-- Discussion containing changed information from previous version are highlighted. --
530
FXUS61 KBOX 261836
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
236 PM EDT Wed Jun 26 2024

.SYNOPSIS...
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Hot and humid today with a risk for showers and thunderstorms
especially late this afternoon into the evening. Though there is
a risk for some spotty activity early this afternoon. Some
storms could turn strong to perhaps severe bringing risk of
strong winds and heavy downpours, which may lead to flooding.
Dry and pleasant weather conditions for Friday as Canadian high
pressure briefly builds in. A more humid and more active weekend is
in store as a slow moving frontal system brings periods of showers
and thunderstorms with localized downpours possible. Drier
weather then returns for early next week with seasonable
temperatures.
-- End Changed Discussion --

&&

.NEAR TERM /THROUGH TONIGHT/...
No major changes to the ongoing forecast this morning. Still
getting some signals for an earlier round of showers early this
afternoon before the main event arrives towards sunset, or even
later. Mainly brought the forecast back in line with observed
trends.

Previous Discussion...

Highlights

* Hot and muggy today. Isolated strong to severe thunderstorms
  possible across southern New England late this afternoon into
  tonight. Main threats are strong to perhaps damaging wind
  gusts and heavy downpours, which may lead to localized
  flooding.

Will be setup in a quasi-zonal flow pattern through a good
portion of this timeframe. Could see a weak/subtle shortwave
lift in from the central/eastern Great Lakes this morning into
the afternoon. A more substantial trough digs into the
central/eastern Great Lakes by this afternoon. This trough becomes
neutral to perhaps slightly negatively tilted as it digs into
the eastern Great Lakes this evening and lifts through New
England tonight. A pre-frontal trough slides across southern New
England late this morning/early afternoon. A cold front slides
through this evening/tonight.

Confidence unfortunately a bit lower than would like being
inside 24 hours, but there is the risk for isolated strong to
severe thunderstorms today/tonight. Main questions at this point
revolve around this earlier pre-frontal trough and if it has
enough juice to trigger showers/storms earlier today. If this
happens what will be the impacts for the activity pushing in
late this afternoon/tonight with the cold front. Details below.

Late morning/early afternoon....

Overall guidance in pretty good agreement on the pre-frontal
trough sliding in late this morning/early afternoon. Guidance
split on if there is enough low level moisture and how unstable
the environment will be as this feature is moving through. There
is also little run to run consistency amongst some CAM, which
somewhat makes sense given the weak-ish forcing. The dry
solutions are the latest GFS/FV3/HRRR (capped with slightly
lower dewpoints), while the ECMWF/GDPS/ARW and NSSLWRF depict a
line of activity developing earlier in the afternoon. The
NAM/NAMNest not as robust with this line as the aforementioned
pieces of guidance, so is essentially the compromise between the
two. Leaned more toward the majority of guidance at this point,
but have only capped us at slight chances heading into the
afternoon due to uncertainty. Should see a tongue of a few
hundred to perhaps 1500 J/kg of MLCAPE along with 7-9 degree low
level lapse rates. There are also hints that there would be a
lingering remnant EML overhead in line with this activity.
However, deep layer shear (0-6 km bulk shear) a bit more meh in
the 20-30 kt range. Any of this activity could disrupt or
enhance anything that develops later today/tonight, so will be
interesting to see what happens.

Afternoon through tonight...

Risk higher for more widespread showers and thunderstorms
across southern New England during this timeframe, especially
late this afternoon/tonight. Any early day activity could
potentially work over the environment, but guidance tends to
struggle with this (though latest NSSLWRF does show this well).
Main question will be can the front arrive quick enough that it
can really tap into waning instability/increasing stability as
we head into the evening. Have leaned on the consensus of
guidance at this point. This brings fairly widespread showers
and thunderstorms to the region. Could have a few hundred to
perhaps 2000 J/kg of MUCAPE to work with. As the front
approaches will see deep layer shear increase to 30-50+ kts. The
mid level lapse rates become quite poor fairly quickly, but
still could see some elevated low level lapse rate until the
evening. Our PWATs also significantly increase to 1.5 to 2
inches and will have warm cloud layer depths of 3.5-4 km, so
heavy downpours will be possible with any storms. Fortunately
the system is fairly progressive, but latest HREF LPMM
indicating could have a few spots that see over 3 inches of
rain. Looks like heaviest would be late evening into the
overnight hours and could cause issues if it falls over some of
our more flood prone areas. Given this setup do think there
could be some strong to perhaps damaging winds as well.

