NWS Forecast Discussion - Boston Area

Forecast Discussion for BOX NWS Office
FXUS61 KBOX 180144

Area Forecast Discussion
National Weather Service Boston/Norton MA
944 PM EDT Tue Jul 17 2018


Scattered showers and a few left over thunderstorms will come to
an end in most locations by midnight...but linger a bit longer
across southeast New England. Other than perhaps some lingering
showers early Wednesday across Cape Cod and Nantucket, dry
weather is anticipated the rest of the work week along with much
lower humidity including warm afternoons and cool nights.
Unsettled weather may return sometime Sunday into Monday along
with increasing humidity.



10 pm update...

Line of widespread light rain with embedded heavier showers,
perhaps a rumble of thunder, continues SE ahead of a sweeping
cold front expected to push offshore of S New England towards
morning. Have toned down on thunder overnight prevailing as
isolated. With isolated thunderstorms, a mention of heavy rain.
Near-term high res guidance, especially HRRR and HRRR-TLE, is
doing a decent job with handling trends.

Low clouds, patchy fog likely to be an issue along the S/SE
coast towards morning with moist SW flow as nocturnal cooling
proceeds. Soupy conditions prevailed, should stay fairly mild
overnight with lows around 70.

However, scouring dry air NW as noted in observations and within
RAP/HRRR models RH fields through the column. Expect improving
conditions across the interior overnight. Yet, with partial
clearing, falling temperatures, light winds behind the front
and antecedent rains, likely to be contending with patchy
shallow ground fog for interior locales. Dry air advection will
be proceeding which should limit the widespread nature, but
wouldn`t be surprised to see lower valley issues per cold air
drainage of a very moist airmass immediately above the damp
ground. Locally dense in spots, around 1 mile possible.

With cold frontal passage, dewpoint discontinuity, likely will
see with further progression into the tropical, moist air a
brief burst of NW flow. By morning, locations across the S-coast,
Cape and Islands may see gusts up around 20 mph with cold frontal




A much better day of weather on tap for Wednesday behind the
cold front. Perhaps a few lingering showers across the
Cape/Islands into mid to late morning. Otherwise...becoming
mostly sunny with high temperatures into the 80s in most
locations although a bit cooler across portions of the
Cape/Islands. Much lower humidity though will make for a much
more comfortable day across the entire region.




* Dry and much less humid later Wed thru Fri, possibly into Sat
* Highs 80-85 each afternoon and 50s at night
* Showers/storms and humid conditions return Sunday/Monday


17.00z ensembles (ECENS & GEFS) show northeast trough Wednesday
departing and deamplifying which gives way to ridging Friday and
likely lingering into Saturday. At the surface this translates to a
cold front slowly exiting the Cape Wed followed by 1018 mb high
pressure building into New England Wed night/Thu and accompanied by
a much less humid airmass. The high then moves offshore Friday but
with ridge axis arcing back into New England thru Saturday.
Ensembles suggest northern stream remains active with robust mid
level trough entering the Great Lakes this weekend and taking on a
negative tilt along the eastern seaboard Sun/Mon. This will support
a return to more humid weather along with risk of showers/T-storms.


850 mb temps begin Wed around +13C/+14C but then lower to about +11C
in the post frontal airmass Thu morning, but modify to +13C by days
end, yielding highs in the low 80s away from coastal seabreezes.
Airmass slow warms to about +15C Friday and Saturday which will
support highs in the mid 80s inland. However humidity remains
comfortable as seen by GEFS PWATs remaining below normal (1 to
2 standard deviations below normal) thru Sat with core of dry
air over the region Wed night into Thu. This dry airmass will
support lows in the 50s overnight, very comfortable. Also given
high pres overhead Thu ngt decided to lean toward the cooler MOS
guidance per ideal radiational cooling conditions. This dry
column will also yield plenty of sunshine Thu and Fri, possibly
Sat as well. 850 mb temps remain about +15C Sunday but do warm
to about +17C Mon. However the main story Sun/Mon will be the
increasing humidity as the low level flow becomes southerly and
advects dew pts into the 60s Sunday and then low to mid 70s


