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Area forecast discussion
National Weather Service Taunton Massachusetts
419 am EDT Wed Mar 29 2017

Synopsis...
rain ends early this morning with dry weather for the rest of
Wednesday and Thursday. Low pressure from the Ohio Valley
passes south of New England Saturday. This brings a mix of
precipitation Friday and Saturday. High pressure brings dry
weather Sunday and Monday, followed by another approaching storm
on Tuesday.

&&

Near term /until 6 PM this evening/...
current msas mass fields suggest the weak parent low pres
responsible for the overnight rain is now about 50-100nm southeast of
Cape Cod and continuing east. At the time of this writing,
deformation rainfall is confined to the bos-pvd corridor and
points southeast with some clearing even already apparent on satellite
infrared/fog product inland. The cold advection has begun also as sfc
dwpts are dropping already. Thankfully, remnant low clouds
should keep temps mainly above freezing for the am commute, but
this will need to be watched for mainly portions of west and northwest Massachusetts.

The trend will be toward some clearing today, however with
robust cold advection as 850 mb temps drop from nearly +2c to +4c
to as low as -6c to -8c by 00z it is likely that cold air advection cloudiness
will commence this afternoon and evening along with peak mixing.
Some ocean effect clouds, given a north-northwest trajectory may impact the
cape/islands, but given SST-850 mb Delta-T magnitude is generally
around 10c or less, not expect much aside from maybe a few light
rain showers.

Temps mainly in the upper 40s and low 50s today in spite of the
cold advection thanks to good mixing and some sunshine. Winds
will increase, peaking this afternoon and evening along with
peak pres rises and modest low level jet. Gusts mainly 25-35 mph can be
expected. Most gusts should remain below Wind Advisory criteria.

&&

Short term /6 PM this evening through 6 PM Thursday/...
overnight...
clearing occurs during the evening hours as diurnal/cold air advection
cloudiness erodes. This will begin to allow some decoupling
although it may take until after midnight for most places to
drop to near calm as pres gradient remains relatively tight
through about 06z. This will limit timing for radiational
cooling. Therefore, the floor for most overnight lows is in the
mid 20s, although a few spots (mainly the most sheltered
valleys) could drop a bit lower.

Tomorrow...
another seasonable day for the most part although with generally
weaker winds. Gusts mainly hold 15-20 mph. Highs per slight
warming to an average of -4c at h85, look to remain in the upper
40s to around 50, although a few spots may do a bit better given
more sunshine and less in the way of diurnally driven clouds.

&&

Long term /Thursday night through Tuesday/...
big picture...

Longwave flow is nearly west-east with a slight trough over the
northeast USA. Shortwave scale flow shows split northern and
southern jet streams directing at least three and possibly four
shortwaves through New England through early next week.

Model mass fields are similar through the weekend, then diverge late
in the forecast period with the approach of the next weather system
Tuesday. Pattern confidence is moderate through the weekend,
diminishing to low by Tuesday.

Details...

Thursday night...

High pressure over the region shifts east overnight. The overall
pattern suggests warm advection precip starting Friday morning. Even
so, the main system return flow continues to focus on New York/PA/NJ
through 12z, even as the models move quantitative precipitation forecast across southern New England
during 06z to 12z. Model Omega fields show weak lift over the region
during late Thursday night, but relative humidity fields show a dry layer below
this lift. All of this leads to concern that the models are
uniformly rushing pcpn in too soon. We have slowed the onset by a
few hours, but continue to bring chance pops to the Western Hills,
CT River Valley, and central hills overnight. Cold air damming
signal in the pressure pattern over the interior, and light
northerly low level ageostrophic flow. Within the context of chance
pops, we expect enough cold air for a snow ptype in the interior and
a rain/snow mix along the coastal plain.

Friday through Saturday...

Models agree on closed low over Illinois/Indiana Friday morning with
surface low over the Midwest. The Midwest low is forecast to
redevelop along the mid Atlantic coast, likely in response to the
cold air over interior New England.

Strong low level southerly jet at 45-50 knots in the Ohio Valley
into western New York. Meanwhile southerly flow into New England is quite
light Friday morning, then slowly increases through the afternoon.
The stronger 25-45 knot onshore flow waits until late in the day and
during Friday night as the coastal low develops. Stronger winds
develop aloft and move over the South Coast and southern waters
Friday night and Saturday morning. There is concern that the models
are not fully accounting for the potential that this wind reaches
the surface at that time. Expect greater confidence for precip
Friday night and Saturday morning.

We will feature increasing pops through the day Friday with highest
values Friday night and Saturday as the coastal redevelopment passes
offshore.

Increasing low level ageostrophic flow from the north during the
afternoon and night...reaching 25-35 knots at night. This should
maintain and build low level cold air for Friday night. This builds
confidence in a snow/ice event for the interior, while the onshore
easterly wind keeps coastal areas above freezing. There is
potential for several inches of accumulation inland, especially in
northern/western Massachusetts.

