562
FXUS61 KBOX 252341
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
741 PM EDT Sat May 25 2024

.SYNOPSIS...
Decaying MCS brings shower and isolated thunder chances to SNE
overnight. Potential for stronger to marginally severe thunderstorms
Sunday afternoon, especially north of the I-90 corridor. Cloudy
and somewhat humid during the morning hours of Memorial Day,
although a frontal system brings increasing chances for rain
with embedded thunderstorms Monday afternoon into early Tuesday.
Localized downpours are possible. Temperatures then trend
cooler than normal as we move into the mid to late week period,
and though dry weather prevails, there is a risk for showers
either Wednesday or on Thursday

&&

.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
720 PM Update:

Stream of high clouds which moved in during the later-aftn hours
has now started to shift offshore with fairly tranquil weather
conditions as we are nearing sundown.

Couple things to talk about for the evening and overnight
period. First is the area of showers associated with a weakened
MCS cluster over central NY through southern PA. The northern
roughly third of this area continues to weaken per radar trends
and warming cloud top temperatures; this area of showers still
looks to advance east during the mid evening to the pre-dawn
hours, although the risk for thunder with this area of showers
has dwindled. Second and finally is the potential for stratus
and/or areas of fog along the south coast of RI and MA and
perhaps a bit further north. Southerly flow should bring rising
dewpoints into the middle 50s as temps fall sharply after
sundown along the south coast. While the risk for fog is by no
means imminent yet, a number of model visby progs show low
visbys developing after 04z Sunday both in the waters and
adjacent portions of the RI/MA South Coast. Further landward
expansion of fog or stratus seems likely into the Plymouth to
Providence to Willimantic corridor but that should delineate the
northern extent. Introduced fog into the forecast with this
update, though the modest southerly breezes could favor more
stratus than fog.

No changes to temps which still look on track. The biggest
perceived change for those outdoors tonight will be a modest
rise in dewpoints but should still be pretty tolerable.

Previous discussion:

Weak high pressure over southern New England this evening eventually
shifts seaward, allowing a weak low pressure/decaying MCS to move in
from the west between around 04Z. Some minor to modest, 200-1000J/kg
CAPE if you buy the NAM who`s dewpoints are running a bit high,
instability will accompany light rain showers overnight, thus, it`s
quite possible a few rumbles of thunder/convective showers initiate
as elevated cold front/weakening MCS sweeps across the region.

Fog development is very likely across the Islands and coastal waters
overnight as surface flow shifts back to the S/SW. Questions
surround how far north/inland fog is able to advect given dewpoints
will take some time to climb into the upper 50s tonight, but its
quite possible portions of the south coast like Newport and New
Bedford are also impacted by locally dense fog by daybreak.
Southwest flow will limit the impact of fog across the interior.

&&

.SHORT TERM /6 AM SUNDAY MORNING THROUGH SUNDAY NIGHT/...
Morning showers and isolated convection should wrap up quickly after
sunrise, but a more stable environment fueled by increased cloud
cover and a surface inversion should yield a period of dry weather
from about 12-18Z Sunday.

The concern for tomorrow is the possibility of afternoon convection
given SBCAPE exceeding 1500J/kg, 0-6km bulk shear nearing ~30kt, low
level lapse rates exceeding 8C/km, in pockets of the region, and mid
level lapse rates, per model soundings, approaching 7C/km. While
these numbers don`t "jump off the page" in alerting for
widespread severe weather tomorrow, there is certainly
potential for a few isolated strong storms, particularly north
of the MA Turnpike between 19-01Z where the above ingredients
can come together. The main concern with any marginally severe
storms would be straight line wind, with DCAPE values
approaching 850J/k combined with the  mid level lapse rates
exceeding ~6.5C/km and near surface dewpoint depressions of
~15F. With lackluster 0-1km SRH of ~60-80m2/s2 and freezing
levels around 12,000ft, not anticipating tornados or large hail
will be a concern but will note, that while any hail that falls
will be well below severe criteria, we can`t rule out some pea
sized hail in the strongest cells as CAMs indicate some brief
periods of updraft helicity swaths exceeding 75m2/s2 combined
with healthy CAPE in the hail growth zone above -10C.

