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Area forecast discussion 
National Weather Service Taunton Massachusetts
125 am EST Tuesday Jan 27 2015

Synopsis...
a crippling and potentially historic winter storm / blizzard will
impact the region tonight into Tuesday night. Travel will become
impossible and life threatening. Drier weather follows Wednesday
afternoon into early Thursday...but low pressure will bring
another chance for snow Thursday night and Friday. Very cold air
likely follows for next weekend.

&&

Near term /through today/...
*/ highlights...

- crippling and potentially historic blizzard
- serious life-threatening storm / travel discouraged!
- 1 to 2 feet of snow with locally higher amounts in snow-banding
- white out conditions / near-zero visibilities at times
- strong to damaging winds / hurricane force across southeast New England
- moderate to major coastal flooding / severe beach erosion

930 PM update...
WV loop shows classic cyclogenesis taking place with well defined
comma head off middle Atlantic coast with warm conveyor belt/trowal
lifting northward into new eng. Still plenty of jet energy diving south
to the southeast US coast with trailing negative tilt shortwave/middle level
circulation off NC coast with scattered lightning strikes noted. It is
this shortwave that will result in deepening middle level low center
and explosive cyclogenesis south of new eng.

First mesoscale band lifting north along the South Coast and into
Rhode Island and southeast Massachusetts this evening associated with enhanced snowfall rates.
Will see occasional bands lift north across southern New England tonight with two
distinct bands likely developing later tonight into Tuesday. First
associated with middle level frontogenesis as the middle level low deepens
and this band will move inland across CT and central Massachusetts and
possibly further west into west Massachusetts during Tuesday. The westward extent of
this band is still uncertain. Then very strong banding signal
associated with low level frontogenesis along the coastal front
lifting across southeast new eng 08-15z. This band will likely be associated
with 2-3"/hour snowfall rates across southeast Massachusetts. Other notable feature
is middle level lapse rates over 6 c/km across southeast new eng suggesting
potential for upright convection and thunder snow with isolated 4"/hour
rates.

Coastal front currently west of i95 extending from between bed-
bos to northwest Rhode Island. As surface low approaches benchmark late tonight and
Tuesday morning expect this front to move along and just east of the
I-95 corridor with temperatures crashing into the teens and lower 20s in
the coastal plain including bos and pvd.

Only change to update was to increase snow across ack. Received
report of 4.5" as of 845 PM and expect snow to continue to at
least 06z before possible changeover.

Previous discussion...

*/ overview...

High confidence forecast for a crippling/historic blizzard impacting
S New England. Negatively-tilting 500 mb trough through which middle-level
energy intensifies surface low pressure southeast of Nantucket near the
40n/70w benchmark NE into the Gulf of Maine tonight-Wednesday along
an offshore baroclinic zone enhanced by high pressure situated north
across east Canada. Classic conveyor belts beneath 300 mb jet coupling axes
of rrq / lfq yields strong diffluent motions aloft. Low vertically
stacks / fills becoming captured / stalling beneath the closing
low aloft resulting in a slow-moving / long-duration storm. Surface
pressure falls of 20-30 mb in 18 hours. Already ongoing as latest
water vapor satellite shows the initial negative-tilt ahead of
which the baroclinic-Leaf structure is evident. Lots of offshore
lightning activity indicative that this storm is getting
beefy/intensifying. Its bombogenesis baby!

Key to the forecast is the location of the closing 850 mb / 700 mb low and
parent cyclonic warm- and dry-conveyor belt motions. Presently an
open-wave ahead of which a SW-NE banding signature is seen per WSR-
88D returns along the leading edge of middle-level f-general forcing aloft
expected to move ashore and impact S/southeast New England from now into
tonight /1-2 inch per hour snowfall rates late/. The band will pivot
south-southwest-north-northeast and trowal rearward of bombing low around midnight into
Tuesday. Middle-level frontogenesis neighbored with -epv /potential
instability/ of high Theta-E air below strong diffluence aloft
yields an intensification of the snowband and the likelihood of
thundersnow. Increasing snowfall rates of 2-4 inches per hour around
midnight into Tuesday. Deformation zone expected of moderate-heavy
snow. Decent Star-signatures within forecast snow-banding with
strong lift through the dendritic growth-zone.

