Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 220007
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
707 PM EST Tue Nov 21 2017

.SYNOPSIS...

A cold front approaches the region tonight, and will combine
with moisture working up the eastern seaboard to bring a period
of rain through tomorrow morning, especially SE New England.
Quiet weather Thursday and Friday. Another shot of wet weather
and breezy conditions Saturday through Monday. Quiet and milder
Tuesday and Wednesday, possibly wet and breezy conditions again
Thursday.

&&

.NEAR TERM /UNTIL 3 AM WEDNESDAY MORNING/...

7 pm update...

Adjusting for trends, not many changes to the going forecast.
Gale warnings were discontinued as winds have dropped, converted
to small craft advisories.

Previous discussion...

Strong subsidence inversion in a regime of continually rising
heights this afternoon, have led to mainly clear conditions
outside of a few high cloud wisps. This has allowed BL mixing
to push into the inversion a bit further aloft than previous
guidance suggested, and led to some upper 50s and even low 60s
for afternoon highs especially away from the coastline, and in
the Merrimack valley where some downsloping is occurring.

The sounding is gradually moisture loading from the top down,
but have noticed a recent gradual increase in surface dewpoints.
The response is a layer of CI building N from offshore, which
combined with the maintenance of a strong pres gradient should
limit evening decoupling somewhat especially S of the Mass Pike.
Mins will likely occur early in the overnight, before 3 AM once
the lower column moistens. Strong temp gradient will result, with
low 30s still possible across the N/NW where some decoupling is
possible. Meanwhile, closer to the SE coastline temps are likely
to remain in the upper 40s and low 50s, given an overall lack
of setup for radiational cooling. Precip will hold off through
3 AM, see below for more information.

&&

.SHORT TERM /3 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/...

Early AM through Wednesday evening...

Acute upper lvl shortwave will slide E late tonight from the
Great Lakes, deepening somewhat as it does so. Merging upper jet
setup will allow for ocean cyclogenesis off the mid Atlantic S
of the Carolinas after midnight in the equatorward entrance
region of the S jet. Meanwhile N jet is associated with slow
moving cold front attendant to low pres sliding through ON/QC
early AM. All models except the ECMWF and many of its ensemble
members were initially keeping each of these features as fully
separate entities, even as coastal low pres develops an inverted
trof which will impact S New England. 12Z guidance a bit more
favorable for at least some phasing of these features, closer to
the slower and more amplified ECMWF, which has been more
persistent. This forecast update will lean most heavily on the
ECMWF given its persistence with this features.

Early morning, moisture continues to increase through the column
as a prefrontal inverted trof and convergence zone develops
mainly across E MA/RI. The moisture plume peaks with PWATs
reaching 1.00-1.20 inches (nearly 2 std deviations above normal
after sunrise Wed). The lift provided by the convergence
boundary and incoming front will lead to a band of light-
moderate rainfall developing early morning then peaking by mid
morning, especially across E MA/RI. Lift/moisture drops off
rapidly to the NW, so am still somewhat uncertain on exact
placement of the cutoff for precipitation. Will focus highest
categorical POPs along the axis convergence in the ECMWF, mainly
E of a line from central CT through the Worcester hills. By
afternoon, punch of dry air following the cold frontal passage
should allow for precip to rapidly end and skies to clear from
W-E, such that much of S New England may see the sun before it
sets. This is a little bit slower, as the more robust ECMWF
suggests development of meso-low pres at the merger point,
slowing the pattern. Final QPF will be focused across E MA and
RI where peak moisture/lift reside. Final totals approaching an
inch are possible, dropping off toward the NW. Will continue to
suggest a risk for TS, mainly in the E, where conditionally
unstable lapse rates occur thanks to anomalously high sfc dwpts
enhance the risk. This also suggests brief heavy downpours
possible during the AM commute. Highs tomorrow will be mainly in
the low- mid 50s.

Wednesday night...

Skies clear rapidly behind the front through the evening, such
that mainly SKC conditions are expected for most of the
overnight hours. Even though winds will briefly increase out of
the NW thanks to mixing enhanced by differential CAA, am noting
enough slackening in the pres gradient by early morning to imply
radiational cooling. Therefore, min temps cooler than previous
night, with several locations dropping into the 20s by early
Thanksgiving morning.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...

*/ Highlights...

 - Quiet weather for Thursday and Friday
 - Showers progged for Saturday night into Sunday, potentially breezy
 - Turning mild Tuesday into Wednesday, could also be breezy
 - A potential near to above seasonable pattern into early December

*/ Overview...

Potential indications of a near to above seasonal pattern into early
December. Noting a +WPO trend with EC/UKmet preference, signals of a
deamplifying N Pacific trof-ridge-trof pattern, becoming more zonal,
a longer, fast fetch of mild Pacific air aimed into Canada. If model
stratospheric forecasts are any indication, the main low shifting
into NW Europe with slightly higher heights over the CONUS, we lose
the Arctic connection via NW fetch, flow more W/SW, the ebb and flow
pattern as of late and likely to continue into next weekend relaxing
indicating possible milder intrusions with any disturbances sweeping
through the continued zonal pattern, lacking an Arctic draw. Neither
Atlantic nor Pacific amplification in the long-range to indicate the
zonal pattern will end anytime soon as all other teleconnections are
near-neutral, subdued.

