518
FXUS61 KBOX 221124
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
724 AM EDT Wed May 22 2024

.SYNOPSIS...

A high pressure remains just southeast of New England, setting
the stage for summerlike warmth through Thursday away from the
immediate south coast. Generally dry weather is expected through
today, but an approaching cold front may bring a round of
scattered showers and thunderstorms Thursday afternoon and
evening. A few strong to severe storms are possible. Mainly dry
on Friday with it remaining mild. Unsettled through this weekend
with hit or miss showers and thunderstorms. Turning more
seasonable early next week with a better opportunity for
widespread rains.

&&

.NEAR TERM /THROUGH TONIGHT/...

715 AM Update...

Main change to the forecast was to eliminate the stratus across
the south coast. Guidance was way too aggressive based on obs,
so have leaned update toward NAMNest, which is doing well at the
moment. Could see the stratus/fog potentially spread northward
this afternoon, but given how guidance did last night with the
low/mid 50 degree dew points sliding in am weary on it occurring.
Since the NAMNest brings it in this afternoon have blended this
with some of the foggier guidance. Rest of forecast looks good
at this time.

Previous discussion...

Still closely tracking the progress of a stratus bank just off
the south coast of New England. Satellite data continued to hint
at this stratus approaching, but as of this writing there has
been no indication of it from the surface observations. Some of
the more reliable near term guidance still suggests it may take
until some time between 4-6 AM for these clouds to overspread
the Cape and islands. Putting these sources together, preferred
the GLAMP guidance for timing the stratus and fog this morning.


Not much change to the synoptic pattern, with a high pressure
still lingering off the south coast of New England. Otherwise,
not much else to drive showers and thunderstorms other than
daytime heating. Most of the convection-allowing models depict
the greatest risk stays to our west into this evening. However,
with high temperatures 85-90 across much of interior southern
New England and dew points around 60 degrees, there is at least
a greater chance towards western MA this afternoon and evening.

Still expecting much cooler conditions towards the immediate
south coast, with highs generally in the 60s. A sharp
temperature gradient should extend across southern RI into
southeast MA.

&&

.SHORT TERM /THURSDAY/...

Mid level lapse rates are projected to be slightly steeper than
moist adiabatic this evening. Thus, despite not having much
synoptic support, showers and thunderstorms could last a while
past sunset. Any such showers or thunderstorms would be
weakening as they moved farther east tonight.

Thursday still appears to be a more active weather day as a
cold front is projected to arrive during the time of peak
heating. Some lingering clouds from the leftover convection
should mean slightly lower high temperatures, but still plenty
warm and humid enough to support some stronger thunderstorms
during the afternoon and evening. Damaging winds and large hail
are the main threats.

&&

.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...

Highlights

* Showers and thunderstorms exiting Thu Night into early Fri as
  a cold front exits.

* Dry and quiet weather for the rest of Fri with  above normal temps.

* Temps remain mild through the weekend. Still a lot of
  uncertainty with how the pattern evolves, but looks unsettled.
  No washouts anticipated at this point.

* Still unsettled early next week with better opportunities for
  more widespread rains. Temps may return to more seasonable
  levels.

Thursday Night into early Friday...

A shortwave trough over the Mid Atlantic Thu night will lift
into southern New England by early Fri. The shortwave moves
offshore by mid/late Fri AM. A cold front will be situated
nearby or over southern New England Thu eve. Some uncertainty
with where the front will be, but it will be pushing further
offshore early Fri.

Fair amount of uncertainty during this period as some guidance
already has the cold front offshore by Thu evening, while others
not until early on Fri. Tough to resolve at this point as
convective influences will also be at play from activity earlier
on Thu. In general have stuck with the NBM at this point in
time, but there are signals that given the slower moving frontal
boundary we could see some heavy downpours.

Due to the uncertainty have largely stuck with probabilistic
guidance, but should note that the UKMET has been consistent
from the last 4 runs and is concerning with a heavy rainfall
risk (1-3+ inches overnight). Though will note the exact
placement of the heavy rain axis has shifted a bit from run to
run. Most of the other guidance clip the south coast or have the
heaviest precip offshore. Both the NAEFS and EPS guidance
indicate PWATs of 1.5-2 STD above model climatology for the BOX
area and a plume of roughly 1.25-1.5 inches impinging on our
area. This matches up well with the latest deterministic
guidance, which also show warm cloud layer depths of 2.5-3.5 km.
Instability will be waning due to the loss of heating, but
there may still be up to a few hundred J/kg of MUCAPE in place.
Deep layer shear may still be high enough that there are a few
lingering strong storms. All sources of ensemble guidance,
GEPS/GEFS and EPS indicate low to mod probs (10-60 percent) of
24 hr QPF AOA 0.5 inches. The risk is highest offshore of the
south coast and probs have been trending offshore from run to
run. The GEPS is the most amped up out of the ensembles and even
has some low probs of 1+ inches of QPF offshore. Based on this
did bump up our QPF a bit, but keep the heaviest over the waters
at this point.

