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Area forecast discussion
National Weather Service Boston/Norton Massachusetts
645 PM EDT Mon Oct 14 2019

Synopsis...
dry and pleasant Fall-like weather continues into Wednesday
morning. Storm system moves up the coast Wednesday and crosses
southeast New England Wednesday night, bringing rain and wind to our
area before moving off during Thursday. High pressure builds in
with cool, breezy conditions during the day and possible
frost/freeze conditions Friday night/Saturday morning.
Conditions modify to seasonable over the weekend as high
pressure moves overhead. The next chance of wet weather comes on
Monday.

&&

Near term /until 6 am Tuesday morning/...

645 PM update...

Low clouds linger across the North Shore and South Shore and are
in and out of Logan Airport in Boston. Low clouds also linger
across Cape Cod and islands with dense fog at Nantucket
Airport. There IA also an area of higher clouds in northwest Massachusetts.
The clouds are associated with a weak cold front approaching the
region.

Satellite imagery shows the clouds on the South Shore, cape,
and islands are slowly pulling offshore. The clouds along the
North Shore have been moving inland toward Lowell and Lawrence.
We will need to watch for some patchy fog developing across
southeast New England this evening with light winds/lingering low
level moisture.

Extrapolation of the cold front shows it moving through the CT
valley 8-10 PM, through the central hills 9-11 PM, and the
eastern coast 11 PM-1 am. This should bring a wind shift out of
the northwest, Flushing the low-level clouds/fog/humidity
offshore.

Low temperatures by daybreak should be mainly in the 40s, but
some upper 30s are possible across western Massachusetts.

&&

Short term /6 am Tuesday morning through Tuesday night/...
Tuesday...

High pressure will build in from the west and result in a mostly
sunny/tranquil day. High temperatures will top out in the lower 60s
in most locations. That coupled with light winds will make for a
very nice fall day across the region.

Tuesday night...

High pressure overhead will set the stage for an ideal night of
radiational cooling. Light winds/mainly clear skies will allow
overnight low temps to bottom out well down into the 30s in many
locales. A few of the normally coolest outlying locations may drop
to or even just below freezing. Frost and freeze headlines will
need to be considered where the growing season has not yet
officially ended. The urban heat islands of Boston/Providence will
bottom out in the 40s.

&&

Long term /Wednesday through Monday/...
big picture...

Broad closed upper low moves off through northern Quebec. But a
shortwave over western Canada slips into the vacated space over the
Great Lakes and closes off, re-establishing a southwest upper flow
over New England. The Great Lakes low then sweeps east over New
England Thursday. A shortwave moves ashore from the Pacific late
week and digs over The Rockies and plains, bringing another
southwest upper flow to our area for the weekend.

Contour heights are near normal Wednesday, but drop well below
normal Thursday as the upper low moves overhead, then climb above
normal for the weekend. Average heights for mid-October range from
567 decameters to 571 dm.

Some differences in placement of the coastal storm Wednesday-
Thursday. High confidence that the storm affects US and on the two
major effects, but low-moderate confidence on the magnitude of those
effects. The mass fields are similar through the weekend with
moderate confidence in the forecast.

Daily concerns...

Wednesday-Thursday...

Shortwave feeding around the Great Lakes upper low heads east to the
coast, generating a coastal low along the Carolina coast Wednesday.
The coastal low then moves north up the coast. Main concern is the
path the storm follows...many details determined by that path. The
GFS continues to carry the path east of Cape Cod as do its ensemble
members, while the 12z ggem tries to bring it up the CT valley. The
12z European model (ecmwf) moves it through the coastal plain while the NAM follows
the east Massachusetts coastline. This forecast favors an eastern solution,
although probably not as far east as the GFS. Something along the Massachusetts
coast or coastal plain seems reasonable.

Starting time in the late afternoon/afternoon rush hour, with
heaviest rain occurring during The Heart of the night, then
tapering off midday Thursday.

Main concern is wind. NAM shows easterly low-level winds (850 to 925
mb) up to 40 kt by Wed evening and 80-90 kt overnight, diminishing
Thursday morning. Ggem is a smidge less but still strong and with
similar timing. The 12z European model (ecmwf) shows 50-60 kt extending to the
coastal plain. The GFS shows 45-55 kt and mainly offshore. These
suggest gusts of 35 to 55 kt at the surface. We will use this
as a guide for the forecast, 35 to 50 kt at the surface.
Coastal/marine areas could reach a little higher.

