Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 242352
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
752 PM EDT Fri Mar 24 2017

.SYNOPSIS...
High pressure moves farther offshore tonight bringing a
southwest flow of milder air to Southern New England. A cold
front will push south across the region Saturday. The front
will stall Saturday night, then will try to push northward
Sunday and Monday. Periods of light rain will fall Saturday,
then a mix of wintry weather will develop Sunday night into
early Monday. Rain may fall heavily at times later Monday as low
pressure passes across the region, then continues into Tuesday
ahead of an approaching cold front. Colder air lags Wednesday
into Thursday with breezy NW winds. Another chance for disturbed
weather Friday into the weekend given the continued active
weather pattern.

&&

.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...

735 PM Update...

Precip associated with warm front has pushed NE into central
Maine and NH as seen on latest NE regional 88D radar imagery.
Based on this and increasing cloud bases, have updated to remove
mention of precip through around midnight. Will start to see
band of light rain shift S after midnight as front begins its
southward return. Light SW winds will also begin to shift to W
across N Mass as well. Will still see gusty SW winds through
this evening along the S coast, but those will start to diminish
as the pres gradient relaxes.

With the front and colder air behind it remaining N of the
region through most of the night, have updated temps/dewpts
to reflect current trends. Readings should bottom out in the
35-40 degree range.

Have updated remainder of forecast to bring conditions current
and incorporated trends through the remainder of the overnight.

Previous Discussion...

Upper level jet will push north of New England overnight,
carrying a wave of low pressure across Quebec. This will in turn
move the front south as a cold front after midnight. Low level
convergence may generate light showers that reach Northern Mass
by morning, but otherwise expect little/no precipitation in
Southern New England tonight due to southwest flow and no low
level forcing.

The mild flow of air will keep temperatures from falling much.

&&

.SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/...
Saturday...

Cold front sweeps across the region first thing in the morning.
High pressure building in behind the front will turn winds from
the north Saturday morning and from the northeast in the
afternoon.

The upper jet referenced earlier will be to our northeast
Saturday with our area under the right entrance region. This may
maintain upper venting over/near the front while precipitable
water values along/south of the front remain at 0.75 to 1.0
inches, which is well above normal for mid-late March...the 90
pct exceed value is about 0.86. This should be enough to support
a chance of light rain, along and a little behind the front.
This rain shifts south of New England toward evening.

With clouds and potential light rain, expect temps to climb just
a little, with max values in the 40s.

Saturday night...

High pressure from Canada builds south into New England and
brings colder air. Expect dry weather most of the night. Dew
points will not fall as much as with most highs because our low
level flow will be out of the east-northeast, more of a marine
flow. Expect these dew points to be in the mid 20s to around 30.
As such, we expect min temps to be in the upper 20s to mid 30s.

With the ridge building in as it shifts toward the Maritimes,
the models show a developing cold air damming signature Saturday
night. This suggests the low level cold air will become
establish in advance of the next weather system.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...
Highlights...

* A period of wintry precip is possible Sunday night into Monday
  morning
* Unsettled weather pattern will continue into Wednesday
* Improving conditions during the later half of next week

Overview...

Confidence with the overall pattern across the northeast U.S.
appears continues to improve, but still issues with details in
timing of weather systems as well as thermal pattern especially
this weekend into early next week based on 12Z model suite and
ensembles.

Active pattern continues for the region with split flow aloft
and several shortwaves ejecting across the lower 48 from the
Pacific. Model guidances continues to signal the deamplification
of H5 cutoff low over the midwest, becoming an open wave as it
moves across the region in the steering flow late this weekend
into early next week. With confluent flow aloft, high pressure
persists over northern New England keeping cool surface
temperatures which will result in a wintry mix of precip at
times late this weekend into early next week. A second wave
will follow a similar path from the Plains towards the northeast
before meshing or partially interacting with the northern jet
stream on Tuesday. This will result in another system for New
England Tues/Wed with high pressure to follow.

Details...

Sunday...Moderate confidence.

A stalled front along the mid Atlantic coast will try to work
northward as high pressure ridge shifts SE into the Gulf of
Maine. Winds aloft are mainly SW, which will allow some low
level moisture well ahead of low pressure over the midwest to
work slowly N during the day as ridge will shift slowly E.

