Area Forecast Discussion
Issued by NWS Boston, MA

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334
FXUS61 KBOX 171102
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
702 AM EDT Fri Aug 17 2018

.SYNOPSIS...
Scattered showers and thunderstorms are expected this afternoon
into Saturday as a cold front approaches southern New England.
A few strong to severe storms are possible along with localized
heavy rainfall. This front moves through late Saturday and
Saturday evening. A wave of low pressure is expected to develop
along the front south of New England which may bring showers at
times along with cooler temperatures Sunday into Monday,
especially near the coast. High pressure returns Tuesday but
another approaching front will bring the risk of showers and
thunderstorms Tuesday night into Wednesday. Dry and seasonable
weather follows Thursday as high pressure builds into the
region.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
* Strong to severe thunderstorms are possible later today into
  this evening
* Potential for localized damaging winds and flash flooding
* Isolated short lived tornado possible late today and evening

7 AM update...

Patchy fog will burn off quickly, otherwise sunshine will give
way to increasing clouds from west to east this morning. Expect
initial pulse of sct showers/t-storms to develop by early
afternoon in the interior with a focus along the warm front.
CAPES increase to 1000-2000 J/kg so a few strong storms likely.
Initially, shear is weak but strengthens late today and
especially this evening as low level jet develops. Various hi-
res guidance sources indicating more robust convection late
today into this evening. A few severe storms appear likely and
given high dewpoints/low LCLs and increasing 0-1km shear, an
isolated tornado can`t be ruled out, especially near the warm
front which will be lifting north across MA this evening. Storms
will need to be monitored closely for rotation. In addition to
the severe weather threat, heavy rainfall and localized flash
flooding will be a concern as PWATs increasing to 2+ inches.

Temps will top off in the 80s at most locations, a tad cooler
across the higher inland terrain and along the coast.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/...
Tonight...The warm front should be NE of the region this
evening, which should take most of the instability with it.
Could still see some strong or even a brief severe thunderstorm
through the evening. However, with a mid level short wave will
move across, there will still be the threat for showers and
scattered thunderstorms through the night. Could still see some
storms produce locally heavy downpours as PWATs remain high.
Temps will only fall back to the upper 60s to mid 70s.

Saturday...Showers and scattered thunderstorms linger across the
region as a cold front slowly approaches. Mid level short wave
moves steadily E during the day, so the front should push across
the region. Decent instability lingers mainly S of the Mass
Pike, so there may be some stronger thunderstorms that develop
ahead of the cold front. PWATs also remain on the high side,
so heavy downpours will still be possible.

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
Highlights...

* Cooler Sun/Mon with showers at times, especially near the coast
* Mainly dry Tue, then increasing risk of showers/thunderstorms Tue
  night/Wed
* Mainly dry and seasonable Thu

Saturday night into Monday...

Cold front will be near the south coast Sat evening and pushing
south of New Eng. Showers and t-storms assocd with the front may
linger into Sat evening, especially south of the Pike within the low
level theta-e axis. Model guidance continues to be more robust with
trailing shortwave which moves from mid Atlc region into New Eng Sun
into Mon. This will stall the front with a wave of low pres
developing along it south of New Eng. The international model suite
is rather robust with sfc low tracking close to New Eng with a good
slug of rain late Sat night into Sun night. Meanwhile the US models
are weaker and further south with the wave with just a few showers
near the south coast. The trend has been a bit stronger and further
north so we will increase PoPs Sun into Sun night but not quite to
the extent of the ECWMF. Expect lots of clouds Sun/Sun night with
some showers at times, especially near the south coast. But if the
stronger and further northward trend from the ECMWF/UKMET is
realized, a cool rainy day would be in store for Sunday. The low
pres begins to pull away Mon but a few showers may linger,
especially eastern New Eng. Much cooler day Sun with potential for
temps to remain in the 60s where showers occur, but into the 70s if
it stays dry.

Tuesday into Thursday...

Mainly dry weather Tue with weak ridging in place, but temps
remaining a bit below normal with easterly flow persisting. Then
risk of showers/t-storms and humidity will increase Tue night into
Wed as tropical PWAT plume moves back into SNE ahead of approaching
mid level trough and cold front. Consensus of long term guidance has
front moving offshore by Thu which should bring dry and seasonable
weather for Thu.

&&

.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/...

Today and Tonight...Moderate to high confidence.
VFR CIGS through most of the day, but conditions lower in any
heavier showers/t-storms this afternoon and tonight. Strong
wind gusts and heavy downpours possible with the stronger
storms. Areas of MVFR cigs developing tonight, with IFR
stratus/fog for ACK.

Saturday...Moderate to high confidence.
Mainly VFR conditions, but will see local MVFR-IFR in
showers/isolated thunderstorms with best chance from midday
through the afternoon as cold front moves across.

KBOS Terminal...High confidence in TAF. Light E-SE wind early,
increasing to around 10 kt at midday then S this evening.
Showers/isold t-storms approach during the evening push with
local MVFR-IFR conditions possible.

KBDL Terminal...High confidence in TAF.

Outlook /Saturday Night through Tuesday/...

Saturday Night: Mainly MVFR, with local IFR possible. Breezy.
Chance SHRA.

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

Sunday Night: Mainly MVFR, with local VFR possible. Chance
SHRA.

Monday: Mainly VFR, with areas MVFR possible. Breezy. Slight
chance SHRA.

Monday Night: Mainly VFR, with local MVFR possible.

Tuesday: Mainly VFR, with areas MVFR possible.

&&

.MARINE...
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...Stalled front across the waters, with E-SE winds on the
eastern waters and S-SW winds across the southern waters. Winds
should remain around 10 kt or less. Front lifts NE as winds
shift to S-SW during the afternoon. Seas 3 ft or less. May see
scattered showers/isolated thunderstorms reach the eastern
waters late in the day with locally reduced visibility.

Tonight...S-SW winds gusting up to 20 kt. Seas 3 feet or less.
Scattered showers/t-storms, may see gusty winds and heavy rain.
Reduced visibility and locally choppy seas in any showers or
storms.

Saturday...SW winds gusting up to 20 kt. Seas 4 ft or less.
Scattered showers/isolated thunderstorms with reduced
visibility and locally choppy seas.

Outlook /Saturday Night through Tuesday/...

Saturday Night: Winds less than 25 kt. Areas of seas
approaching 5 ft. Chance of rain showers, chance of
thunderstorms.

Sunday through Monday: Winds less than 25 kt. Seas up to 5 ft.
Chance of rain showers.

Monday Night: Winds less than 25 kt. Areas of seas approaching
5 ft. Slight chance of rain showers.

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

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...None.

&&

$$
SYNOPSIS...KJC/EVT
NEAR TERM...KJC/EVT
SHORT TERM...EVT
LONG TERM...KJC
AVIATION...KJC/EVT
MARINE...KJC/EVT



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