Given the uncertainty during this timeframe think that SPC
Marginal Risk for severe suffices. Think that the WPC ERO
Marginal is also fine. Though will note for the severe risk
that the Nadocast and CSU ML still really heavily highlighting
a wind threat, maximized from CT into portions of the interior.
For timing the latest 00Z NCAR HRRR Neural Network really
highlights much of our region for a wind threat with the timing
being with the night activity. Could end up seeing more seeing
more scattered vs isolated risk if these ML guidance sources pan
out. Stay tuned as this is a tricky forecast and ultimately
will boil down to the mesoscale details.

Bumped up our high temperatures to the 75th percentile of
guidance given the westerly flow through the day should allow
for downsloping. The result are highs in the 80s for most, but
will have some spots in the low 90s across the Merrimack and CT
Valley. Muggy with dew points into the 60s, but not oppressive
enough where any heat advisories would be needed.

&&

.SHORT TERM /THURSDAY THROUGH THURSDAY NIGHT/...
* Any lingering showers/storms move offshore early in the day.
  Will be dry for most other than perhaps a spot shower across
  the MA/NH/VT border during the afternoon.

Caught in cyclonic flow through this period. A trough over our
area will lift offshore early in the day, but will have another
dig into northern New England during the afternoon. High
pressure will begin to nudge in once the cold front pushes
offshore during the AM.

Main focus of the forecast was in the near term. Only major
change made was to introduce a few spotty showers across the
MA/NH/VT border during the afternoon and try to better time the
AM activity exiting. Overall though for most will be a pretty
nice day with decreasing humidity levels, albeit it may be a bit
breezy at times. High temperatures will generally be in the 80s.

&&

.LONG TERM /FRIDAY THROUGH WEDNESDAY/...
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Highlights:

* Pleasant Friday with clear skies and low humidity levels.

* More active for the weekend, with periods of showers and t-storms
  associated with a slow moving frontal system. Heavy downpours
  appear to be the greatest risk, but it is uncertain if any
  storm(s) may become strong/severe.

* High pressure returns Monday and Tuesday.

Details:

Friday:

Canadian high pressure moves into New England on Fri, moving
offshore Fri night/early Sat. This will usher in a cooler and drier
air mass to Southern New England with full sunshine. Add this all up
and you get the makings of a spectacular Friday summer day, with
highs in the 70s to low 80s and dewpoints in the mid 40s to low 50s.
As high pressure moves offshore by the evening, moisture levels
start to increase again but at least these rises won`t be as
substantial as Sat; lows in the 50s to low 60s.

Saturday and Sunday:

Should be able to eke out a dry Saturday morning at least. But Sat
aftn into Sun evening is looking to be an active period of weather
as a surface frontal system embedded in a regime of seasonably
strong westerly flow aloft will interact with rich deep moisture to
favor showers and thunderstorms. Uncertainties in the forecast stem
from convective details which we can`t resolve at this time range,
as well as the timing, both in terms of when rain/t-storms start and
also when they end. There are indications on Sun into Sun evening
that the axis of a surface cold front becomes more parallel to the
mid/upper flow, which could lead to a slow egress offshore.

Despite the above uncertainties, there`s enough consensus in the
ensembles to indicate PoPs increasing to Likely-Categorical range
Sat night into Sun, then a slow decrease from NW to SE for Sun
night. PWAT values rise significantly up to 2-2.25", which are 2-3
standard deviations above late-June climatology per ECMWF EPS.
Unclear if we would see any storms produce severe weather - it may
end up being too cloudy to really destabilize significantly - but
heavy downpours and possibly hydro could be a concern at some point
in this period, especially if the front slows/stalls. Did boost QPF
a bit over NBM in this period to better reflect potential for heavy
rains, and while it don`t expect it to be raining all the time in
all areas, showers and t-storms producing downpours is a growing
risk for this weekend.

Temps are seasonable with readings in the upper 70s to mid 80s, but
both Sat and Sun look to be quite muggy with dewpoints rising into
the upper 60s to mid 70s.

Monday thru Wednesday:

After what is looking like a soggy weekend, early next week looks
drier/lower humidity levels as we get another infusion of Canadian
high pressure. Both days should feature clear skies with seasonable
temps and modest northerly breezes.
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&&

.AVIATION /19Z WEDNESDAY THROUGH MONDAY/...
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Forecast Confidence Levels:

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

Through 00z... Moderate confidence.