Marginal thermal fields and good cold air damming pattern
remains across the region, which will lead to patchy mixed
rain, freezing rain and/or sleet across the interior for early
Sunday, but they are shallow so temps should rise above freezing
during the morning, However, some sleet/freezing rain may
linger through midday across the higher terrain. Temps will
recover to the upper 30s to around 40 well inland, to the lower
40s elsewhere thanks to low level onshore flow off the milder
ocean.

Sunday night through Monday night...Moderate confidence.

Stalled front to the S tries to shift back N as a warm front
late Sunday-Monday, while surface low pressure works toward the
Great Lakes. Good overrunning works across the region, with
H85-H7 temps rising to +5C to +6C during this timeframe.
Shallow low level cold air damming remains in place, though,
with temps falling the upper 20s to around freezing. This will
cause mixed precipitation to develop during the nighttime hours
both nights. Looks like the best shot for widespread light
sleet and/or freezing rain will occur Sunday night into early
Monday morning. Noting mainly freezing rain signatures on both
NAM and GFS BUFKIT soundings, can`t rule out sleet especially
across the higher terrain of the E slopes of the Berkshires and
northern Worcester hills. At this point, could see up to 0.1
inches ice accretion, but this remains dependent upon how long
the cold air remains in place as well as the low pressure
passage and the QPF amounts that move across.

Low pressure passes across the region during Monday afternoon/
evening so expect good shot of precip to work across. This will
coincide with increased PWAT values of around an inch and, along
with continued confluent pattern, could see around 0.5 to 0.7
inches QPF which will move across when temps should be above
freezing. Also noting some marginal instability across N
CT/RI/SE Mass moving across Monday afternoon, with total totals
around 50 and TQ values in the upper teens. At this point, with
E-SE low level flow continuing, not expecting any thunder at
this time.

Precip should slowly taper off as low moves offshore Monday
evening. With the marginal thermal pattern remaining in place,
could see another round of light freezing rain, though looks to
be primarily across the higher terrain of N central Mass after
midnight through the early morning hours of Tue.

Tuesday into Wednesday...

Continued risk of freezing rain early on, if not at a minimum a
cold rain is forecast. Associated deeper mid to upper level low
over the S Hudson Bay region with a cascading, deamplifying
open wave trough through the S-stream yields an elongated area
of low pressure across the E CONUS with secondary low
development across the Mid-Atlantic. Initial over-running moist
ascent transitioning to frontogenetical, synoptic forcing ahead
of a sweeping cold front. However the colder air lags. So
altogether, PoP chances through Tuesday night, clearing into
Wednesday. A cool, damp day especially for N/NE MA initially,
gradually warming, with a decent slug of rain. The cold air
lagging along with clearing Wednesday, may end up mild as
conditions dry.

Thursday...

High pressure in control however the crux of the colder airmass
is drawn S to the E of New England as the early to mid week low
over the Hudson Bay region of Canada undergoes deeper
cyclogenesis across the N Atlantic. Given a mix of sun and
clouds, and that being a late March sun, leaning mild with more
seasonable conditions. Cold air advection proceeding in the low
levels along with an enhancement of flow aloft, expect some
gusty NW winds the the boundary layer mixing up to around H85.

Friday into the weekend...

Confluence within the mid to upper levels of N/S streams across
the NE CONUS continues. Any closed lows emerging out of the SW
CONUS undergo deamplification. Scenario for continued over-
running setups to which the thermal profiles across the region
require scrutiny. Conflicting signals of low pressure evolution
associated with weak wave impulses neighbored against high
pressure out of Canada. A lot can change and thus a low
confidence forecast. Only confidence is that we`ll continue to
see an active weather pattern into the long term period.

&&

.AVIATION /00Z SATURDAY THROUGH WEDNESDAY/...
Forecaster Confidence Levels...

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

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

Tonight...
Mainly VFR, with patchy MVFR CIGS across central and N Mass. SW
winds will gust to 25 kt through 03Z- 04Z, then diminish.
Patchy -RA may push S out of NH/VT across N/Central Mass after
06Z.

