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FXUS61 KBOX 181201
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
701 AM EST Fri Jan 18 2019

.SYNOPSIS...
Low pressure brings light wintry precipitation to southern New
England early this morning. High pressure then brings a brief
break in the precipitation this afternoon into Saturday. A major
winter storm will impact the region Saturday night into Sunday,
but exact precipitation types and impacts remain uncertain.
Arctic air follows this system into early next week. Another
weather system may bring rain and snow next Wednesday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...

Dew point depressions remain rather large this morning,
indicative of a lot of dry air near the ground. This will keep
much of southern New England from seeing much snowfall this
morning. Adjusted precipitation chances to reflect observed
trends. Made additional tweaks to temperatures and sky cover
into this evening, too.

Previous Discussion...

A weak mid level shortwave and a couple of low pressure centers
to our north and south will continue to bring scattered light
snow showers to southern New England this morning. Given weak
forcing and split flow leading to a lack of moisture, these
showers aren't expected to amount to much. Though the radar
continues to show returns, model soundings show a very dry
boundary layer and observed dewpoint depressions remain around
10-20 degrees. Eventually expecting things to saturate ending
with a dusting of snow. Showers should come to an end after
7-9am for most. The southeast coast will likely see snow
changing to rain showers hang on through late morning as the low
levels continue to warm. Not much in the way of wind with this
event as the pressure gradient remains weak. The rest of today
will be dry with clearing skies as low pressure moves off.
Temperatures will be quite a bit higher than we saw Thursday
with highs in the upper 30s along the coast and low 30s for the
interior high terrain.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/...
A quiet night as high pressure over QC edges into the region
bringing clearing skies and winds turning out of the north.
These northernly winds bring CAA and temperatures dipping into
the teens and low 20s, higher along the south coast. Given
clearing skies and light winds we could see some places radiate
into the single digits in the interior.

Saturday is dry with high pressure still under control. North
winds bring colder air down and we'll see highs in the 20s, low
30s along the south coast. Mostly clear skies to start the day,
then by early afternoon mid and high clouds begin to fill in
ahead of our next system.

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
* Major Winter Storm Sat night/Sun
* Uncertainty remains in regards to exact precipitation types
and impacts

High pressure over eastern Canada will keep the weather dry and
relatively cold across most of the region through mid to late
Saturday afternoon. A major winter storm will impact the region
Saturday evening through Sunday afternoon, but specific
precipitation types/impacts remain uncertain at this time. We
will break things down a bit more below.

1) Snow Amounts and Uncertainty:

Snow will overspread just about the entire region by early Saturday
evening. A strong east to southeasterly low level jet of 50+ knots
will result in strong forcing. So a period of 1 to 2 inch per
hour snowfall rates remain possible, especially N and W of I-495
and I-395. Still thinking the freezing line makes it into
interior southern New England Sunday morning, at least about
4-5 kt up. It will take longer to drive this freezing line NW
lower in the atmosphere.

Where the melting layer sets up will directly modulate the
amount of snow versus sleet. Where it largely remains snow,
looking likely for close to a foot plus of accumulation. There
looks to be about a 6 hour window Sunday morning where sleet may
impact snow accumulations across the interior. Even a couple
hours of sleet during this critical time can drastically reduce
snow accumulation. This is the most difficult portion of this
forecast. Additional changes are possible with later forecasts.

2) Sleet and Freezing Rain Threat:

As we mentioned in the prior section, the amount of warm air
aloft overspreading the region should allow for snow to change
to sleet across a good portion of southern New England, at least
for a time. Significant amounts of sleet are possible given the
depth/strength of the low level cold air near and north of the
MA Turnpike. While we can not rule out some freezing rain north
of the the Pike too, the greater risk remains across portions
of northern CT, and particularly into RI/SE MA. This is where
the warm nose is lower and the depth of the sub-freezing air is
more shallow.

It is a very difficult forecast and tough to pinpoint this far
out. If the coastal front allows temps to climb above freezing,
would expect some light ice accretion as temperatures will
certainly drop quickly Sunday afternoon. However, if those
areas stay below freezing ice accretion of one quarter to one
half inch would be possible across portions of this region. Will
need to be watched for the risk of scattered power outages if
higher ice accretion amounts are realized.

3) Transition Back To Snow/Flash Freeze Potential:

As the low pressure system tracks near or south of the Islands,
any remaining mixed precipitation will transition back to snow
later Sunday morning/afternoon. While some minor additional
snow accumulations are possible, the bigger story will be the
rapid falling temperatures on the coastal plain. High resolution
models indicate that much of the region may fall into the teens
by Sunday afternoon. Areas near, and especially southeast of,
the Boston to Providence corridor may experience a flash freeze.

4) Main Impacts/Concerns:

Besides the heavy wintry precipitation, the greatest concerns
are for scattered power outages where ice accumulation occurs.
Also some concern about black ice as untreated wet surfaces
freeze over Sunday afternoon and evening. Gusty winds will also
make it feel much colder during the afternoon and evening hours.

Sunday night-Monday...

