Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 200754

Area Forecast Discussion
National Weather Service Taunton MA
354 AM EDT Tue Mar 20 2018

Dry but unseasonably cold weather will continue today and much
of this evening. Another Nor`easter tracks south of New England
Wed into Thu but close enough for the potential of 6+ inches of
heavy wet snow, strong northeast winds and coastal flooding.
High pressure brings a drying trend Friday into Saturday but
remaining colder than normal.


Inverted ridging, with MSLP values around 1015hPa will continue
to be the dominant factor for sensible wx. Although most of the
overnight hours have been clear, an advancing band of CI ahead
of the first in a series of mid-Atlantic low pres developments
will continue to slowly spill SW-NE across the region.

This should not completely limit the sunshine. And elsewhere
full sun will be observed. Therefore mixing should be able to
tap the warming low-mid lvl temps along with some downsloping
help thanks to N-NW flow gradually veering toward the NE by late
in the day.

This will yield highs in the mid 40s across the CT Valley where
both a minor downslope component and overall lack of snowpack
will help.  Mid 30s-low 40s elsewhere.

Dry conditions under the inverted ridging.


Upper lvls become cutoff and meandering in the mid-Appalachian.
Upstream and over much of S New England, this enforces ridging
during the evening hours. Noting a very dry column particularly
below H5 (below the building CI shield), where dwpt depressions
remain on the order of 10-15C to start the evening.

These two factors will slow the precip shield advancing during
the overnight hours, with the likelihood of only a very light
snowfall by sunrise Wed AM. POPs reflect this slow timing,
keeping the bulk of the overnight hours dry except for the S


Updated 405 PM...


* Another Winter Storm Wed into Thu with 6+ Snowfall possible
* Heavy Wet Snow & Strong NE winds may result in isolated Power
* Minor to Moderate Coastal Flooding Possible Eastern MA Coastline

Synoptic Overview & Model Preferences...

12z NAM/GFS/UKMET and ECMWF have all trended northward with sub 990
mb occluded/vertically stacked low crossing over or near the 40N/70W
benchmark Wed night and then northeast across Georges Bank Thu. This
northwest trend was original supported by the overnight 00z Euro
ensembles. This more northward solution seems reasonable given the
building upstream ridge over the western CONUS, giving way to
downstream trough amplification off the eastern seaboard. Thus will
follow this northwest trend in the 12z guidance and 00z ensembles.

Snow Potential/model qpf...

As we get deeper into the month March a concern regarding
accumulating snow is the increasing length of daylight and
stronger/higher sun angle. However airmass currently over New
England today and for several days has resulted in unseasonably cold
temps with overnight lows in the single digits and teens. Thus
ground temps are colder than normal. In fact dew pts over the region
remain below zero this afternoon, indicating a modified arctic
airmass in place. The other way to overcome warm ground temps is
elevation but more importantly heavy precip rates are most
effective, providing dynamic and diabatic cooling processes. This
system has that potential with multiple guidance sources offering
over an inch of qpf for the event. Obvious ptype issues across
southeast MA, possibly into RI with potential mid level warm layer
offering risk of sleet/ice pellets and rain closer to the coast with
marginal blyr. However by Wed night column begins to cool with
system moving east, eroding any warm layers and changing rain and/or
sleet over to all snow overnight Wed into Thu across southeast MA.

Regarding the I-95 corridor and points westward into CT and central-
western MA ptype looks to be mainly all snow given cold and dry
airmass on the front end of this system. In fact model 850 temps Wed
cool from -2C to -6C over the region as precip commences and
evaporative cooling achieves wet bulb temps. NAM soundings have some
mid level warming initially but this should erode as mid level low
track near the benchmark, essentially cutting off the northern
progression of mid level warm air. In fact favorable track for
mainly all snow with occluded/closed low tracking near the benchmark
combined with cold confluent flow aloft over New Brunswick,
maintaining cold air over southern New England.

A model blend of 12z NAM/GFS/EC and UKMET offers up to 1 inch of qpf
across most of the region. Typical spring snowstorm with SLR ratio
around 10-1 but lower closer to the rain-snow line and slightly
higher interior MA. Given model consensus of up to 1 inch of qpf,
and fairly cold thermal profiles, greater than 50% risk for 6+
inches of snowfall from the Cape Cod canal northwest thru eastern
MA/RI/CT and central MA. Thus have issued a Winter Storm Watch for
much of the region. Left out extreme western MA as qpf tails off. As
is the case with any spring snowstorm with potential of 6+ inches
there is a risk of power outages. Keep in mind from previous storms
the onset of snapping tree branches is about 3-4" of heavy wet snow.

