Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 241500
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
1000 AM EST Sat Feb 24 2018

.SYNOPSIS...
Unseasonably mild today with a period of showers possible south
of the Mass Pike late today and early this evening. A frontal
system will bring more precipitation late tonight and Sunday
with rain mixed with sleet at times in the coastal plain and
snow sleet and freezing changing to rain in the interior. Near-
seasonable, quiet and dry for the first half of next week,
attention turns towards late week and an anticipated stormy
pattern.

&&

.NEAR TERM /THROUGH TONIGHT/...
10 am update...
Very pleasant Feb conditions this morning. Temps already running
in the upper 40s and low 50s with plenty of sunshine out of
terrain-waves near the Berkshires and lingering stratus across
MVY/ACK, which should be clearing in the next hour or so.
Increased highs a bit as a result as timing of the clouds
associated with a weak leading wave moves in this afternoon
being slower than previously forecast. Many locations make the
50s, or even upper 50s.

Otherwise, narrow corridor of SHRA later this afternoon and
evening, mainly S of the Mass Pike. Not too much with this
initial round as the moisture in the column remains somewhat
low until tonight.

&&

.SHORT TERM /SUNDAY/...
*** Mixed precipitation event for interior late tonight and
 Sunday with some snow and ice accumulations ***

A few showers likely impacting south coast early evening,
otherwise dry weather through midnight before main area of
overrunning precip ahead of the warm front begins to overspread
the region from the SW to NE late tonight into Sunday.

Ptype will be biggest challenge with this event. Initially, the
column is warm but all the guidance is indicating a good cold
air damming signature developing overnight into Sunday as high
pres builds over the Maritimes. In fact, notable cooling in the
925-950 mb layer 06-15z with temps dropping to -2 to -4C before
warmer air moves in from the south. Warming temps above this
layer will set up a sleet and freezing rain scenario in the
interior where low level cold air will be difficult to dislodge
given warm front remaining to the south and a sfc wave tracking
along the boundary to lock in the cold air near the surface.
Some snow is also likely late tonight and early Sunday, north
of the Pike and especially near NH border where cold air just
deep enough as precip moves in. In the coastal plain, mainly
rain is expected, but given cold air below 850 mb, some sleet
will likely mix in with the rain late tonight and Sunday morning
even along the south coast.

Expect 1-3 inches snow/sleet accum in the interior north of the
Mass Pike with highest amounts near NH border. If warmer air
aloft is delayed a few hours it is possible a few locations
could approach 4-5 inches but this is a low probability. We are
concerned with several hours of freezing rain and icy
conditions across portions of the interior during Sunday,
especially over the Berkshires, Worcester hills and portions of
the CT valley. Ice accretion up to one tenth of an inch is
possible with up to 0.25" over the Berkshires. Expect temps to
gradually rise above freezing Sun afternoon in the interior as
precip intensity diminishes.

Winter weather advisories will be issued for all of central and
western MA and northern CT. Some freezing rain is possible in
NW RI and interior NE MA but confidence not yet high enough for
an advisory in this area. Across NE MA may be dealing with more
of a snow/sleet to rain scenario.

&&

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

 - Quiet, dry, near-seasonable early week period
 - Stormy pattern emerging late week for the beginning of March

*/ Overview...

Blocky N Atlantic pattern, associated +3 standard deviation H5 high
retrograding W into SE Canada, parent 1040+ surface high, pressing
the thermal wind axis S through which N/S stream impulses merge, dug
through a prevailing W CONUS H5 trof before ejecting and stretching
E. Energy piling up, cutting off for a time given the block, an area
of favorable storm development emerges off the SE Canadian coast,
evolving S/W towards the NE CONUS per retrograding high, E ejecting
energy. Airmass juxtaposition key as to outcomes towards the end of
the week after a quiet start. Only confidence is persistence of NE
winds of significant duration during high astro-tides, concern with
respect to coastal flooding. Lower confidence otherwise, consensus
forecast guidance heavily weighted with ensemble means. Hit targets
of opportunity below.

*/ Discussion...

Thursday into the weekend...

