-- Discussion containing changed information from previous version are highlighted. --
262
FXUS61 KBOX 270310
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
1110 PM EDT Sun May 26 2024

.SYNOPSIS...
Downpours and thunderstorms persist through early evening.
Widespread shower activity tomorrow ahead of approaching cold
front that will bring renewed heavy rain and thunder chances to
SNE. Cold front moves offshore early on Tuesday, with decreasing
clouds and drier weather conditions. A cooler more seasonable
air mass then builds into Southern New England for the remainder
of the workweek into much of the weekend. Mainly dry weather
prevails as well, though there is a chance for rains on
Wednesday night.

&&

.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
-- Changed Discussion --
Convection had finally moved into northern New England late
this evening, taking it away from from the most favorable
environment per latest SPC mesoanalysis. Expecting a bit of a
relatively quiet period for a few hours.

The last several runs of the HRRR have been quite aggressive in
redeveloping convection after midnight into Monday morning.
However, it seems to be alone with that outcome. Stratus has
overspread that portion of the coastal waters and extended north
across RI and much of southeast MA. Having a hard time accepting
that outcome, but will be wary for the possibility overnight.
Maintained a mainly dry forecast until just before daybreak,
which is when a slow-moving front gets closer to our region.

Patchy fog is also expected, but without the widespread
visibility reduction to less than 1/4 mile, have held off on a
Dense Fog Advisory at this time.


710 PM Update:

Widely scattered and slow-moving heavy showers extend along a
narrow axis of surface wind convergence from roughly Marlborough
to near Amherst to Worthington MA in a regime of weak to
moderate instability (about 500-1500 J/kg of surface-based
parcel CAPE). We haven`t seen any lightning occur in any of
these heavier showers, and seems that shortwave ridging aloft
may be stunting deeper growth as radar all-tilts trends show
storm tops to around 25,000 ft but no higher. Nearing sundown,
waning instability, stabilizing PBL and the rain-cooling effect
over and in vicinity of these heavier showers as they rain
themselves out should cause activity to diminish by 9 PM if not
sooner.

Meanwhile to the south, starting to see the maritime airmass
advance northward, with layer of stratus expanding into eastern
CT, much of central and southern RI and most of southeast MA per
visible imagery. Expect stratus and areas of fog to develop and
expand north and west through the evening and overnight,
expected to take place as soon as temperatures cool off into the
low-mid 60s. It`s still a bit debatable if we`ll see fog -
BUFKIT hydrolapse profiles seem to indicate stratus as being
more favored than fog - but did include that in the forecast as
winds should go light. Removed mention of PoP for the
evening/overnight, think even drizzle could be hard to come by
as moisture layer is very shallow.

Previous discussion:

Downpours have begun to fire along the seabreeze boundary across
the far southern Hudson River Valley with a few rain showers
cropping up along the same boundary across northern CT and
central RI. Fortunately, storms are forming quickly but
transitioning to "orphan anvils" within a few scans, suggesting
our forecast of mainly sub severe storms is on track.


While storms remain innocuous as of this writing (at 19Z), do
continue to anticipate some slow moving strong storms to develop
along the I-90 corridor and perhaps south into northern Bristol
and Plymouth counties. 12Z CAMs continued to show initiation
"prime time" around 21Z with storms persisting through 00-01Z
this evening before subsiding quickly after sunset as
instability wanes. As for instability, models continue to
support 1500 J/kg SBCAPE combined with decent low and mid level
lapse rates of 7.5-8C/km and 6.5-7C/km respectively. Bulk
shear, while lackluster, will increase to between 30-40kt along
the MA/NH border later today, which will likely keep storms on
life support this evening.

Pockets of heavy rain continue to be the greatest threat
associated with storms this afternoon with PWATs around 1.25"
and weak flow aloft to keep storms moving. In general, many
communities will remain dry, but those under a potent cell could
see upwards of 1 to even 2" of rain, which may lead to some poor
drainage flooding especially if storms set up in the Boston
metro.

