086
FXUS61 KBOX 062354
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
754 PM EDT Thu Jun 6 2024

.SYNOPSIS...
A second round of scattered showers and thunderstorms should
cross the region late this afternoon and evening. Unsettled
conditions continue with scattered showers and thunderstorms
Friday through Monday, but it will not be a washout on any of
those days.

&&

.NEAR TERM /THROUGH FRIDAY/...

745 PM Update...

Convective line that impacted the Hudson River Valley this
afternoon and early evening has dissipated considerably, as
expected, as it crosses over the Berkshires into our CWA.
Instability, both surface based and mixed layer CAPE, continues
to wane as we head towards sunset, and a robust cloud deck
preceding individual convective cells helped to stabilized the
environment across western/central MA and CT. With this,
prolific lightning producing cells have lost their luster,
transitioning to light to moderate rain producers. As
precipitation continues to fan out, we expect light rain
showers, particularly north of the MA Turnpike, through about
the midnight hour.

Fog is perhaps the more impactful meteorological condition this
evening as 1/4SM vsbys or less have developed along the eastern
MA coastline, the Cape, and the Islands. Fog is expected to
persist and expand some, particularly across the South Coast, as
we head into the evening given persistent easterly flow. Some
light shower activity may briefly improve vsbys to ~1SM through
~04Z, but improvement will be fleeting until we see a wind shift
after 08-10Z tomorrow morning as weak frontal boundary backs winds
to the northwest. Given anticipated persistence of dense fog, we
have issued a Dense Fog Advisory through 12Z for the immediate
eastern MA coastline. This DFA may be expanded overnight should
we see vsbys drop in other locations.

Previous Update...

Just started to see some breaks in the clouds across western MA.
Visible satellite data showed some convective looking clouds
under development as well. Still not seeing big warning signs
for widespread severe weather. MUCAPE values should be 500-1,500
J/kg, with MLCAPE values less than 500 J/kg. Effective shear
had increased to about 30-35 kt this afternoon. Lapse rates
remained marginally supportive for thunderstorms at all.
Thinking is that our region will be dealing with weakening
thunderstorms from farther west as they move into a less
favorable environment, just before sunset or later. Will monitor
any thunderstorms through this evening. The primary risk would
be locally heavy rainfall, and perhaps some gusty winds.

A weak surface low pressure should move off the coast towards
midnight. Not much of a pressure gradient to speak of, so
expecting rather light winds. Currently forecasting the majority
of showers and thunderstorms to dissipate by shortly after
midnight. Humidity may lower some late tonight, but it will
still fell muggy for most. Stratus and patchy fog should
gradually lift overnight from west to east.

&&

.SHORT TERM /FRIDAY NIGHT/...
Tricky forecast for Friday. it looks like our region will get on
the back side of a cool front to start the day, only to have
another weak low pressure potentially approach late in the day.
While humidity will be considerably less than what we had in
place today, it should still be enough to support a few showers.
Thinking the risk for thunderstorms has diminished across
southern New England. More sunshine will mean higher max
temperatures region-wide.

Humidity should continue to drop slightly Friday night. Mainly
clear skies and light SW to W winds should keep the stratus and
fog at bay. Above normal temperatures continue.

&&

.LONG TERM /SATURDAY THROUGH THURSDAY/...
Highlights

* Unsettled Friday through Sunday with several chances for rain
  and isolated Thunderstorms

* Pattern improvement delayed until early/mid next week as low
  pressure over southern Canada is displaced by building mid-
  level ridge

Friday...

Broad upper level low that has been slowly creeping towards New
England from the northwest sets up shop over the Great
Lakes/southern Canada by early Friday. Several shortwaves will
rotate around the low, the first of which develops mid day
Friday while somewhat of a seabreeze/back door coastal front
drops south out of Maine. While the dynamics of the day don`t
look super impressive, thinking that some scattered showers and
thunderstorms will be able to develop in the convergence zone of
the two "forcing features" where some modest instability,
~600J/kg SBCAPE is present. 0-6km bulk shear also looks to be
supportive of thunderstorm development, topping out around 60kt
Friday afternoon! At present, this convergence zone appears to
set up between Worcester and Boston along I-495, but may shift
over the next 24 hours.

The limiting factors in storm coverage and strength are typical
of southern New England with lackluster mid level lapse rates,
less than 5C/km, and decreasing PWATs as surface cold front
approaches from the west; PWATs drop from near 1.8" early Friday
to 1". So, while the severe potential remains very low, cant
rule out some solid downpours and gusty winds Friday afternoon.

