Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 152039
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
339 PM EST Thu Nov 15 2018

.SYNOPSIS...
A high impact but short duration snowstorm is on tap for areas
away from the very immediate coast this evening. Precipitation
transitions to mainly ice and then rain overnight, except
perhaps portions of the high terrain in Massachusetts. Dry
weather with chilly temperatures move in Friday afternoon
through most of the weekend as high pressure builds across. A
weak low may bring patchy light rain and/or snow late Sunday
into Monday. Another shot of colder air moves in by the middle
of next week, though it should be mainly dry.

&&

.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
235 pm update...

* High impact/short duration snowstorm this evening for many
* Winter Storm Warnings posted for northwest of I-95

A high impact/short duration winter storm is on tap across much
of the region this evening. Potent closed mid level energy
across the Ohio Valley was inducing an 80+ knot winds at 700 mb.
The result has been for heavy snow to break out across portions
of the mid Atlantic States this afternoon.

1) Heavy Snow Away From The Very Immediate Coast This Evening:

This potent speed max/convergence zone at 700 mb will continue
to lift northward resulting in very strong frontogenetic
forcing this evening across southern New England. Heavy snow
should develop after 4 or 5 pm across our northern CT zones and
spread northeastward into northeast MA after 7 or 8 pm. Highest
impact expected in our CT zones...given the heavy snow with 1 to
2 inch per hour rates will impact a large portion of the
evening rush hour. The latter half of the evening rush hour will
likely be impacted in the Springfield...to Worcester to
Providence areas.

The biggest question with this forecast is how well this band of
strong frontogenesis will maintain itself as it lifts northeast.
Confidence is highest across our CT/RI and interior southeast MA
zones. Further north...the guidance is somewhat split on that
potent speed max at 700 mb. The RGEM/NAM maintain it across
northern MA...while the ECMWF/GFS tend to weaken it and shunt it
further east. Regardless...it will snow heavy across much of the
region for a time but this will impact accumulations. We still
feel that 4 to 7 inches of snow looks good for much of the
region. Given the short duration/high impact event and
considering some light icing will persist after the main
event...feel warnings are warranted for much of the region. Snow
amounts will likely be much lighter on the very immediate
coast. SST around 50 with an easterly wind will result in making
it difficult to get more than an inch in these locations before
a quick change to rain.

2) Warming Aloft/Mid Level Dry Air Late This Evening:

Much of the region appears cold enough for mainly snow through
02z or 03z. However...the very potent 700 mb southerly jet will
transport mid level warmth northward across the region. Ptype
should transition from snow to ice/rain from 02z through 06z
across southern New England. At the same time...mid level dry
air will be cutting off our snowgrowth. Regardless...it appears
that the damage will be done across the region but some lighter
rain/ice will linger. Ptype will mainly be rain across eastern
MA/RI...but some light sleet/freezing rain may persist for a
time across the interior.

It does appear that temperatures by daybreak Friday will be
above freezing for most of the region for the morning rush hour.
The exception will be some of the high terrain...especially
interior northern MA where some light sleet/freezing rain may
still result in slippery travel

3) Strong to Damaging Winds:

A strong easterly low level jet of 55 to 65 knots is expected
to develop at 925 mb during the overnight hours along with
impressive pressure falls. Low level lapse rates are fairly
steep along the coast. This will result in a period of easterly
wind gusts of 40 to 50 mph along the eastern MA coast and up to
60 mph across the Cape/Islands. High wind warnings have been
issued for the Cape/Islands with wind advisories along the
eastern MA coast. Will have to watch for some downed trees and
isolated to scattered power outages.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/...
Friday...

The surface low pressure system looks to track near the
southeast New England coast Friday morning. Its associated
closed 700 mb level will also track across the southeast portion
of our region. The result will be periods of rain through Friday
morning...before coming to an end by afternoon. However...colder
air wrapping in behind the system may allow for portions of
interior MA /especially the high terrain/ to transition back to
a period of snow. It is possible that some of these areas see an
additional 1 to 3 inches of snow in this region. However...for
the rest of the region temps should remain above freezing with
perhaps just some wet snow flakes mixing in at the end.

Precipitation petty much ends by Friday afternoon and we may
see some peeks of sun late in the day. Afternoon temps should be
in the 30s to the lower 40s with gusty northwest winds.

&&

.LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/...

Highlights...

*  Below average temperatures persist
*  Weak low pressure brings periods of rain and snow late Sun into
   Mon
*  Dry and cold mid week

Overview...

The extended forecast features a few high pressure systems sandwiched
around a relatively weak low pressure disturbance or two in the
Sunday night/Monday time frame which will bring the return of
both rain and snow chances. Model consensus is good that the
flow pattern becomes much less amplified and more zonal behind
Friday`s system, persisting through late weekend. By then a mid
level shortwave digs south, crossing the region Sunday/Monday
which will generate a weak surface low in the vicinity of SNE
with another round of light snow/rain. Beyond this, models
diverge a bit, with the GFS keeping things unsettled into
Tuesday before the high returns, while the Euro returns a dry
high pressure more quickly. Either way, by mid to late week
heights begin to rise ever so slightly and a cold and dry
pattern returns.

Details...

Friday night and Saturday...

Friday night the low continues to exit to the northeast, relaxing
the surface pressure gradient as it leaves. Winds will take a good
while to come down, however, and the 30-35 H925 jet doesn`t leave
SNE until late Saturday. Thus, expecting gusty west winds to persist
through the period, especially over the Cape, islands, and waters,
decreasing through the day Saturday. Clearing skies early
Saturday will give way to more clouds as mid level moisture
invades from the west. With good CAA behind the system lows
Friday night dip into the low 30s with wind chills in the 20s.
Highs on Saturday won`t get out of the 40s.

