National Weather Service Forecast Discussion

Forecast Discussion for BOX NWS Office
354
FXUS61 KBOX 151201
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
701 AM EST Fri Dec 15 2017

.SYNOPSIS...
High pressure will move offshore today but bring cold and dry
weather for most of the day today. Low pressure off the mid
Atlc coast will track well southeast of Nantucket but close
enough to bring a period of accumulating snow to the coastal
plain this evening, ending overnight. Expect dry but cold
conditions Saturday night into Sunday. Another round of light
snow or a wintry mix before changing to rain is possible early
next week. Cold and blustery conditions look to return around
mid week.

&&

.NEAR TERM /UNTIL 2 PM THIS AFTERNOON/...

645 AM Update...

Noting bands of high clouds streaming eastward across the region
as seen on latest GOES-16 (it`s back but still prelim, non op)
fog product imagery. Temps have fallen back to the single digits
and teens with light/variable or calm winds except W-NW winds at
about 5-10 kt across the higher inland terrain and along the
immediate E coast at 11Z.

Near term forecast on track. Have updated to bring conditions
current.

Previous Discussion...

High pres moves offshore this afternoon. Morning sunshine will
fade behind increasing clouds this afternoon. Developing low
pres emerging off the mid Atlc coast this afternoon but it
should remain dry through sunset as deeper moisture still to the
south. Just a low risk for a few flurries or light snow
developing near the south coast around sunset. 925 mb temps -8
to -9C so another cold day with highs mostly mid-upper 20s with
some lower 30s outer Cape/Islands.

&&

.SHORT TERM /2 PM THIS AFTERNOON THROUGH 6 PM SATURDAY/...
*** A period of accumulating snow likely across RI and
 southeast MA this evening ***

Late today and tonight...

Developing low pres off the mid atlc coast will be tracking NE
late today and this evening. Meanwhile, another digging
northern stream shortwave will be moving through the Gt Lakes
and into the northeast. How these two shortwaves interact will
determine how far north and west accumulating snow gets. Models
have trended stronger and more amplified with this northern
stream shortwave as it is becoming better sampled within the
upper air network. This allows the mid level flow to back enough
for deeper moisture to move further north and west into SNE.

While the sfc low is tracking southeast of the benchmark,
inverted trough extends back to the NW into SNE assocd with
approaching northern stream trough. Models showing large scale
synoptic forcing ahead of the shortwave along with favorable
upper jet dynamics within the left exit region of 170 kt upper
jet. Cross sections indicate a period of modest omega within
the DGZ this evening, especially across SE New Eng. Given the
deep moisture in place, expect a period of light to moderate
snow across much of RI and SE MA this evening where snowfall
of 1-3 inches is expected with a low risk for a few 4 inch
amounts near the south coast and Cape Cod if trough is more
amplified. Onset of the snow will be 5-8 pm so part of the
evening commute may be impacted. Will let next forecast team
examine the 12z model suite to decide whether an advisory is
necessary. Expect light snow to extend north across much of SNE
but amounts will be limited to less than an inch in the distant
interior.

This will be a progressive system and snow will be exiting SE
New Eng coast after midnight.

Saturday...

Another northern stream shortwave will bring a reinforcing shot
of cold air and blustery WNW winds to SNE. 850 mb temps -12C
support highs from upper 20s to mid 30s with a mix of sun/clouds
and wind gusts to 30 mph expected. Mainly dry weather is
expected other than a few flurries in western MA as lake effect
snow showers try to spill over the Berkshires.

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
Overview...

Overall steering pattern across the lower 48 transitions to
relatively flat mid level flow, which remains progressive even
as a long wave trough digs across the western states late this
weekend, then trends toward the southern stream as it slowly
moves along in the flow. The northern stream remains
progressive, with cold temps continuing into early next week.
Another H5 trough moves ashore across the Pacific NW Tue
night/Wed, with rather good agreement amongst the 00Z model
suite. While this moves east through late next week, appears
that rising H5 heights as the return SW flow may bring somewhat
milder temps to eastern areas toward the end of the period.

Models and ensembles continue to show weak short waves moving
across the region in the fast flow aloft, with timing issues
coming into play starting early next week as model solution
spread increases. Also noting some PTYPE issues as the mid level
pattern tries to transition back to near or slightly above
normal temps toward the middle of next week.

Details...

Saturday night and Sunday...

As H5 short wave shifts offshore Sat night, will see any
leftover precip exit as well. Leftover NW low level jet up to
H85 will push offshore. However, will see gusty winds linger Sat
evening, up to 25-30 kt. Good low level lapse rates, on order
8-9C/km mainly across central and eastern areas which will allow
for good mixing. The lapse rates will drop off between 03Z-06Z
as the low level jet also moves offshore, so winds will drop off
rather quickly by around midnight. Skies will be mostly clear
as temps drop to the single digits and teens, except the upper
teens to lower 20s along the immediate coast.

