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Area forecast discussion
National Weather Service Taunton Massachusetts
1006 am EST Wed Dec 13 2017

Synopsis...
a gale center will move into the Maritimes today with windy and
very cold conditions in its wake. A fast moving low pressure
will track south of New England late tonight and early Thursday
bringing a period of light snow to the South Coast and
especially the islands. Another low pressure may bring some
light snow around Friday night then cold and dry for Saturday.
Somewhat milder temperatures return on Sunday.

&&

Near term /until 6 PM this evening/...

10 am update...

Modified Arctic air streaming across the region this morning
with Storm Prediction Center mesoanalysis contouring -20c air at 850 mb over
northern New York state 13z! Thus not much of a temperature recovery
today with highs only in the 20s and blustery west winds 20-30
mph with gusts up to 45-50 mph generating wind chills in the
single digits and teens.

Cyclonic flow aloft combined with some lake effect moisture
will generate scattered-broken cumulus clouds across the area along with
a few snow showers/flurries especially across the upslope areas
of the high terrain of western Massachusetts. Ocean effect snows will
continue off the South Coast of Rhode Island/Massachusetts but should only clip
Nantucket given the west-northwest low level trajectory. Previous forecast
captures these details nicely so no major changes with this
update.

Previous discussion...

Mid level low moves across new eng this morning before lifting
NE this afternoon. Cold pool aloft with 500 mb temps around
-32c combined with some low and mid level moisture moving
through will lead to a period of bkn cu and can't rule out a few
flurries, especially higher terrain. Clearing later afternoon.

The main weather story for today will be wind and cold in the
wake of the departing low pres moving into the Maritimes. 850
mb temps bottom out around -15c to -16c today so only expect
minor recovery from morning lows. Highs will range through the
20s to near 30 Outer Cape/islands, with some upper teens in
the Berkshires. Wind chills will be in the single numbers and
teens. Well mixed boundary layer with steep low level lapse
rates will promote excellent mixing with soundings suggesting
peak gusts 40-50 mph, strongest mid/late afternoon especially
over higher elevations. We expanded the Wind Advisory to include
all of MA/RI.

&&

Short term /6 PM this evening through 6 PM Thursday/...
a period of clear skies expected this evening with gusty winds
gradually diminishing overnight. However, clouds will quickly
return as a rather robust mid level shortwave moves across the
Ohio Valley to south of new eng late tonight and early Thu. Low
pres tracks south of the coast and attempts to spread some
light snow north into portions of southern New England late tonight into early
Thu. Plenty of dry air initially to overcome which will limit
northward extent of snowfall. Best chance for a period of light
snow will be near the South Coast and especially the islands
where better moisture and lift. Guidance indicating a brief
period of modest Omega in the dgz over the islands 09z-12z with
sharp decrease in Omega to the north. Minor accum of a coating
to an inch is possible for the South Coast and especially the
islands with low prob for 2 inches bid-ack. However, a sharp
gradient is expected on the northern edge of the snow shield so
if it shifts slightly south, little or no accum will be the
result.

Light snow may linger into Thu morning immediate South Coast
and especially cape/islands as low pres passes to the south.
Otherwise, drier air and clearing skies behind the departing
low pres will gradually move in from northwest to southeast but it may take
until mid afternoon for clearing to reach far southeast new eng.
A period of gusty northwest winds will develop Thu afternoon with
gusts 20-30 mph. Another unseasonably cold day with highs mid
20s to lower 30s.

&&

Long term /Thursday night through Tuesday/...
overview...

Mid level pattern remains active but progressive across the
lower 48 through this period. High amplitude pattern continues
late this week, with cutoff 500 mb low pressure across eastern
Canada. Another mid level short wave will push out of the Great
Lakes late this week into the weekend. Models continue to signal
some southern stream energy and moisture trying to work up the
eastern Seaboard Fri night into Sat, and may see some moisture
try to link up with the short wave. Model solution spread lends
to lower confidence with this aspect of the forecast.

The amplified mid level pattern forecasted to break down late
this week into the weekend, with the western ridge flattening
out and the 500 mb cutoff out of Ontario breaking down as it shifts
east. The steering flow then opens up, keeping systems moving
cross country while the western ridge may try to re-Orient over
the eastern Pacific by the end of the period.

As for temperatures, expect the coldest air across the region
Thu night into Sat, with 850 mb temps running from -10c to -15c for
a good portion of this period. With the pattern change, the mid
level steering flow flattens out which will allow temps to
moderate with a more westerly flow by late in the weekend into
early next week. Temps will trend close to normal into the
beginning of the work week.

Used a blend of available guidance through Saturday, then trended
toward an ensemble blend through the remainder of the period.

Details...

Thursday night...
expect mainly clear and cold conditions Thu night as high pres
builds east out of the Ohio Valley. Leftover west-northwest winds gusting to
20- 25 kt Thu evening will diminish as the pres gradient
relaxes, though some higher gusts will linger along the
immediate coast. Clouds will start to increase from the S during
the early morning hours.

With 850 mb temps in the -10c to -15c range, will see surface
temps bottom out in the single digits and teens except lingering
in the lower 20s across the Outer Cape and Nantucket.

Friday through Saturday...
500 mb short wave and surface reflection will slide east out of the
Great Lakes, so clouds will continue to increase. Another low
and southern stream energy tries to develop off the mid Atlc
coast Fri afternoon and, as winds shift to light S-SW, some low
level moisture will reach into central and southern areas by
late in the day. Best shot for scattered snow showers looks to
occur along the S coast, Cape Cod and the islands Fri night into
Sat.

