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Area forecast discussion 
National Weather Service Taunton Massachusetts
445 am EST Friday Dec 19 2014

Synopsis...
high pressure moving into eastern Canada will result in dry but
chilly weather today. A weak disturbance and onshore flow will
result in a period of some light rain/snow showers mainly across
eastern new this weekend. Low pressure will pass near the region
on Tuesday...bringing a period of precipitation. A much stronger
storm is likely to bring heavy rain...wind and coastal flooding
concerns to southern New England Wednesday into Wednesday
evening. Dry but very windy conditions follow for Christmas day.

&&

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

4 am update...

Considerable low cloudiness still covered southern New England very
early this morning. This a result of low level moisture trapped
beneath the inversion. Other than the NAM model...most other
guidance does not have a good grasp on these clouds.

The main question is do these clouds hang tough today underneath the
inversion or break up? Sometimes clouds can be very stubborn to
dissipate in these particular situations. Given the
uncertainty...will show an abundance of clouds today but also the
opportunity for peeks of sunshine. Model cross sections show the
best chance for some peeks of sun will be across eastern New
England. Later shifts will have to monitor sky conditions closely
and make any adjustments as needed.

As for temperatures today...high pressure building into eastern Canada will
allow some shallow cold air to ooze south. This should hold high
temperatures in the 30s...but with much less wind than yesterday not bad
for middle December standards.



&&

Short term /6 PM this evening through 6 PM Saturday/...

Tonight...

High pressure across eastern Canada gradually slides east. This
will allow surface winds to turn to a more northerly direction. In
response...ocean effect clouds will move onto eastern New England
coast. Delta T values from the ocean to the mixed layer is around
13c...so may see some light snow showers/flurries develop across the
cape/islands and Plymouth County by daybreak Sat. Low temperatures will
mainly be in the 20s...but lower to middle 30s on the immediate
southeast New England coast.

Saturday...

Interesting setup. Weak inverted trough develops across the region.
Northeast surface winds combined with winds just off the deck
turning east southeast will moisten up the atmosphere across the
southeast New England coast. 850 temperatures around -6c should be cold
enough for ice nucleation...especially given easterly flow bringing
salt particles in off the ocean. Therefore...not too concerned
about freezing drizzle at this time but think will be dealing with
snow showers.

Light snow showers should affect far southeast Massachusetts...especially
Plymouth County where pattern recognition warrants likely probability of precipitation. As
the afternoon wears on we may see these light snow showers develop
across Cape Ann and perhaps even further into eastern Massachusetts by early
evening. Enough mild air should come in off the ocean to result in
ptype changing to light rain showers across portions of the
cape/islands and perhaps onto portions of the immediate southeast
Plymouth County coast.

As for snow accumulations...mainly just looking at a coating to less
than 1 inch across southeast New England through early Saturday
evening. However...low risk for very localized 1 to 2 inch snow
amounts across portions of Plymouth County if the boundary layer
remains cold enough for snow.

&&

Long term /Saturday night through Thursday/...

Highlights...

* scattered snow/rain showers Sunday with average temperatures
* weak wave of low pressure Monday night/Tuesday could bring wintry
mix to interior
* 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. An amplified middle-upper level
trough and precipitation focusing lead surface low/frontal system develops
over the central US by early next week. The supporting and
increasingly negative-tilt trough aloft works over the eastern US and out
through the northestern states into days 5/6 Wed/Thu. 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 night before departing
late Sunday. Longwave digging trough over the central Continental U.S. Will
push a few shortwaves through the flow. First one develops a weak
coastal low moving towards the region late Monday into Tuesday.
There is still some uncertainty with the location of this system
but appears that there will be snow/wintry mix in the interior. As
this system departs appears an anomalous low over the Great Lakes
will develop a secondary low /double barrel low/ closer to New
England...per ensemble guidance. Still a good amount of spread but
if this double low does develop then the rain and wind aspect of the
system increases...details below. Still some uncertainty with this
particular system but with 2 to 3 South Dakota away and general model
agreement have higher confidence this system will impact southern
New England.

Details...

Saturday night into Sunday...high confidence.

