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Area forecast discussion 
National Weather Service Taunton Massachusetts
952 am EST sun Dec 21 2014

scattered snow/rain showers continue through this afternoon. Weak
low pressure will pass south of New England bringing another
period of precipitation Monday night and Tuesday. A much stronger
storm passing west of New England will bring heavy rainfall...
gusty winds and coastal flooding concerns to the region Wednesday
and Wednesday night. Dry but very windy conditions follow for
Christmas day.


Near term /until 6 PM this evening/...
10 am update...scattered snow showers continue across southern New
England this morning. Most of these are too light even to show up
on radar but they are being reported in ground truth reports from
social media and the public.

Previous discussion...

The rain/snow line runs roughly from Taunton and
just north of Marshfield. not expect much change in
the rain/snow line this morning based on thermal profiles and
positioning of the coastal front.

Main uncertainty is just how much snow occurs this morning with the
shortwave moving across CT. Given the track of the
forcing will be located south of the Massachusetts Turnpike and especially
across northern CT/RI. We will forecast 1 to 2 inches of snow
across this region...but a few isolated 3 inch amounts can not be
ruled out. Meanwhile...areas north of the Massachusetts Turnpike will be
removed from the best forcing...but still see some scattered
showers. Accums in this area likely to be a dusting to less than an

Also...some of the precipitation currently across southeast Massachusetts is
being enhanced from the coastal front. A lot of this precipitation will
be in the form of rain showers across Plymouth and even into
portions of Bristol County...points south and east.
least the slightly higher terrain of southeast Massachusetts should see snow
showers. Will have to keep a close eye on if coastal front shifts
slightly this morning. It could mean the difference between an area
getting mainly rain showers or perhaps an inch or so of snow. not expect the coastal front to move too much this
morning...but just a few miles can make difference for a
particularly neighborhood.

Bulk of precipitation will be this morning with the shortwave.
However...enough low level moisture/weak lift will remain this
afternoon for at least this risk of scattered precipitation and will
maintain some probability of precipitation. High temperatures mainly in the 30s.


Short term /6 PM this evening through 6 am Monday/...

A rather cloudy and raw night across southern New England. While
there is not a significant shortwave to trigger a lot of
precipitation...plenty of low level moisture/weak shallow lift
remains. This may be enough to result in a few rain/snow shower
from time to time. Ptype...would be mainly snow showers except to
the southeast of a just north of
Marshfield line. Again...much of the night will probably end up
dry...but just the risk for a few rain/snow showers.


Long term /Monday through Saturday/...

* weak shortwave on Monday night/Tuesday could bring wintry
mix/freezing rain to the interior
* anomalous system across the Great Lakes will bring heavy
rain...gusty winds and coastal flooding concerns around Christmas
* blustery but airmass will be drying out on Christmas day


The models and ensembles offer a reasonably similar and predictable
larger scale middle to upper level pattern evolution for much of the
upcoming week. This active flow regime develops several middle-upper
level trough and associated/unsettling lead surface low/frontal
systems and precipitation swaths progress steadily across the lower 48. The
first amplified/deep lead system works across the eastern/northestern US into
eastern Canada Christmas evening into Christmas allowing for a moist/warmed
airmass in advance of frontal approach with heaviest lead quantitative precipitation forecast
potential over New England. Biggest question is the evolution of the
secondary low pressure system. 00z guidance has backed off on the
strength of the low and is having issues on how to handle its
evolution. The ec and GFS have slowed down the system slightly with
the ec having more of a negative trough vs the GFS. Believe the
UKMET is a good compromise at is was the first deterministic
guidance on tampering down the first main low...while the CMC is out
to lunch. Regardless this is a very anomalous system that is about 4
Standard deviations away.

A secondary system deepens across The Four Corners on Friday and
with the stubborn trough over the central u... will push a
system towards southern New England by Sunday. This system may
support a modest swath of overrunning snow over southern New


Monday into Tuesday...moderate confidence.

A weak wave of low pressure develops along the southeast coast
Monday...and will be on a developing warm frontal boundary. This low
remains weak as the warm front moves slowly north Monday night into
Tuesday. Appears that there is decent warm advection ahead of this
wave. Several hi-res guidance models show that there is the
potential for cold air damming across the interior into Tuesday.
Biggest question will be is there enough moisture for precipitation during
this time frame. Due to a strengthening low level jet providing a good source of
lift anticipate precipitation to develop and overspread across southern New
England. If this plays out with cold air damming and warm air
advection...then we could see freezing rain/drizzle across the I-91
corridor late Monday into Tuesday morning. During the day on
Tuesday...good amount of warm air will push surface temperatures above
freezing by the late morning allowing for any precipitation to be more of
the liquid variety ahead of the Christmas evening storm. However
guidance is always too fast in scouring temperatures out especially across the CT
valley so we will need to monitor this closely...especially due to light
winds and the high in eastern Canada. A Winter Weather Advisory may be
needed for the light icing potential.

We could also see some minor coastal flooding concerns on Tuesday. See
below for more details.

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

Anomalous system will strengthen over the Great Lakes Tuesday into
Wednesday. A secondary shortwave will round the deep trough and
develop a secondary low pressure along the cold front. This
secondary low will strengthen and pass west of southern New England.
This system will continue to deepen as it moves towards eastern
Canada swinging a cold front across the northeast.

