Air Resources Laboratory banner image
Air Resources Laboratory web site National Oceanic and Atmospheric Administration (NOAA)

ct discuss


Office: BOX

FXUS61 KBOX 121529
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
1029 AM EST Tue Dec 12 2017

.SYNOPSIS...
Strengthening low pressure in the Ohio Valley early this morning
redevelops over northeast Massachusetts this evening and then
intensifies into at least a Gale Center over New Brunswick Wed.
This storm system will bring a wintry mix of snow and ice to
the interior with mainly rain in the coastal plain today. A shot
of arctic air wraps around the exiting low Wednesday and
Thursday, with a period of strong winds and bitterly cold wind
chills. Another low may bring some light snow across south
coastal areas Friday night into Saturday if it tracks close
enough to the coast. Somewhat milder weather will arrive by the
end of the upcoming weekend.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
*** A Wintry Mix Across The Interior This Morning ***

1028 AM update...

Surface temperatures continue to slowly rise above freezing for
portions of southern New England. Cold air damming evident at
the usual suspects, particularly in the lower CT River valley
and north central MA. Will need to monitor temperatures closely
over the next few hours. Current thinking is the timing on the
Winter Weather Advisory is fairly good. The timing may need some
later tweaking.

Otherwise, tweaked the ongoing forecast to reflect observed
trends and timing.

Previous Discussion...

Significant trough amplification occurs today over the eastern
Great Lakes, with mid level trough taking on a negative tilt
tonight before closing off to a 506 dam low over western MA by
12z Wed. For this morning, a modest cold airmass in place over
southern New England with temps below freezing inland and dew
pts down to 16F at Worcester. 1020 mb high over Maine early this
morning builds to about 1026 mb later today in response to
upstream trough amplification. This will provide cold air
damming into interior MA and possibly farther south into
northern CT and northwest RI.

Column is cold enough to support snow. However developing low
level southeast jet provides strong WAA and will result in snow
changing over to rain quickly in the coastal plain. However
northwest MA will have the longest duration of snow and could be
moderate to perhaps heavy for a time this morning toward 15z.
In addition, mid level lapse rates approach 6.5-7C/KM combined
with bulk of forcing for ascent occurring in the snow growth
region yields the risk for a period of moderate to heavy snow
12z-15z. Thus this event could overperform a bit across
northwest MA where a brief period of moderate to heavy snow
could produce 1-3" snowfall amounts FROM 12Z- 15Z, possibly a
few 4 or 5 inches in northwest Franklin county.

Otherwise most locations will see snow quickly change to rain
this morning as warm nose at 925 mb advects across the area.
The issue will be across the interior (along and northwest of
I-90 and I-495) where shallow cold air will be stubborn to
dislodge. This will result in a period of freezing rain/ice. A
shorter duration of freezing rain/ice is possible farther south
across northern CT, northwest RI into the I-495 corridor. Given
current temps remain below freezing across interior Essex county
along with dew pts in the low 20s and winds calm, we'll expand
winter weather advisory to this region for a brief period of
mixed wintry precip 7 am - 10 am. Keep in mind it doesn't take
much freezing rain/ice to result in slippery travel.

Farther south into the coastal plain including Boston and
Providence, any snow will quickly changeover to rain given
increasing southeast winds. Rain will likely be heavy at times
this afternoon given trough amplification/strong jet dynamics
with LFQ of upper level jet streak overspreading the area,
enhancing QG forcing. In addition some instability aloft with mid
level lapse rates increasing to 6.5-7.0C/KM (TTs 50-55). This
combined with fairly strong forcing may result in embedded
convection, enhancing rainfall potential.

It will become breezy along the eastern MA coast especially
Cape Ann as low level jet increases as it departs offshore.
This low level jet will warm the coastal plain to 45-50 degs
this afternoon, this includes the Boston to Providence
corridor. Meanwhile farther inland late day highs of 35-40.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/...
Early morning update...