While that is one forecast area of mesoscale snow banding...another
is expected along the coastal front where low-middle level convergence
is maximized beneath favorable dynamics aloft during the day Tuesday.
Consensus of model guidance keeps dry air at -10c east...so ice should
be present at the top of the moist layer through which strong forcing
should occur. This banding expected to occur over southeast New England
adding more snow on top of the initial front-end thump during the
overnight hours.

Activity diminishes west to east as the low pulls out to sea into
Wednesday morning. Additional energy rotating rearward around the
low will lend to localized areas of low- to middle-level convergence
of which will invigorate additional mesoscale banding though not
as widespread...more scattered to isolated as drier air entrains
and the low stacks / occludes lending to its dying stage. All that
will be left is robust northwest-winds drawing colder air S generating
fairly cold wind chill values...especially in wake of a deep-layer
of snow anticipated.

Overall went with mesoscale models /mainly the NAM with a touch
of the WRF/ along with a ec/wpc blend point to the two areas in
question for snow-banding. All other models /GFS-Canadian-UKMET/
were clustered a bit faster. Overall it remains a challenge as
there is a spread in the forecast guidance of low positioning and
there remains forecast uncertainty.

So a word of caution. While there is focus of two areas across S New
England of heavier snowfall amounts with a lull in between...this is
an unprecedented storm for all S New England for which preparations
should have been made and action should be taken on a similar nature
no matter if you're under a blizzard or Winter Storm Warning. This
is a dangerous and life-threatening storm. You should not be out on
the roads...leaving them clear for emergency officials and snow-
removal equipment. You should be taking shelter and making plans in
case you become trapped for days and/or lose power. Consider any and
all possibilities no matter what you think the outcomes may be.

*/ Precipitation-type...

Continued indications that we will see a change-over to rain across
the east-half of the cape and Nantucket...possibly Marthas Vineyard
from around midnight into Tuesday with the transition line gradually
shifting northwest through the timeframe prior to changing back to all snow
as we go into late Tuesday when the storm begins to exit. Precipitation north/west
of h925 0c line / coastal front should remain snow. Model consensus
of the dry-slot around -10c has it confined mainly in areas where
rain is expected. Feel the top of the low-levels especially with the
coastal front and mesoscale banding should be deep and cold enough
to retain the presence of ice at the top of the layer.

*/ Blizzard...

Two areas of focus: southeast and west New England with highlighted mesoscale
snow-banding up above. Anywhere from 2 to 4 inch per hour snowfall
rates will be possible within banding...along with thundersnow
potential. Snowfall amounts exceeding a foot will be possible within
6-hours. Coupled with winds especially east/southeast will make for blizzard /
white-out / near-zero visibility conditions. Storm-total snowfall
graphic updated to account for thinking. Headlines remain unchanged.
Did not go with a mention of thunder with this forecast.

Lower snow-to-liquid ratios towards the northwest in colder air. A wetter
snow closer to the coast to the southeast. Combining with winds will make
southeast New England both vulnerable and the highest risk area for power
outages...as well as downed tree / tree limbs / structural damage.

Widespread amounts of 1 to 2 feet with some locations seeing upwards
of 3 feet...mainly in those aforementioned areas where mesoscale
snow-banding is expected. Travel will be impossible/life-threatening
over the entire forecast region.

*/ Winds...

Strongest NE-winds centered around early Tuesday morning. Agree with
prior forecaster that low-level profiles well-mixed up to h9 support
at least 75-80 percent of momentum within lowest 2 kft to mix-down
to the surface. Hurricane force wind gusts of 70 to 80 miles per hour remain
forecasted for southeast-coastal Massachusetts including Plymouth County...Cape
Cod...islands...and possibly even Cape Ann. Will have to monitor
the situation closely. Such winds are covered within the Blizzard
Warning...thus hurricane wind force warnings are not issued.

So to reiterate...70 to 80 miles per hour for southeast-coastal Massachusetts and even
possibly for Cape Ann. Around 50 to 60 miles per hour gusts inland around the
I-95 corridor / across the coastal plains. 30 to 40 miles per hour gusts across
remainder of the interior.

With strong to damaging winds...downed trees/tree limbs...structural
damage. Risks are greater where snow is expected to be heavy / wet /
more water laden. Scattered to widespread power outages overall with
greatest impacts in areas with strongest winds.

&&

Short term /tonight through 6 PM Wednesday/...
discussion continues below.

