

A cold front will cross the region this evening with temperatures
quickly falling through the 40s and then into the 30s.  However, an
approaching shortwave will keep our region in southwest flow aloft
into Wed morning.  Very strong jet dynamics are in place as southern
New England will be in the right rear quad of a 180 knot jet streak.
This leads to a narrow, but very strong area of mid level
frontogenesis in the 600 to 700 mb layer.  Snowgrowth also looks
favorable especially across portions of CT/RI and SE MA, where NAM
indicates 20+ units of omega in the snowgrowth region for a short
time.

The problem is that the northern extent of the mid level
frontogenesis and sufficiently deep moisture remains uncertain.  It
will also be short-lived and offset by low level drying with WNW
winds just off the deck. This is what makes these anafrontal waves
so difficult to forecast even inside 12 to 24 hours.  There often
can be a narrow area of moderate to heavy snow and a sharp cutoff to
the north.

Based on model consensus and overall pattern recognition, thinking a
general 1 to 4 inches of snow across the region.  Greatest risk for
the higher amounts will be south of the Pike across portions of
CT/RI and SE MA.  If the dry air ends up being a little stronger
than expected, snow accumulations will be on the order of a coating
to 2 inches with little if any accumulations north of the MA
Turnpike.  On the flip side of the coin, if forcing/deeper moisture
ends up a bit further north may see a swath of  4 to 6 inches of
snow.

After collaboration with surrouding offices, opted to go with a
Winter Weather Advisory late tonight into Wed AM for northern CT,
RI, as well as south central and southeast MA.  The biggest concern
is the strong mid level frontogenesis coinciding with the Wed Am
rush hour.  While amounts may not reach advisory criteria across the
entire area, moderate to briefly heavy snow may impact the morning
commute and that can often be quite problematic.  Therefore, felt
the Winter Weather Advisory was warranted.  Fortunately, this system
is progressive with the snow pretty much over by afternoon.  High
temperatures will remain in the 30s, which will be a big change
after today/s anomalously mild temps.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/...

Highlights...

* Colder than normal Thu into Fri but dry except possible ocean
  effect snow showers outer Cape Cod

* Another weather system Sat with heavy rain & potential flooding

Details...

Wednesday night through Friday...

As you would expect pretty cold airmass overspreads the region Wed
night into Thu behind departing Arctic short wave. By 12z Thu 925 mb
temps fall to about -13C over southern New England and 850 temps
down to -20C upstream over northern VT/NH! However mainly dry
weather prevails as 1040+ mb Arctic high builds into the region.
Gusty NW winds will provide frigid wind chills Wed night but by Thu
afternoon gradient relaxes so diminishing winds later Thu into Fri.
However before this occurs low level winds turn to the NNW and may
yield ocean effect snow showers for the outer Cape and Nantucket
Thu/Thu evening. Otherwise dry but cold weather prevails. Given the
magnitude of the cold air (-1 to -2 STD) will derive temps from a
50/50 blend of the coldest guidance and Superblend.

Next Weekend ...

*** Heavy Rain & Flood Potential ***

Another potent system appears to impact the region Saturday with
ensembles showing PWAT anomalies of +1 to +2 STD. Thus heavy rain
threat and given heavy rain from today and tomorrow along with snow
melt contribution, increased risk for river and urban flooding
Saturday. Shallow cold air may linger across the CT river valley of
western MA and may result in freezing rain/wintry mix at the onset
Friday night.

Ensembles suggest pattern remains progressive so some improvement
possible Sunday with a drying trend and cooler weather.

&&

.AVIATION /09Z TUESDAY THROUGH SATURDAY/...
Forecaster Confidence Levels...

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

Short Term /through Wednesday/...

Today...Moderate to high confidence. Bulk of showers exit the
region toward daybreak, but do expect another cluster of showers
to develop later this morning and especially this afternoon
ahead of an approaching cold front. MVFR conditions will
dominate today with IFR-LIFR levels on the Cape/Islands at
times. SW wind gusts of 35 knots across far southeast New
England diminish some toward daybreak. Otherwise, SW wind gusts
of 20 to 30 knots still expected into the afternoon with the
strongest of those across eastern New England.

