Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 220239
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
939 PM EST Wed Feb 21 2018

.SYNOPSIS...
A cold front moves through Southern New England early tonight.
Much colder weather then moves in for Thursday. The front stalls
south of New England Thursday, an area of unsettled weather
moves along the front and across Southern New England. This will
bring a mix of precipitation, with snow and sleet north of the
Mass Pike while rain and sleet falls to the south. An active
wet weather pattern Friday through the weekend during which
there is the possibility of mixed precipitation, freezing rain
late Friday into Friday night and again around Saturday night
into Sunday. Dry weather will follow.

&&

.NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
935 PM update...
Last of the fog confined to Cape/Islands and will be lifting
next 2-3 hours as the front moves through and low level drier
air advects in.

Batch of showers across LI sound is moving east and may impact
the Islands and possibly the immediate RI south coast through
midnight. May see more showers developing to the south
overnight within deeper moisture axis, but low level dry air is
pushing south and will keep highest threat of showers south of
New Eng. Will carry slight chc pops for the Islands.

Colder air will continue to drain south into the region with
temps by daybreak in the 30s, to near 40 along the south coast,
with subfreezing temps higher terrain across northern MA. Temp
change of 35-45 degrees from todays warmest highs.

&&

.SHORT TERM /6 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/...
730 PM Update...

Latest trends in the guidance suggested a cooler scenario for
Thursday event. Thus went ahead and updated the forecast to
reflect these trends. Quick moving wave will ride along the
stalled front south of the region, and with CAA continue to push
through, expect more of an isothermal transfer of temperatures
within the profile. Thus mid and low level temperatures will
cool leading to more of a snow/sleet profile than rain as shown
in BUFKIT model soundings. Still have to watch for a warm layer
around 800mb, but feel that precip will fall hard enough that
the profile should maintain frozen precip (snow/sleet).

All said in done, north of the Pike may see some sleet but will
transition to all snow by the afternoon. Could see around 1-3
inches on elevated surface. Snow will struggle to stick on area
roadways as they are quite warm thanks to the 70+ degree day
today. Along and south of the Pike expect more of a mixed bag of
precip leading to rain to sleet to snow to a low chc of freezing
rain. Lastly along the south coast, could see rain at first, but
if precip falls hard enough then could transition to sleet with
the potential for some freezing rain.

Overall confidence is higher with snow and sleet outcome than
with any freezing rain. But if there is a chc for FZRA than
winter weather advisories may be needed as it only takes a T.


Previous Discussion...

The cold front stalls over the Mid Atlantic coast and south of
New England Thursday. High pressure from the Plains then moves
to the Great lakes Thursday, maintaining the colder temps over
our area and setting up a cool northeast flow off the Gulf of
Maine. The right entrance region of the upper jet will be in
position just as this colder air pushes south.

All of this will provide sufficient lift to generate an area of
precipitation that moves up from the Ohio Valley. The challenge
with this forecast is determining precip type. No cold air
damming signal in the data. there is just the infiltration of
cold air with the high pressure and the northeast wind off the
marine environment. We are maintaining snow and sleet north of
the Mass Pike and rain south of the Pike and along the eastern
coast.

Accumulations will be light, with the best chance of minor
accumulations across the higher elevations of Northern Mass.

Confidence in precipitation is moderate-high, but confidence in
precip type is low-moderate. As noted on the previous forecast,
a warmer solution would mean more rain and less snow/sleet.

Thursday night...

The precipitation moves offshore, and high pressure builds over
Northern New England. The high center moves to the Maritimes by
12Z Friday with a cold air ridge extending southwest through New
England. Low level moisture lingers over Southern New England,
which should keep clouds south of the Mass Pike overnight.
Northern Mass could have partial clearing for a few hours.

&&

.LONG TERM /FRIDAY THROUGH WEDNESDAY/...
Highlights...

* Burst of precip with wintry mix in the interior Fri night
* Dry but warm as Spring returns on Saturday
* Widespread precip on Sunday with wintry mix in the interior
* Dry weather trend for early next week

Overview...

12z guidance is in general agreement with the extended forecast.
Just some minor differences related to thermals which provide
different outcomes in p-type. Deep trough across the western CONUS
with ridging over the Eastern Seaboard. This puts the region in
southwest flow allowing for Gulf moisture to stream into the area.
Ridge will begin to flatten out as subtropical high begins to
retreat back west towards the Gulf of Mexico by the end of the
period. A few waves of low pressure will move through the flow with
the first one Friday night followed by one on Sunday. Thereafter
pattern begins to breakdown as ridge moves over the Great Lakes
region and cut-off low begins to set-up over the Maritimes. This
could result in a return to average to below avg temps mid to late
next week.

Details...

Friday into Saturday...Moderate confidence.

Retreating high pressure as baroclinic zone remains south of the
region. Dry weather to start but with front near by an approaching
WAA anticipate showers to develop and expand by the late afternoon
hours. Thermals will be tricking as WAA moves through bringing in a
good warm nose per BUFKIT soundings. Biggest question is the surface
temps and if they will warm above freezing as high pressure is
retreating resulting in more of a easterly flow rather than the
classic northerly cold air damming. The GFS appears the warmest of
the guidance in terms of 2M temps keeping p-types as mainly rain.
This does appear to be an outlier compared to the NAM/EC and CMC.
While hi-res data would give this forecast more confidence, appears
that the trend is increasing in a period of freezing rain during the
evening commute, esp in NW interior MA.

