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Area forecast discussion 
National Weather Service Taunton Massachusetts
1015 PM EST Thursday Feb 11 2016

Synopsis...
much colder airmass moves into the region tonight om gusty
northwest winds. An ocean storm will track well offshore Friday
night but will bring a period of accumulating snow to the cape
and islands. An Arctic front will deliver bitterly cold weather
this weekend with dangerously cold wind chills. Temperatures
moderate Monday. For Tuesday and Tuesday night...a storm will
likely bring snow quickly changing to rain along the coast and to
freezing rain and then rain inland as much milder air moves into
New England.

&&

Near term /until 6 am Friday morning/...

***bitter cold wind chills of 0 to 10 below zero continue through
daybreak***

1015 PM update...

Although not as extreme as what we will see this weekend...strong
cold advection was resulting in bitterly cold wind chills of 0 to 10
below zero this evening. Northwest winds were still gusting to
between 25 and 35 miles per hour but should gradually diminish as pressure
gradient weakens. Temperatures will continue to fall though and by
daybreak Friday...lows will be well down into the single digits for
most with a few below zero readings possible in northwest Massachusetts where
winds decouple. Skies will remain mostly clear other than perhaps
some ocean effect clouds skirting the Outer-Cape.

&&

Short term /6 am Friday morning through Friday night/...
Friday...
high pressure moves off middle Atlantic coast with sunshine giving way to
increasing afternoon clouds as next potent shortwave moves into
the Great Lakes. With deeper moisture remaining south and west of
the region it should remain dry through the day. There is a low
probability of a few ocean effect flurries/snow showers over cape/islands
toward evening with SW flow and cold air aloft yielding ocean
induced convective available potential energy around 500 j/kg...but moisture is very shallow
which will limit activity. Maximum temperatures will be mostly in the low
and middle 20s...with upper teens higher terrain. Winds will be
light.

Friday night...
*** accumulating snow likely across cape/islands ***

Interesting set up as ocean storm tracks well offshore but potent
middle level trough/shortwave approaching from the west will induce an
inverted trough which extends across southeast new eng. Models are in
fairly good agreement on deepening moisture and area of good low
level convergence and modest Omega in the snow growth
region...focused across cape/islands Friday night which should bring
a period of snow. Some ocean enhancement likely given steep low
level lapse rates and ocean induced convective available potential energy around 500 j/kg yielding
potential for brief heavier snow. Snow accumulate of 1-3 inches
expected with low risk of up to 4 inches. Advisories may be
needed.

Snow showers will likely extend across S coastal Massachusetts with minor
accumulate possible but main focus expected to be across cape/islands.
Otherwise...Arctic front will be approaching from the west late
Friday night and will likely bring a few snow showers to western new
eng.

&&

Long term /Saturday through Thursday/...
have updated Sat night period to increase probability of precipitation over the cape and
islands due to unusual combined middle level trough and ocean effect
interaction. See Sat night discussion below.

Highlights...

* snow showers ending over the cape and islands Sat
* bitter cold Arctic air moves into southern New England for the
weekend
* snow changing to rain likely Tuesday and Tuesday night

Overview...

Closed upper low passes across New England Sat night above coldest
Arctic air witnessed here in quite a while. Strong surface pressure
gradient will most likely result in gales across the ocean waters
and wind chills approaching or exceeding wind chill warning criteria
over much of the forecast area. Upper low and surface high move
east sun and Monday. Short wave trough amplifies long wave trough
position in North America but so sharply that surface low looks to
move north along or just east of the Appalachians as an inside
runner. With the surface high being shown to move off the coast it
is not the right synoptic set up for cold air damming. Thus...any
snow or freezing rain at the outset will likely change rather
quickly to rain even far inland. The only caveat is that this
event is still several days away and could see some variations in
model output as the energy reaching North America becomes better
resolved in the upper air network.

Details...

