-- Discussion containing changed information from previous version are highlighted. --
668
FXUS61 KBOX 261751
AFDBOX

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

.SYNOPSIS...
Potential for stronger to marginally severe thunderstorms
this 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. Generally drier mid to late
in the week with cooler temperatures. Cannot rule out hit or
miss showers/storms on Wednesday and Thursday. Shower activity
more spotty as we head into the weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
930 AM Update...

Shower activity has come to an end across the mainland with just
a few straggling showers across the outer Cape. These showers
too will come to an end over the next half hour to an hour,
yielding a mainly dry mid-day as clouds break up across the
interior. Stratus/fog remain steadfast across Nantucket and will
likely be present all day given a southerly wind direction.

As noted by previous forecasts, do anticipate isolated to
scattered potentially strong, but sub severe, storms will start
to boil up around 3-4pm. Heavy rain may be an issue, leading to
some urban and poor drainage flooding, if one of these slow
moving cells sets up over a more populated area. Current hi-res
guidance highlights the potential for up to 2" of QPF between
18-00Z this afternoon north of the I-95 corridor. Sea breeze
will make conditions too stable along the coast to ~10-20 miles
inland for storms to initiate there, though will likely act as a
fuse for interior convection.

Previous update...

730 AM Update...

Haven`t made any major adjustments to the current forecast.
Though did want to highlight that there were a few thunderstorms
this AM as there was a few hundred J/kg of MUCAPE in place.
Should see this early AM activity taper off as highlighted by
latest HREF. Big question will be if skies clear enough that we
can destabilize, which we are already seeing breaks across
northern New England. Per the latest deterministic guidance we
do have 30-40 kts of deep layer shear along the MA/VT/NH border
this afternoon and latest HREF is indicating there are some low
probs (10-20 percent) of 2-5 km updraft helicity greater than
75. The latest NADOcast guidance also highlighting some probs
for hail/wind. So will be something we keep an eye on this
afternoon. Best shot for activity to fire is roughly 18-20Z.

Previous discussion...

A mid level shortwave moving across northern New England early
this morning will trigger some showers, mainly north of the
Mass Pike. Still not overly concerned about thunderstorms, at
least for this morning. Mid level lapse rates were still steeper
than moist adiabatic, but it is unlikely that updrafts will be
strong or deep enough to produce thunderstorms.

Quite a different story heading into this afternoon. A weak low
pressure is expected to move from the lower Great Lakes into
northern New England, leaving our region on the warmer and more
humid side. This should result in most unstable CAPE values on
1,200-1,700 J/kg. Model soundings are also more supportive of
thunderstorms compared to yesterday, having minimized a mid
level inversion between 700-500 mb. However, there is still not
expected to be much shear to organize this energy.

Thus, we are more likely to see typical rather than severe
thunderstorms today. That said, a few storms could get strong
enough to produce some hail or gusty straight-line winds. The
more sunshine we get this morning, which is not expected to be
much overall, the greater the risk for stronger storms this
afternoon.

Precipitable water values are also expected to be in excess of
1.25 inches, meaning locally heavy rainfall is also possible.
Will need to be wary of how fast any thunderstorms move this
afternoon, and where exactly the rains fall. While most
locations should see rainfall less than one half inch, where it
rains at all, there is a chance for a quick inch of rainfall in
a short time.

Another day of above normal temperatures.

&&

.SHORT TERM /MONDAY/...
Instability drops off quickly tonight as surface flow shifts to
the east. So, while the thunderstorm threat diminishes after
about 11 PM, scattered rain showers are possible overnight.
Higher dew points should keep low temperatures between the mid
50s and lower 60s.

The attention turns to Memorial Day. A low pressure over the
Great lakes is expected to push another warm front our way. With
a lack of sunshine and an easterly flow to start, expecting
high temperatures to be 5-10 degrees lower when compared today.
Outdoor plans are likely to be impacted. Thinking scattered
showers during the morning hours, with more widespread showers
and isolated thunderstorms arriving from the west during the
afternoon. Thinking the heaviest rainfall holds off until Monday
Night.