Generally dry afternoon with VFR CIGs, the low chance of an
isolated and brief shower between 20z-23z across the CWA. Winds
are south to southwest between 10 and 20 knots with gusts upwards
of 20 to 30 knots this afternoon.

Areas of coastal fog and stratus have developed south of the
islands and are propagating northeast with IFR CIGs. Expecting
this to expand across the Cape and Islands. Do have lower
confidence in how expansive this area becomes and duration.

Tonight... Moderate confidence.

Risk higher for scattered showers and storms where conditions
deteriorate to MVFR/IFR with localized LIFR. Some storms could
be strong and produce heavy downpours between 00z and 06z. TAFs
indicate this hazard with PROB30. Winds ease to between 5 and 10
knots, but will quickly shift to the west from west to east as
the cold front slides through (though may not be until Thu AM
for Cape terminals).

Thursday...High confidence in trends, moderate in exact timing.

Any lingering MVFR/IFR improves to VFR between 12z and 15z, ACK
could linger below VFR through 18z. Winds out of the west
between 5 and 10 knots with gusts of 15 to 20 knots.

Thursday Night... High Confidence.

VFR, dry, northwest to north/northwest winds 5 to 10 knots.

KBOS TAF...Moderate confidence in TAF.

VFR for much of today. Could have an isolated shower/storm early
this afternoon, but risk higher tonight where conditions
deteriorate to MVFR/IFR. Have tried to highlight best shot with
PROB30. Winds remain too strong out of the SW for sea breeze to
develop today.

KBDL TAF...Moderate confidence in TAF.

VFR for much of today. Could have an isolated shower/storm early
this afternoon, but risk higher late this afternoon/tonight
where conditions deteriorate to MVFR/IFR. Have highlighted best
shot for thunderstorms with PROB30.

Outlook /Friday through Monday/...

Friday through Friday Night: VFR.

Saturday: Mainly VFR, with local MVFR possible. Breezy. Chance
SHRA, isolated TSRA.

Saturday Night: Mainly MVFR, with areas IFR possible. Breezy.
SHRA, isolated TSRA.

Sunday: Mainly MVFR, with local IFR possible. Breezy. SHRA,
isolated TSRA.

Sunday Night: VFR. Breezy. Chance SHRA, isolated TSRA.

Monday: VFR.
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&&

.MARINE...
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Forecaster Confidence Levels:

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

230 PM update...

* SCA remains in effect thru tonight for southern waters

This evening & overnight...

SW winds 15-20 kt gusting up to 30kt at times. Mainly dry
weather thru the daylight hours, then showers and thunderstorms
over western PA/NY this afternoon, impact RI/MA waters this
evening and overnight. Frequent lightning and heavy downpours
with poor vsby are the main concerns. Secondary concerns is
strong winds.

Thursday..High confidence.

Cold front enters the waters, which will shift winds to W
during the AM along with improving vsbys. However, the front
stalls just offshore. Seas across the outer waters diminishing
to 2-4 ft during the afternoon.

Thursday night...high confidence.

Cold front just offshore finally accelerates out to sea, with
winds becoming NW 15-20 kt, highest across the eastern MA
waters. Dry weather and good vsby.

Outlook /Friday through Monday/...

Friday through Friday Night: Winds less than 25 kt.

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

Saturday Night: Moderate risk for Small Craft Advisory winds
with gusts up to 25 kt. Seas up to 5 ft. Chance of rain
showers, isolated thunderstorms.

Sunday: Moderate risk for Small Craft Advisory winds with gusts
up to 25 kt. Seas up to 5 ft. Rain showers, isolated
thunderstorms.

Sunday Night: Winds less than 25 kt. Local rough seas. Chance
of rain showers, isolated thunderstorms.

Monday: Winds less than 25 kt. Areas of seas approaching 5 ft.
Slight chance of rain showers.
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&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...High Risk for Rip Currents until 8 PM EDT this evening for
     MAZ020>024.
RI...High Risk for Rip Currents until 8 PM EDT this evening for
     RIZ006>008.
MARINE...Small Craft Advisory until 8 PM EDT this evening for
     ANZ231>234-236.
     Small Craft Advisory until 8 AM EDT Thursday for ANZ235-237.
     Small Craft Advisory until 5 PM EDT Thursday for ANZ254>256.

&&

$$

SYNOPSIS...Loconto/Dooley
NEAR TERM...Belk/BL
SHORT TERM...Nocera
LONG TERM...Belk/Loconto
AVIATION...Loconto/Dooley
MARINE...Loconto/Nocera/Dooley