452
FXUS61 KBOX 052340
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
740 PM EDT Wed Jun 5 2024

.SYNOPSIS...
A large cluster of showers and perhaps isolated thunderstorms move
across the region Thursday morning with brief heavy rain. A second
round of more scattered showers & thunderstorms cross the region
Thursday evening.Unsettled conditions with periods of showers
and thunderstorms Friday through Monday.

&&

.NEAR TERM /UNTIL 4 AM THURSDAY MORNING/...

745 PM Update...

Risk for showers and isolated thunder is quickly diminishing
with sunset. Strong storm that produced prolific small hail just
north of the MA/NH border dissipated about an hour ago, leaving
behind a more stable airmass that has inhibited storm growth.
GIven we are headed towards sunset and instability is waning,
not expecting much more than a few scattered sprinkles to drift
over the border into towns like Amesbury and Ipswich over the
next hour.

While the rain threat to our NE is waning, high clouds ahead of
our next precip making shortwave are starting to drift in from
the southwest. While showers will hold off until around 12Z
(west) will likely see a bit of fog formation, particularly
along the south coast, as dewpoint increase tonight. Extent of
the fog is in question as guidance has consistently
overestimated the northern extent of fog over the last several
weeks. FV3/other high res ensembles have the right idea of some
patchy dense fog, but may fall short in the exact placement of
the patches.

420 PM Update...

* Low risk an isolated shower/t-storm in far NE MA this evening
* Mainly dry but quite mild tonight with lows only in the 60s

Plenty of sunshine developed across much of the region this
afternoon. This allowed temperatures to climb well into the 80s in
many locations away from the cooling marine influence near the south
coast. A northern stream shortwave have allowed scattered showers &
t-storms to develop across Maine & NH. The bulk of this energy and
its associated forcing will remain to our north...so generally dry
weather will persist tonight. That being said...there is a low risk
that an isolated shower/t-storm could clip far northeast MA into
this evening.

The main story tonight will be the development of low clouds and
areas of fog from south to north overnight. Timing of this is
somewhat uncertain...but increasing low level moisture and the
cooling boundary layer will allow low clouds and areas of fog to
develop. Overnight low temps should only drop into the 60s given
the increasing clouds and higher dewpoints.

Showers & perhaps isolated t-storms may approach parts of western MA
& CT toward daybreak. We will discuss this in the short term section
of the discussion.

&&

.SHORT TERM /4 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/...
* Large cluster of showers & perhaps iso t-storms and brief
  downpours Thu AM with the highest confidence across CT/RI & SE MA

* A second round of more scattered showers & t-storms Thu evening

Details...

Thursday and Thursday night...

Vigorous shortwave energy approaches from the west toward daybreak
Thu. The amount of forcing should generate a large cluster/fairly
widespread area of showers and perhaps isolated t-storms given
showalter indices dropping near zero. In addition...Pwats on the
order of 1.50 to 2" so expect some downpours with the activity as
well. This activity should move fast enough to prevent any
significant issues...but typical brief nuisance poor drainage street
flooding is possible.

This cluster of activity should exit most of the region by early
afternoon. We should see a lull in the activity for the rest of the
afternoon...although it should remain cloudy in most locations with
onshore flow. It is possible though a few breaks develop in the
distant interior. Mainly skies will hold highs in the 70s...but
might be near 80 in the lower elevations of western MA & CT.

The other concern will be a second round of more scattered showers &
t-storms Thu evening...but not as widespread as what we are looking
at during the morning. This is association with another shortwave
and some leftover diurnal heating/instability. Surface instability
of 1000+ J/KG may develop in the distant interior if we can muster a
few breaks in the clouds. Mid level lapse rates are poor and the
activity is not arriving until the evening will limit the severe
weather potential with this activity. That being said...the HREF
does indicate some updraft helicity swaths and the CSU & HRRR Neural
network machine learning probs do indicate a low risk for a few
severe storms across the distant interior. In a nutshell...thinking
the risk for severe weather is low but can not rule out a few strong
to marginally severe storms in the distant interior. Overall...think
this is a low risk but can not rule out a few strong to marginally
severe storms across interior MA & CT Thu evening. Brief downpours
are possible with this activity too. This activity may reach eastern
MA & RI in a weakened form later Thu evening. Otherwise...drying
trend after midnight with lows in the upper 50s to the middle 60s.

&&

.LONG TERM /FRIDAY THROUGH WEDNESDAY/...
* Unsettled Friday through Sunday with several chances for rain and
  isolated Thunderstorms

* Pattern improvement delayed until early/mid next week as low
  pressure over southern Canada is displaced by building mid-level
  ridge


Friday...

Broad upper level low that has been slowly creeping towards New
England from the northwest sets up shop over the Great
Lakes/southern Canada by early Friday. Several shortwaves will
rotate around the low, the first of which develops mid day Friday
while somewhat of a seabreeze/back door coastal front drops south
out of Maine. While the dynamics of the day don`t look super
impressive, thinking that some scattered showers and thunderstorms
will be able to develop in the convergence zone of the two "forcing
features" where some modest instability, ~600J/kg SBCAPE is present.
0-6km bulk shear also looks to be supportive of thunderstorm
development, topping out around 60kt Friday afternoon! At present,
this convergence zone appears to set up between Worcester and Boston
along I-495, but may shift over the next 24 hours.

