Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 210816

Area Forecast Discussion
National Weather Service Boston/Norton MA
416 AM EDT Fri Sep 21 2018

High pressure moves slowly to the east today. A cold front
sweeps across the region late tonight with showers and a few
thunderstorms. Large high pressure will then bring dry but cool
conditions this weekend lasting into Monday. Another round of
scattered showers and a few thunderstorms are expected Tuesday
afternoon through Wednesday evening as a series of fronts move
across the region. High pressure will bring dry conditions


Showers moving east from eastern Lakes are assocd with modest
instability burst. The higher theta-e air and elevated
instability remain to the north where main focus for showers
will be this morning. There is a low risk for a brief shower or
sprinkle spilling into NW MA this morning, otherwise dry
conditions will continue.

High pres moves offshore today as cold front approaches the
eastern Lakes toward evening. Considerable low level moisture
persists across the interior and especially western New Eng
where lower clouds will likely remain. Less moisture further
east where partial sunshine expected to develop. There is a
good surge of warmer air aloft moving into SNE today but this
will be above a shallow mixed layer so temps will remain on the
cool side. 925 mb temps only around 13-14C so expect highs
upper 60s to lower 70s. Increasing southerly winds develop
this afternoon with gusts to 25 mph by mid/late afternoon.



Cold front moves across SNE late tonight, mainly 06-12z and will
be accompanied by a weakening line of convection from New York
State, mainly after midnight. Surface instability is non
existent, but low level theta-e ridge and KI values into the mid
30s ahead of the front support a line of showers and possibly a
few t-storms. This activity will weaken as it approaches the
coast so best chance for thunder will be in western New Eng.

Gusty S/SW winds will continue tonight, especially near the
coast where gusts 25-35 mph likely. Dewpoints will be climbing
into the 60s with temps holding steady in the upper 60s ahead
of the front, but cooling interior northern and western MA late
tonight behind the front.


The cold front moves south of the coast in the morning followed
by high pres and very dry air advecting into the region. Some
clouds may linger near the south coast early, otherwise lots of
sunshine expected. Gusty post-frontal NW winds expected in the
morning near the coast, then diminishing in the afternoon. Highs
upper 60s to lower 70s with dewpoints falling through the 50s
and into the 40s in the afternoon.




* Large high pressure will bring dry, cool conditions late this
  weekend into early next week
* A slow moving warm front brings scattered showers and a few
  thunderstorms late Tuesday through Wednesday
* Above normal temperatures briefly arrive Wednesday
* Cooler temperatures return late next week as cold front passes


00Z model suite continues trend of fast moving weather systems
into early next week thanks to a nearly zonal mid level
steering flow across the northern tier states. H5 begins to
amplify a trough as it moves out of the northern Rockies Mon
night/Tue timeframe as the long wave ridge tends to suppress
south across the mid Atlc and SE U.S. Some ridging does build
into Quebec in response to the digging trough, which will allow
for a brief period of milder temperatures but will also see
scattered showers and isolated thunderstorms around the middle
of next week with an approaching frontal system. Noting a cold
front working SE by late next week, which should bring another
shot of cooler air, but there are timing issues in play amongst
the model suite, so have lower confidence during the later

With the approaching frontal system around the Tuesday
afternoon through Wednesday timeframe, noting marginal
instability with TQ values in the upper teens and K indices in
the lower 30s. Have mentioned isolated thunderstorms through
this time, but will convection looks to be spotty at this

Expect temperatures through most of this period running close
to or below normal. However, with the ridging that builds in
ahead of the Tue-Wed system, could see temps on Wed run 5 to 10
degrees above seasonal normals.


Forecaster Confidence Levels...

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

Short Term /through Saturday/...

Through today...Moderate confidence.
Mainly VFR in the east with areas of MVFR cigs persisting across
western New Eng. Increasing south winds this afternoon with
gusts to 20-25 kt developing after 20z.

Tonight...Moderate confidence.
Cigs lowering to MVFR with a band of sct showers and isold
t-storms moving west to east across SNE 04-10z. Greatest
coverage across western New Eng with activity weakening as it
moves east. S/SW wind gusts to 20-30 kt, strongest near the

Saturday...High confidence.
Some lingering MVFR cigs early Cape/Islands, otherwise VFR.
Gusty NW winds in the morning diminishing in the afternoon.

KBOS Terminal...Moderate confidence in TAF.

KBDL Terminal...Moderate confidence in TAF.

Outlook /Saturday Night through Tuesday/...

Saturday Night through Sunday: VFR.

Sunday Night through Monday: VFR. Breezy.

Monday Night: VFR. Breezy. Slight chance SHRA.

Tuesday: Mainly VFR, with local MVFR possible. Breezy.
Chance SHRA.


Forecaster Confidence Levels...

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

Short Term /through Saturday/...High confidence.

Through tonight...
Increasing southerly winds developing this afternoon and
peaking tonight as 45-50 kt low level jet develops. Given warm
SST, expect sufficient mixing to bring down some of this wind
with gale force gusts to 35-40 kt over eastern MA waters.
Converted gale watches to warnings for eastern MA waters with
SCA for south coastal waters and BOS harbor. A few showers and
perhaps an isold t-storm is possible late tonight, after

W/NW wind gusts to 25-30 kt in the morning diminishing rapidly
and becoming N/NE in the afternoon as high pres builds over the
waters. Seas subsiding.

Outlook /Saturday Night through Tuesday/...

Saturday Night through Sunday: Winds less than 25 kt.

Sunday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Seas locally approaching 5 ft.

Monday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Seas up to 5 ft.

Monday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Local rough seas.

Tuesday: Moderate risk for Small Craft Advisory winds with
gusts up to 25 kt. Seas up to 5 ft. Chance of rain showers.


MARINE...Small Craft Advisory from 8 PM this evening to 11 AM EDT
     Saturday for ANZ232>235-237.
     Small Craft Advisory from 5 PM this afternoon to 11 AM EDT
     Saturday for ANZ230.
     Gale Warning from 8 PM this evening to 5 AM EDT Saturday for
     Small Craft Advisory from 8 PM this evening to 8 AM EDT
     Saturday for ANZ236.
     Gale Warning from 8 PM this evening to 8 AM EDT Saturday for
     Small Craft Advisory from 8 PM this evening to 2 PM EDT
     Saturday for ANZ255-256.


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