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Area forecast discussion
National Weather Service Boston/Norton Massachusetts
135 am EDT Mon Oct 14 2019

Synopsis...
high pressure moves offshore tonight. Weak area of low pressure
moves up from the southern Appalachians and passes south of our
area Monday. This may bring a few showers late tonight and
Monday morning, mainly along the South Coast. High pressure from
the plains follows with dry weather and seasonable temperatures
Monday afternoon and Tuesday. A coastal storm system on tap
for Wednesday night into Thursday is likely to bring a period
of steady rain and breezy conditions, with lingering cool,
breezy conditions for late Thursday into Friday. Drying out for
Friday, with indications for above-normal temperatures moving
into the weekend and early next week.

&&

Near term /until 6 am this morning/...
130 am update:

Area of showers south of Long Island approaching Cape Cod and
islands but needs to overcome some dry air at mid levels which
will happen by 3 or 4 am. Continuing to lean on high-res models
which keep most of showers on Cape Cod and islands but also
graze South Coast. Best moisture convergence stays offshore
where better coupled jet dynamics are focused overnight.

Dense fog has developed across parts of NE Massachusetts. Dot webcams and
GOES-16 imagery indicate it is patchy and this is expected to
be trend through daybreak. Per coordination with gyx dense fog
is more widespread in S New Hampshire.

Forecast lows in 40s and 50s look good, and we may see a few
upper 30s in northwest Massachusetts.

&&

Short term /6 am this morning through 6 PM Tuesday/...
the South Coast low pressure will move past during Monday
morning. This will maintain a chance of showers during Monday
morning. The low then moves off to the east Monday afternoon and
high pressure/subsidence moves in. Expect drying and clearing
skies from west to east Monday midday and afternoon. Mixing on
Monday should reach to at least 900 mb, where temperatures will
be 10-11c, equiv to 5-6c at 850 mb. Mixing all the way to 850
mb would tap actual temperatures of 7-9c. Morning clouds may
slow the mixing so we will go with the 900 mb values. This
supports Max sfc temps in the 60s and low 70s, warmest in the
Hartford-Springfield area.

A weak cold front moves through Monday evening. Winds turn from
the northwest at night, but remain below 20 kt. High pressure
still to the west of US Monday night will maintain dry weather
through the night. Min temps are similar to tonight, but we cut
forecast guidance by a couple of degrees...upper 30s and 40s.

&&

Long term /Tuesday night through Sunday/...
highlights...

* seasonable/dry weather Tuesday.

* Potent cyclone may bring period of steady, at times heavy rains,
period of strong winds, and building/rough seas Wednesday night
into Thursday. Threat increasing though still considerable
uncertainty in details.

* Cool, breezy Thursday night/Friday, improvement later Friday.

* Warming trend for the weekend and into early next week.

Details...

Tuesday - Tuesday night:

Surface ridge of high pressure to be centered over central PA will
shift eastward off the coast of Long Island and eventually to the
40n/70w benchmark by evening. 500 mb height pattern will also
transition from initially cool cyclonic flow to geopotential height
rises/ridging. Light southwest 925-850 mb winds will contribute to
weak warm thermal advection by afternoon. Though a bit of a cool
start, though highs rebound nicely into the upper 50s to low-mid
60s. Put simply a great fall day areawide, with warm advection
increasing in strength into the overnight.

Late Wednesday - thursday:

Models continue to show an initial primary sfc low associated with
potent northern-stream 500 mb trough/closed low over the Great Lakes
early wednesday; northern stream interacts with less-defined though
still evident subtropical jet energy over the Gulf Coast.
Interaction/possible phasing between the two features aloft to
likely result in secondary cyclogenesis over the Virginia/NC border early
wednesday; this latter feature will become increasingly the dominant
cyclone as it moves northeastward toward southern New England and
into the Gulf of Maine Wednesday night into Thursday.

Speaking broadly, run-to-run consistency over past few model cycles
has been lacking. There's been a decided westward tendency in
surface low track. It is however notable that today's 12z guidance
is in somewhat closer agreement among various deterministic and
ensemble means on an inside/west of or near the 40n/70w benchmark
track by Wednesday night. In fact the 12z European model (ecmwf) now brings the
surface low inland. Will have to see if these trends continue, with
potential still continuing for wind/rain impacts across our area and
adjacent waters. Surface cyclone becomes increasingly wound-up as
mid/upper trough becomes negatively tilted by Wednesday night into
overnight. Into Thursday, international suite of guidance is slower
in terms of forward speed and would briefly stall the cyclone before
progressing into Nova Scotia late Thursday night. Verbatim, this
solution results in lingering rains into a bigger part of
Thursday for our area; GFS on the other hand is still a bit
faster and would barrel the cyclone into downeast Maine and lead
to a shorter period of wrap-around rain. We'll have to monitor
for both better consistency run-to-run and if/where the cyclone
may begin to slow/stall.