Surface inversion that will take quite awhile to mix out, as late as
17Z, as well as a modest rise in heights tomorrow afternoon are
two of the greatest factors limiting widespread convection
tomorrow. Storms will struggle to initiate across much of RI,
SE MA, and central CT given seabreeze development and a more
stable airmass along the coast to ~20miles inland; while the sea
breeze boundary will mitigate the convective threat for the
coast, it may also act as a match in initiating convection
across the interior. Will note that while its likely we see some
isolated strong to perhaps marginally severe storms tomorrow
afternoon, particularly across central and northern
Massachusetts, it`s possible we see little to no convection at
all if the "thunderstorm ingredients" shift north; notably
depicted in the 12Z NAM Nest that keeps convection north of the
MA/NH border tomorrow afternoon.

In addition to the isolated marginal severe threat, convective cells
may be able to drop quite a bit of rain should they remain nearly
stationary. Increasing dewpoints, into the low 60s, as well as PWATs
in excess of 1.25" and warm cloud depths approaching 11,000ft will
likely fuel isolated downpours. Of note, the FV3 shows isolated
pockets of 3-4"+ of rain between 18-00Z tomorrow. While the FV3
remains the outlier, and many localities will see no rain at all,
we would be remiss to neglect the potential of some cells
dropping an inch or more of rain in a short period of time.

Instability drops off quickly overnight with surface flow shifting
to the east. So, while the thunderstorm threat diminishes after ~03Z
Monday, scattered rain showers are possible overnight. As mentioned
previously, dewpoints in the upper 50s and low 60s will yield a warm
and muggy night, with lows struggling to dip to 60F region wide.

&&

.LONG TERM /MONDAY THROUGH SATURDAY/...
Highlights:

* Cloudy and somewhat humid for the AM hrs Memorial Day, then a
  frontal system brings better chances for widespread showers with
  embedded t-storms Mon aftn into early Tue. Heavy downpours
  possible, especially Mon evening.

* Trending drier for Tue with seasonable temps.

* Though drier weather generally prevails for mid to late week,
  still unsettled with chances for rains either Wed or Thurs. Temps
  trend cooler than normal for the second half of the upcoming
  workweek.

Details:

Memorial Day Holiday into Tuesday:

Looks to be a pretty active forecast, as a complex frontal system
with low pressure tracking through the Gt Lakes swings a cold front
or frontal occlusion through SNE in this period. Though there still
is some variance in the models when it comes to timing of rains,
guidance trends indicate cloudy but mainly dry weather (PoP < 20%)
during the Mon morning hours, then increasing during the afternoon.
Appears the biggest risk for widespread showers and embedded
thunderstorms is more focused during the Mon evening into early Tue
AM, with Categorical PoPs shown for that period.

As indicated by prior forecasts, the greatest potential hazard is
localized heavy rain and this risk appears to be the greatest during
the sundown to evening/overnight hours. Due to a pretty robust
southerly LLJ, deep moisture increases with PWAT standard deviations
of 2 to 3 sigma centered around Mon evening, with warm cloud depths
around 11,000 ft. Both are factors which tend to favor heavy
downpours; even though convective instability is pretty limited, it
is a "tall/skinny" type of CAPE profile we tend to see in heavy rain
events. Did note that despite the weak instability progged, we do
have some diffluence around 500 mb Mon evening which could lend
support to some embedded t-storms in light of the weak instability.
NBM 4.2-based QPF probs indicate medium (30-60%) probs for 24 hr
rains over 1 inch, although the EPS and GEFS probs of 24-hr rains
are a bit more bullish, and the 24 hr probs of 2" or greater
rainfall totals in 24 hours are low to zero. QPF amts were around an
inch or less on an areal-average basis, but localized higher totals
are possible wherever persistent or re-developing downpours
develop. Coming out of a very wet cold season, we`ve been pretty
dry of late with one significant rain event on May 23rd, and
NERFC Flash Flood Guidance is pretty high due to that dry
weather spell (about 2 inches needed in an hour). Did include a
mention of heavy rain possible in the forecast grids given the
above ingredients, and that could lead to scattered hydrologic
issues, but would like to see QPF amounts be a bit higher before
considering any Watches. It`s a much smaller threat in light of
the paltry instability, but the one thing we may have to keep
awareness of is that there could be some stronger storms over
eastern/southeast MA and RI. Due to the southerly low-level jet
around 30 to 40 kt and surface winds from the SE to SSE,
hodographs in southeast New England show a curvature to them.

So for Memorial Day itself, most of the morning hrs should be dry
but won`t be ideal with lots of cloud cover. Rain chances then
increase into the afternoon for areas N/W of I-95, and toward later
afternoon in eastern MA and RI; highs in the lower 70s with
dewpoints in the low to mid 60s adding a touch of mugginess to the
air.

