Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 160543
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
1243 AM EST Sat Dec 16 2017

.SYNOPSIS...
Low pressure moves out to sea tonight. Expect dry but cold
conditions into Sunday. Flurries changing over potentially to
freezing drizzle before going to plain rain Sunday night into
Monday, potentially lingering into Tuesday. Cold and blustery
Wednesday, warm rebound Thursday. Quick moving system Friday
into Saturday. An eye on the holiday break for a possible storm
system.

&&

.NEAR TERM /UNTIL 6 AM THIS MORNING/...
1240 AM update...
Back edge of light snow moving through the Cape/Islands and
will exit next hour or so. Otherwise, gradual clearing will move
in from west to east.

&&

.SHORT TERM /6 AM THIS MORNING THROUGH 6 PM SUNDAY/...
Upper lvl shortwave shifts into the Maritimes allowing for
modest mid and upper lvl ridging to take its place through the
early half of the weekend. Soundings gradually dry and give way
to subsidence inversion, which may initially allow for the
development of some CU under W flow during Sat afternoon. These
dissipate diurnally. Otherwise, H92 temps hover near -8C through
the Sat and Sat night timeframe, which, will allow highs to
reach the mid-upper 30s by day. Some breezy conditions possible
during the afternoon as better mixing could tap a nearly 30 kt
LLJ. Sat night, good setup for radiational cooling with light
snowpack and cresting 1025 hPa high pres. Will see several spots
drop back into the single digits and low teens.

&&

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

 - Light wintry mix Sunday night into Monday, possibly into Tuesday
 - Ebb and flow for Wednesday into Thursday
 - Quick moving system Friday into Saturday
 - An eye on the holiday break, possible storm system

*/ Overview...

Transitional pattern emerging with the increasing potential of mixed
precipitation events. Whether the NW Pacific H5 ridge prevailing, or
broken down by a series of mild, Pacific disturbances, accompanying
jet streaks, the pump of milder air from the N Pacific to the Arctic
persists all the way up through the stratosphere. Broad-scale higher
heights building across the CONUS with time, cross polar flow and
dominant region of low pressure / storm development push subtly E,
aimed into E/NE N America however remaining stretched back W. The N
Atlantic high building SW with higher heights, starting to see a La
Nina type pattern emerge with dry and warm conditions building over
the SE CONUS as cooler conditions are rocked back W, wrapping on the
backside, sheared S by the polar jet through the preferred H5 trof
pattern across N-Central America, however not as deep, more flat.

Amplified, then break down, amplified, then break down, NW Pacific
H5 ridge acts like a pump, broken down by a series of mid to upper
level disturbances only to rebound due to the buckling traffic jam
of weather, energy becoming sheared S, cutoff, a constant over the
SW CONUS. Colder air sheared S in concert, cooler and wet pattern
emerges over N-Central CONUS, but up against the prior mentioned SW
building N Atlantic high, a SW to NE transitional zone emerges over
the Central and E CONUS, with a sharp temperature gradient in the
low to mid levels. It then becomes a focus on the depth / magnitude
of individual waves, as to which side of the envelope S New England
will reside with respect to mixed precipitation events as we go into
the holiday week. N flow dominance of colder air is relaxing in the
long term but not absent. Allowance of S warmer flow, over-running,
threats / impacts associated with wintry mixes are forecast.

Closely monitoring the Sunday night through Tuesday timeframe with a
slow moving warm front N parent with weak ascent. Warm nose around
H95 within moist low levels, a drizzly to light wintry mix of snow,
sleet, freezing rain is possible. Could see similar impacts along a
forecast warm front Thursday night into Friday. Threats and impacts
discussed in detail, along with forecast thinking, below.

*/ Discussion...

Sunday through Tuesday...

Potential light wintry mix along a slow moving warm front lifting N.
Broadly speaking over the forecast period, H95 warm-nose protrusion
above which there`s indications of weak isentropic ascent along the
280-300K theta surfaces, possibly some weak convergent forcing along
the nose of a H85-7 jet, all contributing to periods of weak roughly
W-E frontogenetical banding around H7. Greatest forcing more than
likely along the S/W facing slopes of high terrain with orographic
tendencies. N/NE ageostrophic flow N/NE at first turning E maintains
interior sub-freezing wet bulb temperatures especially over the high
terrain. All throughout, BUFKIT soundings showing plenty of moisture
within low-levels to act upon.