Second concern is rain. Precipitable water values climb to around 1.5 inches along
the cape and islands and 1 inch in western Massachusetts. The system will be
progressive, so not expecting widespread flooding, although brief
urban/poor drainage concerns Wednesday night in Rhode Island and southeast mass.

Models show a dry slot moving up to the South Coast. This may
interrupt showers for a few hours Thursday morning. Upper low and
cold pool move overhead Thursday, and may generate more showers
during the day, especially north of the Mass Pike.

Showers should taper off Thursday night as the upper low moves off
to the east.

Friday through Monday...

Possibly a lingering shower Friday morning, but high pressure
surface and aloft move in from the west. This means drier air
moving in at that time so we expect a clearing trend for Friday and
mostly clear skies Saturday and Sunday as the high moves overhead.

Light wind and drier air...dew points 35-40...may bring min
temps of 35 to 40 in interior srn New England.

Next weather system in the Midwest spreads rain our way on Monday.

&&

Aviation /00z Tuesday through Saturday/...
forecaster confidence levels...

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

Short term /through Tuesday night/...

Tonight...moderate to high confidence.

Lingering IFR/LIFR along the Massachusetts East Coast including Logan
Airport and Cape Cod/islands in a light moist southwest flow. A
cold front entering the Berkshires at 6 PM will move across srn
New England early tonight, moving offshore of Boston and the
cape 11 PM to 2 am. Expect any IFR/LIFR to be pushed offshore
when that happens. Winds shift out of the northwest behind the
front.

Tuesday and Tuesday night...high confidence. VFR conditions
expected with light winds.

Kbos terminal...high confidence in taf. Low clouds were noted
at 630 PM with bases at 200 feet. Coverage may vary through 10
PM or 11 PM, but should move offshore by that time.

Kbdl terminal...high confidence in taf.

Outlook /Wednesday through Saturday/... moderate confidence.

Wednesday: mainly VFR, with local MVFR possible afternoon.
Breezy. Rain.

Wednesday night: mainly VFR, with local IFR possible. Windy
with gusts up to 35 kt. Ra, chance rain showers.

Thursday: mainly VFR, with areas MVFR possible. Strong winds
with local gusts up to 40 kt. Chance rain showers.

Thursday night: mainly VFR. Windy with gusts up to 30 kt.

Friday through Friday night: VFR. Breezy.

Saturday: VFR.

&&

Marine...
forecaster confidence levels...

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

Short term /through Tuesday night/...high confidence.

Tonight...a cold front crosses the waters late tonight. Behind this
front a period of northwest wind gusts of 20 to 25 knots along with 4 to 5
foot seas should develop across our far eastern waters. Will hoist
a small craft headlines for our far eastern outer-waters after
midnight.

Tuesday and Tuesday night...any lingering marginal Small Craft Advisory conditions
diminish by mid morning Tuesday as high pressure builds in from the
west. This should result in winds/seas below small craft thresholds
the rest of Tue into Tue night.

Outlook /Wednesday through Saturday/...moderate confidence.

Wednesday: low risk for Small Craft Advisory winds with gusts
up to 30 kt. Seas locally approaching 5 ft. Chance of rain.

Wednesday night: moderate risk for gale force winds with gusts
up to 45 kt. Seas up to 5 ft. Rain, chance of rain showers.

Thursday: moderate risk for gale force winds with gusts up to
45 kt. Rough seas up to 10 ft. Slight chance of rain showers.

Thursday night: low risk for gale force winds with gusts up to
35 kt. Rough seas up to 10 ft.

Friday: moderate risk for Small Craft Advisory winds with gusts
up to 25 kt. Local rough seas.

Friday night: winds less than 25 kt. Seas locally approaching
5 ft.

Saturday: winds less than 25 kt.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...none.
Rhode Island...none.
Marine...Small Craft Advisory from 2 am to 10 am EDT Tuesday for anz250-
254.

&&

$$
Synopsis...wtb/Frank
near term...wtb/Frank

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