Sunday and Monday...

Sunday high pressure sits over the northeast with light winds and
highs in the upper 30s-40s under a cold airmass; should be a mostly
dry day before snow/rain chances increase from the west by the
evening/overnight hours. The upper trough over Canada digs south
helping to generate a surface low in the vicinity of SNE which
brings weak fronts through in quick succession Sunday
night/Monday. Still expecting mainly snow outside the coastal
plain, with rain along the coasts. QPF amounts are lackluster; a
few hundredths to a few tenths of liquid equivalent.

Tuesday through Thursday...

After the system exits zonal flow returns at the base of a large 500
mb trough over the eastern US Tuesday/early Wednesday. Some
disagreement on slight precip chances Tuesday, but overall looks
dry; maybe some showers over the high terrain. By mid week a
surface high returns bringing dry weather. Given the overall
weak longwave trough pattern, cold temperatures will persist
with highs in the 30s and low 40s.

&&

.AVIATION /21Z THURSDAY THROUGH TUESDAY/...
Forecaster Confidence Levels...

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

Short Term /through Friday/...

Tonight...Moderate to high confidence. IFR to LIFR conditions
overspread the region from southwest to northeast roughly from
21z through 01z in moderate to heavy snow. The exception will be
the very immediate coast...where the marine influence will
likely result in a transition quickly over to rain. Across the
rest of the region...ptype will transition to rain across
eastern MA/RI with a period of light icing across the interior
between 02z and 06z. IFR conditions persist overnight with
temps rising above freezing by 12z...except for portions of the
interior high terrain in MA.

In addition...a strong easterly LLJ will result in 40 to 50 knot
wind gusts along the coast, strongest of those across the
Cape/Islands.

Friday...Moderate to high confidence. Low end MVFR-IFR
conditions in the morning in mainly rain with any snow/ice
confined to interior MA. Precipitation ends by afternoon with
improvement to MVFR and even some VFR conditions possible late.

KBOS Terminal...Moderate to high confidence in TAF. Snow or a
mixture of rain/snow develops around 00z or 01z...but quickly
transitions to all rain by 03z. A coating to 1 inch is possible
on runways before that occurs. Easterly wind gusts of 40 knots
expected overnight.

KBDL Terminal...High confidence in TAF. Heavy snow overspreads
the terminal after 21z or 22z. This continues through about 03z
before ptype transitions to a period of sleet/freezing rain.
Ptype should then change to all rain after 6z or so.

Outlook /Friday Night through Tuesday/...

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

Saturday through Saturday Night: VFR. Breezy.

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

Sunday Night: Mainly IFR, with local MVFR possible. Chance SN,
slight chance RA.

Monday: MVFR/IFR conditions possible. Breezy. Chance SN, 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 Friday/...High confidence.

Tonight...Strong Easterly LLJ will result in 50 knot wind gusts
across much of our waters late tonight with strong SCA across
Boston Harbor/Narr Bay. Seas will build to between 15 and 20
feet across our eastern waters by daybreak Friday. VSBYS reduced
at times in mainly a windswept rain.

Friday...Storm force wind gusts should diminish to gales during
the morning and persist into part of the afternoon with still a
decent pressure gradient in place. Seas will remain very high.

Outlook /Friday Night through Tuesday/...

Friday Night: Moderate risk for Small Craft Advisory winds with
local gusts up to 30 kt. Rough seas up to 12 ft.

Saturday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Local rough seas.

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

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

Sunday Night: Winds less than 25 kt. Slight chance of rain.
Local visibility 1 to 3 nm.

Monday: Winds less than 25 kt. Areas of seas approaching 5 ft.
Slight chance of rain.

&&

.HYDROLOGY...
Renewed river flooding is possible across RI and eastern MA with
0.50 to 1.0 inches of rainfall possible tonight into Friday
after a few inches of snow first. Will have to watch this
closely.

&&

.TIDES/COASTAL FLOODING...
Astronomical tides are very low (below 9 ft) in Boston. Thus
with this upcoming storm worse case scenario would be isolated
minor flooding possible.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Winter Storm Warning until 7 AM EST Friday for CTZ002>004.
MA...High Wind Warning from midnight tonight to 9 AM EST Friday for
     MAZ022>024.
     Winter Weather Advisory until 1 AM EST Friday for MAZ017>021.
     Wind Advisory from midnight tonight to 9 AM EST Friday for
     MAZ007-014>016-019.
     Winter Weather Advisory until 2 AM EST Friday for MAZ007-
     013>016.
     Winter Storm Warning until 7 AM EST Friday for MAZ002>006-
     008>012-026.
RI...High Wind Warning from midnight tonight to 9 AM EST Friday for
     RIZ008.
     Winter Weather Advisory until 1 AM EST Friday for RIZ002>007.
     Winter Storm Warning until 7 AM EST Friday for RIZ001.
MARINE...Storm Warning from midnight tonight to 9 AM EST Friday for
     ANZ231>235-237-250-254>256.
     Gale Warning from 10 PM this evening to 4 PM EST Friday for
     ANZ230-251.
     Gale Warning from midnight tonight to 9 AM EST Friday for
     ANZ236.

&&

$$
SYNOPSIS...Frank/BW
NEAR TERM...Frank
SHORT TERM...Frank
LONG TERM...BW
AVIATION...Frank/BW
MARINE...Frank/BW
HYDROLOGY...
TIDES/COASTAL FLOODING...



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