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Area forecast discussion 
National Weather Service Taunton Massachusetts
245 PM EST Thursday Dec 18 2014

Synopsis...
dry and seasonable conditions will prevail Friday. An upper level
disturbance and moist northeast flow may bring some light snow
showers Saturday into Sunday. Low pressure may impact the region
Tuesday...with a much stronger storm likely bringing heavy rain
and wind to southern New England Wednesday into Wednesday evening.
Dry but very windy conditions follow for Christmas day.

&&

Near term /through tonight/...
low pressure moves NE from the Maritimes but cyclonic flow will
persist along with some moisture in the column which will keep a
few clouds across the region although dry weather expected. While
gusty winds will diminish tonight...there is enough pressure
gradient to keep the winds up so temperatures will not drop off that
quickly. Used a blend of available guidance which suggests min
temperatures mostly in the 20s...with lower 30s over the Outer
Cape/islands.

&&

Short term /Friday through Friday night/...
Friday...
high pressure builds across the Great Lakes with northwest flow across new eng.
Tricky cloud forecast as NAM indicating low level moisture trapped
below inversion with widespread low clouds...while GFS and European model (ecmwf)
are drier in the low levels. Given low level flow is still northwest...we
will lean more optimistic than NAM with a mix of clouds and sun
expected. Temperatures will likely be 3-5 degrees colder than today given
cooling at 925 mb...with maximum temperatures ranging from near 30 higher
terrain in northern and western Massachusetts to upper 30s in the coastal
plain.

Friday night...
middle level shortwave moves into west new eng toward daybreak...but
forcing is weak through most of the night and moisture is limited
so expecting a mainly dry night. However...as high pressure builds
eastward across Quebec low level winds become NE and models
indicate increasing low moisture backing in across southeast new eng
overnight. Delta T from SST to the top of the mixed layer is
around 13c which is marginal for ocean effect and ocean induced
convective available potential energy are not impressive...but low level lapse rates are
increasing. Models do show some light quantitative precipitation forecast developing so there may
be a few snow showers developing over Cape Cod and ack overnight
and we have some low chance probability of precipitation.

&&

Long term /Saturday through Thursday/...
highlights...

* scattered snow/rain showers Sat into Sunday with below average temperatures
* weak wave of low pressure on Tuesday
* anomalous system across the Great Lakes will bring heavy
rain...gusty winds and coastal concerns around Christmas evening

Overview...

Overall 00z guidance is in agreement synoptically with issues on
timing and smaller scale features. Amplified pattern set to return
to the U.S. As the strong west Pacific jet translates downstream this
weekend into the Pacific northwest. Building heights over southeast Canada should also
help dig the trough into the Ohio Valley midweek and likely close
off as it swings into southern Canada on Christmas day.
Models/ensembles have been fairly consistent in recent runs on this
amplified pattern and the most recent gefs/European model (ecmwf) ensemble guidance
show good correlation to next Thursday.

During this synoptic transition...high pressure and upper level
ridge will be over the northeast Saturday into Sunday. Digging
trough over the central Continental U.S. Will push a shortwave across the southeast
into the mid-Atlantic. Appears that a weak coastal low will develop
moving towards the region Monday into Tuesday. The GFS brings this
low over the benchmark where the ec pushes well eastward thanks to
building high pressure over eastern Canada. This system is weak so
coastal regions have the best shot for precipitation. As this system depart
appears anomalous low over the Great Lakes will develop a secondary
low closer to New England. The GFS is still a tad faster compared to
the ec but overall excellent model agreement 6+ days out.

Details...

Saturday into Sunday...high confidence.

High pressure in eastern Canada will build across the region Friday
night into Saturday. However it appears on Friday night into
Saturday northerly flow may allow for some ocean effect clouds/snow
showers to impact the cape. Surface to 850 mb temperature difference is 14c
which is unstable enough for snow showers on Saturday for the cape.
Am not anticipate much in the way of accumulations but a dusting
cannot be ruled out.

Weak wave will move through the flow on Saturday night and with high
pressure shifting eastward expect onshore easterly flow. Guidance
continues to show quantitative precipitation forecast moving onshore and with moisture and the lift
could see snow showers moving inland Saturday night into Sunday.
Biggest question is will the moisture saturate enough to -6c. If not
then the potential for freezing drizzle is possible. For now believe
that there is enough moisture so a quick half inch to an inch is
possible per soundings east of the Worcester Hills.

Monday into Tuesday...moderate confidence.

