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Area forecast discussion 
National Weather Service Taunton Massachusetts
1030 am EST sun Nov 23 2014

Synopsis...

Dry and mild today. A warm front will bring a brief period of
heavy rain and gusty south winds along the coast late tonight and
Monday morning followed by unseasonably mild conditions Monday
afternoon. Gusty winds but drier weather follows for Tuesday. A
coastal storm may bring some mixed precipitation to the region
Wednesday into Thanksgiving day...but there is a lot of
uncertainty. Colder weather follows late in the week.

&&

Near term /until 6 PM this evening/...

1030 am update...

The last of the light sprinkles/light showers were exiting the far
southeast New England coast. Otherwise...temporary middle level
ridging/subsidence behind the shortwave will result in partly sunny
skies and mild weather this afternoon. Given 850 mb temperatures around
+5c...feel guidance high temperatures are a bit too cool. High temperatures
should range from around 50 to the lower 50s across the higher
terrain...and middle to upper 50s in most other locations. Winds
generally less than 15 miles per hour...so it will feel quite comfortable
outside this afternoon especially after the recent cold weather.

&&

Short term /6 PM this evening through 6 PM Monday/...
***brief period of heavy rain very late Sun night/Monday morning
impacting the am rush hour***

Dry weather expected through midnight. Low pressure moves into the Great
Lakes late tonight with a warm front approaching from the S. Good
isentropic lift enhanced by a strong low level jet will bring a
period of heavy rainfall late tonight into Monday morning. Gefs
showing precipitable water anomaly increasing to +3sd and low level jet reaches
+4sd in southern New England around 12z Monday so this is a good signal for heavy
rain. However...the system is quite progressive and we are only
looking at about a 6 hour period of heavy rain before tapering
off by midday Monday. This will limit rainfall amounts to
0.50-1.00" with locally higher amounts. Main threat is typical
minor poor drainage flooding but this will be occurring during
the morning commute. Elevated instability parameters are not that
impressive but cant rule out an isolated thunderstorm in the coastal plain
Monday morning at the nose of the low level jet.

The other concern is potential for strong winds Monday morning
along the South Coast. NAM is most robust with the low level
nearing 75 knots at 925 mb while GFS/European model (ecmwf) showing 65-70 knots.
Soundings show a good inversion which will limit these winds from
mixing down. A fine line of convection which could help to bring
down these winds to the surface is not likely as instability is
marginal and there is not a cold front which typically provides a
focus for these fine lines. Taking about 50 percent of the jet
suggests wind gusts could peak at 40-45 miles per hour which is near Wind
Advisory criteria. Best chance of reaching advisory criteria will
be across cape/islands.

Bulk of the rain is over by Monday afternoon...earlier in western
new eng...but cant rule out a few afternoon showers and low clouds
will likely persist through the day. It will be unseasonably mild
with highs well into the 60s in many locations and dewpoints will
approach 60 with gusty S/SW winds to 30-35 miles per hour in the coastal
plain. 925 mb temperatures are quite mild near +15c which would normally
support highs in the lower 70s but think low clouds will limit
heating. If we can get any sun...70+ would be possible.

&&

Long term /Monday night through Saturday/...
highlights...

* mild but windy on Tuesday
* coastal storm with travel impacts possible Wednesday-Thursday with uncertainty
* colder weather by the end of the week

Overview and model preferences...
23.00z model guidance suite continues to show moderate agreement
through the middle term period...including the redevelopment of deep
longwave trough by middle- late week across the eastern third of the Continental U.S..
the primary model issues and disagreements are still regarding the
potential coastal storm for late Wed-Thu. This disagreement is
likely due to the fact that the ejecting shortwave associated with
the frontal wave development is still wrapped in the near
Aleutian Islands low...and may not be better sampled for another
24+ hours. Agree with wpc...the key will be where the low passes
in regard to the 40/70 benchmark...with the GFS leaning about
50-100 nm east and the European model (ecmwf) closer to but west of it. Ecens mean seems
to split this difference well...and given wpc/S preference for it
as well...will lean thermal profiles with this update closer to
it/S thinking...although essentially this is close to a GFS/European model (ecmwf)
blend in spite of the fact the European model (ecmwf) is well west of the benchmark.
Given the uncertainty with the storm itself...how efficiently it
is able to draw in the colder air from the longwave trough to the west
remains still a bit of a question mark toward the end of the
week...but a trend toward colder conditions looks likely.

Details...

Monday night into Tuesday night...
mainly dry forecast here although a few lingering showers are
possible mainly Monday night as upper level shortwave pivots through
along with weakening cold front. Cold air remains to the west and so
in spite of cold advection W-flow...expect temperatures to remain near to
even slightly above normal.