As mentioned, storms wane quickly after sunset but with easterly
flow, marine stratus and fog will expand from the nearshore
waters to at least the coastal plain. Places like the Cape and
Islands can expect to see dense fog, comparative in coverage to
last night. Prevailing wind direction should support more
expansive fog coverage overnight that may overspread inland to
the I-95 corridor.
-- End Changed Discussion --

&&

.SHORT TERM /6 AM MONDAY MORNING THROUGH MONDAY NIGHT/...
-- Changed Discussion --
Low pressure over the Great Lakes continues east-northeast into
Ontario and Quebec as mid level trough digs into the Ohio River
Valley and mid-Atlantic. Warm front lifts from the mid-Atlantic
towards New England tomorrow but will fail to lift of ~central
NJ by mid day tomorrow. While we remain north of the warm
sector, will still have some prefrontal showers beginning as
early as 12Z across the western portions of the CWA before
tracking east through early afternoon with rain holding off in
places like Boston and Providence through around 15/16Z. Cold
front will fuel more potent line of rain after 20Z, with the 12Z
guidance speeding up the front`s arrival and departure by
several hours compared to last evening`s guidance; in fact, had
to stray significantly from the NBM given it`s know lag time to
derive PoPs through early Tuesday morning.

The greatest concern with tomorrow`s forecast will be the threat
for additional heavy rainfall, with PWATs climbing to near 2"
by late tomorrow afternoon and a decent southerly oriented LLJ
of between 30-40kt. Could certainly see some upslope enhancement
across the Berkshires and Worcester Hills, but fortunately the
jet lacks an easterly component that would really "ring out the
rain". HREF PMM and 24 hour MAX QPF ending early Tuesday morning
depicts local max QPF of 1.5-2" across the aforementioned
terrain features, with lesser amounts elsewhere. Warm cloud
depths near 12,000ft combined with very high PWATs should draw
one`s attention to the potential for tropical-like downpours
tomorrow afternoon, but the progressive nature of the front
should limit any flooding concerns to just poor drainage areas,
which is corroborated by WPC placing the entirety of the region
under just a "marginal" excessive rainfall outlook. Given a lack
of evidence pointing to widespread urban or flash flooding, did
not consider a flood watch with this package issuance.

Convective elements will likely be present, particularly along
the cold front, with tomorrow`s rain with modest instability
of 200- 600J/kg CAPE. With that said, convection looks to be
elevated, with really poor low level lapse rates below 5C/km.
Hodographs look quite curly tomorrow, but with no surface based
convection, the severe weather potential remains low. The one
saving grace for more prolonged convective showers would exist
due to bulk shear values approaching 50kt late tomorrow evening,
which may result in some locally gusty winds.

As previously mentioned, cold front should be east of the area
between 06-09Z Tuesday, which will lead to rapidly improving
conditions by sunrise Tuesday. Winds will shift from the E to
the south behind the front, which combined with residual
moisture, may drive another night of coastal fog.
-- End Changed Discussion --

&&

.LONG TERM /TUESDAY THROUGH SUNDAY/...
-- Changed Discussion --
Highlights:

* Cold front moves offshore on Tue with decreasing rain chances
  by later Tue AM and temps warming into the 70s to low 80s.

* Temps cool off to more seasonable levels starting Wed into the
  weekend.

* Other than a chance for rain on Wed night into Thurs, much of
  the rest of the workweek into the early weekend are generally
  dry.

Details:

Cold front from Monday will continue to move offshore during the
morning hrs of Tuesday. There might still be some steadier rain
during the morning commute in eastern and southeast New England
but improving conditions are expected by later-morning, with
decreasing clouds allowing for temps to warm into the 70s to low
80s, with upper 60s/near 70 for the Cape and Islands.

