Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 181509

Area Forecast Discussion
National Weather Service Boston/Norton MA
1009 AM EST Tue Dec 18 2018

High pressure approaches from the west bringing drier weather.
Expect windy and much colder conditions tonight and Tuesday with
bitter wind chill values. High pressure will bring dry
conditions at mid week, with a moderating temperature trend for
Thursday. Strong low pressure will pass west of the region
Thursday night into early next weekend, bringing rain that will
fall heavily at times especially during Friday. Colder
temperatures then return early next week.



1015 AM Update...

Gusty northwest winds continue this morning...recent
observations have been near or below Wind Advisory criteria and
are expected to decrease slowly through the day. For this reason
the Wind Advisory has been shortened, ending at 11 AM. Winds,
though, will remain elevated through the afternoon. As expected
a few mid-high clouds traverse the eastern MA coast as the
shortwave rotates through...these should be brief and scattered
in nature.

645 AM Update...

Upper low pressure south of Nova Scotia. A shortwave in the flow
around the low sweeps across Eastern Mass this morning and then
moves out to sea.

11Z pressure change chart shows the core of pressure rises
directly over Srn New England. Expect this to move offshore
during the morning, at which point pressures rise more slowly
and the pressure gradient relaxes...therefore, less wind.
Observed winds along the coast have been borderline for a wind
advisory, but close enough to keep the advisory in place
through the morning.

Mixing to about 925 mb, with temps at that level around -8C.
That suggests high temps either side of freezing. So the current
forecast looks fine at this time.


High pressure will be our dominant weather feature tonight into
Wednesday. Winds will continue to diminish through this period,
and be more from the W than NW. Dry weather expected through
this period.

Very cold conditions tonight, with low temperatures in the
single digits above zero and teens common away from the
immediate coast. With some sunshine Wednesday and warm air
advection beginning, high temperatures recover some, but will
remain below normal.


Big Picture...

Thursday shows ridges over the Western Atlantic and the Rockies,
while a deep trough digs over the Mississippi Valley. Closed low in
the base of the deep trough will sweep up the East Coast Friday and
across New England Saturday. The flow over the USA then becomes more
zonal, with an embedded shortwave moving through early in the week.

Mass field show good agreement through Friday and possibly Saturday.
MSLP fields show the surface low taking an inside track through
NY/PA Friday. All show the zonal upper flow Sunday-Monday, but with
different timing for the movement of the shortwave. Thermal fields
are similar through Friday with H850 temperatures in the mid teens
briefly moving up across our area Friday.



As noted, the Midwest deep trough ejects up the Eastern USA Thursday
night and Friday, crossing New England Saturday. The Atlantic High
should keep enough low level dry air to keep the weather dry during
the day Thursday.

Deep moisture moves in Thursday night with PW values forecast to 1
inch all the way to the White Mtns while values of 1.5 inches reach
the South Coast. As per the SPC Sounding climatology, even the 1
inch values are at the upper end of what is expected in mid
December while 1.5 inches would be at or near record levels.

South to southeast flow builds Thursday night with 65 to 85 knot
winds forecast at 850 mb and winds near 35 knot forecast at 1000 mb.
The former actually suggests surface winds could gust to 45 knots.

Bufkit soundings show abv freezing air to at least 800 mb.  Based on
this and the expected storm track through New York, expect Southern
New England will have rain developing Thursday night, especially
after midnight, and continuing through Friday. Rainfall should total
at least 1-2 inches with potential for some higher amounts.
Potential for winds at advisory levels, especially in
Eastern/Central Mass and RI.

Rest of the period...

Dry. Height contours at 500 mb are forecast above normal Wednesday
night through Saturday, then seasonable or a little below on Sunday
and Monday. This would mean above normal temperatures late this
week, trending to seasonably cold temps early next week.


Forecaster Confidence Levels...

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

Short Term /through Wednesday/...High confidence.

Today... VFR. Windy with gusts to 30-35 kt, possibly up to 40 kt
along the east coast of MA.

Tonight... VFR. NW wind gusts 25-30 kt early, diminishing to
less than 20 kt after midnight.


KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF. Moderate risk for a
brief snow squall before 06Z, which may coat the ground in
spots. Runways mainly should be wet.

Outlook /Wednesday Night through Saturday/...

Wednesday Night through Thursday: VFR.

Thursday Night: Mainly MVFR. Breezy. RA.

Friday: Mainly IFR, with areas MVFR possible. Windy with gusts
to 35 kt. RA, patchy BR.

Friday Night: Mainly IFR, with areas MVFR possible. Windy with
local gusts to 30 kt. RA, patchy BR.

Saturday: Mainly MVFR, with local IFR possible. Windy with
local gusts to 30 kt. Chance RA.


Forecaster Confidence Levels...

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

Short Term /through Wednesday/...High confidence.

Buoy reports and some of the airport observations on the Cape
and Islands all show NW winds gusting to gale or near-gale
levels. This should continue through the morning, then subside
this afternoon. The winds will maintain rough seas, which will
take longer to subside across the eastern and southern outer
coastal waters of MA and RI. Seas continue to subside Wednesday.

Significantly colder air will move across the waters today into
tonight. With the gusty winds and rough seas, areas of light
freezing spray are likely along the nearshore waters of the MA
coast into this evening.

Outlook /Wednesday Night through Saturday/...

Wednesday Night through Thursday: Winds less than 25 kt.

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

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

Friday Night: Moderate risk for Small Craft Advisory winds with
areas of gusts up to 30 kt. Rough seas up to 12 ft. Rain

Saturday: Moderate risk for Small Craft Advisory winds with
gusts up to 30 kt. Rough seas up to 12 ft. Slight chance of


South to southeast winds Friday will come as astronomical tides
are trending higher. The winds will build seas offshore to 12-14
feet. Forecast guidance also shows a zone of 15-second wave
periods extending from the Gulf of Maine into Massachusetts Bay.
All of this shows a potential for building seas along the South
Coast Friday, where the flow is onshore, and into parts of the
Mass East Coast.

This is still a ways in the future, but it will bear watching
for potential coastal splashover or minor flooding Friday.


MA...Wind Advisory until 11 AM EST this morning for MAZ007-015-016-
MARINE...Gale Warning until 10 PM EST this evening for ANZ232>235-237-
     Gale Warning until 9 PM EST this evening for ANZ230.
     Gale Warning until 11 PM EST this evening for ANZ231.
     Gale Warning until 1 PM EST this afternoon for ANZ236.
     Gale Warning until midnight EST tonight for ANZ250-251-254.


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