202
FXUS61 KBOX 072352
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
752 PM EDT Tue May 7 2024

.SYNOPSIS...

A warm front will bring a round of showers and isolated
thunderstorms very late tonight and Wednesday. Chances for a few
strong thunderstorms capable of producing hail in western MA/CT
Wednesday afternoon. Periods of unsettled weather continue late
this week into this weekend...but not expecting a washout
either with extended periods of dry weather too.
Overall...temperatures will average below normal late this week
into the weekend. Milder/Seasonable temperatures should return
early next week.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...

750 PM Update...

* Areas of low clouds/fog expand onto the south coast overnight
* Showers/embedded t-storms approach western MA/CT near
  daybreak

Areas of low clouds and fog were currently impacting Nantucket
early this evening. Light southerly flow with the cooling
boundary layer should allow the low clouds and fog to
overspread areas near the south coast overnight. The low level
flow will maintain a westerly component overnight...which should
keep the bulk of the low clouds/fog from expanding much further
north than a KPVD-KPYM line.

The other concern will be shortwave energy/mid level warm front
approaching from the west near daybreak Wednesday. This will be
associated with a modest southwest LLJ and a pretty good
instability burst. Showalter indices drop from nearly 10 to
below zero Wednesday morning. There also a band of good
lift/deeper moisture associated with the shortwave/mid level
warm front. Therefore...while dry weather generally prevails
overnight expect showers and possibly embedded t-storms with
brief heavy rain to begin to impact western MA/CT near daybreak.
This activity will then move across the region Wednesday
morning which is discussed in the short term section.

Overnight low temps will bottom out in the upper 40s to the
middle 50s.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/...

350 PM Update:

Wednesday:

Key Points...

* Overcast with warm frontal showers spreading eastward during the
  morning to early afternoon hrs. Low chance of a rumble of thunder
  with this activity.

* Better chance at a few strong t-storms later in afternoon (as
  soon as 2 PM, but more probable ~ 4-7 PM) from Worcester/NW
  RI westward into western MA/CT. Hail possible in the strongest
  of storms, which could reach up to 1 inch in diameter.

Warm front then spreads showers eastward across the remainder of
Southern New England during the morning hours, with
continuing/residual showers around in eastern MA into the early
afternoon. There is a modest decrease in Showalter indices to around
0 with this morning round of showers, and though I can`t rule out a
rumble of thunder with the morning warm frontal showers, in many
cases it would be the exception vs the rule. This will lock in cloud
cover and lead to cooler temperatures with little optimism for
breaks in the cloud cover. With overcast here, expect highs to reach
into the mid 50s to lower/mid 60s.

For the mid to late afternoon...there are indications in a majority
of guidance of some breaks in the cloud cover over western and
central MA, northern CT and into portions of NW RI. As that occurs,
a surge in 700-500 mb lapse rates 7.0-7.5 C/km advects over the
aforementioned area, with a pocket of mid 50s to around 60 degree
dewpoints in the lower Hudson Valley into western MA and portions of
northern CT. With effective shear magnitudes around 40-50 kt, and
with a narrow zone of frontal convergence with weak low pressure
tracking up the CT Valley, we should see scattered t-storms develop
in the lower Hudson Valley/Berkshires, moving ESE during the late-
aftn hours. MUCAPE values for the aforementioned area are
progged around 1000-1200 J/kg per the HREF, with a bullseye in
max updraft progs over western and central MA, northern CT into
northwest RI. It isn`t clear if these storms would be surface-
based here, but the steeper lapse rates aloft combined with
effective shear magnitudes supporting organized updrafts could
favor storms elevated above any surface stable layer/inversion.
It`s a setting which can yield storms capable of hail in strong
storms, and per machine- learning progs from Colorado State and
others, there is a lower probability that storms could produce
hail satisfying severe criteria. SPC has a large portion of SNE
in a Level 1 of 5/Marginal Risk for severe weather, though the
best chance is for areas near and southwest of Worcester. Unless
storms can become surface based, the primary convective threat
would be from hailstones. Opted to include enhanced wording for
small hail in thunderstorms encompassing this general area. The
biggest uncertainties here are on the spatial coverage and
timing of any t- storms, and if storms could become surface-
based. Temps in interior MA/CT and northwest RI have a good
chance at reaching into the mid 60s to lower 70s with any cloudy
breaks.

Wednesday Night:

Weak low pressure initially near the Hudson Valley moves eastward
early tomorrow night, with decreasing rain chances as it moves
offshore. It`s a pretty uncertain period though in the wake of this
wave of low pressure; a number of model guidance wants to clear SNE
out from cloud cover too, but with surface ridge nosing in from
northern New England and light northerly winds cooling the PBL, I`m
a little surprised the guidance is clearing things out to the degree
it is. Pattern recognition seems to favor more cloud cover than
the guidance is offering. I sided the official forecast more
pessimistically, with cloud cover hanging tough and/or filling
back in as the PBL cools off. Will be generally dry, but I think
tomorrow night could feature a good amt of cloud cover and
possible fog, too. Lows in the upper 40s to lower 50s.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...

Key Points...

* Another period of showers sometime later Thu into early Fri
* Seasonable high temps away from coast Thu but quite cool on Fri
* Relatively cool this weekend with another period of showers by Sun
* Milder/more seasonable temps return early next week

Details...

