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Area forecast discussion 
National Weather Service Taunton Massachusetts
404 PM EST Monday Dec 22 2014

Synopsis...
low pressure moving up the coast will bring periods of rain to New
England later tonight into Tuesday. The rain may begin as a brief
period of snow or freezing rain across northwest and north
central Massachusetts. A much stronger storm passing well to our
west Wednesday into Christmas morning will bring heavy rain...gusty
winds and probably minor coastal flooding. Dry weather with above
normal temperatures expected Christmas afternoon into Sat. Unsettled
weather may return for sun and Monday.

&&

Near term /until 6 am Tuesday morning/...
high pressure over the Maritimes with ridging southwest across
interior southern New England. This will hold onto some lingering
colder air at the surface. But the axis of this drainage is
focused more on Vermont and New York...as per low level ageostrophic
flow...and this is allowing moderation of the air over southern
New England.

East flow at the surface continues into eastern Massachusetts and
Rhode Island...keeping clouds and scattered showers over this
region early tonight.

Low pressure off North Carolina moves up the coast tonight. This
will tighten the pressure gradient as the low runs up against high
pressure over the Maritimes...increasing the east to southeast
flow leading the low and approaching southern New England
overnight.

As the east southeast flow moves over our area it will flow over
the remains of the interior cold air promoting low level lift.
This should bring an increased chance of showers from south to
north overnight. The air over northern CT/western and central mass
may be cold enough for light snow or snow/rain mix. A brief period
of freezing rain may take place in spots in northwest mass. But
the trend will be toward rain all areas by morning.

The clouds and east flow will keep temperatures from falling much this
evening...and may help a steady or slowly rising trend overnight.

&&

Short term /6 am Tuesday morning through Tuesday night/...
Tuesday...
the southeast low level jet will move across southern New England
during the day. This will feed moisture to maintain a saturated
airmass through most of the column...and bring our best chance of
rain. The jet moves north of our area during the afternoon which
will bring diminishing chance of precipitation.

As warmer air fills in...temperatures will moderate further with
values mostly in the 40s.

The steady southeast flow will be 15-25 miles per hour with some higher gusts
possible. This winds comes after a steady east flow today which
has pushed the ocean toward the east mass coast. This suggests
building ocean levels nearshore...with some minor coastal
flooding concerns along the east Massachusetts coast. The best
chance would be around Boston Harbor and near Scituate.

Tuesday night...
the southeast jet moves off to the north. We will continue to have
a moist airmass with warm advection and low level lift. The result
will be lingering showers and drizzle Tuesday night. Low level
south jet will move up the Atlantic coast overnight and reach
southern New England after midnight. This will bring increased
lift which will mean increased coverage of showers late at night.
This south jet is also drawing an anomalously humid airmass north
with precipitable water values over 1.5 inches. Showers developing
late Tuesday night will likely have some embedded downpours.

&&

Long term /Wednesday through Monday/...
highlights...

* periods of rain Wednesday into Christmas morning will be heavy at
times
* dry but windy by Christmas afternoon
* temperatures around 60 for some late Wednesday into Christmas morning
* dry but above normal temperatures continue Friday and Sat
* unsettled weather may return for sun but confidence is low

Models are in decent agreement on the first part of the long term
forecast. There has been quite a bit of run to run variability on
the track of the strong shortwave and surface low expected to move
through the Great Lakes Christmas evening. While the overall pattern
has been well modeled since last week...this variability has led to
some uncertainty...particularly with the winds and the quantitative precipitation forecast. Beyond
the Christmas storm...models are in fairly good agreement through
the first part of the weekend. Then all agreement GOES out the
window for the latter part of the weekend into the first part of
next week. The European model (ecmwf) has a strong low pressure system developing
south of the area and moving along the South Coast Sunday while the
GFS has a strong high pressure right over southern New England.
Given the huge difference here...opted for a blend and mention of
the variability here.

