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Area forecast discussion
National Weather Service Boston/Norton Massachusetts
304 PM EDT Wed Aug 21 2019

Synopsis...
scattered strong thunderstorms will diminish this evening. Hot
and humid conditions persist Thursday then a period of showers
and isolated thunderstorms expected Thursday night as a cold
front stalls near the South Coast. Wet weather may linger into
Friday along the South Coast, otherwise improving conditions.
High pressure builds across the Maritimes, with steady onshore
winds bringing cool temperatures this weekend into early next
week. There may also be a chance for a few showers Sunday
through Tuesday.

&&

Near term /until 10 PM this evening/...

Despite marginal mid level lapse rates, moderate instability
across southern New England with MLCAPES 1500-2000 j/kg. Scattered
showers/T-storms will continue to develop through the afternoon
as mid level shortwave moves in from the west.

0-6km shear 30-35 kt so potential for storms to organize into
multi-cellular line segments per the hi-res cams. Damaging wind
is the primary threat with large hail also possible with the
strongest cores and any rotating storms. Modest 850 mb jet
develops later this afternoon/early evening increasing low
level shear so a few supercells can't be ruled out and will
have to watch for potential rotating storms which could produce
an isolated tornado. While all southern New England is at risk for severe storms,
the greatest risk will be across northern CT into interior Massachusetts
where best updraft helicity is focused. Timing of strongest
storms will be through 9 PM.

In addition, pwats pushing 2 inches later this afternoon so
risk of heavy rainfall may result in areas of urban and poor
drainage flooding, especially in areas of thunderstorm training.

&&

Short term /10 PM this evening through 6 PM Thursday/...

Expect convection to diminish later this evening as instability
decreases while focus shifts to the South Coast overnight. Just
a few showers or an isolated T-storm possible late tonight near
the South Coast. Patchy low clouds and fog expected to develop
along the coast. Warm and humid night with lows upper 60s to
lower 70s.

Thursday...

Cold front moves into the interior this afternoon but remains
west of the coastal plain through early evening. Another hot
and humid day as 850 mb temps around 17-18c. Highs in the upper
80s to lower 90s with dewpoints into the lower 70s, although
becoming less humid in the interior in the afternoon as
dewpoints fall through the 60s behind the front. Heat index
values mid/upper 90s in eastern Massachusetts/Rhode Island so close to heat advisory
criteria.

Regarding convective potential, CAPES around 1000 j/kg expected
which may be limited by some drier air aloft moving in during
Thu. This may help suppress convective activity although can't
rule out a few showers or T-storms in the afternoon along the
boundary. Better chance for convection will be late Thu and
especially Thu night as deeper moisture returns from the west
along the frontal boundary.

&&

Long term /Thursday night through Wednesday/...

Highlights...

* showers and a few thunderstorms linger Thursday night as the cold
front slowly crosses
* showers linger on Friday with cooler temperatures
* expect mild temperatures and dry conditions Saturday
* cooler than normal temperatures expected Sunday through Tuesday
along with spotty light rain or showers at times

Details...

Thursday night and Friday...

This period will be a transition between two air masses, as a cold
front slowly makes its way through bringing a renewed chance of
showers overnight into the first half of Friday. The main players
will be the approaching upper trough and surface cold front, which
guidance has begun to suggest will develop another surface low or
wave feature along that frontal boundary as it gets hung up in the
vicinity of the South Coast late. This will bring renewed
showers chances overnight Thursday into the first half of
Friday, especially along and south of the Massachusetts Pike in closest
proximity to the best surface convergence related to the cold
front and secondary low. Ample moisture lingering...pwats are
slow to drop behind the front late Thursday, staying near 2"
along the South Coast through 15z Friday. Good synoptic lift as
well under the rrq of a 115kt h300 jet together with the surface
low/wave could produce an additional 0.25- 0.5" of rain with
locally higher amounts. Instability looks to be limited mainly
to several hundred j/kg cape over Rhode Island and southeast MA, through
about midnight which is where we stand the best chance of seeing
some embedded thunderstorms in the evening/early overnight
hours. The details will depend greatly on the strength and track
of this feature which could potentially take the heaviest rain
offshore or further north if it wavers N-S. Dewpoints will be
dropping from northwest to se, which will keep low temps higher in
southeast Massachusetts/RI, in the upper 60s, with upper 50s for northwest mass.