Sat...snow on the outskirts of an ocean storm should end across Cape
Cod...Martha/S Vineyard and Nantucket during Sat morning. Ocean
effect snow showers may persist for a while longer over the Outer
Cape. Although there will be an impressive low level layer of a very
steep lapse rate...the mean flow in the lower levels has enough
westerly component to keep most of the snow showers offshore
initially but the combination of extreme low level lapse rates
over Cape Cod Bay and approach of a middle level trough may cause snow
showers to redevelop across the middle and Outer Cape toward evening.
Elsewhere across southern New England...kept slight chance snow
showers across the northern half of the area due to the strength
of the Arctic boundary and upper closed low approaching the area.

The main show of the Arctic blast arrives by Sat afternoon with
strong northwest winds driving wind chills dramatically lower during the
afternoon and hence beginning the wind chill watch at 1800 UTC or 1
PM EST. The temperatures themselves may rise a few degrees or so
from early morning to midday and then fall steadily throughout the
afternoon.

Sat night...main story will be the bitter cold. Wind chill watch in
effect for potential wind chills of -25 to -30 across most of the
forecast area. The wind chill watch covers all but the cape and
islands. Later shifts may need to reevaluate to see if the watch
needs to be expanded to the cape...Martha/S Vineyard...Nantucket and
Block Island. Near record low temperatures are forecast for Sun
morning at most southern New England locations including
Boston...Worcester and Hartford/Windsor Locks. Model consensus
brings 850 mb temperatures within a degree or two of -30c Sat
night.

Initially thought that ocean effect snow showers will remain just
offshore given very low level wind flow. However...the low level lapse
rates are so extreme with boundary layer moisture still present
that ocean effect snow showers may be able to generate with just a
trajectory across Cape Cod Bay and an outside chance even just across
Nantucket Sound. In addition...operational computer models depict
a middle level trough...very apparent at 700 mb...extending from the southeast
Gulf of Maine back to southeast New England Sat night. This might provide
an opportunity to seed the ocean effect snow showers and perhaps
make it a more interesting night at least across the Outer Cape
and possibly Nantucket as well. This is an unusual and rather low
confidence scenario...but we see enough ingredients to warrant
chance probability of precipitation for at least the Outer Cape and Nantucket Sat night.

Sun...air mass slowly begins to moderate to -20c to -22c at 850 mb
but still very cold. Anticipate most high to be in the single digits
to lower teens across southern New England. There should be plenty
of sunshine but still a brisk northwest wind into the early afternoon. The
wind chill watch expires at 1 PM.

Sun night...clear skies and lighter winds over the snow cover should
be a good set up for efficient radiational cooling. Temperatures
will likely drop to near or several degrees below zero most interior
locations and not too much above zero across the southeast urban centers.

Monday...temperatures moderate considerably although maximum temperatures
will likely top off below freezing outside of the cape and islands.
High clouds ahead of a vigorous short wave dropping into the mean
long wave position will increase during the afternoon.

Monday night through Tuesday night...model consensus is for a sharpening
trough over the east central Continental U.S. With associated surface low that
runs near or just east of the Appalachians. There is currently good
model consensus for an inside runner...although the event is still
several days away. Current model consensus would favor snow changing
rather quickly to freezing rain and then rain inland and to just
rain along the coast. A strong low level jet at 925 mb in excess of
80 knots may present a wind issue if enough of that momentum can mix to
the surface...a question mark as an inversion is likely to exist.
Significant enough rafl for local street flooding is possible...but
confidence on quantitative precipitation forecast this far out is quite low.

Wednesday through Thursday...for now this period looks to be dry with short
wave energy not sharp enough to produce a significant surface
reflection. However...some prior model runs have shown a clipper
during this time period and so confidence is not all that high. If
nothing else...the short wave trough should help usher in another shot
of Arctic air but not nearly to the extent of what is expected this
weekend.

&&

Aviation /03z Friday through Tuesday/...
forecaster confidence levels...