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. Believe that
with a southerly flow at 850mb and easterly flow at the surface will
allow for the profile to saturate. Usually you would want saturation
up to -8c but because of salt nuclei the snow growth region begins
at -5c or -6c. Appears that there is enough moisture so a quick half
inch to an inch is possible per soundings east of the Worcester
Hills. However because it is a more east-northeast flow...Plymouth County may
see 1 to 2 inches if they remain cold enough for snow...especially during
the day on Sunday.

Snow may overspread across the west during the day if there is
enough moisture. However ridge begins to build into the region late
Sunday into Sunday night so the potential for snow showers will be
limited due to pressure rise.

Monday into Tuesday...moderate confidence.

Weak coastal low pressure system will develop and move close to the
southern New England coastline. There is still descent model spread
on the exact low track but appears that it does weaken as it
approaches from the mid-Atlantic. Regardless there could allow for
shallow cold air from the departing to high to stay trap and with
warm air aloft moving into the region...could see snow or wintry
precipitation especially across the higher terrain late Monday into Tuesday
morning. Because of the model spread still have details to work out
but something to watch closely.

Temperatures will slowly begin to warm on Tuesday as southerly flow
becomes dominate out ahead of stronger system to the east. Easterly
flow will continue to dominate at the surface with a easterly low level jet
strengthen about 20-30kts. Tuesdays high tide at Boston will be 11.6
during the late morning...and with persisent easterly winds
anticipate at least minor coastal flooding. If this surface low
strengthens and the easterly flow increases then the mass east
coastline will have to be watched closely during the Tuesday high
tide.

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

Strong anomalous system will strengthen over the Great
Lakes...developing a potential secondary surface low just west of
southern New England. Still a lot of details to work out as this
system is still 5+ 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. However latest
guidance does show a quick moving dry slot so snowfall will struggle
due to lack of moisture.

This system is quite potent so we will need to watch for potential
hazards. Currently think that we could see possible urban flooding
issues as well as wind issues. Guidance right now is showing a 45 to
55 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 for Boston. Any
easterly componet will push the tide above flood so coastal flooding
is possible as well as beach erosion. The south coastline will also
have to be watch closely. A very strong southerly jet will develop
Wednesday into Thursday and with astronomical high tide could also see
minor to moderate coastal flooding. Another issue is the potential
pressure falls if this system strengthens even more...it could add
another foot onto the surge.

Friday...low confidence.

Once upper low moves into southern Canada...expect a rather fast
upper west flow in place. May see leftover snow showers but temperatures will
fall back close to seasonal norms.

&&

Aviation /10z Friday through Tuesday/...
forecaster confidence levels...

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

Short term /today through Saturday/...

Today and tonight...moderate confidence. A scattered to
broken deck of clouds will probably remain in place today. Ceilings
should generally be VFR but brief periods of marginal MVFR ceilings will
be possible. Otherwise...MVFR ocean effect ceilings should develop
tonight across the southeast New England coast as surface winds turn
more northerly.

Saturday...moderate confidence. MVFR ceilings overspread the rest of
coastal plain and eventually even into portions of the interior by
late afternoon. Scattered ocean effect light rain/snow showers will
overspread the southeast New England coast with northeast surface
winds.

Kbos terminal...moderate to high confidence.

Kbdl terminal...moderate to high confidence.

Outlook...Saturday night through Tuesday...

Saturday night into Sunday...high confidence. Mainly VFR across
central and western areas. East of orh may see MVFR conditions in
snow showers.

Monday-Tuesday...moderate confidence. Mainly VFR. May see local
MVFR in widely scattered rain/snow showers Monday night/Tue.

&&

Marine...
forecaster confidence levels...

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

Short term /today through Saturday/...high confidence.

High pressure building east into Quebec and northern New England
will keep winds/seas below Small Craft Advisory thresholds through Saturday.
Northwest winds today will turn northeast by Saturday with gusts
between 15 and 20 knots possible at times.

Outlook...Saturday through Tuesday...moderate to high confidence.

Saturday through Monday...winds and seas below small craft
criteria.

Monday night and Tuesday...expect east-NE winds gusting up to 25 knots
with seas building up to 5-7 feet...highest on the eastern outer
waters from coastal low.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...none.
Rhode Island...none.
Marine...none.

&&

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

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