Vigorous negatively tilted short wave trough approaches from the
southwest. Moisture is limited with this feature given attending
dry slot. However could be sufficient low level moisture for a
few low top rain/snow showers ahead of the dry slot. Behind this
departing short wave arctic air blast across the region with
very strong CAA as H925 temps crash from +7C over Cape Cod at 7
pm to -7C by 12z Wed. Any standing water and/or slush will
freeze. Thus this afternoon is your chance to remove any
leftover snow/ice.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
Overview...

Deep H5 long wave trough sets up by mid week from the Arctic
Circle southward to the mid Atlantic coast, while high amplitude
ridge continues from the Northwest Territories through the
Pacific Coast. This will bring the coldest air so far this
season to the region along with strong W-NW winds through
Thursday.

As the cold pool retreats into eastern Canada late this week,
appears that the western ridge may break down as a short wave
moves into the Pacific NW late Fri or Sat. Also noting a weak
mid level short wave that may wrap around base of the eastern
Canadian cutoff low and cross the region. May also see a
strengthening southern stream system try to ride up across the
western Atlantic. Rather wide model solution spread at this
point, so track is in question along with the possibility of the
northern fringe of the precip shield approaching portions of
the south coast late in the week or into the weekend. 00Z GFS is
farthest W with this system, while the Canadian GGEM is further
offshore. Something to keep an eye on.

With rather wide solution spreads amongst the model suite from
about Friday into early next week, lower forecast confidence for
this portion of the forecast.

Will use a blend of available model guidance into Friday, then
transition over the model ensembles for the latter portion of
this period.

Details...

Wednesday and Wednesday night...

Strong low pressure will spin across the Maritimes into Labrador
through Wednesday night, bringing a strong surge of arctic air
to the region. Noting H85 temps dropping through the day, due
to -12C to -16C by 00Z Thu. While the core of the arctic air
remains N of the region, will see increasing W-NW winds with the
sharp pressure gradient across the region from the eastern
Canadian vortex. Noting H85 jet up to 40-50 kt moving across the
region by Wed afternoon, with excellent low level lapse rates,
so will see a good chunk of these winds mixing down. Could see
gusts up to 40-50 mph during Wed, so wind advisory headlines
will probably be needed.

Some leftover snow showers may linger across the E slopes of the
Berkshires through midday Wed, with a few lingering along the S
coast early.

Winds may drop off a bit Wed night as better mixing moves
offshore, but temps will tumble with mostly clear conditions.
Wind chills away from the immediate coast will drop to the
single digits, with readings as low as -5 toward daybreak across
the E slopes of the Berkshires.

Thursday...

Expect the gusty W-NW winds to continue through midday Thursday,
but will start to weaken as the eastern Canadian low slowly
exits.

Models continue to signal an H5 short wave passes S of New
England. The northern fringe of the precip shield may clip the S
coast Thursday morning through midday as the weak low passes
close to or just S of the 40N/70W benchmark. Have carried CHC
POPs across Nantucket and Martha's Vineyard. Will see snow
showers or a period of light snow for a few hours as the low
passes. Little if any snow accumulations are expected.

The arctic air will continue across the region, with high temps
only in the upper teens and 20s across the higher inland
terrain, ranging to the lower-mid 30s at the shore. As winds
drop off during the day, wind chills will not be quite as harsh
as on Wed. However, with mainly clear skies Thu night, temps
will fall back to the single digits well inland, ranging to
20-25 across the outer Cape and islands.

Friday through Saturday...

Another weak short wave develops across the Great Lakes Fri,
while stronger low pressure forms off the SE U.S. coast. The
weak low will push E while the coastal low moves NE off the
eastern seaboard. Models showing some spread with their
development and track of the coastal low, with the GFS closest
to the coast but still passing S and E of the 40N/70W
benchmark.