Precip will come to an end by the overnight hours as another front
pushes into the region an stalls just south of the area. With this
front near by, a few showers are possible early Saturday morning,
but strong high and ridging overhead will help result in a dry
forecast.

Despite westerly to northwest flow, surface temps could potentially
warm into the low to mid 50s by the afternoon. However, depending on
where frontal system stalls, temps could actually rise or fall. Thus
a low confidence forecast for temps on Saturday.

Saturday night into Sunday...Increasing confidence.

Last wave will eject out of the desert southwest producing surface
low pressure system up into the northern Great Lakes. Another shot of
overrunning precip as warm front approaches from the south. Again
depending on thermals, we could see a chc for freezing rain at the
onset. This system looks to bring the most QPF out of all the system
so expect widespread precip, esp as we continue to tap into that
Gulf moisture. One thing to watch is the development of a surface
low along the front. This could help drag in more cold air from the
north leading to a changeover from rain to sleet/snow on Sunday.
Still some uncertainty with this forecast because of the thermals
in question. But cannot rule out wintry precip during this period.

The front will quickly push through resulting in a drying trend for
late Sunday night.

Monday and beyond...Moderate confidence.

High pressure will build across the region Monday and last into
early next week. Temperatures will be seasonable to slightly above
average as the pattern begins to shift.

&&

.AVIATION /03Z THURSDAY THROUGH MONDAY/...
Forecaster Confidence Levels...

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

Short Term /through Thursday Night/...Moderate confidence.

Tonight...

IFR fog lingers over Cape Cod and Islands in the evening, but
will improve to VFR 03-06z. Cold front is pushing through the
region switching SW winds to a NW direction. Behind the front
winds will quickly pick up around 20-30 kts, but should only
last an hour or two. VFR conditions remainder of Southern New
England.

Thursday...

VFR in the morning. Winds turn from the northeast. Clouds lower
and thicken as an area of precipitation moves in from the west.
Afternoon cigs/vsbys lower to MVFR in snow/sleet north and
light rain south. -SN accumulation possible north of the Mass
Pike. Potential for -PL/RA south of the Pike. Low confidence in
-FZRA.

Thursday night...

Rain/sleet/snow moves offshore along with the low clouds. Most
of the night should be VFR with continued northeast winds.

KBOS Terminal...High confidence TAF. VFR through Thursday
morning. Cigs/vsbys lower to MVFR in the afternoon with PL and
SN.

KBDL Terminal...High confidence in TAF. Cigs/vsbys lower to
MVFR in the afternoon in rain, possible mixing with sleet.

Outlook /Friday through Monday/...Moderate confidence.

Friday: Mainly MVFR, with local IFR possible. RA, slight chance
FZRA across interior NW MA.

Friday Night: MVFR/IFR conditions possible. Chance RA, chance
FZRA across interior NW MA..

Saturday: VFR.

Saturday Night: Mainly MVFR, with areas VFR possible. Breezy.
RA.

Sunday: Mainly MVFR, with local IFR possible. Windy with local
gusts to 30 kt. RA.

Sunday Night: Mainly VFR, with local MVFR possible. Breezy.
Chance RA.

Monday: VFR. Breezy.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Thursday Night/...Moderate confidence.

Tonight...

Fog is slowly shifting east. This will bring improving vsbys
over the waters this evening and early tonight.

Cold front moves across the waters early tonight, then
stalls south of the waters. Southwest winds shift from the
northwest and then north overnight. Winds gusting near 25 kt
this evening, then 20 knots or less after the front moves
through. Seas at 5 feet will subside overnight. Small Craft
Advisory for wind and rough seas on most waters this evening and
on the outer waters through midnight.

Thursday...

Winds turn from north and then northeast with gusts to 20 knots
at times. Seas mostly below 5 feet, except near 5 feet on the
outer edge of the outer waters. Rain moves in from the west
during midday and afternoon. Could be some mixing with sleet
especially along the North Shore of Mass.

Thursday night...

High pressure centered over Canada moves to Maine and the
Maritimes overnight. Rain moves off to the east. Northeast winds
continue with gusts 20 knots or less. Seas less than 5 feet.

Outlook /Friday through Monday/...

Friday: Winds less than 25 kt. Seas locally approaching 5 ft.
Rain.

Friday Night: Winds less than 25 kt. Seas locally approaching
5 ft. Chance of rain.

Saturday: Winds less than 25 kt.

Saturday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Seas locally approaching 5 ft. Rain likely.

Sunday: Moderate risk for Small Craft Advisory winds with gusts
up to 30 kt. Rough seas up to 11 ft. Rain.

Sunday Night: Winds less than 25 kt. Areas of rough seas.
Chance of rain.

Monday: Winds less than 25 kt. Local rough seas.

&&

.CLIMATE...
Daily record high temps have been set at all four climate site.
Records for all-February have been set at Hartford and
Worcester.

High temperatures through 3 pm...

Boston.....72..Daily record 63 (1906)..Feb record 73 (2/24/2017)
Hartford...74..Daily record 63 (1930)..Feb record 73 (2/24/1985)
Providence 64..Daily record 63 (1930)..Feb record 72 (2/24/1985)
Worcester..71..Daily record 59 (1930)..Feb record 69 (2/24/2017)

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 1 AM EST Thursday for ANZ254>256.

&&

$$
SYNOPSIS...WTB/Dunten
NEAR TERM...KJC
SHORT TERM...WTB
LONG TERM...Dunten
AVIATION...WTB/Dunten
MARINE...WTB/Dunten
CLIMATE...



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