725
FXUS61 KBOX 251129
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
729 AM EDT Sat May 25 2024

.SYNOPSIS...
Warm and mostly dry conditions this weekend, but a few afternoon
showers or thunderstorms are possible Sunday. Frontal system
will bring in widespread showers, especially late Monday into
early Tuesday. Significant, heavy rainfall possible. Turning
drier, milder and breezy on Tuesday. Unsettled with more hit-or-
miss showers possible mid to late in the week with near to below
normal temperatures.

&&

.NEAR TERM /THROUGH TONIGHT/...

720 AM Update...

No changes in the latest update. Do want to mention as it is in
the forecast that there could be some decaying showers and
perhaps a storm or two due to an MCs heading into the interior
tonight. Guidance is all over the place with what happens with
the activity that fires across NY State later today with the FV3
being the most vigorous for our area and remaining CAMs much
more muted. Current forecast reflects the more muted scenario,
which seems most plausible at this juncture, but it isn`t a slam
dunk. Lastly, could see some stratus/fog pushing into the south
coast late tonight with the return of moist southerly flow. For
now thinking will be more of an issue for MVY, ACK and the Cape
before daybreak.

Previous discussion...

High pressure will be in control of our weather today, finally
pushing a weak cold front offshore later this morning. Slightly
cooler today with a weak N to NW flow. This weaker flow will
lead to sea breeze development along both coasts, making it even
cooler there.

&&

.SHORT TERM /SUNDAY/...

Weak high pressure still in place to start tonight. However,
winds just above the surface are expected to switch to the SW as
a weak low pressure moves across northern New England late
tonight into Sunday. This will place our region on the warmer
and more humid side. While there is a low risk for showers or
thunderstorms as the night progresses, the risk increases with
daytime heating Sunday, especially across the interior during
the afternoon.

Most unstable CAPE values are projected to be 1,000-1,500 J/kg,
which could lead to a few stronger thunderstorms. However, model
soundings also depicted a weak mid level inversion between
700-500 mb. Thus, this potential may not be able to be realized.
Overall shear values are also projected to be 15-20 m/s, which
do not suggest widespread sever weather is a concern. Hodographs
should be rather linear, pointing to gusty straight-line winds
and perhaps some hail in the strongest thunderstorms.

Increasing dew points tonight into Sunday, with precipitable
water values projected to peak between 1.2-1.5 inches. While
overall rainfall totals are likely to be less than one quarter
inch for most locations, we cannot dismiss the idea of locally
heavier rainfall from any thunderstorms.

Above normal temperatures continue.

&&

.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...

Highlights

* Unsettled with rain showers Mon into Tue. Rain may be heavy at
  times along with embedded thunder late Mon into early Tue.
  Temps returning to more seasonable levels.

* Drying out briefly Tue into early Wed.

* Turning unsettled once again mid to late in the week with hit-
  or-miss showers. Temps near to below normal.

Memorial Day through Tuesday...

As mentioned yesterday this is our next shot for widespread
showers and potentially impactful weather. Still have a complex
upper level pattern with a cutoff low over Manitoba/Ontario and
a couple shortwaves in the vicinity of the Great Lakes region.
These troughs/shortwaves rotate into the eastern Great Lakes on
Mon. The deepest of the troughs lifts in by late Mon. This lifts
into northern New England by early Tue while another shortwave
lifts into northern New England later on Tue. A frontal system
lifts into and slides through southern New England during this
timeframe.

Given the complex setup confidence lower than typical,
especially on the Tue timeframe. As mentioned previously a dry
slot punches in, which would will act to erode our precip
heading into Tue. Though the exact timing of this is still
tricky this far out, but signals are there that it will be a
pretty nice Tue. Ahead of that dry slot there is the potential
for strong winds and heavy downpours.

Strong wind risk...

Still have an anomalously strong low level jet impinging on our
region. The latest NAEFS and EPS guidance indicate winds of
2.5-3.5 STD above model climatology at 850 hPa. At this point,
looks like a 30-50+ kt S to SW/W low level jet slides through.
The timing appears that it occurs late Mon into early Tue. The V
(north-south) component of the wind continues to increase with
anomalies 3-4 STD above model climo. As mentioned yesterday, am
really not sure how much of these winds will be able to mix
down. The peak timing will be at night and both the NAM/GFS
showing we are pretty inverted. Give the anomalous nature of the
winds and the embedded thunder potential don`t want to
completely rule out the strong wind risk, but am more concerned
with the heavy rain threat.

Heavy rain/thunder risk...