Tonight and Wednesday...Moderate confidence. Winds shift to the
NW behind a strong cold front. Lingering showers should change
to snow across portions of the interior during the evening and
then eastern MA/RI between 5z and 10Z. Once the showers change
to snow, MVFR conditions should lower to IFR levels especially
near and south of the MA Turnpike where the highest risk exists
for a period of moderate to even briefly heavy snow. This may
result in even LIFR conditions for a time. Snowfall amounts look
to be on the order of 1 to 4 inches with the higher of those totals
most likely south of the MA Turnpike.  There also might be a sharp
cutoff in appreciable snow across northern MA.

Bulk of the snow should exit the region from NW to SE between
14z and 18z. Dry weather follows with conditions improving to
MVFR and then VFR.

KBOS Terminal...Moderate to high confidence in TAF. 1 to 3
inches of snow likely between 6z and 15z Wed with an impact to
the morning push.

KBDL Terminal...Moderate to high confidence in TAF. 1 to 4
inches of snow likely between 4z and 14z Wed with an impact to
the morning push.


Outlook /Wednesday Night through Saturday/...

Wednesday Night: VFR. Windy with local gusts up to 35 kt.

Thursday: VFR. Windy with local gusts up to 30 kt.

Thursday Night: VFR.

Friday: MVFR/IFR conditions possible. Breezy. Chance SHSN,
chance SHRA.

Friday Night: Mainly IFR, with areas MVFR possible. Breezy.
SHRA likely, SHSN likely, FZRA likely.

Saturday: Mainly MVFR, with areas IFR possible. Breezy. SHRA
likely.

&&

.MARINE...
Short Term /through Wednesday/...

Today...High confidence.  Gale force southwest wind gusts were still
occurring across many waters as of 09z, but expect these to diminish
over the next few hours as the core of the LLJ shifts east of the
region.  Otherwise, still expect SW wind gusts of 20 to 30 knots so
will need to replace with SCA headlines shortly.  Long southwest
fetch will continue to result in 7 to 13 foot seas across the outer-
waters. Showers become more numerous again late today.

Tonight and Wednesday...High confidence.  Winds shift to the NW this
evening behind a strong cold front.  NW 20 to 25 knot gusts expected
into Wed.  Showers tonight will change to snow for a time Wed
morning before things wind down by mid afternoon.  Reduced vsbys
expected.  SCA headlines will likely be needed for seas and marginal
25 knot wind gusts across the open waters.


Outlook /Wednesday Night through Saturday/...

Tuesday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Rough seas up to 10 ft. Rain likely, sleet
likely.

Wednesday: Winds less than 25 kt. Local rough seas. Chance of
snow, rain likely, sleet likely. Local visibility 1 nm or less.

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

Thursday: Low risk for Small Craft Advisory winds with local
gusts up to 30 kt. Local rough seas.

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

Friday: Winds less than 25 kt. Areas of seas approaching 5 ft.
Chance of rain showers. Local visibility 1 to 3 nm.

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

Saturday: Moderate risk for Small Craft Advisory winds with
gusts up to 30 kt. Rough seas up to 13 ft. Rain showers likely.

&&

.CLIMATE...
Record highs for Tuesday 12/10. Best chance to tie or break
record would likely be at Boston.

BOS ... 64/1907
PVD ... 67/1946
BDL ... 67/1946
ORH ... 63/1946

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Winter Weather Advisory from 10 PM this evening to noon EST
     Wednesday for CTZ002>004.
MA...Winter Weather Advisory from 10 PM this evening to noon EST
     Wednesday for MAZ012-013-015>022.
RI...Winter Weather Advisory from 10 PM this evening to noon EST
     Wednesday for RIZ001>007.
MARINE...Gale Warning until 9 AM EST this morning for ANZ231>235-237-
     250-251-254>256.
     Small Craft Advisory until 5 PM EST this afternoon for ANZ230-
     236.

&&

$$
SYNOPSIS...Frank/Nocera
NEAR TERM...Frank
SHORT TERM...Frank
LONG TERM...Nocera
AVIATION...Frank/Nocera
MARINE...Frank/Nocera
CLIMATE...Staff