NWS Forecast Area Discussion - New England Area

000
FXUS61 KBOX 202317
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
717 PM EDT Sun Aug 20 2017

.SYNOPSIS...
High pressure builds over the region tonight and then offshore
Monday, providing dry and comfortable weather. Hot and humid
weather is forecast for Tuesday as the high moves farther
offshore. A cold front will approach the area late Tuesday, then
sweep across the region by mid-afternoon Wednesday, bringing the
risk of showers and thunderstorms. Any storms could produce
locally heavy rain and gusty winds. Skies clear out later
Wednesday. Then strong high pressure brings fall-like weather
with mainly sunny skies and clear nights Thursday through
Sunday...with highs in the 70s and lows in the 50s.

&&

.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
715 PM update...

Diurnal clouds have dissipated. Clear skies and very pleasant
conditions continued, with temperatures in the 70s and dewpoints
in the mid to upper 50s, except mid 60s Cape Cod and the
islands.

Previous discussion...

High pressure over Pennsylvania builds eastward into region tonight.
This will provide light winds, mostly clear skies and a relatively
dry airmass to promote temps falling fairly quickly with and after
sunset. The most noticeable temp drop will be outside the urban
areas. Given this followed the cooler MOS temps to derive mins
overnight. Most locations outside the urban areas dip into the upper
50s tonight, thus very comfortable. For the cities temps fall back
into the low and mid 60s.

&&

.SHORT TERM /6 AM MONDAY MORNING THROUGH MONDAY NIGHT/...
Updated 4 PM...

Monday...

After a cool start to the day return flow (SW winds) becomes
established by late morning as high pres slips offshore. Low level
WAA results in slightly warmer and more humid conditions tomorrow
with highs in the mid to upper 80s away from the south coast. Dew
pts still tolerable (U50s to L60s) as warm sector airmass remains
west-southwest of New England.

Dry weather prevails along with mostly sunny conditions. Thus good
viewing of the solar eclipse (just remember to wear safe glasses)!
Could be some mid and high clouds arriving late in the day across
western MA/CT, but by then the eclipse will have ended.

Monday night...

Low amplitude lead short wave and attending warm sector approach
from the west. Convection over PA/NY/NJ late Monday likely
weakens/dissipates as it approaches southern New England given loss
of daytime heating and weak mid level lapse rates. Nonetheless low
pops will be included in the forecast to account for any leftover
showers especially western MA/CT.

Much warmer and humid than previous nights given warm sector
overspreads the area. This may be accompanied by stratus/patchy fog
and possibly spotty light drizzle.

&&

.LONG TERM /TUESDAY THROUGH SUNDAY/...
Big Picture...

At upper levels, closed low pressure will be moving slowly
across Canada, from west of James Bay Tuesday to eastern Quebec
by Saturday. The trough axis extending southward from the low
will remain to our west until Saturday. The cold pool of air
will arrive over our region around Thursday and remain in place
into next weekend. A weak ridge begins to build over the area on
Sunday, as the trough finally exits.

At the surface, high pressure will pass southeast of the region
Tuesday. A cold front moves through Wednesday. Then strong high
pressure builds over the Great Lakes states, with its influence
extending eastward into New England Thursday into Saturday.
The strong high pressure becomes centered around New Brunswick
on Sunday.

As the previous forecaster noted, there has been a tendency for
the Canadian model to generate a tropical system off the
southeast coast of the U.S. then head northeast towards us for
the past several runs. Now, the UKMET, GFS, and to some extent
the ECMWF all show the potential for tropical development late
in the week, off the Florida coast. However, the significant
change is that they all now focus the northeastward movement
more along a persistent frontal boundary that currently exists
several hundred miles south of New England. That would appear to
make more meteorological sense, as tropical systems sometimes do
form on the tail end of old frontal boundaries. The strong high
pressure over the northeast states is expected to dominate our
weather. However, it is possible that late next weekend there
could be increasing potential for rip currents once again on
southern beaches. Anyway, that`s a week away.

Details...

Tuesday...

Summer is back. It will be hot and humid as high pressure
shifts offshore and southwest breezes increase, well in advance
of an approaching cold front. With 925 mb temperatures reaching
25-26C and a good deal of sunshine, expecting highs to soar to
90-95. Dewpoints will be rising to near 70 by late afternoon and
heat indices will be reach the mid 90s. Our new criteria for
Heat Advisories is 95 degrees, but it needs to be for 2
consecutive days, unless it hits 100, which would only require
1 day. Since it will probably not be reached on Monday, we may
issue a Special Weather Statement to draw attention to it, as
opposed to a Heat Advisory.

Southwest winds increase to 30-40 kts at 925 mb Tue afternoon,
so expect gusts at the surface to 20-25 mph at times, especially
in southeast MA and RI. CAPE is maximized over NY state, well
to our west, close to the cold front and that is where the best
potential exists for strong/severe thunderstorm activity.
However, a few showers and thunderstorms could form over the
higher terrain of western MA and northwest CT in the mid-late
afternoon hours.

