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Area forecast discussion
National Weather Service Boston/Norton Massachusetts
614 PM EST Sat Dec 7 2019

Synopsis...
cold high pressure will move from the eastern Great Lakes to New
England by late tonight. This will result in a frigid night however
winds will be light. The high moves offshore Sunday, starting a
return flow of milder and moist air for the first half of the week.
This will bring off and on rain from late Sunday night into Monday
and Tuesday. A cold front moves through Tuesday night, and there
might be some snow Tuesday night into Wednesday although confidence
is very low at this point. Dry weather returns late Wednesday and
returns to colder than normal temperatures for early December
Thursday and Friday.

&&

Near term /until 6 am Sunday morning/...
600 PM update:

Increased sky cover a bit through 9 PM across northwest Massachusetts and into
northern CT to account for continued scattered stratocu. These clouds
are expected to fizzle thereafter to clear skies. Near optimal
radiational cooling conditions for tonight still looks on track,
in the single digits to lower teens (teens to low 20s out on
the cape/islands). Quite chilly, but without the wind chill as
winds project to go light/decoupled as high pressure settles
over New England overnight.

Previous discussion from 320 PM follows...

Tonight ...

Chilly night ahead with 1030+ mb high pressure currently over Lake
Erie and Ontario advects east into New England tonight. This will
yield diminishing winds and with diurnal clouds eroding along with a
very dry airmass (dew pts in the teens) in place, temps will fall
rapidly with sunset and mins overnight in the single digits and
teens. Much colder than normal, about 15-20 degs below average.
Given this setup have leaned toward the colder MOS guidance to
derive min temps. Fortunately winds will be light so not much of a
wind chill component tonight.

&&

Short term /6 am Sunday morning through Sunday night/...
320 PM update ...

Sunday ...

Deep layer ridge moves across the area during the morning. Thus dry
weather prevails along with abundant sunshine. However it will be a
very cold start to the day. Not much of a temp recovery as 1030+ mb
high over southeast Massachusetts limits boundary layer mixing given subsidence
inversion. So temps only 3-5 degs higher tomorrow with highs in the
mid to upper 30s, low 30s high terrain. Winds become south-southwest during the
afternoon, fairly light but will increase late in the day especially
from Cape Ann to Cape Cod in eastern Massachusetts.

Sunday night ...

Enough deep layer ridging nearby to support dry weather thru the
evening. However increasing return SW flow aloft will result in the
risk for showers late Sunday night and especially toward Monday
morning on nose of approaching low level jet. Given this low level
warm air advection pattern temps will actually rise overnight thru the 30s and into
the low 40s along the South Coast toward Monday morning.

&&

Long term /Monday through Saturday/...
highlights...

* mild temperatures Mon into Tue with heavy rains Monday
night/overnight. Mainly within-bank river rises anticipated
with rain/snowmelt combo.

* Possible minor accumulations of snowfall Tue nite-Wed that
could impact Wed am commute in interior southern New England
to I-95 corridor. Details and accumulations uncertain.

* Cold and dry for Wed into Fri, moderating late Fri.

* Unsettled weather into the weekend with warming temperatures.

Details...

European model (ecmwf) and GFS ensemble means show good consistency in the mid-
level pattern through midweek, with a pronounced upper-low over
Hudson Bay with a mean trough through the center of the country.
Rising 500 mb heights across southern New England early in the
period transition toward height falls by Tuesday into Wednesday.
Above to well-above normal temperatures along with substantial
reduction in snowpack is probable through Tuesday, given 850 mb
temps in the upper single digits c corresponding to
climatological 90th-percentile values, and 1-1.25" pwats being
2-3 Standard deviations above normal. That should also bring a
period of wet weather ahead of a cold front. While the GFS is
dry Wednesday, international guidance offers potential for a
period of anafrontal snows in the colder air Wednesday as far northwest
as interior southern New England. Considerable spread in
solutions exist (especially the nwern extent of the precip
shield), with varied handling of a shortwave trough over the SW
Texas/Mexico border being a key difference among the guidance.

By Wednesday into late week, the aforementioned mean longwave
trough shifts into New England, with a shot of below-normal
temperatures for Thursday into Friday associated with a cold,
dry Canadian high pressure area. Lower-confidence forecast for
the upcoming weekend with uncertainty in surface low track for
the weekend; however weather pattern should trend unsettled
again by the weekend with moderating temperatures back toward
above-normal levels.