&&

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

* Widespread showers with embedded thunderstorms Mon Night into
  early Tue. Heavy downpours anticipated at times.

* Turning drier and mild on Tue.

* Generally dry for much of the week. Though there could be some
  hit- or-miss showers and thunderstorms Wed/Thu. Spotty
  activity possible late in the week and heading into the
  weekend.

Monday Night into early Tuesday...

Will have a potent shortwave trough initially over the eastern
Great Lakes into the Mid Atlantic Mon evening. This will lift
through much of New England by early Tue. Will likely be within
the warm sector of a frontal system through much of this period.
The trailing cold front may begin sweeping through parts of
western New England early on Tue.

This is the next opportunity for impactful weather across
southern New England. Have an anomalously robust PWAT plume and
low level jet impinging on our area. Main concern at this point
is locally heavy downpours, which could potentially lead to
scattered hydrologic issues. Could also see some stronger storms
given the strong low level jet.

Heavy rain threat...

The latest NAEFS and EPS guidance show that our PWATs will be
roughly 2-3 STD above model climatology. Deterministic and
ensemble guidance show values ranging from generally 1.5 to 1.75
inches, though could get as high a 2 inches. The V component of
the wind is highly anomalous at 3-4 STD above model climo.
Should have a few hundred J/kg of MUCAPE in place with a skinny
profile and warm cloud layer depths of roughly 3-4 km. This
should result in efficient warm rain processes. The latest WPC
Excessive Rainfall Outlook of Marginal still suffices given
we`ve been pretty dry. Though if the heavier rain can fall over
an urban area then there will likely be hydrologic issues.

Ensemble guidance continues to show an interesting spread with
the probs of 1+ and 2+ inches of rainfall. The GEFS still much
more muted than its international counterparts (EPS/GEPS) with
essentially nil probs for southern New England. The GEPS is the
most hot to trot with low to mod probs (10-60+ percent) of 24 hr
QPF AOA 1 inch with the risk greatest for CT, RI and into
western MA. The EPS shows a similar pattern as the GEPS, but is
not as broad. The GEPS even indicating some low probs (10
percent) of 2+ inches of precip. As have previously mentioned am
wondering if the lack of higher probs for heavier precip is due
to the progressive nature of the system and the dry slot
punching through. The dry slot tends to not be well modeled and
slides through quicker than guidance usually indicates.

Severe thunderstorm risk...

There are still some indications that there could be some
strong to perhaps severe storms. The low level jet impinging on
the region is 2.5-3 STD above model climo at the 850 hPa level.
Though should note that the speeds at 925 hPa are not to be
scoffed at either. Should see a 925 hPa jet of roughly 20-50+
kts. Both GFS/NAM Bufkit soundings are indicating that we are
pretty inverted and our instability is somewhat paltry. Despite
this deep layer shear will be right around 40 kts and hodographs
do show some curvature to them as the low level jet is sliding
through overnight. This results in roughly 100-300 units of 0-3
SRH. Concerning signals given we`ve got a closed upper low
setting up over the eastern Great Lakes and feeling somewhat
like a fall pattern setup and we`re solidly in the warm sector.
CIPS and CSU ML guidance somewhat split on the severe risk with
the CIPs analogs not highlighting SNE at all. The CSU ML
guidance does indicate some low probs across western portions of
New England. Something we will need to keep a close eye on.

Rest of Tuesday...

Caught under cyclonic flow through this timeframe. Will have a
broad trough/cutoff setting up over the eastern Great
Lakes/Quebec. Cold front crosses much of southern New England by
late in the day. Though there is some discrepancy on the exact
timing.

