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Area forecast discussion
National Weather Service Boston/Norton Massachusetts
308 PM EDT Fri Sep 20 2019

high pressure remains in control into Monday with Summer-like
temperatures and dry weather. A cold front will bring a period
of showers late Monday and Monday night followed by cooler but
seasonably warm weather through the rest of the week.


Near term /until 6 am Saturday morning/...
shortwave energy passes well to the north and east of southern New England
tonight which will reinforce northwest flow aloft. Quiet night on tap
with mainly clear skies and light winds. Not as cool as
previous nights as airmass has moderated. Mins mostly in the
low/mid 50s, with some upper 40s in the colder spots in western
Massachusetts and interior eastern Massachusetts.


Short term /6 am Saturday morning through Saturday night/...
mid level ridge builds across new eng with warming temps aloft.
Expect lots of sunshine and warmer temps. 925 mb temps 20-21c
so we should see highs into the low/mid 80s, except upper 70s
along the immediate coast as sea breezes develop.

While the swell from Hurricane Humberto will be subsiding, it
will not be entirely gone. Still an elevated risk for rip
currents Saturday. If going to the beach this weekend, be
especially cautious if there are no lifeguards present.

Saturday night...
mainly clear skies with some high clouds spilling into the
region as mid level shortwave approaches. Lows ranging through
the 50s.


Long term /Sunday through Thursday/...

* continued dry weather with mild daytime temperatures Sunday.

* Rain/brief downpours associated with cold front Monday night-
Tuesday. Possible embedded thunder.

* Brief cooldown Tuesday, turning warmer by late week/weekend.



Upper-level ridge crests and shifts eastward into the Gulf of Maine
by early Sunday. Continued mild and dry weather looks to continue
into Sunday under surface high pressure and highs into the 80s. Will
also see some southwesterly breezes, strongest toward the South
Coast (nearing 25 mph). Later Sunday into the overnight,
geopotential heights start to fall and ribbon of mid-high level
moisture on increasing west-southwest flow should lead to an
increase in clouds. Combination of clouds and southwesterly winds
should keep temperatures from falling too far - into the 60s most

Monday - Tuesday...

First part of Monday now looking to be dry, though cloud cover will
be steadily filling/lowering from west to east in advance of a cold
front. Guidance still keying on frontal passing timing beginning
mainly after Sundown (12z NAM is a few hours earlier) through the
first half of the evening across interior Massachusetts/CT and Rhode Island and not until
the overnight toward the Boston-Providence/I-95 corridor, and into
early Tuesday on the cape, islands and coastal waters.

Ahead of and along the front, deep layer moisture quality improves
with precipitable water values rising to 1.5-1.75" and warm cloud depths are
between 10-12kft. These values argue for brief downpours along what
would likely be a strongly-forced band of frontal rain, perhaps
embedded with rumbles of thunder driven mainly from favorable
dynamics versus convective instability. Any embedded thunder would
only serve to enhance rainfall rates, and while wind fields are
strong with straight hodographs, strong storms aren't anticipated in
light of the meager instability progged given unfavorable time of
day and weak mid-level lapse rates. Even the faster NAM only
generates about 500 j/kg or less of cape from a most-unstable parcel

Still some leftover showers into Tuesday on the cape, but across the
interior looking at variable clouds with cold advection bringing
temps closer to seasonal values. With cooler thermal profiles aloft,
may still be enough moisture and shallow instability to produce some
spotty daytime convective showers in northern Massachusetts mainly near and
north of Route 2. Still some lingering clouds but dry
weather anticipated by Tuesday night.

Wednesday thru friday:

By Wednesday, the 500 mb pattern across the eastern roughly two
thirds of Continental U.S. Transitions to one of quasi-zonal flow - governed by
a prominent upper low over Hudson Bay with mean troughing into the
northern plains and northern rockies, a closed low over the Desert
Southwest, and a subtropical ridge over the deep south into the
northern Gulf of Mexico.

Wednesday's looking largely dry, with temperatures moderating amid
850 mb temperatures rising to the 10-12c range supporting 70s to
near 80s. Still some level of uncertainty on a potential frontal
passage Thursday - with the GFS/Canadian indicating a wetter frontal
passage than the European model (ecmwf) is, though limited moisture from the Gulf of
Mexico could lend support to the drier frontal passage indicated by the ec.

Looking ahead, another period of above-normal temps looks to be in
the offing for late next week into next weekend per most models and
teleconnection forecasts centered in the 6-10 day period. It also
jives with cpc's 6-10 day temperature outlook showing a significant
tilt in the odds toward above-normal temperatures across southern
New England.


Aviation /19z Friday through Wednesday/...
forecaster confidence levels...

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

Short term /through Saturday night/...high confidence.

VFR with light winds. Sea breezes developing by midday

Kbos terminal...high confidence in taf.

Kbdl terminal...high confidence in taf.

Outlook /Sunday through Tuesday/...

Saturday night: VFR.

Sunday through Sunday night: VFR. Breezy.

Monday: mainly VFR, with local MVFR possible. Breezy. Slight
chance rain showers.

Monday night: mainly MVFR, with areas VFR possible. Breezy.
Chance rain showers.

Tuesday: mainly VFR, with areas MVFR possible. Slight chance
rain showers.


forecaster confidence levels...

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

Short term /through Saturday night/...high confidence.

Light winds through the period. Peak of the swell from Humberto
will be this evening, then seas will gradually subside through
Sat/Sat night. Small craft advisories due to rough seas
continue across the outer coastal waters into Sat.

Outlook /Sunday through Tuesday/...moderate confidence.

Saturday night through sunday: winds less than 25 kt.

Sunday night: winds less than 25 kt. Areas of seas approaching
5 ft.

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

Monday night: low risk for Small Craft Advisory winds with
gusts up to 25 kt. Areas of rough seas. Rain showers likely.

Tuesday: winds less than 25 kt. Local rough seas. Chance of
rain showers.


Box watches/warnings/advisories...
Massachusetts...high surf advisory until 6 PM EDT this evening for maz007-019-
Rhode Island...high surf advisory until 6 PM EDT this evening for riz006>008.
Marine...Small Craft Advisory until 2 PM EDT Saturday for anz250-254.
Small Craft Advisory until 8 am EDT Saturday for anz251.
Small Craft Advisory until 11 am EDT Saturday for anz255.
Small Craft Advisory until 6 am EDT Saturday for anz256.


near term...kjc

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