While considerable ensemble member / deterministic guidance spread
there`s cohesive signal of a Great Lakes warm occlusion transferring
energy offshore beneath a more favorable region of storm development
evolving towards the NE CONUS. Occluded front transitioning to an
inverted trof, energy pinwheeling round a closed, stacking low well
offshore. Questions surround frontal boundary magnitude buffeting
the pressing N high, how far N/E outcomes slide into New England.
NCEP noting poor run-to-run continuity with ejecting waves, EC, even
the 24.0z, worse than GFS which appears to be locking in. Warm core
low, absent Arctic air, anticipate precip-type issues, rain / snow,
lean to occluded front / inverted trof hanging up into the NE CONUS
as energy transfers, low rolls up offshore. Only confidence is the
persistence of NE winds during high astro-tides, concern of coastal
flooding. Only chance PoPs given spread.

&&

.AVIATION /15Z SATURDAY THROUGH WEDNESDAY/...
Forecaster Confidence Levels...

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

Short Term /through Sunday/...Moderate confidence.

Today...MVFR/IFR eroding over S/SE coastal terminals, while BKN
MVFR decks filter into W New England. With increasing sunshine
should see decks erode, optimistically. However, towards later
in the day, increasing low-mid level decks and -RA, especially
for CT, RI, southeast MA. Light winds overall turning NW late.

Tonight...VFR in the evening, lowering to MVFR from west to east
late. Rain, sleet and freezing rain developing from SW to NE
well after midnight with snow/sleet across northern MA. Mainly
rain mixed with sleet coastal plain.

Sunday...Widespread IFR/LIFR developing. Rain, mixed with sleet
at times in the morning in the coastal plain. Snow, sleet and
freezing rain changing to rain interior.

KBOS Terminal...High confidence in trends. Conditions improving
to VFR early this morning, but exact timing may be off a few
hours.

KBDL Terminal...High confidence in trends. Conditions improving
to VFR early this morning, but exact timing may be off a few
hours.

Outlook /Sunday Night through Wednesday/...Moderate confidence.

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

Sunday Night: Mainly VFR, with local IFR possible. Slight
chance RA.

Monday through Wednesday: VFR.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Sunday/...Moderate confidence.

Today...Diminishing winds becoming northerly late today. Seas
below SCA.

Tonight and Sunday...Increasing easterly winds late tonight and
especially Sunday as low level jet moves across the waters.
Given cold air above the boundary layer, mixing will likely be
sufficient for a period of gale force gusts, especially over
the south coastal waters during Sun. Otherwise, expect SCA
conditions with gusts to 25-30 kt. Gale watches issued where
highest confidence of gales.

Outlook /Sunday Night through Wednesday/...

Sunday: Low risk for gale force winds with gusts up to 35 kt.
Rough seas up to 10 ft. Rain. Visibility 1 nm or less.

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

Monday: Winds less than 25 kt. Seas up to 5 ft.

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

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

Tuesday Night: Winds less than 25 kt.

Wednesday: Winds less than 25 kt. Seas up to 5 ft.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Winter Weather Advisory from 1 AM to 1 PM EST Sunday for
     CTZ002>004.
MA...Winter Weather Advisory from 1 AM to 1 PM EST Sunday for
     MAZ002>004-008>012-026.
RI...None.
MARINE...Small Craft Advisory from 3 AM to 4 PM EST Sunday for
     ANZ232>234.
     Small Craft Advisory from 6 AM to 5 PM EST Sunday for ANZ230.
     Small Craft Advisory from 3 AM to 7 PM EST Sunday for ANZ231.
     Small Craft Advisory from 6 AM to 1 PM EST Sunday for ANZ236.
     Gale Watch from Sunday morning through Sunday afternoon for
     ANZ235-237.
     Small Craft Advisory from 6 AM to 7 PM EST Sunday for ANZ250-
     251.
     Gale Watch from Sunday morning through Sunday afternoon for
     ANZ254>256.

&&

$$
SYNOPSIS...KJC/Sipprell
NEAR TERM...Doody
SHORT TERM...KJC
LONG TERM...Sipprell
AVIATION...KJC/Sipprell
MARINE...KJC/Sipprell



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