Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 192243
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
643 PM EDT Sun Aug 19 2018

.SYNOPSIS...
High pressure over the Maritimes and a stalled front to our
south will combine to bring a northeast flow to Southern New
England through Monday night. A cold front will bring showers
and thunderstorms Tuesday night into Wednesday. High pressure
will build over the region Thursday, bringing fair weather and
comfortable humidity through the weekend.

&&

.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
645 PM Update...

Showers in SW CT are trying to lift NE toward Hartford County,
but presence of surface ridge should cause them to diminish.
Other widely scattered showers have developed near Cape Cod and
Islands this evening, which are too shallow to easily be
detected by KBOX radar. Most of the high-res guidance keeps this
activity across SE MA tonight, although some models try to bring
few showers into Boston and Providence which seems too
aggressive.

Otherwise clouds will spread inland across most of SNE tonight.
Current forecast has everything handled well so only minor
changes made with this update.

&&

.SHORT TERM /6 AM MONDAY MORNING THROUGH MONDAY NIGHT/...
High pressure remains in place over the Maritimes with a
resulting northeast low level flow over Southern New England.
Low level moisture will diminish through the day as the sun
heats the air, so morning clouds should give way to more sun in
the afternoon. Cape Cod and Southeast Mass in general may have
the toughest road to sun. Another shortwave moves overhead and
may generate a light shower or sprinkle mainly along the Cape
and Islands.

Sea temps around the coast are in the upper 60s and 70s, which
will influence values along the coast, with temps in the lower
70s. Inland temps will be more subject to mixing, with mixing
to 900 mb suggested by the forecast data. Temps at that level
are equiv to 9-10C at 850 mb...so max sfc temp potential would
be in the mid to upper 70s. If we were to mix up to 850 mb
directly, slightly warmer temperatures would support max sfc
temps around 80F.

Similar air mass Monday night, with dew points a degree or two
lower. Expect min sfc temps roughly 55-65.

&&

.LONG TERM /TUESDAY THROUGH SUNDAY/...
Highlights...

* Strong/severe storms possible Tuesday night and Wednesday.
* Fair weather Thursday into the weekend with comfortable humidity.

Overview...

Large scale pattern features upper trough over Northeast
through end of week before we see re-emergence of upper ridge
which begins to establish itself closer to SNE by end of
weekend. Aside from brief return of tropical humidity Tuesday
night and Wednesday ahead of cold front, this setup favors near
average temperatures but more importantly brings our first
stretch of more comfortable humidity (dewpoints in 50s and 60s)
later this week and into weekend. It does appear that heat and
humidity may very well return beyond Sunday as upper ridge
becomes more dominant.

Details...

Main concern deals with potential for strong or even severe storms
Tuesday night and Wednesday. Localized wind damage/flash flooding
are main threats but an isolated short-lived tornado is also
possible.

Low CAPE/high shear environment in place ahead of cold front should
be favorable for strong or even severe thunderstorms late Tuesday
night (after midnight) into Wednesday morning. SPC SREF shows decent
probabilities of more than 500 J/kg of MLCAPE and 0-1km helicity of
more than 100. Tropical airmass returns as GEFS shows precipitable
water values climbing above 2 inches (+2-3 SD) and dewpoints rise
back into 70s. 12z NAM/GFS support this thinking and show 30-40kt of
0-6km shear collocated with best instability.

Linear convection looks to be favored but typical of these
environments, we also need to be concerned with possibility of an
isolated, short-lived tornado developing along line due to low LCLs
and high 0-500m helicity of over 100.

For now, this is something to be aware of which will be refined in
later forecasts.

Front moves offshore around midday Wednesday. Drying and subsidence
behind short wave will bring an end to showers/storms by early
Wednesday afternoon. A prolonged stretch of dry and more comfortable
weather follows Thursday and into weekend as high pressure builds
over region.

&&

.AVIATION /00Z MONDAY THROUGH FRIDAY/...
Forecaster Confidence Levels...

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

Short Term /through Monday Night/...Moderate confidence

Tonight...

IFR CIGS near CHH/ACK expand into much of SE MA tonight with
MVFR CIGS across much of RI and central/eastern MA, and VFR CIGS
farther west to Berkshires. Few showers near Cape Cod and
Islands.

Monday...

VFR northwest and MVFR-IFR cigs southeast to start the day.
Expect sun heating to eat away at the sky cover, bringing VFR
conditions to all areas. Exception to this could be Nantucket
and parts of Cape Cod, where 2000-3000 foot cigs may linger
through the day. Northeast gusts to 20 knots mainly along the
coastline.

Monday night...

VFR lingers in the west. MVFR/IFR cigs expands over
Eastern/Central Mass and RI/Northeast CT due to marine flow.

KBOS Terminal...Moderate confidence in TAF.

MVFR ceilings, roughly 2000-2500 feet, build over the airport
and linger through most of the night. Conditions improve to VFR
around 8-9 AM.

KBDL Terminal...High confidence in TAF. VFR.

Outlook /Tuesday through Friday/...

Tuesday: Mainly VFR, with areas MVFR possible.

Tuesday Night: Mainly VFR, with areas IFR possible. Chance
SHRA, TSRA likely, patchy BR.

Wednesday: Mainly VFR, with local MVFR possible. Windy with
local gusts to 30 kt. Chance SHRA, chance TSRA, patchy BR.

Wednesday Night through Friday: VFR.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Monday Night/...High confidence.

Northeast wind through the period. Gusty winds near 25 knots
slowly diminish tonight but 5-6 foot seas linger on the outer
waters. A combination of wind and seas will keep Small Craft
Advisory in effect through early tonight, with areal coverage
slowly diminishing through Monday and Monday night.

Spotty light showers/sprinkles are possible tonight and Monday,
but coverage will be very limited. Mainly good visibility.

Outlook /Tuesday through Friday/...

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

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

Wednesday: Low risk for Small Craft Advisory winds with gusts
up to 25 kt. Local rough seas. Chance of rain showers, chance
of thunderstorms, patchy fog.

Wednesday Night: Winds less than 25 kt. Local rough seas.

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

Thursday Night through Friday: Winds less than 25 kt.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 8 PM EDT this evening for ANZ231-
     232.
     Small Craft Advisory until 10 PM EDT this evening for ANZ230-
     233>235-237.
     Small Craft Advisory until 10 PM EDT Monday for ANZ250-256.
     Small Craft Advisory until midnight EDT tonight for ANZ251.
     Small Craft Advisory until 6 AM EDT Tuesday for ANZ254-255.

&&

$$
SYNOPSIS...WTB/JWD
NEAR TERM...WTB/JWD
SHORT TERM...WTB
LONG TERM...JWD
AVIATION...WTB/JWD
MARINE...WTB/JWD


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