Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 181729
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
1229 PM EST Mon Dec 18 2017

.SYNOPSIS...
A weak wave of low pressure will bring some spotty light snow
today, especially in the interior. Milder temperatures return
Tuesday ahead of a cold front, which will push across the region
Tuesday night. Blustery and colder conditions will move in
Wednesday and continue into Thursday. Low pressure will like
passes west of the region Friday night and Saturday, which could
bring mainly rain and a period of rather mild temperatures
along with gusty winds. Another period of unsettled conditions
may approach late Saturday night into Christmas Eve, but timing
and precipitation types remain uncertain.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
10AM update...
Even though light echos continue on radar this morning, they
generally remain at or below 20dbz. Flurries have really
struggled to reach the sfc, with the likely issue being the
larger magnitude dwpt depressions. Actual values are running
about 5-10F drier than guidance, even those going through their
12Z initiation.

These lower dwpts may factor in precip this afternoon and
evening, but continue to echo the concern of previous forecaster
regarding the risk for FZDZ or even light FZRA as drier air
lingers aloft with a gradual moistening of the lower lvls. Sfc
temps will be marginal, but close enough for some light
accretion especially with some diurnal cooling this evening. No
headlines at this time, but will need to consider at least a
statement for the afternoon and evening.

Previous Discussion...

Weak warm advection pattern ahead of a warm front will bring
risk of spotty light snow to SNE today. Any light snow which
will mix or change to rain across eastern New Eng will be
spotty this morning. Any accum will be rather light, a coating
to a half inch at most, mostly in western New Eng with around
an inch in the E slopes of the Berkshires.

The afternoon looks mostly dry but can`t rule out a continued
threat of spotty light precip. Low level warming below 900 mb
moving up from the south will allow any precip along the south
coast to change to rain by midday. As this low level warming
advances north, there is a low risk of some spotty freezing
rain/drizzle late afternoon/evening in northern and
particularly northeast MA where temps remain below freezing,
but confidence is low.

Expect a north-south gradient in temps today with highs ranging
from near 30 in northern MA to around 40 along the immediate
south coast, with lower 40s over the Islands.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/...
Tonight...
Main concern for the evening period is a low risk for spotty
light freezing rain/drizzle across northern and northeast MA.
Confidence is low as there may not be any precip. However,
there may be some patchy fog around in the evening and with
small T/Td depression tonight and temps settling back near or
below freezing, areas of black ice may develop away from the
south coast.

Tuesday...
Next northern stream shortwave and attending cold front
approaches New Eng toward evening. Low level warming ahead of
the front and increasing SW winds will allow temps to reach into
the 40s. Mainly dry conditions with some sunshine, although
there is a risk of showers in the western MA in the afternoon
as the front approaches.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...
Details...

Tuesday night...

Low pressure will pass across southern Quebec. Most of the
energy and moisture will remain north of the region, so should
see mainly dry conditions. Scattered light rain and/or snow
showers may linger across the E slopes of the Berkshires. A dry
cold front will push across the region, with wind shifting from
SW to W-NW by around midnight. Colder air works in during the
overnight hours. Noting a very strong low level jet, on the
order of 35-40 kt at H925 up to 50 kt at H85. Fairly good low
level lapse rates also move across overnight, so will see gusts
up to 25-40 mph, highest across the higher inland terrain and
along the immediate coast.

Expect temps to bottom out in the upper 20s to around 30 well
inland ranging to 35-40 along the shoreline. The gusty winds
will make it feel much colder early Wednesday morning.

Wednesday and Thursday...

Large Canadian high pressure will build across the region
during this timeframe. Blustery NW winds will continue into
Wednesday, especially across central and eastern areas as the
strong low level jet and very good low level lapse rates will
continue. Could see winds continue to gusts up to 25-40 mph
through the daylight hours Wednesday, then should diminish Wed
night as the low level jet and higher lapse rates move offshore.