A deep longwave trough becomes established over the Northeast
states as we move through the rest of the workweek into the
early part of next weekend. This trough will bring cooler than
normal 925-850 mb temps to Southern New England, bringing temps
closer to late- May/early-June climatological values (upper 60s
to lower to mid 70s). Though it is a generally dry weather
regime, we do have to watch around Wed night into Thurs for
chances for showers as an embedded shortwave disturbance aloft
rotates around the trough. There are differing answers as far as
the placement and track of this shortwave trough, with the ECMWF
the most bullish on bringing steadier rains to SNE for Wed night
into Thurs, with the GFS interestingly being dry while the
Canadian GEM too far south for any significant precip. NBM
guidance was showing 25-40% PoP for rains and opted to stay
close to that until there`s better agreement on the shortwave
trough. Other than the Wed night into Thurs timeframe, overall
it looks to be a decent stretch of weather. Our next chance of
rain may not arrive until very late Sunday or into next Mon.
-- End Changed Discussion --

&&

.AVIATION /03Z MONDAY THROUGH FRIDAY/...
-- Changed Discussion --
Forecast Confidence Levels:

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

Tonight: High confidence in trends, though moderate on timing.

IFR to VLIFR fog and stratus expands north and west as the
evening/overnight progresses. IFR to VLIFT across RI and SE MA
through 12Z. IFR across northern CT into the CT River valley.
Light E to SE winds.

Tomorrow... High Confidence in Trends

IFR/LIFR for most. Scattered showers develop between 12-16Z.
Lull in shower activity expected from 16-19Z or so from west to
east. More robust line of rain associated with a cold front
develops after 19Z. Some embedded Thunder possible with the
second round of rain. Winds remain from the east, may gust as
high as 20kt tomorrow afternoon.

Tomorrow night... Moderate Confidence in timing

Rain will come to an end between ~03-06Z yielding improving
conditions with improvement from IFR to VFR around sunrise.
Winds shift from the E to the S, gusting to 20kt, behind frontal
passage.

KBOS TAF...High confidence in TAF trends, moderate in timing.
VFR initially, though IFR to LIFR stratus develops around 03z
and continues at least through the early afternoon. Guarded
optimism for one- category improvement in the early afternoon
but this isn`t certain; however rain then arrives later in the
day and especially Mon evening. Light E winds become SE 4-8 kt
tomorrow.

KBDL TAF...High confidence in TAF trends, moderate in timing.
VFR initially, though IFR-LIFR stratus develops around 03z and
continues at least thru early afternoon. Could see one-category
improvement to IFR-MVFR range but as that occurs, increasing
chances for steady rain. S winds tonight becoming SE and
increasing to around 10 kt.

Outlook /Tuesday through Friday/...

Tuesday: Mainly VFR, with local IFR possible. Breezy. Slight
chance SHRA.

Tuesday Night: VFR.

Wednesday through Thursday: VFR. Chance SHRA.

Thursday Night through Friday: VFR.
-- End Changed Discussion --

&&

.MARINE...
Forecaster Confidence Levels:

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

Through Memorial Day... Moderate Confidence in fog.

Another night of dense marine fog expected as winds prevail from
the east. Increased shower activity early Monday before a cold
front brings the chance for more potent rain and embedded
thunder to the waters late tomorrow evening and early Tuesday
morning. Winds increase, gusting between 25-30kt from the east
before turning south behind the front. Waves climb in excess of
5 feet across the outer waters. SCA has been hoisted for the
eastern outer, and southern waters, including Nantucket sound
late tomorrow/tomorrow night.

Outlook /Tuesday through Friday/...

Tuesday: Winds less than 25 kt. Seas up to 5 ft. Chance of rain
showers, patchy fog.

Tuesday Night through Wednesday: Winds less than 25 kt. Seas up
to 5 ft.

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

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

Thursday Night: Winds less than 25 kt. Seas locally approaching
5 ft.

Friday: Winds less than 25 kt.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory from 9 PM Monday to 11 AM EDT Tuesday for
     ANZ232-233-235-237.
     Small Craft Advisory from 6 PM Monday to 5 AM EDT Wednesday
     for ANZ250-254.
     Small Craft Advisory from 9 PM Monday to 5 AM EDT Wednesday
     for ANZ255-256.

&&

$$

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