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Area forecast discussion 
National Weather Service Taunton Massachusetts
958 am EST Sat Dec 20 2014

scattered snow/rain showers will gradually overspread the region
from east to west later today into sun. 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 into Wednesday evening. Dry but very windy
conditions follow for Christmas day into Friday.


Near term /until noon today/...
10 am update...made minor updates mainly to the winds and probability of precipitation.
Updated winds using the rap for its higher detail with the coastal
front this afternoon. Snow showers have moved ashore in Plymouth
County as well as portions of the cape and the eastern end of Cape
Ann. Expect these to continue off and on through the remainder of
the morning.

Satellite imagery shows an interesting pattern. Low clouds moving
westward off the ocean have crept in along the East Coast. Then
another area of low clouds have moved northward too about the Mass
Pike. Northwestern mass should remain mostly sunny through the
morning and into the afternoon while clouds continue to move
westward across eastern Massachusetts.


Short term /noon today through 6 PM Sunday/...
this afternoon and tonight...

Interesting mesoscale setup as an inverted trough combines with some
ocean enhancement. Approaching upper level shortwave will allow for
easterly flow just off the ground. This combined with northeast
surface winds will allow low level of the atmosphere to moisten.
Soundings across eastern New England become saturated all the way up
to the -9c layer just above 850 mb. This is favorable for some
decent snow growth...its just lower in the atmosphere than we
typically see with more synoptic snow events. While this is not a
true ocean effect event...Delta T values from the mixed layer to the
ocean are around 15c. Therefore...expect some ocean enhancement.


While flurries/light snow showers may occur across far southeast New
England this morning. The more substantial precipitation should
overspread Plymouth County as well as the cape and islands this
afternoon. Given easterly flow just off the ground...expect snow
showers to overspread most of eastern New England this evening as
low levels moisten. In fact...should see at least some scattered
snow showers affect even our far western zones.


Mainly snow for all but portions of coastal Plymouth County and onto
the cape/islands. A subtle coastal front looks to setup across
southeast coastal Plymouth County. Ptype will be snow to the
northwest of this region...but enough mild air should come in off
the ocean for a change to rain showers southeast of this region.
Not expecting the position of this subtle boundary to change much
through Sunday morning.

Snow accumulations:

We are generally looking at a coating to 2 inches of snow across
most of the region. The main focus for 1 to 2 inches will be across
portions of southeast Massachusetts...away from the southeast Plymouth County
coast. Also...the Worcester Hills into northwest Rhode Island might
be another area as a result of the subtle upslope component to the
wind. You can actually see that in some of the high resolution
model guidance.

There is one area to watch in particular...from northwest Plymouth
County and into Norfolk County based on mesoscale models and pattern
recognition. Would not be surprised to see very localized 3 to 4
inch snow amounts...given some ocean enhancement and a little focus
on the coastal front. Will try to highlight this area in our
snowfall and probabilistic graphics.


Scattered snow showers will continue during the morning with moist
easterly flow in the low levels. Again...ptype will be snow except
across southeast Plymouth County/cape and islands where boundary
layer will probably be warm enough for rain showers. While some
light precipitation may linger into Sunday afternoon...expect a
decrease in areal coverage through the afternoon as low level lift
weakens. It will remain cloudy though with temperatures in the 30s.


Long term /Sunday night through Friday/...

* inverted trough will bring rain/snow showers to the mass east
coastline Monday into Tuesday morning.
* 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 flooding concerns around Christmas


Overall 00z guidance is in agreement synoptically with issues on
timing and smaller scale features. It remains evident that an
amplified middle-upper level trough and precipitation focusing lead surface
low/frontal system will steadily progress to the east-central U.S.
Tuesday as embedded impulses dig to the Lee of an amplified eastern
Pacific/West Coast ridge aloft. The supporting and increasingly
negative-tilt trough aloft works over the eastern US and out through the
northestern states into days 4/5 Wed/Thu. Models/ensembles have been
fairly consistent on this amplified pattern and the most recent
gefs/European model (ecmwf) ensemble guidance show good correlation to next Thursday.

High pressure and upper level ridge over the region will begin to
exit by Monday. At the surface appears another inverted
trough/onshore flow will yield to precipitation along the mass coastline on
Monday. Aside from this quick mesoscale event the main focus is on
the longwave digging trough over the central Continental U.S. Which will push a
few shortwaves through the flow. First one develops a weak wave
moving towards the region late Monday into Tuesday. There is still
some uncertainty with the location/strength of this system but
appears that there is the possibility of a snow/wintry mix in the
interior. As this system departs an anomalous low over the Great
Lakes will develop a secondary low just west of southern New England
per ensemble guidance. Still a some spread but if this double low
does develop then the rain and wind aspect of the system
increases...details below. Still need to work out minor details with
this particular system but with 3 to 4 Standard deviations away and
general model agreement have high confidence this system will impact
southern New England.