Christmas evening and day...strong shortwave moves through the Great
Lakes Christmas evening and into Quebec on Christmas day. In
response...low pressure develops at the surface and moves across the
Great Lakes into Canada on Christmas evening. Deep southerly flow over
southern New England will allow significantly warmer than normal air
to move over the area resulting in temperatures climbing as high as
60 across southeastern Massachusetts and Rhode Island from Wednesday
afternoon through Christmas morning.

In addition...rain will move in across southern New England for all
of Wednesday into Christmas morning. While quantitative precipitation forecast amounts forecast by
the models have varied quite a bit as we move closer to the
event...causing greater uncertainty in how much rain we
anticipate...the gefs has been very consistent in forecasting precipitable water
values 4 to 5 Standard deviations above normal. This presents quite
a concern as rain will be quite heavy at times. At this point...we
are looking at a widespread 1 to 3 inch event with higher localized
amounts. This would certainly be enough to produce typical nuisance
street flooding. However...would not rule out more widespread
flooding given the highly anomalous precipitable waters that are forecast. Will be
monitoring over the next day or so for a potential Flood Watch.

Have kept the slight chance of thunder given k indices over 30 and
showalter indices near zero. The greatest chance of this is south
of the Mass Pike though there is potential for some isolated thunder
along the East Coast Wednesday afternoon. In addition...have added
gusty winds and heavy rainfall wording everywhere.
Conservatively...a 50 to 60 knots low level jet will be over southern
New England...especially late Wednesday afternoon and Wednesday
night. While an inversion looks to keep the strongest of these
winds from reaching the surface...there is potential for some of
these winds to be brought down in any stronger showers or heavy
rainfall.

The rain should come to an end Christmas morning while temperatures
peak out around 60 for much of the region. Despite the stronger low
level jet Wednesday night...there is a much better chance for the
winds to mix to the surface Christmas afternoon and evening. Behind
the system...there are stronger westerly winds and steeper middle level
lapse rates. Advisory level wind gusts are fairly certain...
particularly across southeastern Massachusetts...the cape...and
islands with a low probability of high wind criteria being met.
Will be watching this for any headline issuance over the next day or
so.

Friday and Saturday...high pressure slides south of southern New
England bringing dry weather...and above normal temperatures.

Sunday and Monday...low confidence forecast as mentioned above with
wildly different solutions from the European model (ecmwf) and the GFS. Have opted
for slight chance to chance probability of precipitation for the time being...recognizing
that if the GFS is correct these will be way too high and if the
European model (ecmwf) is correct...way too low. Potentially unsettled weather
during this time period and definitely much cooler temperatures.

&&

Aviation /21z Monday through Saturday/...
forecaster confidence levels...

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

Short term /through Tuesday/...

Through 00z...mixed areas of VFR and MVFR in the continued east
flow across southern New England.

Tonight...moderate confidence. Mainly MVFR conditions in
scattered light rain and snow showers. Patches of light freezing
rain possible late tonight across northwest Massachusetts.

Tuesday...moderate confidence. MVFR conditions may lower to IFR in
showers. The core of the showers will be over our area during the
morning and early afternoon...with visibilities reduced to 2-3sm in the
heavier showers. Showers diminish late in the day with visibilities 5-6
miles but ceilings linger at 1000-1500 feet.

Tuesday night...moderate confidence. After a short pause early in
the night...the chance of showers will increase again overnight.
Ceilings will be IFR or low-end MVFR. Visibilities will be 3-4 miles with
areas of 2 mile visibilities.

Kbos terminal...moderate confidence.

Kbdl terminal...moderate confidence.

Outlook...Wednesday through Saturday...

Wednesday...high confidence. MVFR/IFR conditions in widespread -ra.
Low level wind shear is likely particularly Wednesday afternoon and evening as winds
at the surface are likely to be below 10 kts while a southerly low level jet
screams overhead at 50-60kts.

