NWS Forecast Discussion - Boston Area

Forecast Discussion for BOX NWS Office
000
FXUS61 KBOX 160324
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
1024 PM EST Sat Dec 15 2018

.SYNOPSIS...
High pressure builds across northern New England tonight
providing dry, chilly weather. Low pressure will approach New
England from the south, with rain overspreading the region from
south to north Sunday. The steadiest and heaviest rain will
arrive Sunday evening and overnight. Precipitation will likely
be a mix of sleet and freezing rain across northwest CT into
western and central Massachusetts, then trending toward light
snow Sunday night. Greatest impact will be at the higher
elevations where temperatures will be below freezing. Cold air
will push across the region Monday afternoon through Tuesday as
high pressure builds across the northeast. As the high moves off
the eastern seaboard around mid week, milder air will work in.
Low pressure will develop across the southeast late Thursday,
then will pass west of the region late next week bringing a
swath of rain Thursday night into early next weekend.

&&

.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
1023 PM Update...

Abundant high level clouds across southern New England this
evening, but not much precipitation. The northern extent of the
precipitation shield associated with our next low pressure was
still well to our south. Dew point depressions were still rather
large this evening, so current forecast timing still looks
reasonable. It will take some time to overcome the low level dry
air.

Main changes this evening were to bring the forecast timing back
in line with observed trends. Still expecting risk of
precipitation to increase late tonight across northern CT and
RI. MA is likely to stay dry until after daybreak Sunday.

Previous Discussion...

As northern stream short wave over Maine moves E, frontal
boundary will push farther south. Dewpts will fall into the 20s
and 30s overnight. Thus dry weather prevails and not expecting
any fog given the drier airmass, although may be enough moisture
for frost where temps fall below freezing mainly north and west
of KHFD-KPVD-KBOS.

Later tonight, low pressure approaches from the south later
tonight, while high over Maine will squeeze the pgrad and
result in increasing NE winds along the south coast including
Cape Cod.

&&

.SHORT TERM /6 AM SUNDAY MORNING THROUGH MONDAY/...
Updated 330 PM...

*** Wintry mix of precip inland especially High Terrain ***

Synoptic Overview...

Northern stream short wave currently across ME moves east
tonight and delivers a burst of low level cold air advection
tonight into Sunday, setting the stage for a wintry mix of
precip inland Sunday/Sunday night before ending Monday morning.
The key here is the cold air is fairly shallow (below 850 mb)
Sunday, thus not deep enough to support snow until Sunday night
when the column begins to cool from the top down courtesy of
height falls as the vertically stacked low deepens Sunday night
as it tracks over or near the benchmark. This is typically a
favorable track for heavy snow, however the antecedent airmass
is not very cold as mentioned above and this is a southern
stream system that doe
toward morning sn`t phase with any northern stream
energy. Thus no infusion of cold air.

Sunday...

Model soundings show dry layer between 800-900 mb which delays
northward progression of rain shield during the day. Thus
steadiest rain occurs late in the day across CT-RI and southeast
MA with just spotty light rain elsewhere. Concern is Sunday
morning there may be enough low level moisture for some spotty
drizzle across northern CT and over the border into southwest MA
eastward into southern Worcester county. Therefore there is a
very low risk for some spotty freezing drizzle across this
region. Will issue a Special Weather Statement to address this.

Otherwise rain shield slowly overspreads the region from south
to north Sunday especially during the afternoon. Most location
the column is warm enough along with surface temps to support
all rain. However from northwest Hartford county into western-
central MA a mix of freezing rain and sleet is likely with the
greatest travel impact at elevations above 1000 ft during the
day where temps will be near freezing. Again most of the region
will have temps above freezing so just expecting wet roadways.

NE winds continue to increase especially along the south coast
including Cape Cod and the islands as low pres approaches with
attending low level ENE jet.

Sunday night...

