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Area forecast discussion
National Weather Service Boston/Norton Massachusetts
723 am EST Mon Dec 9 2019

Synopsis...
milder air will push into the region today, accompanied by
showers today into Tuesday. A cold front moves through Tuesday
night, ushering in much colder air that will likely change the
rain to snow and linger into Wednesday with accumulations
possible. Large high pressure will bring cold and dry conditions
Friday. Another system will approach with mixed rain and/or
snow Friday night. Temperatures will slowly rise overnight
allowing the mixed precipitation to change to rain Saturday.

&&

Near term /until 6 PM this evening/...

7 am update...

* spotty light icing will continue through mid morning across north
CT into S central mass *

As southerly winds have been increasing along the S coast of
mass and Rhode Island over the last couple of hours, temps have risen to
the mid 40s to around 50 there at 12z. Further N, winds remain
light/variable or calm, so the warm air is riding up over the
cold dome near ground level across portions of north CT, north Rhode Island into
S central mass. Temps hovering around the freezing mark at
several reporting stations.

Area of precip has been filling in over the last hour or so
especially across central and west CT and into portions of Rhode Island and
interior southeast mass. As the precip becomes steadier, should see
winds mix down and temps slowly rise.

Will continue to see precip lift north-NE through this morning and
continue through the day and become more showery as the milder
air works north.

Previous discussion...

* flood potential *

Southerly flow has allowed temperatures to rise slowly through
the night across most of southern New England. High-res models are in good
agreement that more widespread showers will arrive between 7 and
10 am. Heavier rainfall should hold off until this afternoon as
larger scale lift increases ahead of mid level short wave and
nose of low level jet brings increasing pwats to over 1 inch, with likely
focus south of Mass Pike (and especially near south coast).
Ensembles indicate system is somewhat anomalous in its southerly
wind component and moisture (generally +2 to 3 sd), but not
excessively so.

Temperatures rising into 50s, combined with high dew points and
strong S/SW winds, will lead to rapid snowmelt across much of
southern New England especially in CT, RI, and eastern Massachusetts where nohrsc modeling
erodes much of existing snowpack. Water content in these areas
is no more than 1" on average, so while overall flood threat is
low for rivers/streams per mmefs hydrologic ensembles, there is
a higher potential for urban and poor drainage flooding which
will affect afternoon commute. Not planning on issuing flood
watches at this time.

* Strong winds *

925 mb winds increase to more than 60kt this afternoon across southeast
New England. Forecast soundings show strong inversion between
warmer air aloft and cooler air at surface, which will limit
true gust potential. However, momentum alone should be able to
realize about 2/3 of that, meaning Max gusts up to 40kt seem
reasonable. It looks marginal to reach Wind Advisory criteria on
Cape Cod/islands (may be more likely on sustained winds) but
since impact would be fairly low we will hold off on any wind
headlines, aside from marine as detailed below.

&&

Short term /6 PM this evening through 6 PM Tuesday/...
heavier rainfall continues through early tonight, again focused
south of Mass Pike where we have larger scale lift from both
low level and upper level jets, as well as strong low level
convergence and deep moisture. Activity should subside overnight
as short wave moves offshore.

Strong southerly flow will keep temperatures nearly steady
tonight, in 50s for much of sne, which will lead to further
erosion of snowpack. Per nohrsc modeling it's likely that we
lose all of the snowpack by Tuesday morning except across the
hills of western/central Massachusetts and northern CT. Expect continued
urban/poor drainage flood threat.

Other concern is for areas of dense fog, especially across
interior where deeper snowpack is in place.

Next round of showers arrives later Tue afternoon with approach
of cold front. By that time pwats are on the decrease and
moisture convergence is less impressive, so not anticipating
significant rainfall. We spend most of day in warm sector so
despite thick cloud cover highs will top out in the 50s to some
lower 60s.

&&

Long term /Tuesday night through Sunday/...

Highlights...

* rain moves in early Tuesday night, then changes over to snow
as temperatures fall

* confidence increasing for a plowable snow across the interior
and urban corridor after midnight Tuesday night through
Wednesday morning...conditions late Wed/Wed evening

* cold and dry Wednesday night through Friday. Moderating
temperatures and increasing moisture levels Friday night.

* Another weather system approaches from the southwest with
mixed rain/snow to start late Friday, changing to rain
overnight Fri night into Sat morning then continues through
most of the weekend

Details...

Tuesday night and Wednesday...

As the cold front pushes offshore, will see some spotty showers
lingering. However, with the SW flow aloft in place with the
long wave trough across the Great Lakes and Ohio Valley, next
area of rain lifts NE on the backside of the exiting front.
Excellent moisture plume (2-3 South Dakota above normal on the 00z gefs)
working across the region. Rain will back in and, as the colder
air filters back in, will become a brief wintry mix before
changing to snow.