Tuesday night...

Precipitable water values increase to 2.0 to 2.2 inches across
the region Tue night, becoming maximized in eastern sections
toward daybreak. We will have lost some heating as the front
moves into western sections Tue night. Nevertheless the 12Z ECMWF
keeps 1000+ J/kg CAPE overnight. There will be increasing wind
fields aloft, mainly unidirectional. The Storm Prediction Center
has a "marginal risk" of severe weather the western part of our
area. Expect that we will mainly have remnants of the strong
convection that had been to our west/southwest during the day,
but there should be some new development as well. Any storm
could produce locally heavy rainfall and gusty winds. Any
training of storms or clusters of storms could lead to a
flooding/flash flooding threat.

Wednesday...

Cold front swings across southern New England. Timing is still in
question, but consensus would have the front exiting the coast
in the afternoon. Southern New England will be in the right rear
quadrant of a 300 mb jet max of 100-110 kts, which would be
favorable for strong thunderstorm development, especially given
high PW values still in place in eastern MA and RI through early
afternoon. Skies will become mostly sunny from west to east
during the mid and late afternoon. Highs in the 80s.

Thursday through Saturday...

High pressure builds in at the surface while an upper trough digs
south from Quebec. The trough will carry a cold pool with cold
advection aloft on Thursday and Friday. Temperatures at 500 mb
will minimize at around -16C early Saturday morning. Moisture
fields show a moist layer between 850 and 700 mb during this
period, but dry air above and below. This looks like a mostly
sunny and dry pattern, but the instability shows some potential
for a few showers on Saturday. The mixed layer reaches to
between 850 and 800 mb, with temps at the top of the layer
supporting max sfc temps in the 70s...giving a fall-like feel.
Overnight low temperatures will be in the 50s each night.

Sunday...

High pressure becomes centered over New Brunswick. Sunny, cool
weather will continue across southern New England with highs in
the 70s and northeast to east wind flow. It is possible that
southernmost coastal waters could see increasing swell and
potential rip currents if there is any tropical development
several hundred miles south of the region.

&&

.AVIATION /23Z SUNDAY 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/... High confidence.

715 PM update...

Tonight...VFR. Light WNW winds becoming calm. Patchy MVFR fog
possible across interior valleys.

Monday...VFR and winds becoming SW. Afternoon T-storms across
NY/PA should diminish before entering western MA/CT late in the
day.

Monday night...VFR to start but lowering to MVFR and/or IFR as
low clouds and fog overspread the area after midnight and
especially toward Tue morning. Low risk of spotty light drizzle
late.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Tuesday through Friday/...

Tuesday into early Wednesday afternoon... Moderate confidence.
VFR except areas of MVFR/IFR in showers and scattered strong
thunderstorms mainly Tuesday night and Wednesday morning to
early afternoon. Patchy fog.

Later Wednesday afternoon through Friday... High confidence.
VFR. The only exception will be patchy IFR in fog Wednesday
night and early Thursday morning.

&&

.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.

715 PM update...

Tonight...Modest west winds become light and variable
overnight. Good vsby and dry weather prevails.

Monday...High pres moves offshore yielding modest SW winds.
Dry weather and good vsby continue.

Monday night...Modest SW winds continue and will likely be
accompanied by low clouds/patchy fog along with a low risk for
spotty drizzle. Thus vsby may be reduced at times especially
southern waters.

Outlook /Tuesday through Friday/...

Tuesday...High confidence.

High pressure shifts east of the waters. Winds 20 knots or less, and
seas 4 feet or less.

Tuesday night-Wednesday...Moderate confidence.

A cold front approaches from the Great Lakes, crossing the waters
late Wednesday afternoon. Showers and scattered thunderstorms
will move across the waters Tuesday night and Wednesday, with
locally heavy rain and gusty winds.

Southwest winds increase with gusts 25 knots. Winds shift from
southwest to northwest Wednesday night and diminish to 15 knots
or less. Seas build Tuesday night and Wednesday reaching 5 to 6
foot heights on the outer waters. The southwest flow may also
nudge those higher seas into RI Sound as well. Once the cold
front moves through and winds become northwest, seas will
subside to 3 feet or less Wednesday night.

A Small Craft Advisory may be needed on some of the waters Tuesday
night and Wednesday.

Thursday-Friday... High confidence.

Generally light north to northeast wind flow with seas
below 3 ft.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...None.

&&

$$
SYNOPSIS...Nocera/Field
NEAR TERM...Nocera/Field
SHORT TERM...Nocera
LONG TERM...Field
AVIATION...Nocera/Field
MARINE...Nocera/Field

NWS BOX Office Area Forecast Discussion