092
FXUS61 KBOX 121956
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
356 PM EDT Wed Jun 12 2024

.SYNOPSIS...

Sunny and mild Thursday aside from the chance for some patchy south
coast fog. An approaching cold front will bring a period of
scattered showers and thunderstorms Friday afternoon to the mid
evening. Some of these storms could become strong to severe, as
well as produce isolated instances of urban and flash flooding.
Pleasant weekend with mild days, cooler nights and dry weather.
Significant warmup towards hot and humid weather in the Tuesday
to Thursday timeframe, with excessive heat and humidity
possible.


&&

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

Weak upper level disturbance/shortwave that brought renewed diurnal
cumulus to the region will shift seaward this evening, allowing mid
level ridging to build in overnight and into the first part of
Thursday. Diurnal cumulus will dissipate quickly after 23Z this
evening as well as any spotty shower activity that developed this
afternoon, though most hi-res guidance overestimated the coverage of
spotty showers with most of the shower activity remaining to our
north and east over NH and Maine. Lows tonight will be seasonable,
in the 50s.

&&

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

Thursday looks like a beautiful early summer day across much of the
region with mid level ridging in place and drying flow aloft as
winds above 800mb shift to the NW. This wind shift will result in
decreased diurnal cumulus, compared to Tuesday and Wednesday. While
winds aloft shift to the NW, surface flow will remain out of the SW,
which will help enhance WAA and increase dewpoints through the day.
High temperatures should tack between 5 and 10F onto Wednesday`s
highs as 850mb temperatures increase from 8/9C to ~13/14C by late
afternoon. Winds will increase tomorrow afternoon as weak 925mb LLJ
develops along and SE of the I-95 corridor, which will mix gusts to
as high as 20kt to the surface.

So, while clouds will be more sparse across much of the region, do
anticipate some stratus/fog to develop along the south coast, Cape,
and Islands tomorrow afternoon as dewpoints climb into the 60s.

Shortwave trough, that will bring more significant weather Friday,
approaches southern New England from the Great Lakes/southern
Ontario late Thursday and Thursday night. This will allow dewpoints
to further increase as surface flow shifts to the south (PWATs
increase to near 1.25" by daybreak Friday), yielding muggy, mild
night with lows generally in the mid 60s.


&&

.LONG TERM /FRIDAY THROUGH WEDNESDAY/...

Highlights:

* Cold front brings potential for isolated/scattered strong to
  severe t-storms along with isolated instances of urban and flash
  flooding Friday midafternoon to mid-evening.

* Beautiful weekend with mild days, cooler nights, dry weather and
  low humidity levels.

* Growing confidence in a period of hot and humid weather with
  excessive heat possible Tue thru Thurs, with little nighttime
  relief. Potential exists for heat indices in the low-100s in some
  spots with nighttime lows in the low to mid 70s.

Details:

Friday:

Forecast models continue to point to Fri being a active day as a SW
to NE oriented surface cold front and associated weak-amplitude
shortwave disturbance act on increasing moisture content and temps
in the 80s through strong insolation. While the first roughly half
or so of the day should be rather warm and dry, our weather turns
more active during the afternoon to mid-evening hours with the risk
for strong to perhaps severe t-storms. There are still some
uncertainties to iron out, including the timing of greatest threat
and the degree to which we can destabilize, as well as the typical
sub-synoptic-scale uncertainties inherent to a Day-3 thunderstorm
forecast.