Should see the dry slot continuing to work its way through our
region. Thinking the bulk of shower/storm activity will be
tapering off in the morning before drying out in the afternoon.
Though wondering if the dry slot pushes in/through a bit faster
than am currently indicating. Regardless, not anticipating much
precip wise during this period. Should see a decent SW to W low
level jet setting up with 925 hPa winds of 15-30 kts. Should be
somewhat mild given our 925 hPa temps range from 16-19 degrees
Celsius. Bumped up our highs toward the 75th percentile of
guidance given some uncertainty on cloud cover. Will be mild
with highs generally in the 70s for most, but there could be
some low 80 degree readings across the Merrimack and CT River
Valley.

Wednesday through Saturday...

Blocky cyclonic flow pattern in place through much if not this
entire timeframe. There is some uncertainty on how things will
evolve especially as we head late into the week. The question
will be on where the cutoff is located.

Overall think much of this timeframe will feature dry weather
for much of southern New England. Think that the best
opportunities for more scattered showers/storms comes on Wed/Thu
with the trough/cutoff overhead. When it comes to Fri/Sat
should be drier especially as a ridge builds into the Great
Lakes, but cannot rule out some spotty activity at this point.
The NBM handled this period pretty well, so have not deviated
from it at this point. Temperatures will be turning near to
cooler than seasonable.

&&

.AVIATION /18Z SUNDAY THROUGH THURSDAY/...
-- Changed Discussion --
Forecast Confidence Levels:

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

Today...High confidence in trends, moderate in timing.

VFR across the interior today, but will need to keep an eye out
for hit or miss showers/storms this afternoon. Timing wise am
thinking things initiate at roughly 18-21Z. If a shower/storm
can move over a terminal then will have risk of gusty winds and
and small hail. IFR in low clouds across the south coast. Though
think we improve for PVD-FMH-HYA with some heating around
roughly 14-16Z. Winds out of the S/SE at 5-10 kts. Will have
localized sea breezes developing roughly 14-17Z.

Through tonight... moderate confidence in storm location, high
confidence in trends

Thunderstorms will bubble up along the I-90 corridor, between
Springfield and Boston, north, after 21Z and will be scattered
in nature. Sea breeze will likely keep storms away from the
immediate coast, including Logan, but it is possible for a
strong cell to produce heavy rain and gusty winds at several
terminals through 01Z tonight.

IFR develops quickly after storms subside overnight with stratus
and fog advecting from the east. Uncertainty regarding how far
inland fog develops, but it is almost a surefire thing for
terminals on the Cape and islands.

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.

Generally VFR through 00Z except if a thunderstorm is able to
develop over the terminal. Thunderstorms more likely just
northwest of the city of Boston. Winds from the E/ESE through
the period building late tomorrow to around 20kt. Several rounds
of rain for Monday. IFR developing quickly after sunset this
evening as stratus develops.

KBDL TAF...High confidence in TAF trends, moderate in timing.

Generally VFR, hit or miss Thunderstorms likely stay north of
the terminal but cant rule out impactful TSRA between 21-01Z.
IFR develops overnight, winds from tthe south shift to the ESE
tomorrow.


Outlook /Monday Night through Thursday/...

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

Tuesday Night: VFR.

Wednesday: VFR. Chance SHRA.

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

Thursday: VFR. Breezy. Slight chance SHRA.
-- 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.

Low risk for showers and a few thunderstorms this afternoon,
mainly across the eastern coastal waters. Greater risk for
showers and a few thunderstorms during the afternoon on Memorial
Day.

Fog less likely tonight as winds shift to the E/NE. Seas remain
between 1-3ft through this portion of the forecast.

Outlook /Monday Night through Thursday/...

Memorial Day: Winds less than 25 kt. Chance of rain showers,
patchy fog. Local visibility 1 nm or less.

Monday Night: Moderate 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. Chance of rain
showers, 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...Belk/BL
NEAR TERM...Belk/BL/KS
SHORT TERM...Belk
LONG TERM...BL
AVIATION...KS
MARINE...Belk/BL