&&

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

* colder weather arrives late this week
* clipper low may bring more measurable snow Thursday night into Friday
* much colder early next week

Overview...

26/12z guidance has come into better agreement with their overall
synoptic handling for this portion of the forecast. There are still
some crucial details to be worked out though. Will favor a consensus
approach.

Details...

Wednesday...lingering snow showers should come to an end during the
morning across most of eastern New England as powerful low pressure
lifts into the Canadian Maritimes. A few ocean-effect snow showers
may linger into the afternoon or perhaps early evening across the
cape and islands. Otherwise...partial sunshine develops as the day
wears on across the region.

Wednesday night and Thursday...high pressure builds in Wednesday
night. With a fresh...deep snowpack in place with diminishing
wind...some outlying locations should fall below zero. Model
guidance is still likely too high. Will make further adjustments
later this week to dial this in.

Thursday night and Friday...another low pressure quickly moves in
from the west. This system won't compare to what we will see into
Wednesday...but shortwave looks fairly impressive. The primary track
of a clipper low should be to the north of southern New England.
This should result in more scattered snowfall. Plowable amounts are
possible...even with the more northern track.

This weekend into Monday...a shot of very cold air approaches
southern New England this weekend into early next week. There is
some question if another coastal low pressure can develop and get
close enough to our region. 29/12z European model (ecmwf) has come closer to the GFS
solution...both in track and timing. At present...will place a
chance of snow in the forecast late Sunday into Monday. There is
still too much uncertainty at this point to say more than that at
this time.

&&

Aviation /06z Tuesday through Saturday/...
forecaster confidence levels...

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

Short term /through Wednesday/...

Through 12z...moderate confidence.
Expect LIFR-vlifr ceilings/visibilities across east Massachusetts/Rhode Island with periods of
+sn/blsn. May see 2-3 inch/hour snowfall rates as well. North-NE wind
gusts up to 60-70 knots on Cape Cod and the islands...locally up to
80 knots on the east end of Nantucket and possibly Outer Cape
cod...35-50 knots across remainder of east Massachusetts into east Rhode Island. Even stronger
winds aloft with low level wind shear at 2kft. Isolated thunder remains possible.
Further west...MVFR-IFR ceilings/IFR-LIFR visibilities with periods of vlifr in
any +sn bands moving across. North gusts up to 30-40 knots.

Today...moderate confidence.
Periods of IFR-vlifr ceilings/visibilities in periods of +sn/blsn through
16z-18z across east MA/RI...then slowly improving. North-NE wind gusts
continue up to 50-60 knots across east coastal areas...slowly
diminishing during this afternoon except on Cape Cod and the
islands. Central-west areas...MVFR-IFR ceilings/visibilities with local LIFR
visibilities in periods of +sn through midday. Low level wind shear continues across east Massachusetts
through early afternoon.

Tonight...moderate confidence.
Central-west areas...visibilities improve across to VFR by around midnight
with ceilings MVFR- IFR early...slowly improving to VFR. North-northwest wind
gusts 25-30 knots early slowly diminishing. East MA/RI...IFR-LIFR ceilings/
visibilities early...slowly improve around midnight or so.

Wednesday...moderate to high confidence. VFR.

Kbos taf...high confidence in taf trends...lower confidence in
exact timing.

Kbdl taf...high confidence in taf trends...lower confidence in
exact timing.

Outlook /Wednesday night through Saturday/...

Wednesday night...high confidence in trends...moderate confidence
in timing. Conditions improve to VFR from west to east...although
MVFR ceilings may linger cape/islands. Northwest wind gusts to 25
knots.

Thursday and Friday...moderate confidence. Another round of MVFR-
IFR conditions in some snow sometime Thursday night into Friday.

Saturday...moderate confidence VFR. Gusty northwest winds.

&&

Marine...
forecaster confidence levels...

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

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

*** Powerful storm produce life threatening conditions ***

A rapidly strengthening low pressure system will move up the
coast...slowing near southeast New England through tomorrow. This
will generate wind gusts of 55-65 knots out of the NE. Expect seas to
approach 30 feet over the east waters as well. Hurricane force wind
warnings continue for the open waters...with storm warnings closer
to the shore. It is encouraged that mariners return to port by
this afternoon.

Conditions improve somewhat Tuesday night into Wednesday...although a period
of gales are possible until conditions drop down to Small Craft
Advisory thresholds.