Any organized showers/T-storms will be focused across Cape Cod and
Nantucket Wed morning as slow moving cold front and frontal wave
move across this region in the morning. Trailing short wave sweeps
across the region later in the day along with attending cold front
moving offshore. Thus drying trend Wed which persist Thu, Fri and
likely into Sat. Next chance of showers/T-storms likely arrives Sun
and Mon as negative tilt mid level trough dives out of the Great
Lakes into the Mid Atlc region. Heavy rainers possible as
subtropical stream gets captured by negative tilt trough sending
PWATs of +1 to +2 standard deviations up the eastern seaboard into
New England Sun/Mon.



Forecaster Confidence Levels...

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

Short Term /through Tonight/...

Tonight...Moderate confidence.

Line of widespread -RA with embedded RA/+RA, perhaps a rumble of
TSRA. Given isolated nature of TSRA, have removed from terminals.
Focused lowest conditions associated with -RA SE of LWM-ORH-HFD
with potential IFR-LIFR CIGs and MVFR VSBYs. Expected mixed outcomes
S/E with terminals wobbling VFR-IFR.

Should see clearing SE towards morning with a sweeping cold front.
Lowest conditions pushed towards Cape and Islands. However, across
the interior, light winds, recent rains, partial clearing, likely
will be dealing with patchy IFR-LIFR shallow ground fog towards
morning. Noted 3SM BR roughly 8-11z for higher confidence terminals.

With cold frontal passage, could see a brief burst of NW winds
around 320 with 10G20KT, pushing offshore around 12z over Cape
and Islands.

Wednesday...High confidence.

Lingering low clouds/fog patches across the Cape/Islands during
the morning lift by late morning/early afternoon. Otherwise...VFR
conditions. Gusty N winds roughly 15-21z over Cape and Islands
with 10-15KT sustained, gusts up to 25 kts.

KBOS Terminal...

Hold BKN008 for the first couple of hours but should scour out
towards midnight then hold VFR with winds turning N.

KBDL Terminal...

Ground FG possible towards morning. Will hint at the potential
9-11z with 3SM BR for now. Increase if greater confidence is

Outlook /Wednesday through Sunday/...

Wednesday Night through Saturday: VFR.

Saturday Night: MVFR/IFR conditions possible. Slight chance

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

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

Monday: Chance SHRA, chance TSRA.



Forecaster Confidence Levels...

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

Short Term /through Tonight/...High confidence.

Cold front pushing offshore overnight towards morning. With
passage by morning could see a brief uptick in winds out of the
NW with sustained values 10-15 kts with gusts up to 25 kts.

Otherwise, through the remainder of this evening, SW flow around
10 kts, widespread light rain with embedded heavier showers,
perhaps a rumble of thunder. Contributions to low clouds and
lower visibility for most of the time.

Waves above 5 feet on the outer waters, small craft headlines
continue towards midnight. Will evaluate whether extension is
necessary with cold frontal passage with later forecasts.

Winds and seas should remain below small craft thresholds. Some
northeast wind gusts up to 25 knots or so are possible across
the Cape/Islands by mid-morning continuing into afternoon,
diminishing towards sundown.

Outlook /Wednesday through Sunday/...

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

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

Thursday Night through Saturday: Winds less than 25 kt.

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

Sunday: Winds less than 25 kt. Seas locally approaching 5 ft.
Chance of rain showers, chance of thunderstorms.

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

Monday: Winds less than 25 kt. Areas of rough seas. Chance of
rain showers, chance of thunderstorms.



MARINE...Small Craft Advisory until 11 PM EDT this evening for



NEAR TERM...Frank/Sipprell
LONG TERM...Nocera

NWS BOX Office Area Forecast Discussion