Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 141518
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
1018 AM EST Fri Dec 14 2018

.SYNOPSIS...
High pressure moves E of New England through today with a mild
south flow developing. A series of low pressure systems
tracking south of New England may bring a period of rain Friday
night into early Saturday ... and again Sunday into Sunday
night with mixed precip and ice possible in the interior. A cold
front sweeps across the region Monday followed by blustery and
cold weather Tuesday. Moderating temperatures by Wednesday into
Thursday. Another storm system possible by late week into
weekend.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...

10 am update...

Sunshine across much of the region this morning including
central-eastern CT into RI and most of central-eastern MA will
give way to increasing clouds this afternoon in developing warm
air advection pattern. SPC mesoanalysis shows 0C isotherm at 925
mb over the region and moving northbound via southerly flow.
Although 1034 mb high over the area providing subsidence
inversion that will limit boundary layer mixing below 925 mb. So
perhaps forecast of temps rising into the 40s seems reasonable
especially along the coast and into the I95 corridor with winds
off the ocean and water temps in the 40s. Previous forecast
captures these details nicely so no major changes planned with
this update.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/...

It still appears that confluent mid level flow will develop over our
region later tonight. The main impact of this will be to steer a
low pressure to our south, limiting the risk for significant rainfall.
Until this confluence develops, we will have to contend with a weak
shortwave and weak front. These two features should be enough to
trigger scattered areas of light rain early this evening. Winds
should remain light S to SW, so temperatures should hold nearly
steady overnight.

High pressure moving into the Saint Lawrence Valley Saturday should
bring drier weather from N to S during the day. Winds turn more
from the N Saturday afternoon, starting the return of colder air.

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...

*/ Highlights...

 - Wintry mix around Sunday
 - Turning colder Monday through Tuesday
 - Mild conditions return Wednesday into Thursday
 - Another storm system possible late week / weekend

*/ Overview...

Ensemble mean waffling of the N Pacific H5 trof lends to variable
trends within WPO / EPO teleconnections. Of certainty, continuation
of a progressive, mild Pacific jet lends to reloading shots of warm
air across N America echoed up through the stratosphere, seemingly
shunting the stratospheric vortex and colder Arctic air to the other
side of the N hemisphere. Persistent stream of Pacific energy, an
active mid to late December pattern is likely, however juxtaposition
of ridging / troughing across the CONUS is not. Present transition
towards a ridge-trof regime may reverse come the holidays, N-winds
initially, more S further out in time. Some signal gleaned from EC
weeklies and CFS of holiday warmth, but no true confidence at this
time.

For now, a H5 ridge-trof setup, downshearing potential vorticity and
colder air, tapping into the S-stream sub-tropical Pacific jet. Yet
Arctic air bottled up N. Cohesion between the two going into this
weekend, there is the potential for winter weather outcomes which
are highlighted in the discussion below along with forecast model
preference. As noted by the prior forecaster, key to the discussion
is the cohesion / phase timing with respect to cyclogenesis relative
to the 40N/70W benchmark.

*/ Discussion...

Saturday night through Sunday night...

Possible interior wet snow. Consensus trend, downsheared PV-streamer
and attendant trop-fold from Canada into the NE CONUS draws inward a
sub-tropical Pacific S-stream closed low thru cyclonic convergence
and ascent beneath lowering heights. However, individual forecast
model guidance struggling on this transition particular to locations
of surface to mid-level features relative to the 40N/70W benchmark,
as well as subsequent interior impacts related to deformation, low
level convergent forcing, and broadscale ascent. Aside, good model
agreement on thicknesses illustrating an Arctic air disconnect, the
storm having to manifest its own cold towards wintry precip-types,
only doing so through dynamic cooling and surface wet-bulbing that
is proportional to greater storm strength. Downstream progressive
flow pattern, bombogenesis into SE Canada, consistent model signals
of increasing lift, deformation, trowaling, steep lapse rates, even
potential instability wrapping around through the snow growth region
as the storm lifts out, becoming cyclonic negatively-tilted, closing
off into a deeper low with broader diffluence aloft. Perhaps with,
however, some dry air issues and subsequent ice presence lending to
mixed precip-types, the possibility of sleet and/or freezing rain.