Strong Winds...

All model guidance has low level northeast jet of 50-60 kt Wed and
Wed night with core of the jet along the south coast then pivoting
up the eastern MA coast. Thus will likely need at least wind
advisories for the coastline including the Boston to Providence
corridor. In addition, these strong winds could result in blowing
and drifting snow if 6+ inches of accumulation comes to fruition.

Friday through Sunday...

Colder than normal continues as northeast trough reloads and further
amplifies. Will have to watch next wave late in the weekend if it
tracks close enough to the coast for any impact.


Forecaster Confidence Levels...

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

Short Term /through Tonight/...High confidence.

Through 04Z tonight...
VFR. Increasing high then mid clouds through the day and early
evening hours. NNW winds shift to NE this evening.

After 04Z tonight...
Gradual reduction to MVFR first due to low CIGS then with low
vsbys in -SN from S-N timing may be off a bit in TAF, but the
overall forecast is expected.

KBOS Terminal...High confidence in TAF overall. MVFR CIGS will
precede SN early Wed AM. Timing of this may be off a bit in TAF.

KBDL Terminal...High confidence in TAF overall. MVFR CIGS will
precede SN early Wed AM. Timing of this may be off a bit in TAF.

Outlook /Wednesday through Saturday/...Moderate Confidence.

Wednesday: MVFR/IFR conditions possible. Strong winds with
gusts to 45 kt. Snow possibly heavy at times.

Wednesday Night: MVFR/IFR conditions possible. Strong winds
with gusts to 45 kt. Snow possibly heavy at times especially in
the evening.

Thursday: Mainly VFR, with local MVFR possible. Strong winds
with local gusts to 40 kt.

Thursday Night: VFR. Breezy.

Friday through Friday Night: VFR.

Saturday: Mainly VFR, with local MVFR possible.


Short Term /through Tonight/...High confidence.

Quiet winds and seas are expected through this evening. Winds
increase and seas begin to build overnight ahead of
intensifying surface low pressure east of the Delmarva
Peninsular. By daybreak Wednesday, NE wind gusts will be in the
lower end of storms with Gales in the sheltered Bays/Sounds.

Outlook /Wednesday through Saturday/...Moderate Confidence.


NE winds will ramp up Wednesday with gale force sustained winds or
gale force gusts overtaking all waters by mid morning. Storm force
gusts are likely late Wednesday morning through evening across the
outer southern and eastern waters. Anomalously cold air will help
with fairly efficient mixing through the boundary layer to bring the
stronger wind gusts to the ocean surface. Have increased the SWANGFS
wave height guidance field looks like a fairly good first cut and
increased a relatively modest 5 to 10 percent. This results in waves
of 15 to 20 feet forecast for east of Cape Ann, Stellwagen Bank, and
Massachusetts Bay with a little over 20 feet projected for just east
of Cape Cod and Nantucket.

Winds back to the N Wednesday night and very gradually diminish
overnight but still remaining as gale force through Thursday
morning. Winds and seas gradually subside Thursday afternoon and
night with seas projected to drop below 5 feet across the outer
waters by Friday afternoon.


The combination of a series of low pressure passages along or
just south of the 40/70 benchmark will lead to building
seas/surge thanks to NE-E fetch through the day Wed and Wed
night. This will culminate around the time of high tide early
Thu AM, which at Boston is running about 10.7. Final surge
values are likely to approach 2.0-2.5 ft, which would yield low-
moderate coastal flooding. This is focused where a Coastal Flood
Watch is currently running, from the S Shore of Mass down to
the Cape/Nantucket.


CT...Winter Storm Watch from Wednesday morning through Thursday
     morning for CTZ002>004.
MA...Coastal Flood Watch late Wednesday night for MAZ019-022>024.
     Winter Storm Watch from Wednesday morning through Thursday
     morning for MAZ003>007-010>021-026.
RI...Winter Storm Watch from Wednesday morning through Thursday
     morning for RIZ001>007.
MARINE...Gale Watch from late tonight through Wednesday evening for


LONG TERM...Nocera
TIDES/COASTAL FLOODING...staff is the U.S. government's official web portal to all federal, state and local government web resources and services.