318
FXUS61 KBOX 072004
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
404 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/...
350 PM Update:

Chamber-of-commerce weather conditions underway across Southern
New England with high pressure dominating, with wall- to-wall
sunshine and temps into the 70s, and a few locations were
pushing 80 degrees in the CT Valley. Sea breezes near both
coasts were supporting cooler temps in the 60s. Though it is
mostly clear, satellite shows marine stratus in the waters south
of Nantucket, which will likely play a role on weather
conditions tonight along the south coast. Well to our west over
the OH Valley is a warm front which moves into western New
England toward early Wed AM.

Clear skies to prevail for most areas at least through overnight.
However we`re expecting the area of stratus to expand a bit in
spatial coverage over the southern waters first, then expand a
little bit landward into southeast New England. Fog and stratus
could spread into coastal areas as soon as sundown but think late-
evening/midnight is more likely as the PBL cools off. This
fog/stratus layer then gradually expands northward up toward a PVD-
PYM line but likely no further. Toward daybreak, will see a general
west to east increase in clouds with showers developing over
interior western MA/CT by early morning. Lows mainly in the lower to
mid 50s, but with upper 40s along the south coast, Cape and Islands.

&&

.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 aformentioned 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 /20Z TUESDAY THROUGH SUNDAY/...
Forecast Confidence Levels:

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

18z TAF Update:

Through 00z Wednesday: High confidence.

VFR. N winds 5-10 kt with sea breezes on the coasts lasting
thru ~23-00z Wed.

Tonight: High confidence in TAF/trends, though moderate on
arrival of south-coastal fog/stratus.

Mainly VFR for most, though IFR-LIFR stratus and fog develops
in the waters potentially as soon as 00z but more likely after
03z, which then advances landward into Cape Cod and south-
coastal RI/MA. Possible it could make it as far north as PVD.
Toward early Wed AM, lowering VFR warm frontal cloud cover
moves in from west to east, but could reach MVFR levels at BAF-
BDL-CEF with accompanying -SHRA 09-12z Wed. Light south winds
to trend E/ENE 4-8 kt by pre-dawn Wed.

Wednesday: Moderate confidence.

MVFR-IFR expands eastward thru 14-16z with areas of SHRA;
possible LLWS. We could see some breaks/scattering in western
MA/CT after 18z Wed and that could lead to SCT TSRA mainly from
ORH south and west. Mentioned PROB30 TSRA at BDL after 20z; any
TS could be capable of lightning, brief downpours and small
hail. Afternoon TS unlikely further north and east but the risk
isn`t zero. SE to S winds 5-10 kt, through may become variable
at times in the afternoon in interior CT/MA.

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. VFR. Sea breezes to continue
through 00z, then trend light and variable early
tonight/overnight. Easterly flow resumes after 09z Wed; with
developing sub-VFR by Wed AM although the onset timing is still
a bit unclear.

KBDL TAF...High confidence in TAF. VFR with light north winds
today, become briefly light/calm early tonight before trending
SE to S overnight. Sub-VFR comes in early Wed AM although onset
timing is still a bit unclear.

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