Wednesday into Thursday...
as mentioned above...there is still too much uncertainty here for
a high confidence forecast even with the potential impacts at
play. In any case...do have confidence in a more rapidly deepening
coastal low pressure moving out of the middle Atlantic to the southeast Massachusetts
coastline...making it/S closest pass especially Wednesday
night...dipping below 1000mb. The exact track is the main question
and as such...what the thermal profile is most likely to look
like. At this time...0c h92 line varies between just west of the bos-
pvd corridor to offshore...so exactly where any rain/snow line
develops remain in flux. Given the dynamics as pointed out by the
previous forecaster...wetbulb/dynamic cooling are likely to lead
to cooler temperatures than The Straight 2m model temperatures would
indicate...so even with the further east track...snow is possible.
How much is just too uncertain to call given not just the thermal
profile issues... there is a banding signal and trowal signature
toward the NE quadrant. Which could provide dynamic cooling.

What this all means is that there is the chance for a coastal
storm...with wintery precipitation and possibly even plowable snow
depending on the final track for the Wednesday afternoon into early
Thanksgiving morning timeframe. Exactly how much snow and where
rain/snow line sets up remains in question. But precipitation is likely to
lean east given signals for heavy rain on the rain side as well as
gusty winds if the track is closer. Could also be significant
gradient between western Massachusetts/CT/NH and earn Massachusetts/Rhode Island in snow/rainfall.
Also...as mentioned by previous forecaster...coastal flooding threat
appears relatively low given the rapid movement /lack of downstream
blocking/ and low astronomical tides. All these features will have
to be watched given they will begin during a very busy travel period
on Wednesday.

Friday...
colder air begins to build in behind the exiting low
pressure...expect a trend toward drier but colder weather by late in
the week.

Next weekend...
as core of cold air trough begins to move over...expect temperatures below
normal...and possibly even a weak wave of low pres/clipper.

&&

Aviation /15z Sunday through Thursday/...
forecaster confidence levels...

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

Short-term /through Monday/...

15z update...

Today...high confidence. VFR despite some middle level cloudiness.

Tonight into Monday...moderate confidence.

Deteriorating conditions to MVFR-IFR as widespread rain...locally
heavy moves west to east across the region...beginning around 06z
CT valley and reaching East Coast by 10z. Isolated thunderstorm possible Monday
morning near the South Coast. Rain tapers off 12-16z west to east but
IFR ceilings will likely persist through the day along with patchy
fog as higher dewpoints move in. A period of S wind gusts to
30-40 knots possible 12-16z Monday along the S coast and especially
cape/islands...then SW gusts to 30 knots coastal plain in the
afternoon. Low level wind shear will also be a concern late tonight CT valley
shifting to southeast new eng Monday morning as southerly low level jet
50-60 knots develops at 2k feet.

Kbos terminal...high confidence.

Kbdl terminal...high confidence.

Outlook...Monday night through Thursday.

Monday night into Tuesday night...moderate confidence. Mainly VFR. But a
few leftover showers possible Monday night especially may yield MVFR
conditions. West-SW wind gusts about 20-25 knots at times each day.

Wednesday into Thursday...low confidence. A coastal low pressure will move
somewhere between just onshore of southeast Massachusetts or offshore. Given this
uncertainty in track...there is uncertainty in snow/rain
potential...but there is the possibility of each with accumulations
of snow. NE winds could also gust 25+ or more either day.

&&

Marine...
forecaster confidence levels...

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

Short term /through Monday/...

Today...high confidence.

SW gusts 25-30 knots southeast waters early...otherwise diminishing wind
through the morning with gusts dropping below Small Craft Advisory thresholds this
afternoon.

Tonight and Monday...moderate confidence.

Developing low level jet will result in gusty S winds late tonight
into Monday. There is an inversion over the waters which will
likely limit gusts and prevent strong winds from mixing
down. However given strength of the jet...gale force gusts to
35-40 knots seem plausible and gale warnings may be needed. Seas
will likely build to between 10 and 15 feet across the southern
waters with very long fetch.

Outlook...Monday night through Thursday.

Monday night through Tuesday night...high confidence. At least Small
Craft Advisory conditions with winds mainly SW-west and gusts 25-30 knots
into early Tuesday. Seas also around 8-10 feet. Both winds and seas drop
off late Tuesday tonight with a brief lull expected into early Wednesday. Low
probability of some gale force gusts on Tuesday.

Wednesday and Thursday...low confidence. A coastal storm will pass somewhere
near 40n/70w...this will yield building swell and winds shifting
from NE late Wednesday to west-northwest by late Thursday. Depending on how close the
storm comes...heavy rain and at least Small Craft Advisory
conditions are possible. Gales also a possibility. Visibilities
drop in rain/fog.

&&

Tides/coastal flooding...
astronomical tides are fairly high during the Monday morning high
tide cycle. With southerly gales expected...a storm surge of 1-2
feet is possible along the South Coast which may bring some
locations to minor flood. The main threat for minor coastal
flooding will be along Narragansett Bay and Westport Harbor due
to southerly fetch. Coastal flood advisories may eventually be
needed. No flooding is expected along the East Coast.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...none.
New Hampshire...none.
Rhode Island...none.
Marine...Small Craft Advisory until 1 PM EST this afternoon for
anz231>235-237.
Small Craft Advisory until 3 PM EST this afternoon for anz250-
254>256.

&&

$$
Synopsis...kjc/doody
near term...Frank
short term...kjc
long term...doody
aviation...Frank/kjc/doody
marine...kjc/doody
tides/coastal flooding...kjc

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