Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 161816
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
116 PM EST Sun Dec 16 2018

.SYNOPSIS...
Low pressure tracks south of New England today and tonight then
east of the area Monday. Spotty light rain becomes more
widespread and heavy at times after sunset into tonight. Across
northwest Connecticut into western-central Massachusetts spotty
light rain this afternoon transitions to sleet and freezing rain
especially at elevations above 1000 ft, then changes to snow
overnight. Any leftover precip at sunrise Monday gives way to
dry weather by midday. Much colder air follows late Monday into
Tuesday with blustery northwest winds, bitter wind chills. High
pressure follows, return mild conditions Wednesday afternoon
into Thursday. Potential slug of rain Friday with very mild
conditions and strong strong winds, followed by a return to
seasonable conditions next weekend.

&&

.NEAR TERM /UNTIL 5 PM THIS AFTERNOON/...

1 PM update...

New 12z guidance including the NAM/HREF and GFS support spotty
light rain this afternoon, then becoming steadier and heavier
from southwest to northeast after 21z or so as vertically
stacked low approaches the region. Model soundings have cold/dry
layer around 875 mb now but descending as heavier precip lowers
wet bulb heights with core of cold air settling around 3 kft
with freezing level heights down to about 1 kft or so. Thus
could see freezing rain at elevations above 1 kft after 21z or
so. Otherwise all other locations should be plain rain with
surface temps above freezing and warm nose aloft around 5-6kft.
Chilly/raw day for sure.

NE winds increase along the south coast, Cape Cod and Islands
after 21z as low level jet comes onshore. This will yield a wind
swept rain for the south coast this evening and overnight.

&&

.SHORT TERM /5 PM THIS AFTERNOON THROUGH 6 PM MONDAY/...

This is the period of greatest risk for wintry precipitation,
especially across the higher terrain, say about 1,000 ft or
higher. Very tricky and subtle precipitation type forecast.
Thermal profiles suggest temperatures are cold enough for snow,
despite the influx of colder air. The ingredient which seems to
be missing the most will be moisture within the dendritic snow
growth region. It`s marginal, and right around a tipping point.

Most of the 16/00Z guidance suggest more of rain/freezing rain
mix across the higher terrain, with some snow or sleet thrown
in at times. This does not look a big icing event. However, it
doesn`t take much ice to cause travel issues. Hence, the Winter
Weather Advisory will continue.

If more humidity remains in the snow growth region that current
expectations, obviously would mean more snow accumulation. This
will need to be monitored closely through the day and tonight.

Gave some thought to a Wind Advisory for the Cape and Islands,
but am thinking gusts will come up a bit below criteria.

The storm remains progressive, and moves farther offshore.
Precipitation expected to end during the morning. Winds turn NW
and slowly become less gusty. Partial clearing during the
afternoon. Colder air will start to arrive, but high
temperatures should still be near normal.

&&

.LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...

*/ Highlights...

 - Cold and blustery late Monday through Tuesday night
 - Coldest conditions Wednesday morning
 - Late-week warm-up
 - Potential soaking rains, possible flooding around Friday
 - Seasonable pattern weekend into early next week with showers

*/ Overview...

Very open Pacific flow with sub-tropical connection to a +ENSO and
lingering phase 3-4 MJO lends to a continual W/SW heat pump into
Canada echoing right up thru the stratosphere pushing the attendant
vortex to the other side of the hemisphere. Wiggles in N Pacific H5
troughing echoed in ensemble mean WPO / EPO trends through which
Pacific energy translates, expecting alternating downstream trends
with respect to storm-center tracks, however, with an overall trend
towards above seasonable conditions as we approach the holidays as
emphasized by CFS and EC weeklies. Ensemble mean preference along
with WPC forecasts, stronger NE Pacific H5 troughing will lend to W
CONUS ridging, E CONUS troughing thru this week into the following
weekend. Going into late December, may see the shift of N/NW Pacific
H5 troughing contributing to NE Pacific ridging, W CONUS troughing,
and E CONUS ridging, with subsequent above average temperatures for
our holiday season. Details in the discussion below.

*/ Discussion...

Late Monday through Tuesday night...

Looking at unseasonably cold conditions with NW winds potentially
exceeding wind advisory criteria (45 mph). Wind chills feeling some
10 to 15 degrees colder than actual. This resultant of NW gradient
flow between a SE Canada bombing low pressure and high pressure that
builds E. Strongest winds Monday night through Tuesday beneath the
coldest H85 air -12 to -14C allowing dry-adiabatic, unstable lapse
rates to H8, the top of the mixed layer with NW winds around 50 mph.
Only through a lowering warm, dry inversion with the building high
eastward will diminish winds into Wednesday morning. Lake effect
machine ongoing with scattered to broken cloud decks, possibly snow
shower / flurry activity over the Berkshires, dissipating as the
high builds E, leaning coldest guidance Wednesday morning with light
winds and mostly clear conditions. Highlight the potential WIND
ADVISORY wind threat in the HAZARDOUS WEATHER OUTLOOK.

Wednesday through Thursday...