The limiting factors in storm coverage and strength are typical of
southern New England with lackluster mid level lapse rates, less
than 5C/km, and decreasing PWATs as surface cold front approaches
from the west; PWATs drop from near 1.8" early Friday to 1". So,
while the severe potential remains very low, cant rule out some
solid downpours and gusty winds Friday afternoon.

Overall, should see a mix of sun and showers with a moderate
mugginess to the air with dewpoints in the low and mid 60s the first
half of the day.

Saturday and Sunday...

Unsettled conditions continue Saturday and Sunday, though neither
day appears to be a washout with a few more shortwaves rotating
around the persistent upper level low.

Saturday looks to be the drier of the two days as PWATs fall to
around 0.8" behind surface cold front that passes through overnight.
Guidance is split on the shower potential, with more coarse
resolution global guidance, like the GFS, trending towards a drier
day and hi-res guidance like the NAM initiating more widespread
shower activity Saturday afternoon. Dynamics appear to be more
supportive of thunderstorms, compared to Friday, as mid level lapse
rates increase to ~6C/km and low level lapse rates approach 8.5C/km,
but instability appears to be quite poor, around 200J/kg.

PWATs surge back towards 1.25-1.4" early Sunday with a similar
dynamic setup, which will yield more showers and isolated
Thunderstorms compared to Saturday. Still, not expecting a washout,
with a mix of sun and showers.

Early next week and beyond...

Very persistent mid level trough/low pressure over Canada, remains
near stationary through early next week. Guidance shows stronger
shortwave development in the Monday period that could bring a more
substantial shot of QPF to the region, before a brief reprieve from
unsettled conditions develops Tuesday/Wednesday with some indication
that strong mid level ridging develops during that timeframe. While
ridging looks robust, temperatures look seasonably mild rather than
hot, with highs in the 80s.

&&

.AVIATION /00Z THURSDAY THROUGH MONDAY/...
Forecast Confidence Levels:

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

00z TAF Update...

Tonight into Thursday night...High Confidence on trends but
lower confidence on specific timing.

Light moist southerly flow will allow for low stratus and areas
of fog to overspread the region tonight from south to north.
Specific timing uncertain...but do expect most locales to see
IFR-LIFR conditions develop overnight. These conditions will
continue through much of Thursday. Improvement will occur Thu
night across the interior...but lower conditions will persist
for most of Thu night across the eastern half of the region.

The other concern over the next 36 hours will be for a band of
widespread showers and perhaps isolated t-storms that will cross
the region from southwest to northeast Thu morning. Brief
downpours are expected with this activity too. A second area of
scattered showers & t-storms will impact the region from the
west Thu evening...but this activity will be more scattered than
what we are anticipating during the morning. As for wind...they will
be from the S generally 10 kts or less...but will shift to the
E on Thu across northern MA.

Prob30 groups have been added to the TAFs, away from the south
coast/Cape, to account for two periods of potential convection;
the first between ~13-17Z west to east, and the second between
21-01Z (Friday) west to east. The second round of convection
looks to be shorter lived, associated with a thin line moving
from the SW to NE, this currently looks to miss the Cape and
Island terminals completely.

KBOS TAF...High confidence in trends, but lower confidence in
specific timing.

KBDL TAF...High confidence in trends, but lower confidence in
specific timing.

Outlook /Friday through Monday/...

Friday: VFR. Chance SHRA, isolated TSRA.

Friday Night: VFR. Slight chance SHRA.

Saturday: VFR. Breezy. Chance SHRA, isolated TSRA.

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

Sunday: VFR. Breezy. Chance SHRA, isolated TSRA.

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

Monday: VFR. Slight chance SHRA.

&&

.MARINE...
Forecaster Confidence Levels:

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

Tonight through Thursday night...Moderate Confidence.

Southerly winds of 10 to 15 knots will persist tonight into Thu
before shifting to more of the SE across the northern waters.
However...S-SW winds will persist across the southern waters into
Thu night. This long S-SW fetch will build seas to around 5 feet
across our southern outer-waters and we may see gusts on the order
of 20 to perhaps up to 25 knots. Therefore...have hoisted small
craft headlines for our southern waters Thu afternoon and night. The
other concern for mariners will be areas of fog developing
especially late tonight into Thu morning. A cluster of showers &
perhaps isolated t-storms will cross the region Thu morning into the
early afternoon.

Outlook /Friday through Monday/...

Friday: Winds less than 25 kt. Areas of seas approaching 5 ft.
Slight chance of rain showers, isolated thunderstorms.

Friday Night: Winds less than 25 kt. Areas of seas approaching
5 ft.

Saturday: Winds less than 25 kt. Areas of seas approaching
5 ft. Slight chance of rain showers, isolated thunderstorms.

Saturday Night: Winds less than 25 kt. Areas of seas
approaching 5 ft.

Sunday through Sunday Night: Winds less than 25 kt. Areas of
seas approaching 5 ft. Slight chance of rain showers, isolated
thunderstorms.

Monday: Winds less than 25 kt. Seas locally approaching 5 ft.
Slight chance of rain showers.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory from 2 PM Thursday to 8 AM EDT Friday for
     ANZ254>256.

&&

$$

SYNOPSIS...Frank/KS
NEAR TERM...Frank/KS
SHORT TERM...Frank
LONG TERM...KS
AVIATION...Frank/KS
MARINE...Frank/KS