An overall quiet pattern, any disturbances progressive, separated to
a degree by the split-flow regime governed and slowly sliding E in
concert with a trof disturbance over the SE CONUS extending into the
Caribbean. Warm intrusions ahead, cold air following behind. As it
was mentioned above, ebb and flow pattern continues. N/S streams
mostly split, however closely watching a Saturday night into Sunday
disturbance. Touch upon any threats and/or impacts in the discussion
below.

*/ Discussion...

Thursday and Friday...

Quiet. Cooler Thursday, NW winds, colder aloft. Highs below average.
Upper 30s to low 40s. High pressure, light winds, clear conditions,
opportune radiational cooling into Friday morning with lows in the
20s. Mild Friday, rebound with S winds, highs in the 40s, thickening
clouds late.

Saturday through Monday...

Watching offshore storm development and parent winds. Progressive
flow through the split-flow regime, taking a page from the oncoming
disturbance within the next 24 hours. Wet weather late Saturday into
Saturday night, chance PoPs, could see decent rains. Breezy SW winds
in advance, not anomalously strong. More concerning is NW winds rear-
ward, rapid cyclogenesis occurring outside of the 40N/70W benchmark.
Undercutting colder air, steeper lapse rates into Sunday, possible
that with H85 winds in excess of 50 mph that we could potentially
mix-down gusts that suffice WIND ADVISORY criteria, gusts 46-57 mph.
An isallobaric component with pressure couplet. Cold air advection.
Additional energy acting upon any moisture, perhaps both showers and
winds linger into Monday.

Tuesday into Wednesday...

Warm up. Return S flow as high pressure shifts E, have to closely
monitor its interaction with upstream disturbances, a subsequent SW
jet with tightening pressure gradient and whether it can effectively
mix down to the surface. Greater threat Wednesday. Mild, highs into
the 40s and 50s.

Thursday onward...

Potential sweeping weather disturbance Thursday, however absent an
Arctic connection as pointed out in the discussion above. Attendant
high pressure to follow of CONUS rather than Canada origin.

&&

.AVIATION /00Z WEDNESDAY THROUGH SUNDAY/...

Forecaster Confidence Levels...

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

Short Term /through Wednesday Night/...High confidence.

Through tonight...
VFR through at least 09-10z, lowering MVFR to IFR with -RA/RA
with SW flow prevailing.

Wednesday...
Mixed MVFR/IFR CIGS through the day with RA and VSBYS 1-3SM at
times in RA/+RA, low risk TSRA, especially E of the same line
(HFD-ORH-MHT). Areas W may remain mixed MVFR/VFR with mainly
-RA. RA clears and rapid improvement to VFR occurs after 18Z
from W-E as winds shift to the NW.

Wednesday night...
VFR. NW flow around 10 kt.

KBOS Terminal...
Bulk of -RA/RA progged 12-20z, mainly MVFR CIGs, with +RA/IFR
threats 14-18z. Feel TSRA will remain SE of the terminal.
Clearing after 20z.

KBDL Terminal...
Feel RA/+RA threats will remain E. Looking at mostly -RA
developing around 11z persisting through 18z. MVFR CIGs with
lesser impact to VSBY. Clearing out shortly after 18z.

Outlook /Thursday through Sunday/...

Thanksgiving Day through Friday: VFR.

Friday Night: VFR. Breezy.

Saturday: VFR. Breezy. Chance SHRA.

Saturday Night: VFR. Slight chance SHRA.

Sunday: VFR. Breezy. Slight chance SHRA.

&&

.MARINE...

Forecaster Confidence Levels...

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

Short Term /through Wednesday Night/...

Overnight into Wednesday...High confidence.

Gales dropped to small crafts. Winds gradually diminishing but
will remain brisk with the threat of gusts up to 30 kts into
Wednesday morning. Once winds shift to the NW Wednesday late
afternoon/evening gusts increase once more, but should remain
generally below gales. Seas drop overnight but remain at or
near 5 ft mainly on the SE waters. Rain anticipated 7a - 4p
which will be heavy at times, along with the risk of thunder-
storms. Likely some visibility impacts along the waters.

Wednesday night...High confidence.
The cold front having swept through late Wednesday, NW winds
continue to gust to around 25 kt and allow seas to briefly build
to about 8 ft on the outer ocean waters. Small craft advisories
are likely to continue.

Outlook /Thursday through Sunday/...

Thanksgiving Day: Winds less than 25 kt. Local rough seas.

Thursday Night through Friday Night: Winds less than 25 kt.
Areas of seas approaching 5 ft.

Saturday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Areas of seas approaching 5 ft. Chance of rain
showers.

Saturday Night: Winds less than 25 kt. Seas locally approaching
5 ft. Chance of rain showers.

Sunday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Seas locally approaching 5 ft. Slight chance of rain
showers.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...

CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 7 AM EST Wednesday for ANZ232>234.
     Small Craft Advisory until midnight EST tonight for ANZ231-251.
     Small Craft Advisory until 4 AM EST Wednesday for ANZ236-250.
     Small Craft Advisory until 10 AM EST Wednesday for ANZ235-237.
     Small Craft Advisory until 6 AM EST Thursday for ANZ254>256.

&&

$$

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



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