Thursday and Thursday night...

Enough subsidence behind Wednesday/s shortwave to result in mainly
dry weather to start off the day on Thursday. However...another
piece of shortwave energy will be approaching from the west later
Thu and Thu night. So we expect more showers to arrive...but
specific timing is uncertain. Right now thinking later Thu into Thu
night...but this will need to be refined. High temps are tricky
too...but we might have enough of a dry window during the first half
of the day to allow highs to reach the 60s to perhaps near 70 in CT
River Valley. It probably will be tough though to break 60 in the
immediate coast with onshore flow.

Friday...

A couple waves of low pressure pass to our south on Friday. This
generates an easterly flow of cool air into southern New England.
High temps will probably be held mainly in the 50s on Friday.
Thinking the main shower threat will be during the first part of the
day, but perhaps trending drier for the second half of the day as
the low pressure system moves east of the region.

This Weekend...

Upper trough sets up across our region this weekend. This will
result in relatively cool/below normal high temps for most of the
weekend along with onshore flow. Thinking is that highs will mainly
be between 55 and 65...with the coolest of those readings on the
immediate coast. It will also be a bit unsettled at times with the
risk for some showers...but timing is uncertain and a washout is not
expected. Current indications are that much of Sat may turn out dry
with the better chance for some showers being sometime on Sun.

Early Next Week...

The upper trough will push east of our region early next week. This
should allow for rising height fields and seasonable high
temperatures.

&&

.AVIATION /00Z WEDNESDAY THROUGH SUNDAY/...

Forecast Confidence Levels:

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

00z TAF Update...

Tonight...High Confidence for most of the region but moderate
confidence near the south coast.

We generally expect VFR conditions to prevail for most areas
tonight. The exception will be areas near the south coast, Cape
and Islands...where IFR-LIFR conditions in low clouds and fog
are expected to overspread the region. This as the boundary
layer begins to cool with light southerly flow. We do not expect
these low clouds/fog to advance much further north than the
PVD-PYM corridor given a westerly component to the wind.

Wednesday...Moderate confidence.

Low end MVFR-IFR conditions will tend to overspread the
remainder of the region Wed morning. This will also be
associated with a cluster of showers and possibly embedded
t-storms impact the region roughly in the 10z to 16z time frame.
Timing is tough and areal coverage/confidence in thunder
remains somewhat uncertain. So we did include Prob30 groups for
thunder at most terminals and brief heavy rain is also possible
with the activity. A second round of scattered showers/t-storms
is possible later in the day and early evening. For now we did
include some VCTS in BDL/BAF/ORH TAFS and later shifts may need
to expand on that depending on model trends. Some hail is
certainly possible too with the afternoon/early evening
activity.

Wednesday Night: Low to moderate confidence.

SHRA/TS exits into the waters early Wed night, and that could
lead to a period of clearing/brief VFR. However there`s
potential for IFR to re-develop later in the overnight into
Thurs as high pressure ridges southward from ME/NH. Light winds
to trend light NE by overnight.

KBOS TAF...High confidence in TAF tonight and moderate
confidence on Wednesday.

KBDL TAF...High confidence in TAF tonight and moderate
confidence on Wednesday.

Outlook /Thursday through Sunday/...

Thursday: Mainly VFR, with local IFR possible. Breezy. Chance
SHRA, patchy FG.

Thursday Night: MVFR/IFR conditions possible. Breezy. SHRA
likely, patchy BR.

Friday: Mainly MVFR, with areas IFR possible. Breezy. Chance
SHRA.

Friday Night: Mainly MVFR, with local IFR possible. Breezy.
Chance SHRA.

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

Saturday Night: Mainly MVFR, with areas VFR possible. Breezy.
Slight chance SHRA.

Sunday: Mainly MVFR, with areas VFR possible. 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.

Winds and seas below SCA thresholds through the period.

Mainly easterly winds around 10-15 kt tonight into Wednesday;
winds then turn WNW to NW at similar speeds for Wednesday night.
Seas mainly 4 ft or less all waters. Fog is likely tonight over
the southern waters. Periods of showers Wednesday, with
possible embedded thunderstorms. Some storms could produce small
hail and locally rough seas.

Outlook /Thursday through Sunday/...

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

Thursday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Areas of seas approaching 5 ft. Rain showers
likely, patchy fog. Local visibility 1 to 3 nm.

Friday: Moderate risk for Small Craft Advisory winds with gusts
up to 30 kt. Seas up to 5 ft. Chance of rain showers.

Friday Night through Saturday: Winds less than 25 kt. Seas up
to 5 ft. Chance of rain showers.

Saturday Night: Winds less than 25 kt. Seas up to 5 ft. Slight
chance of rain showers.

Sunday: Winds less than 25 kt. Areas of seas approaching 5 ft.
Chance of rain showers.

&&

.TIDES/COASTAL FLOODING...
High astronomical tides coupled with onshore flow may result in
some splashover/very minor coastal flooding late this week.
However...wind/waves do not appear high enough to result in a
significant issues.

&&

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

&&

$$

SYNOPSIS...Loconto/Frank
NEAR TERM...Frank
SHORT TERM...Loconto
LONG TERM...Frank
AVIATION...Frank/Loconto
MARINE...Frank/Loconto
TIDES/COASTAL FLOODING...Frank