Strong southerly jet out ahead of the front with a Gulf moisture
connection will impact southern New England on Christmas evening. This
strong jet will yield for precipitation in the liquid variety. Precipitable water anomalies
are between +4 to +5 Standard deviations above normal which is quite
anomalous for this time of year. In fact current precipitable waters are prognosticated
above 1.6 inches with model quantitative precipitation forecast over 2 the very least
could see urban and poor drainage flooding. Latest ec even shows
over 2.5 inches of quantitative precipitation forecast...and with the tropical connection and strong
southerly flow could see higher amounts due to topography
enhancements. Rainfall records may fall if everything falls into

Ahead of the front...southern New England will warm sector Wednesday into
Thursday morning allowing for temperatures to be well above average. In
fact we may be close at breaking record high temperatures if temperatures warm to
60f or above on Christmas evening. Since we will be in the warm
sector...and showalters close to zero...isolated thunder is still

Strong low level winds will also be a concern especially Wednesday
night. The European model (ecmwf) shows east-southeast winds 30-40 knots on Wednesday
increasing to 60-75 knots by Wednesday night. Strong downpours may
overcome the low level inversion and bring strong gusts of at least
30-40 knots especially along the South Coast. A Wind Advisory may be
needed in some areas.

Christmas day...moderate confidence.

As the cold front passes through early Christmas day...the west
winds will help dry the airmass allowing for partly sunny skies.
Strong cold air advection on Thursday will allow descent mixing. Winds in the surface-
850 mb layer will be 30-40 knots and possibly a little
higher...which would require a Wind Advisory for much of the area.

Friday-Sunday...lower confidence

Weak high pressure builds over the area Friday into Saturday. The
next shortwave approaches late Saturday into Sunday and should bring
increasing clouds from west to east along with a chance of rain or
snow showers mainly in the afternoon and at night.


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 /today and tonight/...

Today and tonight...moderate confidence. Mainly MVFR conditions
south of the Massachusetts Turnpike with borderline VFR to marginal MVFR ceilings
north of the Pike through tonight. An area of snow showers will
expand mainly across areas south of the Pike this morning...but a
few snow showers will still affect northern Massachusetts. Visibilities/ceilings may
briefly drop to IFR levels in northern CT/RI/se Massachusetts. Ptype will be
rain showers across most of Plymouth County and onto the
cape/islands with snow showers north and west. Bulk of precipitation this
morning...but a few lingering rain/snow showers still possible this
afternoon and evening.

Kbos terminal...moderate confidence. A few snow showers possible
this morning but bulk of activity should be south of the terminal.

Kbdl terminal...moderate confidence. Period of snow showers this
morning may drop an inch or two of snow. MVFR conditions dominate
today....but a brief period of IFR conditions are possible this
morning in snow showers.

Outlook...Monday through Thursday...moderate confidence.

Monday...MVFR ceilings continue in Rhode Island/eastern mass in marine east flow.
VFR in the west.

Tuesday...VFR with scattered MVFR in any rain/snow showers.

Wednesday...conditions lowering to IFR in rain/heavy rain. Isolated
thunderstorms possible especially Wednesday night. East-southeast
surface winds may gust to 35-45 knots. Winds at 2000 feet above ground level will
reach 50-60 knots Wednesday afternoon and early night creating low
level wind shear concerns.

Christmas day...west wind gusting 30-40 knots. Conditions improving
to VFR as the wind brings in cooler drier air.


forecaster confidence levels...

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

Short term /today and tonight/...high confidence.

High pressure across Quebec will result in northeast winds across
the waters that will gust to between 15 and 20 knots. This will
result in 2 to 4 foot seas. headlines planned as
conditions should remain below Small Craft Advisory thresholds.

Outlook...Monday through Thursday...moderate confidence

Monday...east to northeast winds gusting around 15 knots or less.
Seas will hold around 3-4 feet...may increase to 5ft in northeast
flow as guidance may be underdone.

Tuesday... increasing east winds with frequent gusts 25 to 30 knots.
Seas will build to 5-6 feet on the exposed waters. A Small Craft
Advisory may be needed.

Wednesday...rain will be heavy at times producing poor visibility.
Increasing southeast winds 20-25 sustained and 40 knot gusts. Winds
should shift to south Wednesday night and southwest toward Thursday
morning. Rain will taper off with the wind shift to southwest. Seas
will continue to build with 7-11 feet heights on the exposed waters.
A Gale Warning will be needed...with a low probability of a few storm force
gusts across the eastern outer waters.

Christmas day... west southwest winds will continue to gust 35-40 a continued Gale Warning may be needed. Seas of 7-15 feet
are possible on the exposed waters...and 5 feet is possible on some
of the nearshore waters.


Tides/coastal flooding...
high astronomical tides will occur Tuesday through Thursday. Breaking
it down...on Tuesday high tide at Boston will be 11.6 feet. There will
be onshore flow during this high tide allowing for minor splashover.
If wind gusts increase then could possible see minor coastal

On Wednesday...winds will switch to a more southeasterly flow during
the day and increase quickly from approaching system and decent
pressure falls. Wednesday afternoon high tide in Boston is 11.7 feet
and with any strength on the easterly wind flow will push the tide
over. Right now believe minor coastal flooding is possible along the
east coastline.

Wednesday evening...the wind GOES in a more southerly direction and
the tide will be increase across the South Coast. High tide in
Providence will be 5.2 feet. 925 mb low level jet strengthens to 50-60 kts which
could push a decent surge up along the South Coast during the time
of high tide. Thinking that minor coastal flooding could occur but
cannot rule out moderate along the South Coast...especially if low level jet
strengthens even more.


Box watches/warnings/advisories...
Rhode Island...none.


near term...Frank/dunten/rlg
short term...Frank
long term...dunten
tides/coastal flooding...

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