Weak coastal low pressure system will develop and move somewhere
between the benchmark and well offshore. Believe a solution father
out to sea is possible due to strong high pressure in eastern
Canada. Regardless a round of showers will move across the region
with the coastal areas having the best shot to see precipitation. Thermal
profiles will allow for snow at night and rain during the day.

Temperatures will slowly begin to warm as southerly flow becomes
dominate out ahead of stronger system to the east.

Tuesday high tide will be 11.6 in Boston during the late morning and
with any easterly flow may allow for minor coastal flooding.

Wednesday into Thursday...high confidence in system low confidence
on details.

Strong anomalous system will strengthen over the Great
Lakes...developing a secondary surface low just west of southern New
England. Still a lot of details to work out as this system is still
6 days away...however ensembles show that this system is around 2 to
3 Standard deviations allowing for increasing confidence that a
potent storm will occur. Latest 00z guidance shows that most precipitation
will be liquid with a deep subtropical connection back across
Mexico... precipitable water anomalies forecast to rise to between +1 to +3
/or higher/...and temperatures well above average. In fact we may be
close at breaking record high temperatures if temperatures warm to 60f or above on
Christmas evening. As the system wraps up into Canada...cold air
advection should circulate around and begin a transition from
southwest to northeast in turning p-type to snow.

This system is quite strong so we will need to watch for possible
urban flooding issues as well as wind issues. Guidance right now is
showing a 40 to 50 knots jet at 925 mb which may warrant wind
advisories but this system could strengthen even more so a stronger
jet is possible. Lastly coastal flooding issues will need to be
address as high tide on Wednesday and thus are 11.7 and 11.5 respectively.
Any onshore component will push the tide above flood so coastal
flooding is possible as well as beach erosion.

&&

Aviation /19z Thursday through Tuesday/...
forecaster confidence levels...

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

Short term /today through Friday night/...

Through 00z...low VFR/MVFR ceilings around 3000-3500 feet. Trend should
be for cloud deck to become scattered toward 00z..especially near the
coast. West/northwest gusts to 25-30 knots.

Tonight...moderate confidence. Mainly VFR...but lingering MVFR
ceilings possible in the west. Diminishing wind.

Friday...low confidence due to uncertainty with extent of any MVFR
ceilings. We have mainly VFR in the tafs with pockets of MVFR...but
it is possible MVFR ceilings could be more expansive if low level
moisture becomes trapped below the inversion. Later forecasts
will need to reevaluate this.

Friday night...moderate confidence. Mainly VFR...but MVFR ceilings
from the ocean expanding west across east Massachusetts. Low probability for some ocean
effect snow showers over Cape Cod and ack after midnight.

Kbos terminal...moderate confidence. Mainly VFR...but low probability
for MVFR ceilings Friday.

Kbdl terminal...moderate confidence. Should see MVFR ceilings scatter
out this evening. MVFR ceilings may return Friday but this is uncertain.

Outlook...Saturday through Monday...

Saturday into Sunday...high confidence. Mostly VFR with MVFR
conditions any scattered snow/rain showers.

Monday...low confidence. VFR conditions.

&&

Marine...
forecaster confidence levels...

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

Short term /today through Friday night/...high confidence.

Tonight...W/NW gusts 25-30 knots will linger over the waters into
this evening before diminishing overnight. Small Craft Advisory headlines will
continue tonight.

Friday...northwest winds will be below Small Craft Advisory thresholds...but hazardous
seas will continue through the morning over the outer eastern
waters...before subsiding below Small Craft Advisory. Will likely need to extend
the Small Craft Advisory for seas into Friday morning.

Friday night...N/NW winds and seas below Small Craft Advisory.

Outlook...Saturday into Monday...moderate to high confidence.

Friday night into Sunday...anticipate diminishing winds and seas
subsiding late Thursday night into Saturday leading to below Small Craft
Advisory levels.

Monday...expect NE winds gusting up to 20 knots. Low probability of 25 knots
gusts as well as 5 feet seas.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...none.
Rhode Island...none.
Marine...Small Craft Advisory until 1 am EST Friday for anz231-232-251.
Small Craft Advisory until 11 PM EST this evening for
anz233>235-237.
Small Craft Advisory until 10 PM EST this evening for anz230-
236.
Small Craft Advisory until 6 am EST Friday for anz250-254>256.

&&

$$
Synopsis...kjc
near term...kjc
short term...kjc
long term...dunten
aviation...kjc/dunten
marine...kjc/dunten

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