Large high pressure, extending from Quebec down the eastern
seaboard, will move across the region late Sat night and Sunday.
Light W-NW winds will become variable as ridge axis crosses the
during Sunday. Mid and high clouds start to stream across the
region Sunday afternoon. It will be another chilly day, with
temps remaining below freezing across the interior, reaching to
lower-mid 30s along the immediate coast.

Sunday night into Monday...

Some question about next approaching system from the Ohio
Valley during this timeframe. Both the GGEM and ECMWF were
pretty robust with the development of overrunning precip, while
the GFS and NAM keeps the mid level energy rather diffuse in the
nearly zonal flow.

Went along with an overall model blend, which has precip
developing overnight Sun night and continuing into Monday.
Should see some light snow break out, but overall this is
another moisture starved system with QPF amounts generally less
than 0.1 inches, which yields an inch or less of snow as it
mixes with or changes to rain before ending across eastern
areas. Have most of the precip pushing offshore Mon afternoon,
though timing is in question due to model solution spread with
this system.

Monday night through Tuesday night...

Another short wave moves along in the zonal flow, but again
more questions about whether there is enough energy and moisture
to bring another round of precipitation during this time. Could
also see PTYPE issues with this system as low and mid level
winds appear to back to S-SW as long wave trough starts to dig
across the northern Plains states, with possibly a wintry mix
developing late Mon night into early Tue before changing to rain
during Tue. Current forecast suggests temps will actually rise
during the day, with highs running around 5 degrees above
seasonal normals.

Will see the strong low pushing NE across the Great Lakes into
Quebec. Attendant cold front should push through, bringing a
return to colder temps Tue night.

Wednesday and Thursday...

Blustery and cold conditions return Wed as the strong low will
exit into the Maritimes. May see some snow showers develop in
the W-NW flow, possibly lake effect induced, moving across the E
slopes of the Berkshires. May also see ocean effect snow
showers develop offshore.

High pres will push eastward Wed night and Thu, with winds
diminishing and temps close to or slightly below seasonal
normals.

&&

.AVIATION /12Z FRIDAY THROUGH TUESDAY/...
Forecaster Confidence Levels...
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.

Short Term /through Saturday/...

Today...High confidence.
VFR through most of the day. Some late day MVFR cigs developing
near the south coast after 19Z with chc -SN.

Tonight...
High confidence in a period of snow with IFR impacts developing
across RI/SE MA this evening. Localized LIFR in moderate snow
possible near the south coast. Accums 1-3 inches. Northern and
western extent of the snow still uncertain, but it will likely
reach BDL-ORH-BOS for a short time with brief MVFR possible and
a coating to less than an inch. Snow exiting SE New Eng coast
around 06z with improving conditions to VFR thereafter.
Increasing NW winds along the coast 08Z-12Z with gusts to 25
kt.

Saturday...High confidence.
VFR with SCT-BKN 050-080 developing. WNW gusts to 25 kt.

KBOS Terminal...Moderate confidence. Period of light snow with
MVFR conditions this evening. Brief IFR vsbys possible.

KBDL Terminal...Moderate confidence. Period of light snow with
MVFR conditions this evening. Low risk for brief IFR vsbys

Outlook /Saturday Night through Tuesday/...

Saturday Night: VFR. Breezy.

Sunday: VFR.

Sunday Night: Mainly MVFR, with areas VFR possible. Chance SN.

Monday: Mainly MVFR, with areas VFR possible. Chance SN, slight
chance RA.

Monday Night: Mainly VFR, with local MVFR possible. Slight
chance FZRA, slight chance SN.

Tuesday: Mainly MVFR, with local IFR possible. Breezy. Chance
RA, chance FZRA, chance SN.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Saturday/...High confidence.

Today...
W-NW winds 10-15 kt shifting to SW this afternoon. Seas below
SCA. Vsbys may lower in light snow south coastal waters late in
the day.

Tonight...
Increasing NW winds developing after midnight as coastal low
moves offshore. Gusts to 25-30 kt expected late tonight. Vsbys
reduced in developing snow this evening, especially south
coastal waters, improving after midnight.

Saturday...
SCA conditions with WNW gusts to 30 kt with low risk for
marginal gale force gusts over southern waters.

Outlook /Saturday Night through Tuesday/...

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

Sunday: Winds less than 25 kt.

Sunday Night through Monday: Winds less than 25 kt. Chance of
rain, chance of snow.

Monday Night: Winds less than 25 kt. Slight chance of rain.

Tuesday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Seas locally approaching 5 ft. Chance of rain.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...None.

&&

$$
SYNOPSIS...KJC/EVT
NEAR TERM...KJC/EVT
SHORT TERM...KJC
LONG TERM...EVT
AVIATION...KJC/EVT
MARINE...KJC/EVT

NWS BOX Office Area Forecast Discussion