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000 
FXUS61 KBOX 131506
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
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 SPC mesoanalysis contouring -20C air at 850 mb over
Northern NY 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 SCT-BKN cumulus clouds across the area along with 
a few snow showers/flurries especially across the upslope areas
of the high terrain of western MA. Ocean effect snows will 
continue off the south coast of RI/MA but should only clip 
Nantucket given the WNW 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 SNE 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 NW to SE but it may take 
until mid afternoon for clearing to reach far SE New Eng.
A period of gusty NW 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 H5 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 H5 cutoff out of Ontario breaking down as it shifts 
E. 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 H85 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 E out of the Ohio Valley. Leftover W-NW 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 H85 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...
H5 short wave and surface reflection will slide E 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 E 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 E. Will see W-NW 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 MA/RI should just
clip Nantucket with bulk of activity remaining offshore. See 
previous discussion below.

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

Today...High confidence. 
VFR with SCT-BKN 040-050. Local MVFR CIGS across the outer Cape
and KACK at times. Strong, gusty W-NW 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 NW to SE. W/NW 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 W-NW winds peaking this afternoon into early this 
evening with gusts 40-45 kt, then diminishing through tonight. 
Another pulse of W-NW 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.
MA...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.
RI...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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