Sunday night into Tuesday...moderate confidence.

High pressure and upper level ridge will begin to move offshore as
digging trough begins to set-up over the central Continental U.S.. appears
another mesoscale-scale inverted trough will set up on Monday and last
into early Tuesday morning. Guidance shows that rain/snow showers
will develop along the mass east coastline thanks to onshore
easterly flow. Appears the column will saturate per BUFKIT soundings
and southerly flow aloft. Both the NAM and GFS show quantitative precipitation forecast making it
onshore during this time frame. However the ec is slightly slower
and keep the bulk of the precipitation just offshore. Add a chance of probability of precipitation
due to the NAM/GFS model continuity.

By late Monday night into Tuesday a southern branch storm system
forms along the southeast coast and takes a track off the middle Atlantic
coast. This system gets caught up with the overall large scale
amplification of the upper air pattern. There is decent warm
advection with cold air damming across the interior. Biggest
question will be is there enough moisture for precipitation during this
time frame. If precipitation overruns then could see freezing rain/drizzle
across the interior late Monday into Tuesday morning. During the day
on Tuesday...good amount of warm air will push surface temperatures above
freezing 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.

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.

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 continue to show
strong agreement so have 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 the Gulf of
Mexico...precipitable water anomalies forecast to rise between +3 to +4. Current
precipitable waters are prognosticated above 1.6 inches so have continued the categorical
probability of precipitation. Temperatures will be well above 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 possible urban flooding
issues as models prognosticated well over 1.5 inches of quantitative precipitation forecast. Latest ec even
shows 2.5 inches of quantitative precipitation forecast is possible. The gefs show a moisture flux
at 850 mb being 4 to 5 Standard deviations away...and with the
tropical connection and strong southerly flow could see higher
amounts due to topography enhancements. Rain fall records may fall
if everything falls into place.

Also believe that we may see some wind issues with this system.
Guidance right now is showing a 45 to 55 knots jet at 925 mb...however
it will be hard to break the inversion with the southerly flow.
Right now believe that wind advisories may be needed but if this
system strengthen even more so a stronger jet is possible.

Continued the mentioned of isolated thunder in the forecast per previous
forecaster suggestion. Believe it could be possible as southern New
England will be in the warm sector...with the anomalous jet and
showalters close to zero.

Friday...low confidence.

Once upper level 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 /15z Saturday through Wednesday/...
forecaster confidence levels...

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

Short term /through Sunday/...

700 am update...

Today...moderate confidence. MVFR ceilings will continue to overspread
eastern New England this morning and eventually into the interior
this afternoon. Scattered precipitation will overspread
southeast Massachusetts/cape/islands this afternoon. Ptype mainly snow showers
northwest of the southeast Plymouth County coast and rain showers
southeast of that region. Localized IFR conditions possible just
inland from the southeast New England coast...where ptype is snow.

Tonight and Sunday...moderate confidence. MVFR conditions dominate
in lower clouds and scattered precipitation. Localized IFR
conditions possible with the best chance in snow showers.
Precipitation affects mainly eastern New England but still some
scattered activity across western New England. Ptype mainly snow
showers northwest of the southeast Plymouth County coast...with rain
showers southeast of that region.

Kbos terminal...moderate confidence in taf. Snow showers may arrive
towards 00z this evening.

Kbdl terminal...moderate confidence in taf.

Outlook...Sunday night through Wednesday...

Monday into Tuesday...moderate confidence. Expect mostly
VFR...isolated MVFR along the mass east coastline in rain/snow

Tuesday night into Wednesday...moderate confidence. Conditions will
be deteriorating as heavy downpours...isolated thunder and gusty southerly
winds move into the region by Wednesday. Could see MVFR to IFR
conditions in ceilings and visibilities.


forecaster confidence levels...

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

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

High pressure across Quebec will result in persistent northeast
winds across the waters through the weekend. Wind gusts up to 20
knots with seas between 2 and 4 feet are expected for most no headlines planned. There is a low probability that a
brief surge of northeast wind gusts near 25 knots with marginal 5
foot seas could affect our northern waters late tonight into Sun
morning...but not confident enough to issue any headlines at this

Outlook...Sunday night into Wednesday...moderate to high

Sunday into Monday. Continuous northeast flow will keep seas choppy.
Lingering northeast swell may bump seas above 5ft especially across the
northeastern waters. Small Craft Advisory may be needed...but lower confidence.

Tuesday into Wednesday...NE winds will increase...gusting to 30 knots
Tuesday then shift to southeast gusting to 30-35 knots Wednesday...highest on the outer
waters. Seas build up to 7-11 feet by Wednesday. Gales will be needed.


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 descent
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. However Wednesday evening...the wind GOES in a more
southerly direction and high tide in Providence will be 5.2 feet. 925
mb low level jet strengthens to 50-60 kts which could push a descent 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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