Weekend storm departs through the Maritimes Sunday night, but
continues to draw colder air south into Southern New England. High
pressure over the Great Lakes and the storm in the Maritimes will
mean a strong pressure gradient over New England, maintaining the
cold advection and resulting mixing over our area.

Tuesday through Thursday...

High pressure moves over New England Tuesday. Radiational cooling
Tuesday morning will allow min temps in the single numbers and below
zero. The high pressure will continue dry weather through Tuesday.

Low pressure that moves on shore on the Pacific coast this weekend
will move across the country next week, bringing rain or snow to
Southern New England Wednesday and possibly Thursday.

&&

.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/...High confidence.

Today...Mainly VFR. Localized IFR/MVFR conditions in the heavier
scattered snow showers. Expecting two separate areas of snow.
One generally remains west of the Berkshires, and the other
grazes the South Coast, Cape and islands this morning. Some
runways may receive a coating of snow. SN may briefly change to
-RA before ending over RI/SE MA.

Tonight...VFR. Could become breezy along east coastal
MA/Cape/Islands w/NW gusts 15-20 kts.

Saturday: Mainly VFR, with local MVFR possible.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Saturday Night through Tuesday/...

Saturday Night: Mainly IFR, with local MVFR possible. Strong
winds with gusts to 40 kt. Wintry mix of SN and FZRA.

Sunday: Mainly IFR, with local MVFR possible. Strong winds with
areas gusts to 40 kt. Wintry mix of RA and PL, with a chance of
FZRA or SN.

Sunday Night: Mainly VFR, with local MVFR possible. Strong
winds with local gusts to 40 kt. Slight chance for a wintry mix
of FZRA or SN.

Martin Luther King Jr Day through Monday Night: VFR. Windy with
gusts to 35 kt.

Tuesday: VFR. Breezy.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Saturday/...

Today...Light snow and rain over the waters this morning. SE
winds around 10 kts tonight becoming SW 10-15 kts during Friday.
Seas below 5 feet.

Tonight...Winds become NW and increase, with gusts to 25 kts
possible, especially over the eastern waters. Seas building to
4 to 5 feet on the eastern outer coastal waters.

Saturday: Winds less than 25 kt. Areas of seas approaching
5 ft.

Outlook /Saturday Night through Tuesday/...

Saturday Night: Moderate risk for gale force winds with gusts
up to 40 kt. Rough seas up to 11 ft. Rain and snow. Areas of
visibility 1 nm or less.

Sunday: Moderate risk for gale force winds with gusts up to
40 kt. Rough seas up to 15 ft. Rain, with snow likely. Local
visibility 1 nm or less.

Sunday Night: Moderate risk for gale force winds with gusts up
to 40 kt. Rough seas up to 14 ft. Freezing spray, with a chance
of snow.

Martin Luther King Jr Day: Low risk for gale force winds with
gusts up to 40 kt. Rough seas up to 12 ft. Freezing spray, with
a chance of snow showers.

Monday Night: Low risk for gale force winds with gusts up to
35 kt. Rough seas up to 11 ft. Freezing spray.

Tuesday: Low risk for Small Craft Advisory winds with gusts up
to 30 kt. Local rough seas. Freezing spray.

&&

.TIDES/COASTAL FLOODING...
An approaching deep low pressure has been trending on a
somewhat southern track with the last few model runs. The
consequences are twofold: 1. Extended period of long fetch
onshore flow for the E coast and 2. Low level jet energy becomes
oriented with a stronger E component, compounding the surge.

By the time of high tide Sunday along the east coast (roughly
10 AM local), surge should be approaching 2.0 ft, which
combined with offshore wave action around 15 ft should lead to
localized minor to moderate E coastal flooding given a high
astronomical base tide in Boston around 11.6 ft MLLW. There are
still track variances, so these will be better resolved with
later forecasts. Confidence remains high enough to continue a
Coastal Flood Watch, with a particularly focus from Scituate to
the New Hampshire border.

For the south coast, surge will not be as high given the wind
trajectory, likely between 1.0-1.5 ft. While this may lead to
some pockets of minor coastal flooding around the 7 AM Sunday
tide, confidence is not as high. So will hold off on headlines
for the time being.

Will also need to monitor Mon AM high tide (11.9 ft MLLW in
Boston Harbor). As continued wave action and lingering swells
may lead to pockets of minor flooding along the E coast as well,
even as winds shift to a stronger westerly component.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Winter Storm Watch from Saturday afternoon through Sunday
evening for CTZ002>004.
MA...Coastal Flood Watch Sunday morning for MAZ007-015-016-019-022-
024.
Winter Storm Watch from Saturday afternoon through Sunday
evening for MAZ002>021-026.
RI...Winter Storm Watch from Saturday afternoon through Sunday
evening for RIZ001>007.
MARINE...Gale Watch from Saturday evening through Monday morning for
ANZ231>235-237-250-251-254>256.

&&

$$
SYNOPSIS...Belk/BW
NEAR TERM...Belk/BW
SHORT TERM...BW
LONG TERM...Belk
AVIATION...Belk/BW
MARINE...Belk/BW
TIDES/COASTAL FLOODING...Staff
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