Saturday...
A cold front moves north to south across the region during the
morning. An area of light rain may develop, with cigs and vsbys
lowering to MVFR. Winds shift from the north in the morning and
northeast during afternoon/evening.

Saturday night...
Mainly VFR as high pressure builds from the north. MVFR will
lurk to our southwest, preparing to move north again. East flow
through the night.

KBOS TAF...High confidence in TAF.

KBDL TAF...High confidence in TAF.

Outlook /Sunday through Wednesday/...

Sunday-Sunday night...Low to moderate confidence.
VFR CIGS/VSBYS early Sunday. CIGS lower to MVFR-IFR mainly
across the interior by midday Sunday and continue Sunday night.
Mainly VFR VSBYS Sun, then lower to MVFR-IFR Sun night in rain
and patchy fog. Areas of -FZRA/-PL develop across the interior
Sun night, especially across the higher terrain.

Monday-Monday night...Low to moderate confidence.
Any leftover -FZRA/-PL Monday morning will end across higher
terrain. Otherwise MVFR-IFR CIGS in light rain. VSBYS should be
mainly VFR, but may see local MVFR at times. Low chance for
-FZRA Mon night across higher terrain of N Central Mass, mainly
N of Route 2.

Tuesday...Low confidence.
MVFR-IFR mix with -RA/RA, low risk FZRA early over N/NE MA. N/E
winds turning out of the N/W.

Wednesday...Moderate confidence.
Improving. Cigs lifting VFR. N/W winds continue.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Saturday Night/...

Tonight...High confidence.
SW winds gusting 25-30 knots early will diminish overnight, and
become west toward morning. Rough seas linger across the outer
coastal waters, and ocean exposed south coastal sounds.

Gales early on the eastern outer waters. Small Craft Advisories
will lower on Boston Harbor and Narragansett Bay, but linger on
the remaining waters.

Saturday...High confidence.
W winds shift to N in the morning and early afternoon as a cold
front moves north to south. Winds then shift from the northeast
by afternoon as high pressure moves in from Quebec. Winds will
be less than 25 knots through the day. Seas will linger at 5
feet on the southern outer waters and parts of RI Sound much of
the day. Small Craft Advisory will be needed where seas linger
near 5 feet.

Saturday night...Moderate-High confidence.
Northeast winds turn from due east overnight. Speeds remain
below 20 knots through the night. Seas will remain less than 5
feet through the night.

Outlook /Sunday through Wednesday/...

Sunday-Sunday night...Moderate confidence.
E-SE winds will increase especially on the southern outer
waters. Expect gusts up to around 25 kt on the open waters. Seas
may reach 5 ft on the outer waters S of RI briefly Sunday
afternoon/evening. Visibility reductions in rain and patchy fog
will develop Sun afternoon and continue Sun night.

Monday-Monday night...Low to moderate confidence.
E-SE winds continue. Winds gust to around 25 kt mainly over the
eastern waters as seas build to around 5 ft. May need small
craft headlines there. Local visibility restrictions continue in
rain and patchy fog mainly during Monday.

Tuesday through Wednesday...Low to moderate confidence.
Rain continues across the waters with E winds. Could see some
visibility restrictions ahead of a cold front sweeping the
region late into Wednesday morning. Behind the front winds
become more westerly and could see some gusts. Waves over 5 feet
possible on the outer waters.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 6 AM EDT Saturday for ANZ232>235-
     237.
     Small Craft Advisory until 8 PM EDT this evening for ANZ230.
     Small Craft Advisory until midnight EDT tonight for ANZ231-251.
     Small Craft Advisory until 10 PM EDT this evening for ANZ236.
     Gale Warning until 10 PM EDT this evening for ANZ250-254.
     Small Craft Advisory until 8 PM EDT Saturday for ANZ255-256.

&&

$$
SYNOPSIS...WTB/Sipprell/EVT
NEAR TERM...WTB/EVT
SHORT TERM...WTB
LONG TERM...Sipprell/EVT
AVIATION...WTB/Sipprell/EVT
MARINE...WTB/Sipprell/EVT


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