Overall, should see a mix of sun and showers with a moderate
mugginess to the air with dewpoints in the low and mid 60s the
first half of the day.

Saturday and Sunday...

Unsettled conditions continue Saturday and Sunday, though
neither day appears to be a washout with a few more shortwaves
rotating around the persistent upper level low.

Saturday looks to be the drier of the two days as PWATs fall to
around 0.8" behind surface cold front that passes through
overnight. Guidance is split on the shower potential, with more
coarse resolution global guidance, like the GFS, trending
towards a drier day and hi-res guidance like the NAM initiating
more widespread shower activity Saturday afternoon. Dynamics
appear to be more supportive of thunderstorms, compared to
Friday, as mid level lapse rates increase to ~6C/km and low
level lapse rates approach 8.5C/km, but instability appears to
be quite poor, around 200J/kg.

PWATs surge back towards 1.25-1.4" early Sunday with a similar
dynamic setup, which will yield more showers and isolated
Thunderstorms compared to Saturday. Still, not expecting a
washout, with a mix of sun and showers.

Early next week and beyond...

Very persistent mid level trough/low pressure over Canada,
remains near stationary through early next week. Guidance shows
stronger shortwave development in the Monday period that could
bring a more substantial shot of QPF to the region, before a
brief reprieve from unsettled conditions develops
Tuesday/Wednesday with some indication that strong mid level
ridging develops during that timeframe. While ridging looks
robust, temperatures look seasonably mild rather than hot, with
highs in the 80s.

&&

.AVIATION /00Z FRIDAY THROUGH TUESDAY/...
Forecast Confidence Levels:

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

Tonight...Moderate confidence.

Isolated thunderstorms dissipate quickly thru 02Z tonight with
little chance of making it east of ORH. Light shower activity
possible through 04/05Z, primarily north of the MA/Turnpike.
VLIFR along the immediate eastern coastline, including the Cape,
with a gradient towards MVFR and perhaps pockets of VFR at
western terminals like BDL. E winds back to the NW behind
frontal passage tonight, which will allow vsbys to improve close
to sunrise.

Friday...Moderate confidence.

Any lingering dense fog stratus will dissipate in the morning,
otherwise VFR. Scattered showers developing in the afternoon,
mainly north of the Mass Pike. SW wind 5-15 kt, with sea-breezes
developing along the coast.

Friday Night...Moderate confidence.

Mainly VFR. SW to W winds 5-15 kt.

KBOS TAF...High confidence in trends, moderate in timing. TEMPO
between dense fog with 1/4SM and 1SM as light showers impact the
terminal between 02-04Z tonight. No significant improvement
until winds shift from the E to the NW behind frontal boundary.
MVFR to VFR will develop on Friday but may take some time with
light winds. Seabreeze possible after 17Z.

KBDL TAF...Moderate confidence. MVFR to VFR, light shower
activity wanes by 03Z. Patchy fog possible.

Outlook /Saturday through Tuesday/...

Saturday through Saturday Night: VFR. Breezy. Slight chance
SHRA.

Sunday: VFR. Breezy. Chance SHRA.

Sunday Night through Monday: VFR. Slight chance SHRA.

Monday Night: VFR.

Tuesday: VFR. Slight chance SHRA.

&&

.MARINE...
Forecaster Confidence Levels:

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

Moderate confidence through Friday Night.

Main concern across the coastal waters will be reduced
visibility in fog tonight, visibility improves Friday into
Friday Night. Marginal 5-foot seas continue, so maintained the
Small Craft Advisory.

Outlook /Saturday through Tuesday/...

Saturday through Saturday Night: Winds less than 25 kt. Areas
of seas approaching 5 ft.

Sunday: Winds less than 25 kt. Areas of seas approaching 5 ft.
Chance of rain showers.

Sunday Night: Winds less than 25 kt. Seas locally approaching
5 ft. Slight chance of rain showers.

Monday through Monday Night: Winds less than 25 kt.

Tuesday: Winds less than 25 kt. Slight chance of rain showers.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...Dense Fog Advisory until 8 AM EDT Friday for MAZ007-015-016-
     019-022>024.
RI...None.
MARINE...Small Craft Advisory until 8 PM EDT Friday for ANZ254.
     Small Craft Advisory until 7 AM EDT Saturday for ANZ255-256.

&&

$$

SYNOPSIS...Belk/KS
NEAR TERM...Belk/KS
SHORT TERM...Belk
LONG TERM...RM
AVIATION...Belk/KS
MARINE...Belk/KS