223
FXUS61 KBOX 211410
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
1010 AM EDT Tue May 21 2024

.SYNOPSIS...
A high pressure remains just south of New England, setting the
stage for summerlike warmth today through Thursday, at least
away from the immediate south coast. Generally dry weather is
expected through Wednesday, 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. Still mild and drier on Friday. Unsettled through this
weekend into early next week with more seasonable temperatures.

&&

.NEAR TERM /THROUGH TONIGHT/...

10 am update...

Fog and stratus has cleared out for much of the region, save for
a smattering of spots along the immediate south coast and the CT
valley. Temperatures are on track, in the upper 60s to low 70s
at this hour.

Previous discussion...

High pressure to our south will remain the dominant weather
feature impacting our forecast. abundant stratus and fog trapped
beneath a subsidence inversion is expected to start the day
across RI, a large portion of eastern MA, and even parts of
northeast CT. Last several runs of the GLAMP guidance seem to
have a rather good handle on the status now. As such, used that
as the basis for the forecast today.

This high pressure to our south should maintain dry weather
across most of southern New England today. A mid level shortwave
is expected to arrive this afternoon into this evening across
northern New England. However, it may have enough influence when
combined with daytime heating to lead to a few showers across
portions of northwest MA.

Temperatures will be complicated. Much will depend on how quickly
the stratus breaks. Thinking areas away from southern RI and
southeast MA should break out into sunshine rather quickly.
Towards the south coast, it could be early afternoon before
getting appreciable sunshine. There is also the issue of the
light south to southwest wind coming in off ocean waters in the
50s. Expecting much of interior southern New England to reach
the 80s, with a sharp gradient towards the south coast, where
temperatures should struggle to get out of the 60s.

&&

.SHORT TERM /WEDNESDAY/...
Not much change to the overall synoptic pattern tonight into
Wednesday. Expecting more stratus and fog tonight, but with more
of a southwest wind at the surface, it should not extend as far
inland. More sunshine and a higher starting point from overnight
lows, thinking high temperatures should be about 3-5 degrees
higher than Tuesday. This could mean some readings around 90
across the interior, with another sharp gradient into the 60s
towards the south coast.

Dry weather should prevail, with just a low risk for an
afternoon shower or thunderstorm towards western MA.

&&

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

* Strong to severe thunderstorms possible on Thu as a cold front
  sweeps through. Temps remain above normal.

* Dry and quiet weather Fri with above normal temps persisting.

* Turning more seasonable temperature wise this weekend. Lot of
  uncertainty at this point on how things evolve, but it could
  be unsettled. No washouts anticipated.

* Still unsettled as we head into early next week.

Wednesday Night through Thursday...

A cutoff low will be situated over the Upper Mississippi River
Valley/western Ontario Wed Night. The cutoff will rotate
northeastward into northern Ontario by late Thu. A shortwave
trough rotating around the cutoff will lift from the central
Great Lakes into the eastern Great Lakes by early Thu and into
New England by late in the day. In response to this forcing a
cold front will slide through the region.

The main concern of the extended is this timeframe as there
will be a risk for strong to severe thunderstorms. Should note
that there is uncertainty in the forecast as convective cloud
debris from the previous day could limit the instability that we
realize. Need to keep this in mind as all pieces of guidance
show mid level clouds (RH in 500-700 hPa layer). The current 00Z
NAM clears this very quickly and allows for a large amount of
destabilization, whereas the GFS is much more expansive and does
not clear us, which results in less destabilization. The
GEFS/EPS guidance are muted with low/mod (10-50 percent) probs
of SBCAPE AOA 1000 J/kg, which generally highlights CT to RI.
The GEPS guidance is much more amped up with mod to high (40-70+
percent) probs of SBCAPE AOA 1000 J/kg, which highlights our
interior. The GEPS even shows some low probs of SBCAPE AOA 2000
J/kg. Given the uncertainty in the earlier day cloud cover will
definitely be something we need to keep an eye on. This also
impacts what low/mid level lapse rates we realize, but there is
indication of 7-9 degree C low level lapse rates in a well mixed
boundary layer. The mid level lapse from the GFS/ECMWF/GEM are
not bad at 6-8, but the NAM is pretty poultry.

The moisture will be more than sufficient for thunderstorm
development along with deep layer shear. Models are in pretty
solid agreement with deep layer shear increasing to 20-40 kts on
Thu. Didn`t stray from the NBM dew points given the
southwesterly warm air advection. Will feel quite muggy out Thu
with dew points in the 60s for a good chunk of southern New
England. Could also potentially have some heavier downpours as
these showers/storms are moving through. Both the NAEFS/EPS
guidance indicate our PWATs are around 2- 2.5 STD above model
climo, which is roughly around values of 1-1.5 inches. Our warm
cloud layer depths are right around 2.5-3.5 km, so there could
be efficient rain processes going on. The only hindrance is that
the front is pretty progressive, so think the flooding threat
will be pretty minimal outside of our typical urban/nuisance
spots.