Moderating temperature trends towards seasonable conditions. High
pressure shifting E / SE, return S flow. Looking at dry conditions
with scattered cloud decks.

Thursday night through Friday night...

Potential soaking rains and flooding. Anomalous situation. Upstream
pronounced W CONUS SW ridge enhancement downshears Pacific energy
into the sub-tropics. Inside-runner low occludes into the OH River
Valley with secondary low development and attendant warm frontal
boundary along the Mid-Atlantic coast NNE through SE New England
around Friday. S plume of +3-4 precipitable waters exceeding 1.5
inches per +2-3 S low level jet, pushing +10C H85 and around mid 50s
surface dewpoints into S New England. Convective indices exceeding
thresholds along with negative showalters, weak instability, and
high shear. Can`t rule out elevated thunderstorm activity along with
moderate to heavy rain. Potential +1 inch rainfall storm totals. But
a low confidence forecast per juxtaposition of synoptics as ensemble
members are quite spread with secondary low development and location
of S sub-tropical plume, slug of heavy rain, with some suggesting an
immediate offshore pass, New England falling beneath the dry slot
after a quick initial slug. Further W synoptics stack with the low
occlusion, the greater likelihood of a NE CONUS heavy rain event.

Weekend into early next week...

Over the weekend, the low lifting out, a sweeping secondary cold
front to the occlusion with some scattered shower activity. Brief
high pressure, seasonable, prior to a lifting warm frontal system
early next week based on preference to ECens guidance.

&&

.AVIATION /18Z SUNDAY THROUGH FRIDAY/...
Forecaster Confidence Levels...

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

Short Term /through Monday/...Moderate confidence.

18z update...

Thru 21z...

Marginal MVFR/VFR with spotty light rain and gusty ENE winds up
to 30 kt along the south coast including Cape Cod and Islands.

After 21z...

Marginal MVFR/VFR at 21z trends to IFR/LIFR in rain with lowest
conditions across eastern MA. Ptype mainly rain but
transitioning to sleet and snow (possibly freezing rain
elevations above 1000 ft) during the evening and overnight
across northwest CT into western - central MA. A coating to 2
inches is possible in this region. NE winds up to 35 kt or so
across southeast MA.

Monday...

Any leftover light snow inland, light rain along and near the
coast ends 12z-15z with IFR/LIFR eastern MA and MVFR elsewhere
improving to MVFR/VFR by 18z along with dry weather. Gusty north
winds up to 35 kt over Cape Cod and Islands at 12z diminish and
shift to the NW by 18z.

Monday night...

Marginal MVFR/VFR but dry weather except MVFR outer Cape Cod in
scattered snow showers. Gusty NW winds 25-35 kt.

KBOS Terminal...
Spotty light rain becoming more widespread and steadier/heavier
after 22z. Rain may briefly end as a mix of rain/snow late
tonight/very early Mon morning. Temps should remain above
freezing so runways will remain wet.


KBDL Terminal...
Spotty light rain becoming more widespread and steadier/heavier
after 21z. Rain mixes or changes to snow overnight but little if
any accumulation.

Outlook /Monday Night through Thursday/...

Monday: MVFR, with local IFR possible. Windy with local gusts
to 30 kt. Chance RA, chance FZRA.

Monday Night: Mainly VFR, with local MVFR possible. Strong
winds with areas gusts to 40 kt.

Tuesday: VFR. Strong winds with local gusts to 40 kt.

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

Wednesday through Wednesday Night: VFR.

Thursday: VFR. 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 Monday/...High confidence.

A gale center will move ENE across the southern coastal waters
of New England today and tonight, then continue to move farther
offshore Monday. Winds and rough seas will build this morning
across the waters, especially the outer coastal waters. Gusts
diminish late tonight as winds shift N, then NW. Rough seas will
linger across the outer coastal waters through Monday.

Outlook /Monday Night through Thursday/...

Monday: Moderate risk for Small Craft Advisory winds with local
gusts up to 30 kt. Rough seas up to 11 ft. Chance of rain,
patchy fog.

Monday Night: Moderate risk for gale force winds with gusts up
to 40 kt. Rough seas up to 11 ft. Chance of rain, slight chance
of snow.

Tuesday: Moderate risk for gale force winds with gusts up to
40 kt. Rough seas up to 11 ft.

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

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

Wednesday Night through Thursday: Winds less than 25 kt.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Winter Weather Advisory until 7 AM EST Monday for CTZ002.
MA...Winter Weather Advisory until 7 AM EST Monday for MAZ002>004-
     008>011-026.
RI...None.
MARINE...Gale Warning until midnight EST tonight for ANZ231>235-237-255-
     256.
     Small Craft Advisory until 10 AM EST Monday for ANZ230-236.
     Gale Warning from 5 PM this afternoon to 7 AM EST Monday for
     ANZ250.
     Small Craft Advisory until 7 PM EST Monday for ANZ251.
     Gale Warning until 7 AM EST Monday for ANZ254.

&&

$$
SYNOPSIS...Belk/Sipprell
NEAR TERM...Nocera
SHORT TERM...Belk
LONG TERM...Sipprell
AVIATION...Nocera/Sipprell
MARINE...Belk/Sipprell


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