Coastal low continues to slowly deepen as it tracks over or near
the benchmark. This causes the column to cool from top down in
response to height falls with mid level warm layer eroding
across northwest CT into western-central MA. This is where sleet
and freezing rain will transition to mainly light snow
overnight. Thus a winter weather advisory has been issued for
this region for a period of wintry mix (sleet and freezing rain)
followed by a coating to 2 inches of snow possible. Greatest
forecast confidence for impact will be across the higher terrain
where temps will be at or below freezing, so expecting slippery
travel here. Thus the advisory has been confined to the highest
elevations. At lower elevations in the advisory area roads may
just have a minor slushy accumulation or perhaps just wet in the
valleys. If later trends suggest a colder scenario the advisory
may have to be expanded south and east. But for now any travel
impacts should be confined the high terrain of northwest CT into
western-central MA.

Elsewhere especially from HFD-PVD-BOS to the coastline just
expecting periods of rain overnight and wind gusts up to 40 mph
over Cape Cod and the Islands. Held off on a wind advisory as
wind gusts will be marginal given lack of cold air and steep low
level lapse rates in addition to strongest winds still beyond 36
hrs. Later shifts will have to monitor trends here.

Monday morning...

Vertically stacked low just NE of the 40N/70W benchmark remains
progressive and moves farther offshore with morning precip
ending and giving way to at least partial clearing by midday or
so.

&&

.LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...
Highlights...

* Mainly dry, blustery and cold Monday afternoon and Tuesday
* Mild temperatures return Wednesday through late next week
* Rain, heavy at times from Thursday night through Friday
* A few lingering showers on Saturday with somewhat cooler
  temperatures

Overview...

H5 trough across the northeast U.S. pushes E across the
Maritimes late Monday into Tuesday. Northern stream storm track
lifts into southern Canada by mid week, while H5 trough digs
across the central Plains during the Wed-Thu timeframe. This
will allow milder temperatures to push northward as W-SW wind
flow at the surface and aloft sets up ahead of digging long wave
trough from the western Great Lakes down the Mississippi River
valley. Noting cutoff H5 low develops around 12Z Fri on long
range models, riding across the Ohio/Tennessee valleys. Deep
trough pushes slowly E, allowing southerly flow to work up the
eastern seaboard late Thu into Fri, bringing deep moisture feed
up the coast as low pressure rides up the Appalachians into
Quebec into next weekend.

Details...

Monday afternoon through Tuesday night...

H5 cutoff low S of Nova Scotia Monday afternoon will weaken as
another cold pool shifts SE out of northern New England into the
Maritimes Monday night. Persistent cyclonic flow will keep low
level moisture across eastern areas through Monday afternoon.
Could see lingering light rain across portions of Cape Cod,
while light snow lingers across the E slopes of the Berkshires
into Monday night. Rain across the outer Cape will taper to snow
showers Monday night, but will linger into Tuesday with cold,
blustery NW winds in place.

Cold pool works into NY state and New England by Monday night
which will spread across the region overnight. H85 temps will
drop down to -12C to -15C by 12Z Tuesday. Expect highs to only
reach the mid to upper 20s across the higher terrain to the
lower 30s across the coastal plain, running 5 to 10 degrees
below seasonal levels. Lows Tuesday night will be mainly in the
teens to around 20, except in the 20s across the coastal plain.

Wednesday and Thursday...

Large high pressure builds across the eastern U.S. Wednesday,
with dry conditions and seasonal temperatures. As this high
moves offshore, the return SW flow will bring milder conditions
on Thursday with readings in the upper 30s across the E slopes
of the Berkshires to the upper 40s across E RI into SE Mass.

Clouds will increase during Thursday well ahead of developing
low pressure across the SE states. Scattered light rain may
reach into the CT valley by late in the day.

Thursday night through Saturday...

Surge of moisture works up the eastern seaboard with deep
southerly wind flow at the surface and aloft. Also noting PWAT
moisture plume working up the coast by around midnight Thursday
night, up to 2 to 3 SD above seasonal normals by Fri morning.
Could see some heavier rainfall as this plume moves across
during Friday, with the best shot across central and eastern
areas.

As the low shifts NE across the Appalachians Friday night into
early Saturday, dry slot will work into the coastal plain on
Saturday, though showers may linger across the central and
western areas.

&&

.AVIATION /03Z SUNDAY THROUGH THURSDAY/...
Forecaster Confidence Levels...

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

Short Term /through Monday/...Moderate confidence.