Monday through tuesday:

Overall an unseasonably mild and rather wet period, with two
periods of rain: first with the isentropic/warm frontal rains
Monday afternoon into evening and another into Tuesday. Of the
two, the first rainy period looks to be the most substantial in
terms of expected rains.

Pops increase into the categorical range early to mid Mon aftn
across western/central Massachusetts into Rhode Island/northern CT (and areawide by
Monday late afternoon) amid persistent southwest warm/moist
advection, with enhanced 925-850 mb SW jet pumping in elevated
moisture/pwats into southern New England. Though models vary on
quantitative precipitation forecast magnitude, most of them have increased forecast quantitative precipitation forecast for
Monday into Monday night; appears the wettest quantitative precipitation forecast is confined to
areas south of the Mass Pike and along/southeast of I-95 perhaps
starting as early as the Monday afternoon commute. With the
influx in moisture, did note a 925 mb Theta-E ridge which noses
into coastal/eastern Rhode Island and into the South Coast/South Shore
areas Monday evening; in that period, while showalter indices
are on the stable side, k-index values are around 30. Didn't
include thunder in the forecast as the Prospect appears too
remote but nonetheless feel these indicate offer the potential
to enhance rainfall with somewhat greater rain rates into the
Monday PM commute into Monday evening period. Should see a
decrease in pops toward solid chance by the second half of the
overnight, essentially a break of sorts, ahead of the cold front
for Tuesday with an increase back into the likely range by
afternoon.

So a pretty good slug of moisture/precip through Tuesday, to
the tune of 1-1.5" north of the Mass Pike, and 1.5-2" south of
the Mass Pike into much of northern CT, Rhode Island into southeast mass.
Leftover snowpack will be significantly reduced and likely
nonexistent by Tuesday with several hours of >40 degree
dewpoints. Current snow-water equivalent (swe) per the Dec 7th
nohrsc snow analysis indicates there is still about 2" of snow-
water locked up in the snowpack across northwest Massachusetts into the eastern
Berkshires/Litchfield Hills where snow depth is greatest, with a
rapid southeast taper to less than an inch towards I-95 and less than
that across Rhode Island into southeast mass. Some of this swe will likely be
reduced slightly by sublimation over the next couple days. Did
look at mmefs-based river guidance for our hydrologic service
area, with some rise to action stage levels shown Monday into
Tuesday on some of our northern rivers such as the north Nashua
and shawsheen basins. Unless the forecast quantitative precipitation forecast shifts further to
the north/west than currently progged, probably not looking a rain W/
snowmelt river flood scenario as the heaviest rains don't look
aligned with where snow-water is significant. Certainly might be
some urban or poor drainage issues Monday night, though.

Winds will also increase Monday night towards the South Coast,
Cape Cod and the islands. European model (ecmwf) has 925 mb southwest jet of 50
to 60 kts by Monday night, though rising temperatures through
warm advection aloft may limit the mixing potential/depth both
on water and over land. Ended up capping wind gusts at 40 mph
across the cape and the islands - see this as a sub-advisory
scenario, with gusts 20-25 mph more into the interior as the jet
decreases in magnitude. Potential for gale-force winds on the
waters late Monday into Monday night.

Monday and Tuesday should prove unseasonably mild and will
likely feel far from typical early December weather with the
elevated dewpoints. Have highs into the 50s (nearing 60 towards
southeast mass) each day, with lows Monday night into the upper 40s to
lower-mid 50s.

Tuesday night through wednesday:

850 mb front surges through southern New England Tuesday night,
with temperatures falling quickly into the 20s to near
freezing. There remains some concern for anafrontal snows
(falling on the cold side of the cold front) Tuesday night into
Wednesday. This potential is hit the hardest by the European model (ecmwf) and
the ec ensemble, but also has support from the Canadian Gem.
Even drilling down into the 12z European model (ecmwf) ensemble membership, there
remains a lot of uncertainty with nearly as many members
indicating little to no snow while a few others show some
notable accumulations. Ensemble 24-hr probabilities of snow > 3
inches are greatest across Windham thru Hartford counties north/northwest
toward western and central MA, but those probabilities are no
better than about 30 percent. Light snows are possible towards
southeast mass and the cape but temperatures are more marginal. Also
Worth noting that the 12z European model (ecmwf) now shows less interaction with
southern-stream trough over the Rio Grande region in SW Texas which
shrinks the nwern extent of the precip shield but still brings
some light accumulations to the I-95 corridor into southeast mass/Rhode Island
overnight into Wednesday. Snow potential is something that will
need to be monitored. Unless a wave low can develop on the
front, though, anafrontal type snow situations aren't usually
substantial snow producers in general.