Provided we clear out there will be a risk for strong to severe
thunderstorms. The CIPs guidance still from last evening, but
think it highlights the risk well with a 15-30% risk across
southern New England with the highest probs across western
MA/CT. Could have both a wind and hail threat given the probs,
but the wind appears to be the higher risk at this point in
time. The latest CSU ML guidance also highlights the wind/hail
risk, but with a 5 to 15 percent probability. Both the NCAR MPAS
and C-SHiELD guidance from last night show low to mod probs of
2-5 km Updraft Helicity > 75 units, which highlights interior
southern New England. Main threats for Thu at this point would
be strong/damaging winds and hail.

Expecting temps to be quite mild with the SWly warm air
advection. This will park 17-22 degree Celsius 925 hPa temps
overhead. The end result will be widespread 80s away from the
immediate south coast. Temps across the south coast will
generally be in the 70s.

Friday...

A ridge axis builds from the mid/upper Mississippi River Valley
Thu Night into the eastern Great Lakes/Mid Atlantic by Fri
afternoon. A weak high builds into southern New England during
this timeframe.

Dry and quiet weather anticipated during this timeframe. Will
have another mild day across southern New England with 925 hPa
temps ranging from 19-21 Celsius. There is some uncertainty on
the temps as some guidance keeps our wind SWly and other WNW/NW.
For now the NBM looked reasonable with fairly widespread
low/mid 80 degree highs away from the immediate south coast.

This weekend into early next week...

Caught in quasi-zonal flow pattern with waves sliding through.
There is considerable uncertainty how a the blocky pattern over
the Northern Plains/Pacific NW breaks down. At this point looks
like we could have several waves swing through with perhaps a
more substantial trough early next week. The result is an
unsettled pattern. At this point have stuck with the NBM, which
keeps spotty shower chances through the weekend. Though it does
not look like there are any washouts at this point. Better shot
for the washout/heavier precip comes early next week. The
NAEFS/EPS guidance indicate PWATS around 1-1.5 STD above model
climo. Temperatures returning to more seasonable values during
this period.

&&

.AVIATION /14Z TUESDAY THROUGH SATURDAY/...
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 exact timing.

Mix of VFR conditions across portions of the interior to
IFR/LIFR stratus and fog across the CT River Valley, RI and
eastern MA. Should see the IFR/LIFR stratus and fog burn off for
most roughly 12-14Z. The only exception is across the
Cape/Islands where may not be until 16-18Z, though ACK may only
briefly improve to VFR - but the low ceilings linger. Winds out
of the S/SW at 5-10 kts. Could briefly see the sea breeze
develop across eastern coastal locations roughly 16-20Z, but
some uncertainty in if winds will be light enough in lowest 2kft
per RAP soundings.

Tonight...High confidence in trends, moderate in exact timing.

Conditions deteriorate to IFR/LIFR across the south coast,
especially near and SE of the I-95 corridor. The only exception
is for the Boston metro area where am anticipating the SWly
component to keep the fog/stratus at bay. Some uncertainty if
this stratus/fog moves up the CT River Valley, so have only
hinted at BAF. Winds relatively light (<10 kts) out of the SW.

Wednesday...High confidence.

Should see any IFR/LIFR stratus and fog burning off roughly
12-15Z, though may linger longer for ACK. Elsewhere VFR with SW
winds 5-10 kts and 15-20 kt gusts during the afternoon.

KBOS TAF...High confidence.

VFR through the day, though there will be some IFR ceilings
lingering through roughly 13-14Z. Already in the process of
burning off, so think we remain VFR. Winds S to SW at 5-10 kts.
Winds shift more SW tonight. Could briefly sea breeze between
16-20Z, but some uncertainty as winds within the lowest 2 kft
are borderline.

KBDL TAF...High confidence.

IFR stratus/fog burns off by 14Z and will remain VFR through
rest of TAF. Not out of question stratus/fog comes back in
tonight, but confidence not high enough to include in latest
update. Winds S 5-10 kts today shifting to the SW tonight.

Outlook /Wednesday Night through Saturday/...

Wednesday Night: VFR. Breezy. Slight chance SHRA.

Thursday: Mainly VFR, with local MVFR possible. Breezy. Chance
SHRA, slight chance TSRA.

Thursday Night: VFR. Chance SHRA, isolated TSRA.

Friday through Friday Night: VFR.

Saturday: VFR. Slight chance SHRA.

&&

.MARINE...
Forecaster Confidence Levels:

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

Through Wednesday...High Confidence.

High pressure south of the waters maintains relatively light
winds and seas through Wednesday. The main concern will be poor
visibility in areas of fog reducing visibility to less than 1
mile at times, especially early this morning, and again Tuesday
night.

Outlook /Wednesday Night through Saturday/...

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

Thursday: Winds less than 25 kt. Seas locally approaching 5 ft.
Chance of rain showers, slight chance of thunderstorms.

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

&&

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

&&

$$

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