Roughly daytime into evening event, mild thicknesses, uncertainty
with storm track and associated synoptic features, expect interior
winter precip going with high-res 2m temperatures and wet-bulb
processes yet however lower confidence any wet snows will meet or
exceed the 3-inch threshold for advisory-level criteria. Not much
confidence gained from ensemble means and probabilistics, and left
feeling as if CIPS Analogs are contaminated by poorly associated
analogs to the forecast mass fields.

A mix-bag, wintry-precip event given marginal thermal fields as
described above by thicknesses, and the possible presence of dry
air. Non-GFS preference with a closer track to S New England. A
worthy mention in the HAZARDOUS WEATHER OUTLOOK. Continued track
uncertainty with respect to exact specifics, stay tuned for later
forecast updates.

Monday into Tuesday...

Storm lifting out, bombing over SE Canada. Isallobaric / gradient
wind response, expect blustery NW winds mixing down easily to the
surface as cold air advection proceeds. Potential for wind advisory
headlines with gusts at or exceeding 45 mph. Lake effect machine
ongoing, snow shower / flurry activity into the Berkshires that
dissipates with time as high pressure slowly builds in from the W.
Considering coldest 2m temperature guidance going into Wednesday
morning as winds turn light under mostly clear conditions.

Wednesday into Thursday...

Moderating temperature trend as high pressure shifts E and S-flow
returns. More seasonable conditions expected with increasing clouds
late ahead of a storm system for the late-week timeframe.

Friday into the weekend...

Series of storm systems followed by shots of colder air. Far out in
time and discrepancies among deterministic guidance. Preference to
ensemble means with low confidence.

&&

.AVIATION /15Z FRIDAY THROUGH TUESDAY/...
Forecaster Confidence Levels...

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

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

15z update...

Today...VFR except western CT/MA MVFR which will overspread RI
and eastern-central MA this afternoon. Dry weather prevails with
developing S-SE winds. Earlier discussion below.

=================================================================

Tonight...
BKN-OVC low-end VFR CIGs lowering into MVFR. SCT -SHRA mainly S
of the MA Pike.

Saturday...
Improving towards VFR N to S as S winds shift NW. -SHRA pushing
S as well, out over the waters, with time towards Saturday evening.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Saturday Night through Tuesday/...

Saturday Night: VFR. Breezy.

Sunday: Mainly VFR, with local IFR possible. Windy with gusts
to 35 kt. Chance RA, chance FZRA, slight chance SN.

Sunday Night: MVFR/IFR conditions possible. Windy with gusts to
35 kt. Chance RA, chance FZRA, chance SN.

Monday: Mainly VFR, with local IFR possible. Windy with local
gusts to 30 kt. Slight chance RA, slight chance FZRA.

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

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

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Saturday/...High confidence.

Tranquil boating conditions to prevail through Saturday across most
of the coastal waters. The exception will be a risk for some gusts
up to 25-30 kt across the coastal water east of MA as a front crosses
late this evening into tonight. After more consideration, decided
to issue a Small Craft Advisory for the coastal waters east of MA
for tonight.

Chance for light rain tonight into Saturday across the southern
waters.

Outlook /Saturday Night through Tuesday/...

Saturday Night: Moderate risk for Small Craft Advisory winds
with gusts up to 30 kt. Local rough seas. Slight chance of
rain.

Sunday: Low risk for gale force winds with gusts up to 40 kt.
Rough seas up to 9 ft. Chance of rain.

Sunday Night: Low risk for gale force winds with gusts up to
35 kt. Rough seas up to 9 ft. Chance of rain.

Monday: Low risk for Small Craft Advisory winds with gusts up
to 30 kt. Rough seas up to 9 ft. Slight chance of rain.

Monday Night: Strong winds with gusts up to 35 kt. Rough seas
up to 9 ft. Slight chance of rain.

Tuesday: gale force winds with gusts up to 40 kt. Rough seas up
to 9 ft. Slight chance of snow.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory from 9 PM this evening to 8 AM EST
     Saturday for ANZ250.
     Small Craft Advisory from 11 PM this evening to 6 AM EST
     Saturday for ANZ251.

&&

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


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