ECens exhibits high probability of 0.01" in 24 hours from 0z Monday
to 0z Wednesday prior to a sweeping cold front. Similar signals in
GEFS/SREF plumes. More than likely the S/W slopes of high terrain
under greater risk seeing a wintry mix of precipitation outcomes.

Favoring the NAM trends with 2m wet bulb temperatures, retreating N
to NE, leaning towards high-chance PoPs with light, drizzly outcomes
with the wintry mix of initially flurries changing to freezing light
rain / drizzle prior to rain Sunday night into roughly late Monday,
the high terrain and NE MA seeing the changeover to plain late the
latest. Snow to freezing rain subsequent of drying indications with
drying occurring within the dendritic growth zone.

Even with a trace of ice, WINTER WEATHER ADVISORIES required. Will
note the possibility in the HAZARDOUS WEATHER OUTLOOK. Monitoring
the strength and depth of the rather flat feature approaching late.
If it can become more pronounced and exhibit a stronger S flow early
on out ahead, could see more significant impacts, especially if it
speeds up. Otherwise cold front sweeping through Tuesday night with
some showery weather, cold air blasts right back in for midweek.

Wednesday into Thursday...

Ebb and flow. Cold blast Wednesday into Thursday morning with breezy
NW winds. Rebound during Thursday with S flow. Dry throughout.

Friday onward...

Quick sweeping system. Warm front lifting on Friday with potential
onset of wintry mix precipitation types. A mild rebound overnight
prior to the cold front sweeping through Saturday. A dry forecast
but the cold front stalling along the aforementioned SW building
Atlantic high in the OVERVIEW section. Energy out of the SW CONUS
acting upon, the potential for an ice storm over the SE CONUS and
potentially on up the E coast exists as the cold front is slow to
push out while taking on anafrontal characteristics. There`s still a
lot of uncertainty, but can`t ignore the general synoptic pattern.

&&

.AVIATION /06Z 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/...High confidence.

Through 12z...MVFR Cape/Islands will improve to VFR 07-08z as
light snow ends, otherwise VFR rest of SNE.

Today...VFR. Bkn cigs 050-080 developing in the afternoon. Low
risk of a snow shower western MA/CT. West wind gusts to 25 kt
developing.

Tonight and Sunday...VFR with diminishing wind.

KBOS Terminal...High confidence.

KBDL Terminal...High confidence.

Outlook /Sunday Night through Wednesday/...

Sunday Night: Mainly VFR, with local MVFR possible. Chance SN,
slight chance FZDZ.

Monday: Mainly VFR, with areas MVFR possible. Chance SN, slight
chance DZ, slight chance FZDZ.

Monday Night: Mainly VFR, with local MVFR possible. Slight
chance FZDZ.

Tuesday: Mainly VFR, with local MVFR possible. Breezy. Chance
DZ, slight chance FZDZ.

Tuesday Night: VFR. Windy with local gusts to 30 kt. Chance RA,
slight chance SN.

Wednesday: VFR. Windy with gusts to 30 kt.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Sunday/...High confidence.

Tonight into Saturday...High confidence.
Low pressure sweeping S/E of the waters into Saturday, deepening
out to sea. Winds shifting W behind the system, strengthening to
near gale force Saturday into evening. Seas peaking 7 to 9 feet
on the SE ocean waters. SMALL CRAFT ADVISORIES issued.

Saturday night...
Winds and seas diminish, this will allow any lingering small
craft advisories to be dropped through the overnight hours.

Outlook /Sunday Night through Wednesday/...

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

Sunday Night through Monday: Winds less than 25 kt. Chance of
drizzle, chance of snow, slight chance of freezing drizzle.

Monday Night: Winds less than 25 kt. Slight chance of drizzle,
slight chance of freezing drizzle.

Tuesday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Chance of drizzle, slight chance of freezing drizzle.

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

Wednesday: Strong winds with gusts up to 35 kt. Areas of rough
seas. Slight chance of rain.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory from 4 AM early this morning to 10 PM EST
     this evening for ANZ231>234.
     Small Craft Advisory from 9 AM this morning to 10 PM EST this
     evening for ANZ236.
     Small Craft Advisory from 4 AM early this morning to 1 AM EST
     Sunday for ANZ235-237-250-254>256.
     Small Craft Advisory from 7 AM this morning to 7 PM EST this
     evening for ANZ251.

&&

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


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