The system out of the lakes will push E, with most of what's
left of its energy pushing across northern New England.
However, some leftover moisture could reach the east slopes of the
Berkshires Fri night through about midday Sat before
dissipating. Have slight chance to low chance pops going there.

As the coastal low exits by midday Sat, pressure gradient
tightens again as high pres pushes east. Will see west-northwest winds
become gusty once again, upwards to 20-25 mph. So, even with
slight milder temps, wind chill values will range from the
single digits across the higher terrain to around 20 along the
immediate coast by Sat afternoon.

Highs on Friday will remain below freezing for most areas,
except may reach the lower-mid 30s along the S coast, Cape Cod
and the islands. It will be a bit milder on Saturday, with highs
from the mid 20s across the higher terrain to the lower-mid 30s
across the coastal plain.

Saturday night...
high pressure ridge will build across the eastern Seaboard, so
expect mainly mostly clear skies. Some gusty winds will linger
Sat evening then diminish, but it will be another cold night.
Expect low temps in the teens for most areas, except in the 20s
along the immediate coast and islands.

Sunday and Sunday night...
as the high ridge shifts E, winds shift to S-SW which will
bring a bit milder air northward. However, will also see
moisture ahead of an approaching warm front moving NE. At this
point, looks like precip will start off as snow showers Sunday
afternoon, mixing with rain at times before changing to snow
showers Sun night. Not a whole lot of moisture expect with this
system. Some question how fast the warm front moves amongst the
model suite.

Monday and Tuesday...
at this point, the front should push offshore Monday, but again
rather wide model solution spread with the departing low. Not a
lot of cold air behind the front, though, as a nearly zonal
flow at the surface and aloft in place through Tue. Current
forecasted temps suggest readings close to seasonal levels
during this timeframe.

&&

Aviation /15z Wednesday through Sunday/...
forecaster confidence levels...
low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.

Short term /through Thursday/...

15z update...

Not much change from previous 06z and 12z tafs. Mainly VFR but
marginal MVFR at times in brief snow showers/flurries. Ocean
effect snow showers off the South Coast of Massachusetts/Rhode Island should just
clip Nantucket with bulk of activity remaining offshore. See
previous discussion below.

=====================================================================

Today...high confidence.
VFR with scattered-broken 040-050. Local MVFR ceilings across the Outer Cape
and kack at times. Strong, gusty west-northwest winds peaking this
afternoon with gusts 35-40 kt, locally around 45 kt across the
higher terrain.

Tonight...moderate confidence.
Mainly VFR cigs, but lowering to MVFR over the islands and
South Coast after 06z as area of light snow spreads north. Low
risk for brief IFR vsbys over the islands. Gusty winds to 35 kt
in the evening, diminishing overnight.

Thursday...high confidence.
Areas of MVFR in lingering light snow in the morning
cape/islands and South Coast, otherwise VFR with gradual
clearing from northwest to southeast. West/northwest gusts to 25 kt in the afternoon.

Kbos terminal...high confidence in taf.

Kbdl terminal...high confidence in taf.

Outlook /Thursday night through Sunday/...

Thursday night: VFR. Breezy.

Friday: VFR.

Friday night: mainly VFR, with areas MVFR possible. Breezy.
Slight chance shsn.

Saturday: mainly VFR, with local MVFR possible. Breezy.

Saturday night: VFR. Windy with local gusts to 30 kt Sat
evening.

Sunday: VFR. Breezy. Chance shsn.

&&

Marine...
forecaster confidence levels...

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

Short term /through Thursday/...high confidence.

Gale force west-northwest winds peaking this afternoon into early this
evening with gusts 40-45 kt, then diminishing through tonight.
Another pulse of west-northwest gusts to 25-30 kt developing Thu
afternoon. Rough seas. Vsbys lowering in developing snow late
tonight and early Thu over south coastal waters, improving in
the afternoon.

Outlook /Thursday night through Sunday/...

Thursday night: moderate risk for Small Craft Advisory winds
with areas of gusts up to 30 kt. Areas of rough seas.

Friday: winds less than 25 kt. Seas locally approaching 5 ft.

Friday night: winds less than 25 kt. Seas locally approaching
5 ft. Chance of snow showers, chance of rain showers.

Saturday: moderate risk for Small Craft Advisory winds with
gusts up to 30 kt. Local rough seas. Slight chance of rain
showers, slight chance of snow showers.

Saturday night: low risk for Small Craft Advisory winds with
gusts up to 30 kt. Local rough seas.

Sunday: winds less than 25 kt. Seas locally approaching 5 ft.
Chance of rain showers.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...Wind Advisory until 10 PM EST this evening for maz007-019-
022>024.
Wind Advisory until 8 PM EST this evening for maz002>006-
008>018-020-021-026.
Rhode Island...Wind Advisory until 10 PM EST this evening for riz008.
Wind Advisory until 8 PM EST this evening for riz001>007.
Marine...Gale Warning until 1 am EST Thursday for anz232>235-237.
Gale Warning until 1 am EST Thursday for anz230-231-251.
Gale Warning until 9 PM EST this evening for anz236.
Gale Warning until 3 am EST Thursday for anz250-254.
Gale Warning until 3 am EST Thursday for anz255-256.

&&

$$
Synopsis...kjc/evt

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