Could still see some light snow or snow showers push into SE
Mass/E RI, but this is still up in the air for exact track and
impacts later Friday into early Saturday. Should see improving
conditions with slowly moderating temps Sat.

Sunday and Monday...

High pressure ridge builds across the region Sunday. Winds back
from W-NW to SW, which will bring milder temps, actually near or
just a few degrees below seasonal normals. May see another short
wave bring some light precip by Sun night and continuing into
Monday, but timing and track of this system is in question.

&&

.AVIATION /16Z TUESDAY THROUGH SATURDAY/...
Forecaster Confidence Levels...
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.

Short Term /through Tonight/...

1528Z update...

Through Today...High confidence on trends but lower on exact
details, particularly timing of precipitation type changes.

MVFR CIGS across central and western areas will push E, with
local IFR-LIFR at times across N central and western areas.
Light snow/sleet transitions to rain along the coast, with some
freezing rain at times inland. S-SE winds increase along the
coast.

Tonight...High confidence on trends, lower on details.

MVFR CIGS/MVFR-IFR VSBYS improve to VFR by around 05Z-07Z,
except lingering lower CIGS across higher terrain. Gusty W-NW
winds develop.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Wednesday through Saturday/...

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

Wednesday Night: VFR. Strong winds with gusts to 40 kt.

Thursday: VFR. Windy with local gusts to 30 kt.

Thursday Night: VFR. Breezy.

Friday: VFR.

Friday Night: Mainly VFR, with local IFR possible. Breezy.
Slight chance SN.

Saturday: VFR. Breezy.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Tonight/...

Today...

SE winds increase to 20-30 kt with strongest winds across
eastern MA waters. Snow early changes to rain with reduced
vsby. Low pres intensifies as it moves from W NY to northern
MA.

Tonight...

Arctic cold front sweeps across the area with SE winds shifting
to W-SW up to 20-30 kt. Vsby improves after evening rain/snow
showers.

Outlook /Wednesday through Saturday/...

Wednesday and Wednesday night: Moderate risk for gale force
winds with gusts up to 45 kt. Rough seas up to 15 ft. Slight
chance of snow showers Wednesday.

Thursday: Moderate risk for Small Craft Advisory winds with
gusts up to 30 kt. Local rough seas. Chance of snow showers.

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

Friday: Winds less than 25 kt. Seas locally approaching 5 ft.
Chance of snow. Areas of visibility 1 to 3 nm.

Friday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Areas of seas approaching 5 ft. Chance of
snow. Visibility 1 to 3 nm.

Saturday: Moderate risk for Small Craft Advisory winds with
gusts up to 30 kt. Local rough seas. Slight chance of snow.
Areas of visibility 1 to 3 nm.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...Winter Weather Advisory until 1 PM EST this afternoon for
     MAZ005-006-010>012.
     Winter Weather Advisory until 4 PM EST this afternoon for
     MAZ002>004-008-009-026.
RI...None.
MARINE...Gale Warning from 9 AM Wednesday to 6 AM EST Thursday for
     ANZ232.
     Gale Warning from 9 AM Wednesday to 4 AM EST Thursday for
     ANZ233-234.
     Gale Warning from 1 PM Wednesday to 1 AM EST Thursday for
     ANZ230-236.
     Gale Warning from 10 AM Wednesday to 6 AM EST Thursday for
     ANZ231.
     Gale Warning from 8 AM Wednesday to 6 AM EST Thursday for
     ANZ235-237-251.
     Gale Warning from 6 AM Wednesday to 6 AM EST Thursday for
     ANZ250-254.
     Gale Warning from 5 AM Wednesday to 6 AM EST Thursday for
     ANZ255-256.

&&

$$
SYNOPSIS...Nocera/EVT
NEAR TERM...Belk/Nocera/EVT
SHORT TERM...Nocera
LONG TERM...EVT
AVIATION...Belk/Nocera/EVT
MARINE...Nocera/EVT