Will see about 0.25 to 0.4 inches of precip overnight, which
could yield a few inches of snow as the colder air works in.
Will likely see impacts on roadways for the Wednesday morning
commute. Especially on secondary roads and other untreated
surfaces. The steady snow will continue through around midday
Wednesday before starting to taper off as the precip shifts
offshore during the afternoon.

At this point, could see several inches of snow mainly across
the interior and higher terrain. Will likely need snow headlines
for late Tuesday night into Wednesday for the central part of
the interior from the east slopes of the Berkshires and CT valley
(including the Hartford/Springfield area) to Worcester and
interior NE mass.

Wednesday night through Thursday night...

Large high pressure shifts east out of the Great Lakes across New
England during this timeframe. Expect the coldest air to move
across Wednesday night and Thursday as 850 mb temps drop to -9c
to -17c from S-north overnight Wed night. Temps will bottom out in
the teens well inland to the mid 20s to near 30 near the coast.
Highs on Thursday will only reach to 20s well inland, ranging to
the lower to mid 30s across the coastal plain.

Expect gusty northwest winds along the coast Wed night into Thu, up to
25-30 kt, so could see wind chills down to the teens.

Friday through Sunday...

Model solution spread increases during this timeframe as high
pres pushes toward the Maritimes Friday.

Will see light/variable winds to start on Friday, but some
question how quickly the next system will lift NE out of the southeast
U.S. Leaned toward an overall model blend. Could see some
moisture start to push across the region during the day Friday
along with milder air on S-SW winds. However, colder air may be
trapped at the onset of the precip during the day Friday, so
may start as light snow showers before becoming mixed with rain
or possibly a wintry mix mainly across the higher terrain.
Noting a non-diurnal temp trend through the night, so the min
should occur Friday evening.

Looks like temps will run close to or above seasonal normals for
mid December, but will also keep the chances for rain through
most of the weekend. Temps on Saturday will range from around 40
across the east slopes of the Berkshires to the lower 50s across southeast
New England, with readings on Sunday just a few degrees cooler.

&&

Aviation /12z Monday through Friday/...
forecaster confidence levels...

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

Short term /through Tuesday/...high confidence.

12z tafs...

Mainly MVFR ceilings/VFR visibilities well inland with local VFR conditions
across north mass taf sites. Further S, mix of VFR-MVFR conditions
at most sites, except IFR-LIFR across portions north CT into S
central mass. Expect lowest conditions across north central/northwest mass
with deep snowpack in place.

IFR-LIFR conditions will continue early tonight, then visibilities may
slowly improve as SW winds will increase overnight. Ceilings will
remain low.

Scattered -shra linger across region Tue with MVFR-IFR ceilings and
areas of MVFR-IFR visibilities mainly along S coast.

Low level wind shear this morning inland due to light winds at surface and 2kft
winds 19040kt. Later today and tonight, focus of low level wind shear shifts
into Rhode Island and eastern Massachusetts as southerly low level jet increases to
50-60kt at 2kft.

Kbos terminal...high confidence in taf.

Kbdl terminal...high confidence in taf.

Outlook /Tuesday night through Friday/...

Tuesday night: mainly VFR, with local MVFR possible. Breezy. Rain
likely, snow likely, pl likely.

Wednesday: mainly VFR, with local MVFR possible. Breezy. Chance
sn, isolated shsn.

Wednesday night through thursday: VFR. Windy with local gusts
up to 30 kt.

Thursday night: VFR.

Friday: VFR. Breezy. Slight chance shsn.

&&

Marine...
short term /through Tuesday/...high confidence.

** Gale warnings for most of waters through Tue morning **

Increasing S-SW winds today between high pressure well offshore
and warm front lifting north of waters. Onset of gale force
gusts is early to mid afternoon, with peak gusts of 35-40kt
expected from late afternoon through tonight, before winds fall
below gale force later Tue morning.

Steadier showers arrive this morning, with heaviest rain this
afternoon through early tonight. Patches of dense fog possible
mainly on south coastal waters. More in way of scattered
showers Tue before next round of steadier rainfall arrives
later in the day.

Outlook /Tuesday night through Friday/...

Tuesday night: low risk for Small Craft Advisory winds with
gusts up to 25 kt. Rough seas up to 10 ft. Rain likely, patchy
fog.

Wednesday: winds less than 25 kt. Local rough seas. Rain
likely, snow likely, scattered rain showers, scattered snow
showers, patchy fog.

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.
Slight chance of rain showers, slight chance of snow showers.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...none.
Rhode Island...none.
Marine...Gale Warning from 6 PM this evening to 9 am EST Tuesday for
anz231>235-237-250-251-254>256.
Small Craft Advisory from 2 PM this afternoon to 9 am EST
Tuesday for anz230.
Small Craft Advisory from noon today to 9 am EST Tuesday for
anz236.

&&

$$
Synopsis...evt/jwd
near term...evt/jwd

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