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Area forecast discussion 
National Weather Service Taunton Massachusetts
1020 am EDT Tuesday Oct 21 2014

Synopsis...

A long duration coastal storm will bring periods of rain through
Thursday with lingering showers Friday as the storm slowly pulls
away from New England. Mainly dry weather expected this weekend
with seasonable temperatures.

&&

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

***scattered hit and miss showers through the afternoon but not
expecting a washout**

1020 am update...

Band of rain exiting Nantucket as of late morning. Otherwise...just
some widely scattered showers were affecting the region at late
morning.

Hit and miss showers will continue into the afternoon...but may
become a bit more prevalent. The reason is that were quite cold
aloft given the 500 mb closed low to our west. This should generate
enough instability for scattered showers to increase a bit in
coverage by afternoon...but not expecting a washout. There is even
the low probability for an isolated thunderstorm or two across
eastern New England.

All in all...while scattered hit and miss showers will increase in
coverage this afternoon not expecting a washout. High temperatures
will range from the upper 50s to the middle 60s with an abundance of
cloud cover.

&&

Short term /6 PM this evening through 6 PM Wednesday/...
tonight...

Dry slot overspreads the region. This will result in periods of
drizzle as low level moisture becomes trapped beneath dry air aloft.
Farther inland across southern New Hampshire and western Massachusetts a steadier rain is
possible as comma head rains may clip this region while bulk of rain
is confined to central and northern New England.

As for temperatures...not much of a drop off given onshore flow/clouds and
light precipitation.

Wednesday...

Trailing short wave energy rounds the base of the trough and then
takes aim at southern New England later in the day. Strong synoptic
scale forcing with this feature and results in coastal low off New Jersey
coast to intensify. Thus slug of heavy rain likely to move onshore
into southern New England late in the day. In addition as dry slot
approaches Theta-E lapse rates begin to decrease. Thus embedded
convection possible and would enhance heavy rainfall potential. This
will increase the risk urban/poor drainage flooding and may impact
the evening commute.

Temperatures similar as today with highs in the l60s over southeast Massachusetts and
cooling into the 50s elsewhere.

&&

Long term /Wednesday night through Monday/...
overview and model preferences...

With fairly good model agreement through the middle term regarding a
slow meandering and stacked low pressure system moving just to the S
of southern New England...a general consensus blend will be used as a
baseline for this forecast. For the remainder...toward the end of
the week...the operational European model (ecmwf) continues to exhibit a bit of a
higher amplification bias /somewhat typical of this model/. While
other operational models and several of it/S own ensembles are
more progressive with a secondary kicker wave and high pressure.
Therefore...will add less weight to the European model (ecmwf) by the late week and
weekend-early next week time frame.

As mentioned above the key feature initially is a slow moving
cutoff low pressure with attendant occluding surface low through late in
the week. It/S not until a kicker wave /deepening wave ejecting
out of the Canadian west/ moves in on Sat that the low pressure moves
fully into the Maritimes and allows drier air to fully build in.
Therefore...expect the wet/gray conditions to continue through Friday
with gradual improvement from then into early next week.

Details...

Wednesday night into Thursday...
at this point all features line up for the potentially wettest
period of the forecast. In essence a reinforcing low level jet /about 40+ knots
mainly north of the box County Warning Area/ moves in combined with upper level dry slot
and near 6.0+ c/km lapse rates. Therefore...looks like a decent
setup for convective elements with this low level jet particularly between
00z and 12z on Thursday. With modest precipitable waters up to 1.5 inches /about 2+
Standard deviations above normal/. Could see pockets of heavier rain
through the period...gradually diminishing into the daylight hours
on Thursday as the low level jet and dry slot shift north. Right now the primary
threat would be the typical urban areas where drainage is poor for
any nuisance flooding...but will need to keep an eye on this over
the next 24-36 hours. Otherwise...the gray/damp conditions
continue with onshore flow and plenty of moisture/cloud cover. Due
to this cloud cover...suspect that lows remain mainly in the 50s
and highs only rise into the middle 50s and low 60s.

Friday...
gradual improvement as the upper level shortwave ejecting out of
Canada begins to force the upper level cutoff to the east and weakens
it. Still enough upper level cool air and moisture to support a few
more showers. So the gray/wet conditions continue...but with a
more north component to the flow...these showers are likely to be
fewer and farther between. Temperatures a bit warmer...mainly upper 50s
to middle 60s.