Will be mild across southern New England under SWly flow ahead
of the front and W/WNW behind it. Low temperatures range from
the mid 50s to the low 60s.

Friday...

A ridge axis extends from the western/central Great Lakes into
the TN Valley early on Fri. The ridge builds into the eastern
Great Lakes/Mid Atlantic by late Fri. In response a weak high
nudges into southern New England.

Dry and quiet weather anticipated during this timeframe. Expect
another mild day across southern New England, but in wake of
the front will be a bit less muggy. Will have WNW to NW flow at
925 hPa with temps of 19-21 Celsius. Based on this and consensus
amongst models have bumped up our highs to the 75th percentile
of guidance as we should have good downsloping. The result are
fairly widespread 80s. Though winds may stay enough to the SW
across the immediate south coast where temps will struggle to
get out of the 70s.

This weekend into early next week...

Still a considerable amount of uncertainty through this
timeframe. Especially with how the blocky pattern evolves out on
the Northern Plains/Pacific NW. There are several waves that
lift through southern New England, though the better shot for a
deeper trough comes early next week.

Given the uncertainties mentioned have just stuck with the NBM.
Though should note temps have taken a swing from 24 hrs ago to
milder readings through the weekend, before we turn more
seasonable early next week. Both days over the weekend feature
spotty shower opportunities, but no washouts are anticipated.
Ensemble guidance not showing much signal wise for impactful
weather with PWATs around normal relative to model climo.
Temperatures still mild.

Best shot for the washout/heavier precip comes early next week.
The NAEFS/EPS guidance indicate PWATS around 1-2 STD above
model climo along with that deeper trough lifting in. The EPS
indicates there are low to mod probs (10-40 percent) of 24 hr
QPF AOA 1 inch. The GEPS/GEFS are more muted and firmly in low
probs at this point. Will be another period we will have to
monitor in the coming day, but for now have stuck with the NBM.

&&

.AVIATION /11Z WEDNESDAY THROUGH SUNDAY/...

Forecast Confidence Levels:

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

Today...High confidence away from the immediate south coast,
where confidence is low.

Any stratus/fog across the interior burns off quickly and
conditions will be VFR. Winds out of the SW at 5-15 kts and
could see some 20-25 kt gusts across the Cape/Islands.

The stratus never materialized across the south coast overnight,
and guidance continues to hit it too hard based on obs. Have
leaned toward the NAMNest, which is currently doing well. This
keeps ACK/FMH/HYA VFR through much of the day. Could see the
IFR/LIFR stratus/fog spread in 21-00Z or later.

Tonight...Moderate confidence.

Light SW winds continue 5-10 kts. IFR/MVFR cigs in stratus/fog
possible over southeast MA terminals, but low confidence in
areal extent. Could also see some status in the lower CT River
Valley. Areas MVFR in weakening showers and thunderstorms,
mainly across western and central MA.

Thursday...Moderate confidence.

Mainly VFR, with areas MVFR/IFR in heavier showers and
thunderstorms.

KBOS TAF...High confidence in TAF.

KBDL TAF...High confidence in TAF.

Outlook /Thursday Night through Sunday/...

Thursday Night: VFR. Slight chance SHRA, isolated TSRA.

Friday through Friday Night: VFR.

Saturday through Saturday Night: VFR. Slight chance SHRA.

Sunday: VFR. Slight chance SHRA, isolated TSRA.

&&

.MARINE...

Forecaster Confidence Levels:

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

Through Thursday...High Confidence.

High pressure south of the waters maintains relatively light
winds and seas through Thursday. The main concern will be poor
visibility in areas of fog reducing visibility to less than 1
mile at times into tonight.

Outlook /Thursday Night through Sunday/...

Thursday Night: Winds less than 25 kt. Chance of rain showers,
isolated thunderstorms.

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

Friday Night: Winds less than 25 kt.

Saturday through Saturday Night: Winds less than 25 kt. Slight
chance of rain showers.

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

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...Air Quality Alert from 11 AM this morning to 11 PM EDT this
     evening for MAZ004>007-010>016-026.
RI...None.
MARINE...None.

&&

$$

SYNOPSIS...Belk/BL
NEAR TERM...Belk/BL
SHORT TERM...Belk
LONG TERM...BL
AVIATION...Belk/BL
MARINE...Belk/BL