Christmas...moderate confidence. Conditions improve to VFR as
system clears out. Westerly winds increase during the afternoon
gusting to around 40 kts.

Friday and Saturday...moderate confidence. VFR.

&&

Marine...
forecaster confidence levels...

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

Short term /through Tuesday night/...moderate to high confidence.

High pressure across Quebec and the Maritimes will result in
persistent northeasterly to easterly winds across the waters
tonight. Low pressure moving up the Atlantic coast will tighten
the pressure gradient a little tonight...and may allow winds to
gust around 20 knots or in the low 20s. The persistent east-southeast flow
will allow seas to build to 5 feet on the eastern outer
waters...with 2-4 feet most other waters. Small Craft Advisory
continues.

Approaching surface low pressure from the middle-Atlantic tonight will
help increase an easterly low level jet over the water Tuesday. This will
build the seas between 5-8 feet. Could see wind gusts between 20-30
kts as the front passes. The Small Craft Advisory has been expanded
accordingly.

Southeast winds will diminish but maintain transport of ocean
water toward the New England coastline especially along eastern
Massachusetts. Expect seas at or above 5 feet on many of the
southern New England waters through the night. Winds should
generally be below small craft thresholds.

Outlook...Wednesday through Saturday...moderate confidence

Seas increase Wednesday into Christmas morning as a cold front
approaches and then crosses the waters. High pressure then builds
over the waters Friday and Saturday allowing seas to subside.
Southerly gales Wednesday into Christmas morning will give way to
westerly gales as the cold front moves across.

Visibilities will be reduced Wednesday into Christmas morning in
rain and there is a slight chance of thunder Wednesday.

&&

Tides/coastal flooding...
high astronomical tides occur Tuesday and Wednesday.

East Massachusetts coast:

Tuesday...high tide at Boston will be 11.6 feet at midday with a
surge of 1 to 1.5 feet. Winds will be from the east at 10 to 15
knots...so there will be onshore flow during this high tide. While
this is nothing too strong...given the high astro tide and
persistence of east winds we expect some splash over/minor
coastal flooding. Certainly do not expect to see any significant
issues. A coastal Flood Advisory has been issued for the east mass
coastline for this midday high tide.

Wednesday...high tide at Boston will be at 11.7 feet at midday
with a surge of 1 to 1.5 feet. Winds will be from the southeast
near 10 knots but there will be some pressure falls with
intensifying low pressure to our west. This should be enough for
more splash over/minor coastal flooding with the high astro tides.
However...do not expect any significant problems given winds/seas
should be held in check during the time of high tide.

South coast:

Wednesday evening...southerly winds are expected and astronomical
tides increase along the South Coast. High tide in Providence will
be 5.2 feet Wednesday evening. 925 mb low level jet strengthens to 55-65
kts which could push a 1 to 2 foot surge up along the South Coast
during the time of high tide. The low level inversion will
probably hold the strongest winds just above the ground...so
thinking the risk is for splash over and minor coastal flooding.
There still is a low risk for moderate coastal flooding...but that
is often hard to get in a non-tropical system along the South
Coast.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...coastal Flood Advisory from 9 am to 3 PM EST Tuesday for
maz007-015-016-019-022.
Rhode Island...none.
Marine...Small Craft Advisory from 1 am to 7 PM EST Tuesday for anz231-
232.
Small Craft Advisory from 1 am to 10 am EST Tuesday for anz233-
234.
Small Craft Advisory from 1 am Tuesday to 6 am EST Wednesday
for anz235-237-251-256.
Small Craft Advisory until 6 am EST Wednesday for anz250-254.
Small Craft Advisory from 10 PM this evening to 6 am EST
Wednesday for anz255.

&&

$$
Synopsis...wtb/rlg
near term...wtb
short term...wtb
long term...rlg
aviation...wtb/rlg
marine...wtb/rlg
tides/coastal flooding...

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