Sat and sun...
surface cold front associated with the shortwave will be moving
through the region...which should finally release the region from
the grips of the cutoff. At this point with drier air moving
in...may only see cloud cover with this frontal passage...but an
isolated shower is not out of the question mainly Sat night into
early sun. The differences in amplification between models make
it difficult to tell exactly how deep the cold air gets Sat night
into sun...but with full mixing highs of low-middle 60s are not out
of the question with lows falling back into the 40s.

Monday and Tuesday...
high pressure builds in across the area with drier air and mostly
clear conditions. Temperatures may be near seasonal normals...if not
slightly above depending on how deep the cool air becomes at this
point.

&&

Aviation /14z Tuesday through Saturday/...
forecaster confidence levels...

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

Short term /through Wednesday/...

1020 am update...

Today...moderate confidence.
Mainly VFR through the day today except for the southeast New
England coast...where MVFR ceilings may affect the region with
southeast onshore flow. In addition...scattered showers developing
by afternoon may result in brief localized reductions. There is even
a low probability of an isolated thunderstorm or two across eastern
New England this afternoon.

Tonight...

Main area of rain lifts northward into central New England.
Spotty light rain and drizzle will be common across southern New
England terminals with VFR/MVFR trending toward IFR late tonight.

Wednesday...

IFR/MVFR with most of the rain confined to the interior. However
a new area of rain should move onshore to the South Coast late in
the day. East-NE winds will increase late in the day as well.

Kbos...high confidence in taf on trends but lower confidence on
exact timing and details.

Kbdl...high confidence in taf on trends but lower confidence on
exact timing and details.

Outlook...Wednesday night through Saturday...moderate confidence.

Wednesday night through Thursday...
mainly IFR/MVFR with periods of heavy rain possible. Low ceilings and
visibilities in heavier rain and fog. Slight chance of thunder through
the period. Chance for low level wind shear especially overnight into early Thursday
morning with up to 40 knots possible at around 2kft especially
terminals north of the Mass Pike.

Friday and Saturday...
improving to VFR. Areas of MVFR ceilings/visibilities in showers on
Friday...with the best chance in southern New Hampshire and eastern mass.

&&

Marine...
forecaster confidence levels...

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

Short term /today through Wed/...

Today...

Strongest southeast winds will occur over the eastern Massachusetts
waters...with gusts up to 25 knots. Thus Small Craft Advisory posted for that area.
Heavy rain with low risk of T-storms across this area. Elsewhere
lighter winds and showers.

Tonight...

East to northeast winds. Low risk of 25 knots over Cape Ann waters.
Visibility reduced in areas of fog and drizzle.

Wednesday...

Heavy rain and possible embedded thunder moves southeast to
northwest across the waters and then onshore late in the day.
Northeast winds 15-20 knots. Visibility reduced in areas of fog and heavy
rain late in the day.

Outlook...Wednesday through Saturday...moderate confidence.

Wednesday night into Thursday...
mainly east-NE flow along the waters Wednesday night into early Thursday. Then
shifting to northwest during the afternoon and evening Thursday. A few wind
gusts may approach gale force Wednesday night mainly across the north
waters...with Small Craft Advisory level winds elsewhere. The
winds drop off as they shift Thursday...but then gust again to Small
Craft Advisory levels late day Thursday. Otherwise...seas build Wednesday
night into Thursday morning with at least 5-9 feet seas possible. Some
heavy rain possible Wednesday night into early Thursday morning.

Friday...
low pressure moves further offshore...winds and seas may still reach
Small Craft Advisory levels during the day Friday and into Friday
night...but expect gradual improvement.

Saturday...winds and seas should remain below small craft levels.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...none.
New Hampshire...none.
Rhode Island...none.
Marine...Small Craft Advisory until 8 am EDT Wednesday for anz250-254.
Small Craft Advisory from 3 PM this afternoon to 8 am EDT
Wednesday for anz251.
Small Craft Advisory until 7 am EDT Wednesday for anz255-256.

&&

$$
Synopsis...nocera/doody
near term...Frank
short term...nocera
long term...doody
aviation...Frank/nocera/doody
marine...nocera/doody

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