Sunday and Monday...

Northern stream shortwave moves overhead and carries an appreciable
cold pool aloft, especially over northeast Massachusetts. Meanwhile the surface
layer shows less cold advection than aloft. So with clearing skies
mixing should bring temps into the 40s and near 50. This combination
should destabilize the airmass and support diurnal cumulus during
Sunday. It is possible we are underestimating the sky cover for
Sunday...this will depend on the amount of lingering moisture for
clouds.

High pressure and lighter wind will move in for Monday with sunny
skies. Mixing will support Max sfc temps in the upper 40s and lower
50s.

Tuesday...

Low pressure ejecting from the southwest USA is projected to
approach our area on Tuesday. This system bears some similarity to
the Friday-Saturday system, including a cold air damming signal and
a coastal redevelopment. This could be another coastal rain/inland
mix situation, but currently expected temperatures support all rain.
Much too early to lock onto a solution for this.

&&

Aviation /07z Wednesday through Sunday/...
forecaster confidence levels...

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

Short term /through Thursday/...

Through 18z...high confidence in trends, lower confidence in
exact timing.
Rain and low clouds will be gradually shifting east through late
morning. This will allow LIFR/IFR to gradually give way to MVFR
then VFR as the low clouds break from W-E. Timing may be off a
bit in tafs.

This afternoon and tonight...high confidence.
VFR except for maybe some spotty MVFR ceilings across Cape Cod and
the islands late this afternoon and evening. Otherwise, breezing
northwest flow with gusts 25-30 across east Massachusetts/Rhode Island and 20-25 kt across west
Massachusetts and CT. These dissipate somewhat late tonight.

Tomorrow...high confidence.
VFR. Northwest flow weaker than today.

Kbos taf...high confidence in taf trends, a return to brief IFR
this morning possible before improvement after sunrise. Timing
may be off a bit.

Kbdl taf...high confidence in taf trends, a return to brief IFR
this morning possible before improvement after sunrise. Timing
may be off a bit.

Outlook /Thursday night through Sunday/...

Thursday night through Saturday...moderate confidence.

VFR Thursday evening will lower to MVFR Friday and IFR/LIFR Friday
night and Saturday. Inland areas will see a rain/snow mix Friday
change to snow/sleet/freezing rain Friday night with several inches
of snow/sleet possible in the western and central hills. More likely
rain in the coastal plain with a period of snow/rain mix Friday
night. Precipitation tapers off west to east Saturday afternoon.
Gusty easterly winds in the coastal plain Friday night and Saturday,
becoming northerly Saturday night.

Sunday...moderate confidence.

Conditions trend to VFR with at least partial clearing. Northwest
winds with gusts 20 knots or less.

&&

Marine...
forecaster confidence levels...

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

Short term /through Thursday/...

Today and tonight...high confidence.
Winds will increase across the waters through the day, but will
peak by late this afternoon and early in the overnight. Northwest gusts
at peak will reach 25-30 mph with a few gusts just shy of gales.
Seas increase with this wind as well, peaking early Thu morning
at 8-10ft on the southeast ocean waters. Small craft advisories remain
as issued.

Thu...high confidence.
Northwest winds diminish and waves slacken through the day. Small craft
advisories are likely to be dropped through the daylight hours.

Outlook /Thursday night through Sunday/...

Thursday night... high confidence.

Lingering 5 foot seas on the outer waters. Diminishing wind as high
pressure builds in. A Small Craft Advisory is possible for leftover
seas Thursday evening.

Friday-Saturday...moderate confidence.

Midwest weather system redevelops along the mid Atlantic coast
Friday night and passes the waters on Saturday. Winds remain below
25 knots Friday, but become east and may increase to 30 knots Friday
night and Saturday. Winds turn from the north as the system moves
past. Expect seas to build Friday night and Saturday with heights 5
to 10 feet possible.

Winds of 40-50 knots are possible aloft, about 2000-3000 feet above
the surface. There is at least a low concern for 35 knot gusts
Friday night and Saturday primarily over the southern waters.

Sunday...moderate confidence.

Low pressure moves away from New England, with diminishing winds
over the waters. Lingering 5-6 foot seas on the outer waters may
require a Small Craft Advisory there.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...none.
Rhode Island...none.
Marine...Small Craft Advisory from 11 am this morning to 2 PM EDT
Thursday for anz231>234-251.
Small Craft Advisory from 5 PM this afternoon to 8 am EDT
Thursday for anz230.
Small Craft Advisory from 8 PM this evening to 8 am EDT
Thursday for anz235>237.
Small Craft Advisory until 8 PM EDT Thursday for anz250-254.
Small Craft Advisory until 2 PM EDT Thursday for anz255-256.

&&

$$
Synopsis...wtb/doody
near term...doody

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