Wind potential still looks considerable from late Wednesday night
into Thursday. Wind fields increase into the evening, with
southerly/warm-conveyor belt 850 mb S/southeast winds into the 40-50 kt
range early Wednesday night, and becomes even stronger overnight out
over the waters to impressive 60-70 kt values. Stronger southerly
jet Wednesday night may not fully mix as southerly jet core will
likely be coincident with a period of steady/at times heavy rains,
and it may take heavier downpours to transport some to the surface.
Cold advection/dryslot behind the low overnight-early Thursday may
offer a better potential for strong northwesterly gusts across a
larger area, potentially from Rhode Island into southeast mass, the cape and islands.
May need to watch for possible tree damage given wet ground and
fully leafed trees and a potential need for wind headlines in coming
days, but too low-confidence at this point. Less robust, though
still breezy northwesterly gradient winds then more likely across
the interior.

Start timing of rains appears to have shifted slightly later, with
rain potentially coinciding with the Wed PM commute for CT into
western/central Massachusetts. Steady rains then appear to ramp up into the
evening/overnight hrs especially eastern Massachusetts/RI, and heavy rains may
be possible in this area. Leftover/potential wrap-around rains
mainly north of the Pike Thursday, though timing when these rains
end is unclear. Used a consensus model/wpc blend for qpf, which
produces a large area of 1.5-2" of rain Wednesday into Thursday.
Given uncertainty on when rains end, confidence in the rainfall
forecast is pretty low.

On the waters, potential still exists for gale force winds on most
coastal/offshore waters, and increasingly rough seas especially
offshore waters for late Wednesday into Thursday.

Thursday night - friday:

Less confidence into Thursday night; may still have lingering wrap-
around rain/showers north of the Mass Pike into at least the first
part of Thursday night. Even as showers begin to dissipate, a
continued cool/raw evening with mostly cloudy skies and continued
northwesterly gradient winds.

Continued breezy northwest winds to open Friday along with at least
partial cloudiness, though improvement trends in both aspects as
surface ridge builds in for later Friday. Most areas should turn
clear by Friday evening.

Into the weekend:

Still am seeing indications for a warming trend in global models as
500 mb geopotential heights rise. 850 mb temps rise to +8 to +10c
Saturday, and into the lower teens celsius for Sunday. Could be
looking at highs nearing 70 by Sunday with greater coverage of 70s
potentially into early next week.

Warming trend is also consistent with the 6-10/8-14 day
temperature probability outlook from the climate prediction
center, which also calls for a significant tilt in the odds
toward above- normal temperatures centered over late
weekend/early next week.

&&

Aviation /06z Monday through Friday/...
forecaster confidence levels...

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

Short term /through Tuesday/...

06z taf update: high confidence.

Areas of LIFR ceilings/visibilities across NE Massachusetts due to fog, which will
lift around 12z.

Otherwise ceilings lower to MVFR early this morning from Cape Cod
and islands into much of Rhode Island with IFR ceilings around daybreak and
lasting most of the day. Showers should be confined to immediate
South Coast, Cape Cod, and islands through midday. Ceilings lower
to MVFR into bos but remain VFR farther north and west. Lower ceilings
improve 21z- 00z as light S/southeast winds shift to W/NW.

VFR tonight and Tuesday. Gradient weakens enough to allow for
coastal sea breezes Tuesday afternoon.

Kbos terminal...high confidence in taf.

Kbdl terminal...high confidence in taf.

Outlook /Tuesday night through Friday/...

Tuesday night: VFR.

Wednesday: MVFR/IFR conditions possible. Windy with local gusts
up to 30 kt. Rain.

Wednesday night: mainly MVFR, with areas IFR possible. Strong
winds with local gusts up to 45 kt. Rain.

Thursday: mainly MVFR, with areas VFR possible. Strong winds
with gusts up to 45 kt. Chance rain.

Thursday night: mainly VFR, with local MVFR possible. Windy
with gusts up to 30 kt.

Friday: VFR. Breezy.

&&

Marine...
forecaster confidence levels...

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

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

730 PM update:

Seas of 5 to 7 feet continue on the eastern waters, though will
subside during the first half of tonight. A Small Craft
Advisory will continue for these waters through midnight, and
possibly a little later in the night on the waters east of
Massachusetts Bay.

Seas remain below 5 feet on Monday. Seas may approach 5 feet on
the outer waters Monday night.

Outlook /Tuesday night through Friday/...moderate confidence.

Tuesday night: winds less than 25 kt.

Wednesday: low risk for Small Craft Advisory winds with local
gusts up to 30 kt. Local rough seas. Rain.

Wednesday night: low risk for gale force winds with gusts up to
45 kt. Rough seas up to 12 ft. Rain.

Thursday: moderate risk for gale force winds with areas of
gusts up to 45 kt. Rough seas up to 12 ft. Chance of rain,
slight chance of rain showers.

Thursday night: low risk for gale force winds with gusts up to
35 kt. Rough seas up to 11 ft.

Friday: moderate risk for Small Craft Advisory winds with gusts
up to 30 kt. Local rough seas.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...none.
Rhode Island...none.
Marine...Small Craft Advisory until 6 am EDT early this morning for
anz250.

&&

$$
Synopsis...wtb/loconto
near term...jwd

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