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Area forecast discussion
National Weather Service Boston/Norton Massachusetts
1023 am EDT Tue Aug 20 2019

Synopsis...
high pressure builds over southern New England today, bringing
warm but drier/less humid air. A warm front moves north across
the region Wednesday. Very warm and humid conditions through mid
week with scattered showers and thunderstorms. A cold front
will cross the region Thursday, then will slow down along the
South Coast into Thursday night with lingering showers and a few
thunderstorms there. Once the front moves offshore Friday, dry
and less humid conditions will move in through early next week
with near or cooler than normal temperatures for late August.

&&

Near term /until 8 PM this evening/...

1030 am update...

Wow, what an airmass change from yesterday ... dew pts falling
into the mid to upper 50s at 10 am from hfd-orh-owd-bos and
points northward. Combination of northerly flow/anticyclonic
flow around 1020 mb high over PA/New York will continue to advect
these low dew pts southward but also as blyr deepens drier air
from aloft (pwats under an inch!) Will mix to the surface. Thus
have lowered dew pts a bit more than guidance indicates based
on above reasoning. However along and near the coast afternoon
seabreezes will advect maritime air into the coastal plain
keeping dew pts in the 60s. Still a big improvement over
yesterday 70+ dew pts.

Otherwise previous forecast on track with highs in the mid to
upper 80s, about 5 degs warmer than climo.

Previous discussion...

High pressure builds over New England today. This brings
subsidence, drying, and lots of sunshine. Dew points across New York
state are in the mid 50s to low 60s, which will be much more
comfortable than the values experienced Monday.

Temperatures aloft of 16c-17c should support Max sfc temps in
the mid to upper 80s, with a few warm spots touching 90. Light
wind will favor a developing sea breeze from late morning to
evening, and this should keep temperatures along the coast a few
degrees cooler.

&&

Short term /8 PM this evening through 6 PM Wednesday/...

Tonight...

High pressure lingers over the region early tonight, then moves
offshore late night. This should keep the area dry for most of
the night. But there will be potential for showers late at
night, especially in western Massachusetts and western CT (including
hartford).

Precipitable water values climb above 1.5 inches after midnight,
signaling the return of deeper moisture and humidity. Expect
dew points to climb overnight into the mid and upper 60s. This
will keep min temperatures in the 60s, with some urban centers
around 70. The increasing humidity and light wind should allow
patchy fog late.

Wednesday...

Shortwave moving east from the Great Lakes initially drives a
warm front north Wednesday, plunging southern New England back
into the warm sector. Precipitable water values continue to climb, reaching 2
inches Wednesday afternoon. This returns US to the oppressive
airmass of Monday with dew points in the low 70s.

Rel humidity fields also show lots of moisture and suggest
cloudy skies. But cross sections show many breaks...enough to
suggest temps warming into the 80s again Wednesday. Stability
parameters are impressive with SBCAPE 1500-2500 j/kg, Li of -5
to -8, totals around 50. Winds at 850 mb are forecast at 25-30
kt while 500 mb winds are forecast around 35 kt. The 0-3km
helicity is forecast at 150 on the European model (ecmwf) and 220 on the GFS.

As such, the forecast continues to feature a warm humid day with
showers and a chance of thunderstorms. Thunderstorms will have
the potential for gusty winds and local downpours. The favored
area for heavy rainfall is from Hartford County up across
western and central MA, but all areas have potential for the
strong gusts and local downpours.

Mixing will be hampered a little by the additional clouds, but
should still favor Max temps in the mid 80s.

&&

Long term /Wednesday night through Monday/...

Highlights...

* showers and scattered thunderstorms continue Wednesday night
through Thursday
* a cold front will slow its progress, keeping the threat for
showers and a few thunderstorms into early Friday morning
across southern areas
* large high pressure will slowly push across New England into
early next week, bringing dry and cool conditions

Details...

Wednesday night through Thursday night....

Low pressure crosses Quebec, with its trailing cold front slowly
pushing eastward out of western New York/PA Wed night, entering
western Massachusetts/north central CT by around 12z Thu. Will still see the
threat for showers/scattered thunderstorms, with the best
chances through around 03z-04z, then with lack of heating
should see pops lower though can not completely rule out spotty
convection overnight. K indices linger in the lower-mid 30s
early Wed night, then will slowly fall back from NW-se.

On Thursday, however, will see a renewed chance for showers/
thunderstorms as the main cold front pushes across. Noting
another spike in k indices, up to the lower 30s close to or just
ahead of the front, with CAPES increasing to 800-1000 j/kg by
mid to late morning across NE CT/RI/se mass. As winds shift to west
with the passage of the front, should see conditions dry out
across most areas expect the immediate S coast where showers may
linger.