Friday showers linger in the morning and potentially into early
afternoon as the wave moves off. Post frontal air mass is much drier
with dewpoints back squarely in the comfortable zone by late Friday,
in the 50s for most, save for potentially the extreme South Coast.
Surface pressure increasing and skies clearing through the day. A
big airmass change in the works, with cooler (slightly below
average) temps in the 70s.

Previous discussion...

Saturday through Tuesday...

Quite a bit of model solution spread amongst the members this
morning, with lends to lower confidence on the forecast during this
timeframe.

Rather confident with the slow but steady movement of the high
pressure across northern New England, setting up across Maine into
eastern Quebec and New Brunswick late this weekend. This will set up
a NE and eventually easterly wind flow by the start of the work
week. Could see some gusts up to 20-25 mph across Cape Cod and the
islands, possibly into S coastal areas on Sunday. The big question
will be whether patchy light rain and/or showers could develop from
time to time from Sunday to Tuesday. Where these showers and/or rain
set up is in question, but have kept slight chance pops going for
now. At this point, looks like there might be a somewhat better
chance for more organized showers Monday into Tuesday, but not a lot
of confidence.

With the steady onshore flow, expect temps to run around 5 degrees
or so below seasonal normals, mainly from Sunday to Tuesday. Highs
on Sunday and Monday may not break 70 at some locations along east
coastal mass.
&&

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

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

Short term /through Thursday/...moderate confidence.

Through 00z...
mix of VFR/MVFR cogs. Scattered showers/T-storms developing
through the afternoon will be accompanied by brief IFR and
heavy downpours/gusty winds.

Tonight...
a few showers/T-storms this evening shifting to South Coast
later tonight. MVFR/IFR stratus and patchy fog developing after
midnight, especially near the South Coast and over portions of
the CT valley.

Thursday...
areas of MVFR/IFR early, then improving to VFR by midday.
Isolated showers/T-storms possible in the afternoon.

Kbos terminal...moderate confidence in taf. A few T-storms
possible 4-8 PM. Brief downpours, gusty winds and IFR possible
in any strong storms. Mostly VFR tonight.

Kbdl terminal...moderate confidence in taf. A few T-storms
likely through 8 PM. Brief downpours, gusty winds and IFR
possible in any strong storms. Mostly VFR tonight.

Outlook /Thursday night through Monday/...moderate confidence

Thursday night: VFR. Chance shra, slight chance thunderstorms and rain.

Friday: VFR. Slight chance rain showers.

Friday night through saturday: VFR.

Saturday night: IFR. Breezy.

Sunday: mainly VFR, with local IFR possible. Breezy. Slight
chance rain.

Sunday night: mainly VFR, with local IFR possible. Breezy.

Monday: breezy.

&&

Marine...
forecaster confidence levels...

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

Short term /through Thursday/...high confidence.

A period of marginal 20-25 kt wind gusts possible tonight over
southern waters along with building seas to around 5 ft so have
continued the Small Craft Advisory. Winds diminish by early Thu. A few
showers/T-storms possible this evening shifting to south
coastal waters overnight. Vsbys may also be reduced at times in
fog late tonight and Thu morning.

Outlook /Thursday night through Monday/...moderate confidence.

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

Friday: winds less than 25 kt. Seas locally approaching 5 ft.
Chance of rain showers.

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

Saturday: winds less than 25 kt.

Saturday night: winds less than 25 kt. Seas locally approaching
5 ft. Visibility 1 nm or less.

Sunday: low risk for Small Craft Advisory winds with gusts up
to 25 kt. Areas of seas approaching 5 ft. Slight chance of
rain. Local visibility 1 nm or less.

Sunday night: winds less than 25 kt. Seas up to 5 ft.

Monday: low risk for Small Craft Advisory winds with gusts up
to 25 kt. Areas of seas approaching 5 ft. Slight chance of
rain.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...none.
Rhode Island...none.
Marine...Small Craft Advisory from 8 PM this evening to 8 PM EDT
Thursday for anz254>256.
Small Craft Advisory from 11 PM this evening to 8 am EDT
Thursday for anz235-237.

&&

$$
Synopsis...kjc/bw

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