Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 161735
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
135 PM EDT Thu Aug 16 2018

.SYNOPSIS...
Showers and thunderstorms will be on the increase Friday into
Saturday as a cold front approaches southern New England. This
front moves through late Saturday, followed by mainly dry and
cooler weather Sunday into Tuesday as high pressure builds in
from the north. The humidity will return by Wednesday leading to
the risk of more showers and thunderstorms.

&&

.NEAR TERM /THROUGH TONIGHT/...
135 PM Update...

Airmass is destabilizing over SNE with MLCAPE over 1000 J/kg in
parts of the interior. Cap has broken away from coast where we
are seeing an increase in building CU but so far not much in way
of shower or thunderstorms aside from some isolated activity in
southern VT. While high-res models continue to point toward
activity developing in E MA ahead of weak front dropping south
across region, limiting factor appears to be lack of deep
moisture as seen in K index. Still, there is sufficient
effective shear and decent mid level lapse rates (6.5-7.0C/km)
to support thunderstorm development so will keep mention of
widely scattered activity through sunset.

Otherwise mostly clear skies expected tonight, before clouds
begin to approach from west overnight. Lows will fall back into
60s and lower 70s.

&&

.SHORT TERM /6 AM FRIDAY to 6 AM SATURDAY/...

* Potential for strong/severe storms later Fri into Fri evening
* Localized wind damage/flash flooding possible
* Isolated short-lived tornado also possible

Showers and thunderstorms become more likely Friday, especially
during afternoon and evening across interior, as front lifts
back north thru SNE as a warm front.

Models show good instability with MLCAPE up to 1500 J/kg but
0-6km shear is minimal, however it does increase later in day
and especially Friday evening. Setup favors potential for a few
strong/severe storms with localized wind damage. Also have
potential for localized flash flooding as precipitable water
values climb back to 2 inches or higher. We also need to be
concerned with an isolated short-lived tornado late Friday
afternoon/Friday evening given rather high 0-500m helicity near
100, low LCLs, and warm front nearby. Later Friday night,
conditions become less favorable for organized strong/severe
storms as warm front lifts into northern New England but
showers/storms should make their way to coast due to approach
of mid level short wave.

Warm and humid conditions return with highs Friday in 80s and
lows Friday night only falling back into upper 60s to mid 70s.

&&

.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
 Highlights...

* Showers/t-storms likely Saturday
* Locally heavy rainfall possible along with the potential for a
  few strong to severe storms Saturday
* Mild to warm days and cool nights Sun/Mon/Tue
* More humid with increasing risk of showers/t-storms Tue night/Wed

Saturday...

On Saturday, there is still some uncertainty with timing of
fropa but consensus of model guidance favors slower timing with
cold front moving south across SNE during the afternoon. This
will keep tropical airmass across the region with threat of
showers/t-storms continuing. Deep layer shear is marginal but
enough instability for a few strong to marginally severe storms
with heavy rainfall threat as well. Some of the guidance doesn`t
push the front to the south coast until Sat evening. Drying
trend possible across northern MA in the afternoon depending on
exact timing of fropa. If front is delayed, temps will reach
well into the 80s, especially south of the Pike. Some showers
may linger near the south coast in the evening, otherwise,
cooler and drier conditions will filter into the region from
north to south Sat night with gusty NE winds developing,
especially along eastern MA coast.

Sunday into Monday...

Uncertainty with location of frontal boundary south of New Eng leads
to a lower confidence forecast for Sun/Mon. ECMWF trended further
north with boundary and surface wave track closer to New Eng which
keeps it unsettled Sun into Mon. GFS and ensembles are further south
and drier and have support from UKMET so we leaned toward drier
solution. Cooler and drier airmass overspreads SNE Sunday and Monday
as high pres builds to the north. Looks mainly dry during this
period although will have to watch moisture to the south Sun into
Mon. Coolest day will be Sunday, especially SE coastal MA where
temps may struggle to reach 70 with gusty NE winds, but ranging
through the 70s elsewhere. Clouds may linger through the day Sunday
near the south coast. Gradual airmass moderation Mon.

Tuesday into Wednesday...

Model differences with timing and amplitude of next upstream trough,
but it appears humidity and risk of showers/t-storms return by Tue
night or Wed. It is possible showers could impact region as early as
Tue, but more likely Tue night into Wed.

&&

.AVIATION /17Z THURSDAY THROUGH MONDAY/...
Forecaster Confidence Levels...

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

Short Term /through Friday Night/...

High confidence. Isolated showers/thunderstorms this afternoon
mainly in E MA with brief MVFR/IFR conditions, otherwise VFR
through tonight. Scattered showers/storms develop in western
New England Friday morning and head east, reaching coast Friday
evening and continuing Friday night. VFR conditions expected
most of the time but areas MVFR/IFR in showers/storms.

KBOS Terminal...High confidence. Sea breeze remains offshore this
afternoon as W winds veer to N/NW by evening push. Brief
shower/thunderstorm possible 18z-21z.

KBDL Terminal...High confidence in TAF.

Outlook /Saturday through Monday/...

Saturday: Mainly MVFR, with areas VFR possible. Breezy. Chance
SHRA, chance TSRA.

Saturday Night: Mainly VFR, with local MVFR possible. Breezy.

Sunday through Sunday Night: VFR. Breezy.

Monday: VFR.

&&

.MARINE...
Forecaster Confidence Levels...

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

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

Weak cold front pushes south across waters tonight and stalls
south of New England. Winds become light N/NE tonight before
veering to S/SE Friday as seas remain calm. Increasing S winds
expected Friday night with gusts up to 20kt on south coastal
waters, which may bring seas up to 5 ft offshore.

Outlook /Saturday through Monday/...

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

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

Sunday: Winds less than 25 kt. Local rough seas.

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

Monday: Winds less than 25 kt. Seas locally approaching 5 ft.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...Air Quality Alert until 11 PM EDT this evening for MAZ020-
     022>024.
RI...Air Quality Alert until 11 PM EDT this evening for RIZ006>008.
MARINE...None.

&&

$$
SYNOPSIS...JWD
NEAR TERM...JWD
SHORT TERM...JWD
LONG TERM...KJC
AVIATION...Belk/JWD
MARINE...Belk/JWD


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