986
FXUS61 KBOX 220253
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
1053 PM EDT Tue May 21 2024

.SYNOPSIS...
A high pressure remains just south of New England, setting the
stage for summerlike warmth through Thursday 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 warm
and mainly dry on Friday. Continued warm but unsettled through
this weekend, then cooler early next week with more seasonable
temperatures.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
Continuing to monitor the progress of a stratus bank south of
the islands. This far, this bank has been lower to move north
when compared to last night. Temperatures are slightly higher,
meaning they were not as close to the ocean temperature. This,
not expecting fog to be as widespread or dense as this morning.
Otherwise, just minor tweaks to bring the forecast back in line
with observed trends.

Previous Discussion...

We continue to monitor some scattered convection which has been
firing off in NY this afternoon and inching closer to SNE.
However, due to lack of strong bulk shear these cells are going
up and down fairly quickly without reaching severe levels. This
is expected to be the case with a smattering of pop-up
thunderstorms over northwest MA this afternoon and early
evening, even as bulk shear is expected to increase marginally
to 20-25kts lapse rates are poor and only sub severe activity is
expected at this time. After a warm Tuesday temperatures will
cool tonight but remain several degrees warmer than the night
previous, in the mid to upper 50s and even low 60s thanks to
increasing moisture/dewpoints on SW flow. The surface high
pressure in place to our south will remain overnight leading
to prolonged warmth and fog/low stratus on the immediate south
coast and southeast MA. The more southwesterly trajectory of the
wind should help the those low clouds/fog to be limited further
south than last night, and is not expected to again extend into
the CT Valley.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/...
Wednesday expect more of the same, but starting off with a
warmer low temp will help us achieve the hottest day of the
week, with highs reaching into the upper 80s inland (lower 90s
in the hottest locations like the Merrimack Valley and northern
CT valley). Those along the south coast will be spared the hot
temps thanks to onshore flow keeping temps in the 70s. During
the afternoon we may once again see a pop up shower or
thunderstorm, but less coverage is expected than today; not
seeing any strong source of forcing other than diurnal heating
and shear is even weaker than today. Overnight lows temperatures
will once again be quite mild owing to the high dewpoint airmass
overhead, in the upper 50s and low 60s.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
Key Points...

* A few strong to severe t-storms possible Thu. Damaging wind
  and large hail are the main threats

* Warm and dry Fri with above normal temps persisting

* Continued warm this weekend but turning more unsettled, then
  cooler early next week with risk of showers

Thursday...

The main forecast challenge revolves around the potential for
severe weather as a cold front moves into a rather warm and
humid airmass in place across SNE. While there is some
uncertainty with extent of cloud cover and how that will impact
instability, we expect at least partial sunshine which will
allow temps to reach mid-upper 80s, except 70s near the south
coast. Dewpoints will climb into the mid 60s. These conditions
beneath decent mid level lapse rates will contribute to SBCAPES
of 1000+ J/kg with a low prob of 2000 J/kg. Effective shear is
progged around 25-35 kt which is on the lower end of what would
be considered sufficient for organized severe convection. The
one negative factor which may limit areal coverage of severe
storms is limited synoptic forcing as main shortwave energy and
best upper jet dynamics will be well to the north. This is
reflected in the NAM3k and FV3 simulated reflectivity which
shows rather widely scattered storms. The CSU ML probs are still
highlighting SNE for a low risk for severe. Still not within
the window of the full suite of hi-res CAMs which will provide a
better idea of coverage and intensity of storms. Current
thinking is we will see a few strong to severe storms with best
chance across interior MA into CT which is where NCAR ensembles
show best updraft helicity swaths. Favorable low level lapse
rates suggest damaging wind will be the primary threat with any
severe storms, but hail is a secondary risk given decent mid
level lapse rates and marginal shear. SW flow off cooler SST
will limit instability and severe potential across RI and SE MA.


Weakening storms will move to the south coast Thu evening then
drier air from the north will gradually overspread SNE overnight
Thu night.

Friday...

SNE will be in between systems Fri with northern stream trough
and attendant cold front well to the north and weaker shortwave
to the south. Rather dry air in the mid levels will lead to lots
of sunshine although clouds may linger along the south coast in
the morning before sunshine increases. It will be another warm
day as 925 mb temps reach 20-21C. Highs will reach well into the
80s, but sea breezes will keep coastal locations cooler. It
also won`t be quite as humid as mixing lowers dewpoints into the
50s.

This weekend into early next week...

Messy pattern sets up with numerous shortwaves within a quasi-
zonal flow before pattern amplifies early next week. A few of
these shortwaves will be rotating through New Eng and PWATs will
be above normal which will result in an unsettled pattern with
showers at times although no washouts are expected. Timing of
these shortwaves will be a challenge but it looks like there
will be a risk of showers somewhere in SNE each day. Warm
pattern continues this weekend with above normal 925 mb temps
leading to 70s and lower 80s for highs, then cooler early next
week.

&&

.AVIATION /03Z WEDNESDAY THROUGH SUNDAY/...
Forecast Confidence Levels:

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

Tonight...High confidence

Showers and thunderstorms from earlier have dissipated.
Expecting drier weather to prevail the rest of tonight.
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.

Wednesday...High confidence.

Should see any IFR/LIFR stratus and fog burning off roughly
10-13Z, though may linger longer for ACK. Elsewhere VFR with SW
winds 5-10 kts and 15-20 kt gusts during the afternoon.

Wednesday night...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.

KBOS TAF...High confidence in TAF. Low risk for TSRA at terminal
between 00-02Z. Would expect showers/storms currently over
Franklin county to weaken by the time they get to the coast,
but remaining elevated instability may be enough to support
continued thunder as has been the case in southeastern NH.
Otherwise VFR conditions overnight and tomorrow.

KBDL TAF...High confidence.

Outlook /Thursday through Sunday/...

Thursday: VFR. Breezy. Chance SHRA, chance TSRA.

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

Friday through Friday Night: VFR.

Saturday: VFR. Slight chance SHRA.

Saturday Night: VFR. Chance SHRA.

Sunday: 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 night...High Confidence.

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

Outlook /Thursday through Sunday/...

Thursday: Winds less than 25 kt. Slight chance of rain showers.
Local visibility 1 to 3 nm.

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

Friday through Friday Night: Winds less than 25 kt.

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

Saturday Night: Winds less than 25 kt. Chance of rain showers.

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

&&

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

&&

$$

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