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

Short term /through Friday night/...

Overnight...high confidence in VFR conditions. Northwest wind gusts
of 25 to 35 knots will gradually diminish as the night wears on.

Friday...high confidence. Mainly VFR...but some MVFR ceilings may
develop in the afternoon north of the Mass Pike.

Friday night...moderate confidence. Conditions lowering to MVFR/IFR
cape/islands as snow overspreads the region with some accumulate
likely. Scattered snow showers may extend into southeast Massachusetts...otherwise a mix
of VFR/MVFR ceilings.

Kbos terminal...high confidence. VFR through the period.

Kbdl terminal...high confidence. VFR through the period.

Outlook /Saturday through Tuesday/...

Sat through sun...moderate to high confidence. MVFR ceilings/visibilities in
snow showers over the cape and island...improving during Sat afternoon.
Otherwise...VFR for the period but gusty northwest surface winds.

Monday...high confidence. VFR.

Monday night through Tuesday...low to moderate confidence. Becoming IFR
with snow changing to rain with areas of fog late Monday night into
Tuesday. Strong southerly winds aloft may result in significant low
level wind shear Tuesday afternoon.

&&

Marine...
forecaster confidence levels...

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

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

Overnight and Friday...W/NW gales of 35 to 40 knots in strong cold
advection late this evening. Winds slowly diminish overnight and
drop below Small Craft Advisory Friday morning with light SW winds developing Friday
afternoon. Areas of moderate freezing spray over east Massachusetts waters and
Cape Cod Bay overnight. Also possible in some waters near Buzzards
Bay entrance. Freezing spray advisory overnight in these
locations...but should be allowed to expire by early Friday morning
as winds diminish.

Friday night...winds and seas below Small Craft Advisory. SW winds becoming northwest after
midnight...increasing toward daybreak. Visibilities reduced in areas of
snow.

Outlook /Saturday through Tuesday/...

Moderate to high confidence.

Strong surface pressure gradient and cold air advection is expected
to result in gale force northwest winds across all waters Sat afternoon
into sun. In addition...there is a risk for heavy freezing spray Sat
night into Sun morning.

Winds and seas subside Sun night and Monday. A developing low moving
along or just east of the Appalachians may produce southerly winds
of gale force across at least some of the waters Tuesday along with
building seas.

&&

Climate...
bitterly cold temperatures are expected this weekend. Record cold
high and low temperatures may be tied or broken. Here are the
current records for the weekend.

Record cold highs for Feb 13th - 14th...

Boston.......15/1979 - 14/1916
Hartford.....11/1979 - 11/1979
Providence...10/1979 - 10/1979
Worcester.... 8/1899 - 7/1979
Blue Hill.... 9/1899 - 10/1979

Record cold lows for Feb 13th - 14th - 15th...

Boston...... -3/1967 - -3/1934 - -14/1943
Hartford.... -7/1967 - -9/1979 - -18/1943
Providence.. -5/1967 - -7/1979 - -14/1943
Worcester...-12/1967 - -11/1979 - -19/1943
Blue Hill...-10/1967 - -9/1979 - -18/1943

&&

Box watches/warnings/advisories...
CT...wind chill watch from Saturday afternoon through Sunday
afternoon for ctz002>004.
Massachusetts...wind chill watch from Saturday afternoon through Sunday
afternoon for maz002>021-026.
Rhode Island...wind chill watch from Saturday afternoon through Sunday
afternoon for riz001>007.
Marine...Gale Warning until 4 am EST Friday for anz231>235-237-250-251-
254>256.
Freezing spray advisory until 7 am EST Friday for anz231-
233>235-250-251.
Small Craft Advisory until 4 am EST Friday for anz230-236.

&&

$$
Synopsis...kjc/Thompson
near term...Frank
short term...kjc
long term...Thompson
aviation...Frank/kjc/Thompson
marine...Frank/kjc/Thompson
climate...staff

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