Some model members signal another short wave in the flow along
the front Thu night, which could slow its progress and keep the
chance for showers from the Boston-Providence corridor S and E,
and enough instability could keep the threat for a few
thunderstorms along the S coast from 04z-06z before pushing
offshore.

Expect highs on Thu to run 5 to 10 degrees above normal, topping
off in the 80s across most areas. With dewpoints around 70
degrees across coastal areas, it will feel rather sultry. Temps
will begin to fall back to the mid-upper 50s well inland and
especially across the higher inland terrain Thu night, but will
linger in the 60s closer to the coast.

Friday through Sunday...

As the cutoff 500 mb low pres across Quebec and its associated long
wave trough slowly crosses the region Friday, will see drying
conditions move in with west-northwest winds. Large high pressure at the
surface and long wave ridging from Ontario to the western Great
Lakes will slowly shift east. Expect the high to push from Quebec
into northern New England this weekend into the start of the
work week.

With the cyclonic flow around the high, expect northwest winds on
Friday to veer to north to NE during the weekend. This will bring
close to or below normal temperatures especially on Sunday, with
forecasted highs only in the 70s.

&&

Aviation /14z Tuesday through Saturday/...
forecaster confidence levels...

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

Short term /through Wednesday/...moderate confidence.

14z update ...

VFR and dry weather prevail. Light northerly winds becoming
light and variable inland but onshore/seabreezes for the coastal
plain.

================================================================

Today...

VFR. Dry conditions. Late morning and afternoon seabreezes near
the coast.

Tonight...

VFR. Mainly dry through the night. Cloud bases lower and
thicken after midnight. May see spotty showers move into
portions of the CT valley around or after 09z.

Wednesday...

Showers and scattered thunderstorms, with local downpours.
Ceilings/bases at VFR most of the day, but lowering to between
1000 and 3000 feet in any thunderstorms. Vsbys mostly
unrestricted, but lowering to 1 to 3 nm in thunderstorms.

Kbos terminal...high confidence in taf. Sea breeze develops
around 10 am/14z, lingers until the evening. Initially from NE,
turning from southeast later in the afternoon.

Kbdl terminal...high confidence in taf.

Outlook /Wednesday night through Saturday/...moderate confidence
through Thursday night, then high confidence.

Wednesday night: mainly VFR, with areas MVFR possible. Chance
shra, chance thunderstorms and rain.

Thursday: mainly MVFR, with areas VFR possible. Chance shra,
chance thunderstorms and rain.

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

Friday: VFR.

Friday night: mainly VFR, with local MVFR possible. Slight
chance rain showers.

Saturday: VFR.

&&

Marine...
forecaster confidence levels...

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

Short term /through Wednesday/...high confidence.

1030 am update...

This afternoon ...

Ideal late Summer boating weather with 1020 mb high pres over
New York/PA providing light north-northwest flow across the New England waters.
Winds become light and variable this afternoon and onshore near
the coast with afternoon seabreezes. Dry/less humid airmass
provides vsby to the horizon.

==================================================================

Tonight...

Tranquil dry weather through midnight, then lowering vsby
toward daybreak in fog and chance of showers and thunderstorms
around daybreak.

Wednesday...

South winds gusting to 20 kt at times. Showers and scattered
thunderstorms. Poor visibility in fog and heavier downpours.
Strong wind gusts possible in thunderstorms.

Outlook /Wednesday night through Saturday/...moderate confidence.

Wednesday night: winds less than 25 kt. Areas of seas
approaching 5 ft. Chance of rain showers, chance of
thunderstorms.

Thursday: winds less than 25 kt. Local rough seas. Chance of
rain showers, chance of thunderstorms.

Thursday night: winds less than 25 kt. Areas of seas
approaching 5 ft. Chance of rain showers, chance of
thunderstorms.

Friday through Friday night: winds less than 25 kt. Areas of
seas approaching 5 ft. Slight chance of rain showers.

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

&&

Climate...
Boston set a high temperature record Tuesday, reaching 95
degrees. Old record was 92 degrees.

Boston tied a high minimum temperature record Tuesday, reaching
74 degrees.

Hartford/Windsor Locks tied a high temperature record Tuesday,
reaching 95 degrees.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...none.
Rhode Island...none.
Marine...none.

&&

$$
Synopsis...wtb/evt
near term...wtb/nocera/evt
short term...wtb

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