1023 PM update...

Tonight...
VFR conditions. Patchy MVFR CIGS may approach N CT/RI around
10Z. NE wind gusts increase to 25-30 kts along south coastal
MA/RI around or after 05Z.

Sunday...
A mix of MVFR and VFR trending toward IFR/MVFR as the day
progresses as rain overspreads the area from south to north.
Freezing rain and sleet likely across the higher elevations
including KORH. NE winds up to 35 kt possible late in the day
along south coast of RI/MA including Cape Cod and Islands.

Sunday night...
IFR/LIFR in rain except freezing rain and sleet changing to all
snow across the interior with minor accumulations possible
higher elevations. NE winds up to 35 kt Cape Cod and Islands.

Monday Morning...
MVFR/IFR in rain trending up to MVFR/VFR by midday or so. Gusty
north wind shifting to NW around noon.

KBOS Terminal...
Quiet weather tonight with lowering cigs Sunday. Much of the
day may be dry with rain possibly holding off until after 21z
Sunday. Rain may change to a period of sleet and snow before
ending Monday morning.

KBDL Terminal...
Quiet weather tonight but lowering cigs toward 12z Sunday.
Spotty light freezing drizzle possible Sunday morning.
Otherwise all rain Sunday then possibly mixing with sleet and
snow Sunday night.

Outlook /Monday Night through Thursday/...

Monday Afternoon: MVFR-IFR CIGS and MVFR VSBYS across central
and eastern areas improve during the afternoon, though may
linger across Cape Cod through 22Z. NW wind gusts up to 25 to
30 kt across Cape Cod.

Monday Night: VFR. Windy with gusts to 35 kt.

Tuesday: VFR. Strong winds with local gusts to 40 kt.

Tuesday Night: VFR. Windy with gusts to 35 kt.

Wednesday through Wednesday Night: VFR. Breezy.

Thursday: VFR. Slight chance RA.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Monday/...High confidence.

*** NE Gales Likely late Sunday into Sunday night ***

Tonight...Dry weather with NE winds increasing to 30 kt by
Sunday morning.

Sunday...NE winds increase to 35 kt late southern waters. Vsby
lowering in rain from south to north.

Sunday night...NE winds up to 35 kt with strongest winds around
the Cape Cod waters.

Monday...Gale center just NE of 40N/70W moves seaward with
Northerly gales diminishing and becoming NW by midday.

Outlook /Monday Night through Thursday/...

Monday Night: Moderate risk for gale force winds with gusts up
to 40 kt. Rough seas up to 10 ft. Chance of snow showers.

Tuesday: Moderate risk for gale force winds with gusts up to
40 kt. Rough seas up to 11 ft. Slight chance of snow showers.

Tuesday Night: Low risk for gale force winds with gusts up to
35 kt. Areas of rough seas.

Wednesday: Winds less than 25 kt. Seas locally approaching
5 ft.

Wednesday Night: Low risk for Small Craft Advisory winds with
gusts up to 20 kt. Seas locally approaching 5 ft.

Thursday: Winds less than 25 kt.

&&

.TIDES/COASTAL FLOODING...
330 PM update...

High Tide 4 am Monday is only around 9 ft and only expecting a
storm surge of 0.75 ft to 1.5 ft, along with seas only 7 to 15
ft offshore. This will result in total water levels below flood
stage and insufficient wave action to yield any coastal
flooding.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Winter Weather Advisory from 6 AM Sunday to 7 AM EST Monday
     for CTZ002.
MA...Winter Weather Advisory from 6 AM Sunday to 7 AM EST Monday
     for MAZ002>004-008>011-026.
RI...None.
MARINE...Gale Warning from 9 AM Sunday to midnight EST Sunday night for
     ANZ231>235-237.
     Small Craft Advisory from 4 PM Sunday to 6 AM EST Monday for
     ANZ230.
     Small Craft Advisory from 2 AM Sunday to 6 AM EST Monday for
     ANZ236.
     Gale Warning from 3 PM Sunday to 6 AM EST Monday for ANZ250-
     254.
     Small Craft Advisory from 5 AM Sunday to 6 AM EST Monday for
     ANZ251.
     Gale Warning from 6 AM Sunday to midnight EST Sunday night for
     ANZ255-256.

&&

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

NWS BOX Office Area Forecast Discussion