Area Forecast Discussion
Issued by NWS Boston, MA

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480
FXUS61 KBOX 192101
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
501 PM EDT Mon Aug 19 2019

.SYNOPSIS...
Very warm and humid conditions Monday will be accompanied by
scattered thunderstorms as a weak cold front sweeps across the
area. Behind the front, a brief period of drier and less humid
weather Tuesday. The front returns Wednesday as a warm front
with humid and very warm conditions overspreading the region Wed
afternoon through Thursday. The return of this summer-like
airmass will be accompanied by the risk for scattered
thunderstorms Wed and Thu. A cold front sweeps across the area
late Thu setting the stage for dry and less humid weather Friday
into the weekend.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...

445 PM update ...

*** Severe Thunderstorm Watch til 8 PM for much of MA/RI/CT ***

Weak short wave and attending surface trough moving across the
region this afternoon. Some storms have produced severe weather
including damaging winds and large hail in response to a very
unstable environment with SB CAPES up to 3500 j/kg and 2000 ML
CAPE per latest meso analysis. Special 18z sounding from GYX
revealed very steep mid level lapse rates from about 850 mb to
700 mb of nearly 8C/KM, owing to very high CAPE values. Thus
expecting convection over CT, northern RI to remain strong and
enter south-central RI into southeast MA next few hours, with
storms feeding on dew pts in the mid 70s (77 at WST)!

Main hazards remain strong to damaging winds, large hail given
high CAPE and mid level rotation and torrential downpours with
PWATs almost 2 inches.

Mid level short wave and attending surface trough fairly
progressive so expecting convection to exit eastern MA including
Cape Cod by 9 pm / 10 pm.

Behind the short wave and surface trough turning drier and less
humid with dew pts falling into the 60s and upper 50s northwest
MA late tonight.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH TUESDAY NIGHT/...

445 PM update ...

Tuesday ...

Post frontal airmass settles over the region tomorrow with good
column drying as PWATs fall below 1 inch, providing abundant
sunshine. This dry air mixes to the surface with dew pts in the
mid to upper 50s, although afternoon seabreezes will keep dew
pts in the low to mid 60s. Still much more comfortable than the
mid 70 dew pts of today.

Not as hot as today but warm with highs in the mid to upper 80s,
just more tolerable with dew pts in the mid to upper 50s.
Overall a real nice late summer day by New England standards.

Tuesday night ...

Pleasant during the evening hours with dry weather and
comfortable humidity hanging on. Second half of the night warm
front approaches from the southwest with risk of showers and
thunderstorms toward daybreak. Also patchy fog possible late.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...

*** Will update after thunderstorms exit ***

Highlights...

* Shower and thunderstorm chances Wednesday-Thursday
* A cold front brings dry & cooler conditions Friday through the
  weekend

Details...

Tuesday night through Thursday....

A series of shortwaves and a strong cold front will bring periodic
shower and thunderstorm chances through Thursday. Starting late
Tuesday/early Wednesday a warm front will begin to make its way
north across the region. With this warm front comes a very humid
airmass (dewpoints rising from the 50s Tuesday to the low 70s early
Wednesday, and PWATs back up near 2"). As mid level energy
associated with a weak shortwave moves through and interacts with
the warm front it initiates showers and thunderstorms. There is
potential for some strong thunderstorms given the moist warm sector
and CAPE values 2000-3000 J/kg with moderate to good bulk shear (35-
45kts). Any storms that form could produce gusty winds and heavy
downpours given how much moisture is in the column. At the moment
the best chance for heavier rainfall looks to be areas north and
west, with less QPF over southeast MA/coastal RI. However, we`re
still too far out to nail down specifics with much confidence.

The surface low and upper trough over the Great Lakes region then
slides north and east late Wednesday to Thursday dragging its cold
front across southern New England overnight Thursday. Along and
ahead of the front we again see shower and thunderstorm chances with
>2000 J/kg CAPE and good shear parameters (>40 kts bulk shear).
Temperatures remain relatively steady through this period in the
80s before a cool down arrives to end the week.

Friday through Sunday...

The cold front exits off the south coast or in the vicinity by
Friday morning leaving a cooler and much drier post frontal airmass
in its wake. PWATs drop to 0.5" while surface dewpoints in the low
50s will make the air feel refreshing compared to the previous week.
Surface high pressure and mid level ridging builds in through the
period bringing dry and pleasant weather through the weekend. Cold
pool aloft (850mb temps 7-10C) will allow for surface highs in the
70s each day.

&&

.AVIATION /20Z MONDAY THROUGH SATURDAY/...
Forecaster Confidence Levels...

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

Short Term /through Tuesday Night/...Moderate confidence.

21z update ...

Thru 00z ...

VFR but brief MVFR/IFR in localized TSRA with strong winds, hail
and brief heavy rain. Storms should be weakening and exiting 00z
or shortly thereafter.

After 00z ...

VFR but may MVFR/IFR may linger over Nantucket.

Tuesday ...

VFR and dry weather. Afternoon seabreezes near the coast.

Tuesday night ...

VFR and dry to start but becoming MVFR with chance of showers
and thunderstorms toward morning.

KBOS Terminal...TSRA risk over by 23z.

KBDL Terminal...TSRA threat has now shifted east of BDL as of
21z.

Outlook /Wednesday through Saturday/...Moderate confidence.

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

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

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

Thursday Night: Mainly VFR, with areas MVFR possible. Slight
chance SHRA.

Friday: VFR.

Friday Night: Mainly VFR, with local MVFR possible. Slight
chance SHRA.

Saturday: VFR.

&&

.MARINE...
Forecaster Confidence Levels...

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

5 pm update ...

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

Tonight ...

Scattered T-storms ending 7 pm to 10 pm from west to east. Then
improving.

Tuesday ...

Very pleasant weather with light winds and good vsby.

Tuesday night ...

Tranquil dry weather in the evening, then lowering vsby toward
daybreak in fog and chance of showers and thunderstorms.

Outlook /Wednesday through Saturday/...Moderate confidence.

Wednesday: Winds less than 25 kt. Chance of rain showers,
chance of thunderstorms.

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.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Heat Advisory until 8 PM EDT this evening for CTZ002-003.
MA...Heat Advisory until 8 PM EDT this evening for MAZ005-011-013-
     014-017-018.
RI...None.
MARINE...None.

&&

$$
SYNOPSIS...Nocera
NEAR TERM...Nocera
SHORT TERM...Nocera
LONG TERM...BW
AVIATION...Belk/Nocera
MARINE...Belk/Nocera



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