The S/SW low level jet impinging on the region will pump in
anomalously high moisture values. Should see PWATs appearing to
increase to 1-2 inches. The NAEFS/EPS are more muted, but
continue to increase from run to run and are now at 1.5-1.75
inches, which is 2-3 STD above model climo. LOoks like there
will be a few hundred J/kg of MUCAPE as the LLJ is sliding
through and warm cloud layer depths of 3.5-4 km. The result
should be efficient rainfall processes. Given the setup and
these values the WPC Marginal Excessive Rainfall Outlook still
suffices, but will need to watch how things evolve closely. The
one kicker in the heavy rain risk is that dry slot, which from
past experience tends to come in faster than forecast guidance
indicates.

Ensembles continuing to show some spread. The GEFS still not as
amped up for the heavy rain risk with nil probs of 24 hr QPF AOA
1 inch and low to mod probs (10-40 percent) of 24 hr QPF AOA 0.5
inches. The EPS/GEPS more in line with a heavier rain risk as
there are low to mod probs of 24 hr QPF AOA 1 inch, with the
risk greatest across the south coast and western MA. Both
GEPS/EPS also showing some low probs of 24 hr QPF AOA 2 inches.
Given the discrepancy have just stuck with WPC for QPF for now,
but heaviest precip would be late Mon into early Tue.

Will also note that there is roughly 30-50 kts of 0-6 km bulk
layer shear in place. The low/mid level lapse rates are quite
poor. Given the GFS/NAM inverted profiles am thinking will be
tough for any stronger storms, but CSU MLP and CIPS Analogs
still indicating some low probs for strong/severe storms. Stay
tuned!

Given the cloudiness on Memorial day should see temps around
seasonable levels, which is the generally the mid 60s to mid
70s. Looking drier, mild and breezy on Tue. Temps should rebound
into the 70s for most, but there may even be some low 80 degree
readings across the Merrimack and CT Valley.

Wednesday through Friday...

Caught under cyclonic flow through this timeframe with a cutoff
nearby or a trough overhead. Still unsettled through this
period, but with considerable uncertainty on how things evolve,
especially given the blocky pattern.

Due to the high uncertainty in this period have just stuck with
the NBM. This keeps slight chances to chances of precip, which
suffices for now. Temperatures still around seasonable levels on
Wed, but we turn cooler as we head late into the week.

&&

.AVIATION /11Z SATURDAY THROUGH WEDNESDAY/...

Forecast Confidence Levels:

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

Today...High confidence.

VFR. Will have light/variable winds to start for most, but will
shift to the S/SW with speeds of 5-10 kts. Localized sea breezes
developing on the early-ish side today around 14-16Z.

Tonight...High confidence away from the immediate south coast
where confidence is moderate.

Generally VFR, but could have IFR/LIFR stratus/fog after 06Z for
ACK and the Cape/Islands. More confident in ACK at this point
given winds turn southerly. Could see some spotty showers
perhaps a rumble of thunder across the interior, but confidence
low as will depend on how things evolve in NY State this
afternoon. Winds out of the S at 10 kts or less.

Sunday...Moderate confidence.

Generally VFR, but will have spotty areas of MVFR to perhaps IFR
in the hit-or-miss showers/storms. Best shot for this activity
is during the afternoon. Winds out of the S/SW at 10 kts or
less. Localized sea breezes developing across eastern MA after
17-19Z.

KBOS TAF...High confidence in TAF.

KBDL TAF...High confidence in TAF through the afternoon.
Moderate for tonight as there could be a few decaying showers
moving in. Low risk that there is a rumble of thunder.

Outlook /Sunday Night through Wednesday/...

Sunday Night: Mainly VFR, with local IFR possible. Slight
chance SHRA.

Memorial Day: Mainly VFR, with local IFR possible. Chance SHRA.

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

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

Tuesday Night: VFR. Breezy.

Wednesday: VFR. Slight chance SHRA.

&&

.MARINE...

Forecaster Confidence Levels:

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

Through Sunday...High Confidence.

Relatively light winds and seas expected across the waters
during this time. Good visibility expected as well. Low risk for
showers and a few thunderstorms Sunday afternoon, mainly across
the eastern coastal waters.

Outlook /Sunday Night through Wednesday/...

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

Sunday Night: Winds less than 25 kt.

Memorial Day: Winds less than 25 kt. Slight 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, chance of
thunderstorms.

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

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

Wednesday: 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...Belk/BL
NEAR TERM...Belk/BL
SHORT TERM...Belk
LONG TERM...BL
AVIATION...BL
MARINE...Belk/BL