The coldest air will push across later Wed and especially
Thursday with another surge of cold air. H85 temps will drop
back to -8C to -12C by early Thursday morning. Temps will
struggle to reach the 20s across the interior, ranging to the
lower-mid 30s along the shore. Some higher wind gusts may linger
across the immediate coast early Thu, but will diminish as the
ridge axis crosses the region.

Friday and Saturday...

As a digging H5 trough develops across the central U.S., low
pressure will form during Friday, moving toward the Great Lakes.
Responding mid level ridge will build from Quebec and down the
eastern seaboard during this timeframe. The S-SW winds
associated with this ridge, and out ahead of the developing
system to the N and W, will bring milder temperatures as S-SW
winds may gust up to 25-35 mph especially along the S coast Fri
night and Sat. Strong warm air advection will work northward, so
temps on Saturday could reach into the mid or even upper 50s.

Models having trouble handling the development of the surface
and upper level features, as well as the track and timing of the
low and associated arctic front across the region. At this
point, with colder air still trapped across N central and W
Mass, may see a mix bag of wintry weather at the onset of the
precipitation sometime Friday afternoon. As the milder
temperatures move in, will see any mixed precipitation change
over to rain sometime Friday night.

Current timing suggests that the arctic front sometime
Saturday, depending upon how fast the front pushes eastward.

Sunday...

With widening model solution spread, have low confidence with
this portion of the forecast.

As the cold front settles S of the region and stalls in the
continued SW flow aloft, may see another low develop and,
depending upon its track and timing, could bring another round
of rain or mixed precipitation into the region. For now, have
brought in the mainly rain later Sunday, but steadier rain, with
mixed precip across northern and western areas, will develop
during Christmas Eve. A lot of uncertainty as to the exact track
and timing of the precip, as well as impacts especially during
Christmas Eve. Stay tuned.

&&

.AVIATION /17Z 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 night/...

Through tonight...High confidence.
Mainly MVFR due to CIGS through this evening. Occasional light
SHSN and even some light FZRA possible, especially SE MA this
evening. Ice accums around a T where it occurs. Otherwise, some
fog/freezing fog possible overnight away from the coast,
dissipating during the morning hours as winds shift to the SW
and CIGS lift to VFR.  Timing may be off by a few hours in TAF.

Tomorrow and tomorrow night...High confidence.
Mainly VFR, although with some occasional FEW-SCT MVFR clouds
during the day tomorrow. Main issue will be W-SW breezes which
could gust to around 20-25 kt especially late day and overnight,
peaking near shore where gusts may approach 30 kt tomorrow
night.

KBOS Terminal...High confidence. MVFR CIGS return through this
evening. FZ DZ should stay mainly NW of the terminal, but there
is a low risk this evening, mainly between 23Z and 04Z if it
were to happen.

KBDL Terminal...Moderate confidence in TAF.

Outlook /Tuesday Night through Friday/...

Tuesday Night through Wednesday: VFR. Windy with gusts to
35 kt.

Wednesday Night: VFR. Windy with local gusts to 30 kt.

Thursday: VFR. Breezy.

Thursday Night: VFR.

Friday: Mainly VFR, with local MVFR possible. Breezy. Slight
chance RA.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Tuesday/...High confidence.

Today...Winds becoming southerly and increasing in the
afternoon, but gusts below 20 kt. Light seas. Spotty light snow
or rain possible. Good vsbys.

Tonight...SW winds below SCA. Gusts to 20 kt. Seas below SCA.

Tuesday...Increasing SW winds with gales likely developing by
late afternoon. Will issue a gale watch for late afternoon
through Tue night. Gusts to 40 kt possible.

Outlook /Tuesday Night through Friday/...

Tuesday Night: Moderate risk for gale force winds with gusts up
to 35 kt. Rough seas up to 10 ft.

Wednesday: Low risk for gale force winds with gusts up to
35 kt. Areas of rough seas.

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 gusts up
to 25 kt. Areas of seas approaching 5 ft.

Thursday Night through Friday: Winds less than 25 kt.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Gale Watch from Tuesday afternoon through Wednesday morning
     for ANZ231>235-237-250-251-254>256.

&&

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


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