Outlook /Wednesday through Saturday/...

Wednesday...high confidence. Any leftover gale force northwest wind
gusts early in the morning will quickly diminish to Small Craft
Advisory levels. Seas will continue to subside...but still remain
well above small craft thresholds across the eastern waters.

Thursday and Friday...moderate confidence. High pressure nearby will
keep winds/seas below small craft thresholds Thursday into early Friday.
Depending on the track of an approaching low pressure...we could see
Small Craft Advisory conditions develop later Friday or Friday night.

Saturday...pressure gradient between an offshore low pressure and a
high pressure over the northeast intensifies. This could result in
gale force wind gusts with rough seas...especially across the outer
coastal waters.

&&

Tides/coastal flooding...
*** moderate with pockets of major coastal flooding along the
eastern Massachusetts coast for both the Tuesday early morning and Tuesday
late afternoon high tides ***

Warnings/advisories...significant coastal flooding is expected for
both the early Tuesday morning and late Tuesday afternoon high tides
along the Massachusetts East Coast. Coastal flood warnings remain in
effect for the Massachusetts East Coast for both the Tuesday early morning and
late afternoon high tides. Have also issued a coastal Flood Advisory
for the ocean exposed coastline of southern Rhode Island...including
Block Island.

Changes...no significant changes to the coastal flood forecast with
the latest issuance. Have tweaked wave and storm surge forecasts
only very slightly. We still expect the coastal flooding impact for
this storm to be on par with that from the February 2013 event. Keep
in mind...however...that no two storms are ever exactly alike. Some
areas will likely be impacted a little less and others a little more
with this storm.

Tide times...along the Massachusetts East Coast...high tide occurs between 4 and 6
am and again between 5 and 7 am along most of the coastline. High
tide specifically in Boston is around 430 am early Tuesday morning
and near 5 PM for Tuesday late afternoon. For the Rhode Island coastline...high
tide tonight is generally between midnight and 2 am.

Tuesday am high tide...the storm surge is still anticipated to be 3
to 3.5 feet and still rising at the time of the Tuesday early am
high tide. The actual peak storm tide could be 15 to 30 minutes
after the scheduled high tide in some locations due to the
increasing surge. By the time of the early am high tide...anticipate
seas to be between 20 and 25 feet just offshore...highest east of
Cape Cod and Nantucket. Seas will be high enough to cause
significant overwash in typically vulnerable locations...especially
in but not limited to Hull...Scituate...and Marshfield. NE surface
winds will likely gust to between 50 and 60 knots by the time of the
early Tuesday am high tide...perhaps even between 60 and 70 knots
along the Chatham and Nantucket coast. Severe beach erosion is
likely in some spots given the combination of elevated water levels
and wave runup. This is a storm that could produce one or more new
inlets along exposed east and northeast facing barrier beaches. We
are especially concerned with the erosion potential for east facing
shorelines along Plum Island...Plymouth...Orleans...Chatham and
Nantucket.

Tuesday late afternoon high tide...this tide is about a foot lower
astronomically but the surge could be about the same or a few tenths
higher than at the Tuesday am high tide. Also seas may still be 25
to 30 feet just offshore at the time of the Tuesday late afternoon
high tide. Areas of severe erosion are expected again for the late
Tuesday afternoon high tide. Besides continuing concerns regarding
erosion along Plum Island...Orleans...Chatham and Nantucket...severe
erosion is likely to be also a concern along the north side of Cape
Cod from Sandwich to Eastham as winds will shifted to a more
northerly direction by that time. Note also that some areas
compromised by the early morning high tide may be pounded a second
time by the late afternoon high tide.

Southern Rhode Island South Coast...portions of the ocean exposed
Rhode Island coast...including Block Island...will likely experience
pockets of minor coastal flooding and minor erosion for the high
between midnight and 2 am tonight due to swells on top of elevated
water levels. At this time...it looks like the wind flow will be
sufficiently offshore by Tuesday afternoon to preclude any coastal
impacts.

Wednesday early morning high tide along the Massachusetts East
Coast...there may be enough residual surge and swell for minor
coastal flooding and some erosion along north and northeast facing
shorelines.

&&

Box watches/warnings/advisories...
CT...Blizzard Warning until 1 am EST Wednesday for ctz002>004.
Massachusetts...blizzard warnin

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