Lows Tuesday night to run into the 20s to near freezing, while
highs Wednesday to range only in the upper 20s to mid-upper 30s.

Wednesday night through Friday night:

Large/broad 1040 mb high pressure builds neward into northern
New England by Thursday into Thursday night, with related
ridging staying in place through Friday night. This will create
dry conditions and a couple nights of excellent radiational
cooling Wednesday and Thursday nights. 850 mb temps through
Thursday night are around -11 to -14c, which supports lows in
the single digits to the teens each night, and highs on Thursday
only in the 20s to near freezing out on Cape Cod.

Flow becomes swly by Friday, affording more moderation in
temperatures to values around freezing to the the upper 30s.
Lows in the 20s to mid 30s. These are still somewhat below
normal for mid-December.

The weekend:

Will likely see temps moderate back toward near/above normal
levels by the weekend with another period of pronounced S/SW
flow pumping in warmer temperatures and higher moisture levels.
Greater uncertainty in handling another coastal runner cyclone,
however, that should be the next weather maker after the early-
midweek system. It is looking like an unsettled weekend, but
details are uncertain.

&&

Aviation /23z Saturday through Thursday/...
forecaster confidence levels...

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

Short term /through Sunday night/...high confidence.

00z taf update:

Tonight:

Few-scattered stratocu mainly in the interior to erode shortly after
00z. High pres crests over the region resulting in winds
becoming calm. VFR and dry weather prevail.

Sunday...VFR. Light SW winds slowly increasing to 10-15 kt later
in the day.

Sunday night ... VFR to start becoming lowering to MVFR in
scattered showers after midnight. Low level wind shear possible late.

Kbos terminal...high confidence in taf.

Kbdl terminal...high confidence in taf.

Outlook /Monday through Thursday/...

Monday: MVFR/IFR conditions possible. Windy with areas of gusts
up to 30 kt. Rain showers likely, patchy br.

Monday night: MVFR/IFR conditions possible. Windy with gusts up
to 35 kt. Rain showers likely.

Tuesday: MVFR/IFR conditions possible. Windy with gusts up to
30 kt. Rain showers likely.

Tuesday night: mainly VFR, with areas MVFR possible. Breezy.
Rain showers likely, chance shsn.

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

&&

Marine...
short term /through Sunday night/...high confidence.

600 PM update:

Tonight ... northwest winds 10-15 kts with gusts 20 kts early, though
with diminishing winds, dry weather and good vsby as high pres
builds in and settles over New England overnight.

Sunday ... 1030+ mb high overhead during the morning but drift
offshore in the afternoon shifting winds from light/variable to south-southwest
over southern New England in the afternoon. Dry weather and good
vsby continue.

Sunday night ... high pressure offshore with south-southwest winds increasing to
near 25 kt especially eastern Massachusetts waters. Dry weather and good vsby
in the evening but showers likely develop late and especially toward
Monday morning lowering vsby.

Outlook /Monday through Thursday/...

Monday: low risk for gale force winds with gusts up to 35 kt.
Local rough seas. Chance of rain showers.

Monday night: moderate risk for gale force winds with gusts up
to 35 kt. Rough seas up to 14 ft. Rain showers.

Tuesday: low risk for gale force winds with gusts up to 35 kt.
Rough seas up to 14 ft. Rain showers likely.

Tuesday night: moderate risk for Small Craft Advisory winds
with gusts up to 25 kt. Areas of rough seas. Rain showers
likely.

Wednesday: moderate risk for Small Craft Advisory winds with
gusts up to 30 kt. Local rough seas. Chance of rain showers.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...none.
Rhode Island...none.
Marine...none.

&&

$$
Synopsis...nocera

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