It looks like the front moves offshore on Tue afternoon, and while
there could be some lingering showers early on Tue in southeast New
England, most areas should trend drier by Tue aftn. Kept highs on
Tue a little cooler than NBM out of deference to some of the slower
frontal timing guidance in the 70s to around 80 (mid/upper 60s Cape
and Islands), though an earlier passage would favor temps closer to
NBM`s lower to mid 80s.

Wednesday through Friday:

SNE`s weather in this period becomes governed by broad troughing
aloft with cooling temps aloft, which is pretty well agreed upon
across the guidance on the larger-scale. There is at least one
shortwave disturbance rotating around the broader trough which could
bring a better opportunity for precip but unclear if that would be
more centered on Wed or Thurs, so kept a 20-40% PoP mention for both
days until that becomes more in focus. Though it should be drier
more often than not, temps trend near to cooler than normal as 850
mb temps drop down into the mid single digits C range.

&&

.AVIATION /00Z SUNDAY THROUGH THURSDAY/...
Forecast Confidence Levels:

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

00Z TAF Update:

Tonight: High confidence.

VFR from BDL to BOS northward; period of unrestricted SHRA
possible from 04-08z until about 10-12z associated with
weakening showers now over central NY. For the South Coast and
PVD, will start VFR until about 04-05z, then start to see fog
and stratus develop at IFR to VLIFR levels, which will tend to
expand northward to the IJD-PVD-TAN-PYM corridor. Winds light
southerly.

Sunday...Moderate confidence in Thunderstorms.

Coastal IFR dissipates by 15/16Z, with generally VFR conditions
expected. Localized MVFR possible in afternoon and early evening
thunderstorms, most likely along and north of the I-90 corridor
after 20Z. PROB30 groups introduced into the TAF to account for
uncertainty in thunderstorm coverage during the late afternoon.

Localized sea breezes developing across eastern MA after
17-19Z.

Sunday night...

Winds become east overnight which will generate reduced
cigs/vsby IFF/MVFR for the high terrain of central and western
MA due to upsloping. Fog anticipated to be much less widespread
compared to Saturday night. Light hit or miss showers possible
at any location overnight.


KBOS TAF...Moderate Confidence in Thunderstorm potential on
Sunday. VFR but with passing generally unrestricted SHRA
08-12z. Uncertainty in possibility of thunderstorms at the
terminal tomorrow afternoon, so introduced PROB30 between
20-23Z. Seabreeze today dissipates around sunset, tomorrow`s
seabreeze develops around 16/17Z.

KBDL TAF...Moderate Confidence in Thunderstorm Potential. VFR
with light showers later tonight into overnight (04-08z).
Better thunder chances late Sunday after 19Z but storms will
likely remain north of the terminal. Winds generally from the
south.

Outlook /Monday through Thursday/...

Memorial Day: MVFR/IFR conditions possible. Breezy. Chance
SHRA, slight chance TSRA.

Monday Night: MVFR/IFR conditions possible. Breezy. SHRA,
chance TSRA.

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

Tuesday Night: VFR.

Wednesday: VFR. Chance SHRA.

Wednesday Night through Thursday: Mainly VFR, with local MVFR
possible. Chance SHRA.

&&

.MARINE...
Forecaster Confidence Levels:

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

18Z Update...

Through Sunday Night... Moderate Confidence in fog.

Dense fog possible for the waters tonight as winds shift to the
S/SW and dewpoints rise to near 60. Low risk for showers and a
few thunderstorms both tonight and tomorrow afternoon, Sunday
afternoon, mainly across the eastern coastal waters. A few
rumbles of thunder are possible with either round of rain.

Fog less likely Sunday night as winds shift to the E/NE. Seas
remain between 1-3ft through the entire period.

Outlook /Monday through Thursday/...

Memorial Day: Winds less than 25 kt. Chance of rain showers.

Monday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Seas up to 5 ft. Rain showers, slight chance
of thunderstorms.

Tuesday: Winds less than 25 kt. Seas up to 5 ft. Rain showers
likely, isolated thunderstorms.

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

Wednesday: Winds less than 25 kt. Seas up to 5 ft. Slight
chance of rain showers.

Wednesday Night through Thursday: Winds less than 25 kt. Areas
of seas approaching 5 ft. Slight chance of rain showers.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...None.

&&

$$

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