On the larger-scale, have relatively weak low-level flow but quite
strong mid to upper level winds leading to SWly sfc-6 km shear
magnitudes around 35-50 kt, with higher values lagging the frontal
boundary. CAPE values could reach into the 1000-1200 J/kg range per
the SPC SREF mean, although those probabilities are on the lower
side, greatest in interior CT/MA. Biggest potential limitation is
the progged instability being on the lower side, but it could still
be sufficient for at least an isolated strong/severe t-storm risk.
Hydrologically, there are some meteorological ingredients which
could favor isolated to scattered instances of flash flooding even
if antecedent conditions aren`t necessarily favorable. For one,
there is a decent pool of PWAT values around 1.5-1.8 inches.
Secondly, warm cloud depths reach into the 10,500-11,000 ft
range which favor potentially efficient rains driven through
warm cloud rain processes. The speed of the mid-upper level flow
favors faster- moving individual storms, although the boundary-
parallel orientation of the steering flow to the cold frontal
boundary itself suggests re- development or backbuilding is a
possibility. So besides the isolated severe weather potential in
the form of localized straight line wind damage, we also appear
to have a favorable meteorological setting for street flooding
and some potential for flash flooding in a
backbuilding/redeveloping convective situation. It appears the
greatest potential for t-storms for areas north and west of
I-95, perhaps into the Boston to Providence corridor in a
weakening state. Added enhanced wording for heavy rain and gusty
winds, meshing well with the Colorado State Machine Learning
progs and SPC Convective Outlook and WPC Excessive Rain Outlook.

Front moves offshore later Friday night into the pre-dawn hrs, with
decreasing rain showers and falling dewpoints. Lows mid 50s to low
60s.

The Weekend:

Large 1020+ mb high pressure cell builds into New England in the
wake of the frontal passage from Fri/Fri night. It sets the stage
for a beautiful weekend with mild days, cooler nights and low
humidity levels. Highs mainly in the 70s, perhaps near 80 in some
interior spots, with lows in the upper 40s to mid 50s. Thus a really
nice weekend in store. If you haven`t already done so, the weekend
would be a good time as any to put in any window air-conditioning
units as our weather pattern changes toward very warm and humid
weather into next week.

Next Week:

There remains strong multi-model/multi-ensemble agreement on a
significant warmup toward very warm to hot weather along with
increasing humidity levels. This stretch of heat and humidity looks
centered during the Tue-Wed-Thurs timeframe, and we could be talking
about the heat and humidity turning to excessive levels on these
days. By Wed and Thurs, high temps away from the south coast could
reach well into the 90s with potential for 100 degree heat in the
Merrimack and CT Valleys with little nighttime relief.

There remain details that could greatly shape the heat and humidity
forecast in this time period that are unknowns, including potential
for seabreezes, if we can fire off any t-storms given the warmth and
humidity, as well as the potential effect of cloud cover. It looks
like SWly winds are strong enough to keep the eastern MA seabreeze
offshore, but we`ll have to see in subsequent guidance on that
aspect.

Still, what we can say is that there is quite a signal for
increasing heat and humidity next week - in fact, low-level temps
are approaching values which are outside model-forecast climatology
for late June. Although there are still uncertainties on cloud cover
and potential convection, the strong model signals elevates the
potential for an anomalous heat event by mid/late June standards,
and we could be talking heat indices in the lower 100s in some
locations.

&&

.AVIATION /19Z WEDNESDAY THROUGH MONDAY/...
Forecast Confidence Levels:

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

18z TAF Update...

Today: High Confidence

VFR. Light west winds with localized sea breezes this morning on
the immediate coast. A few spot showers later today especially
in eastern MA in the vicinity of the coastal front.

Through Thursday...

Spot shower with diurnal cumulus around 4kft this afternoon will
dissipate quickly after 23Z. Sea breeze will also dissipate
after that time, with winds becoming S/SW across the entire
region overnight. SW winds prevail into Thursday with drier air
aloft, which will mitigate the coverage of diurnal clouds. The
exception to this will be along the south coast and islands
where some stratus/fog is possible mid day Thursday. VFR
everywhere except perhaps Nantucket which may go IFR to LIFR in
fog.

KBOS TAF...High confidence in TAF.

VFR with diurnal cumulus until 23Z. Very brief seabreeze
possible at BOS around 16Z tomorrow but will be short lived,
with winds increasing out of the SW tomorrow afternoon.

KBDL TAF...High confidence in TAF.

VFR with WSW/SW winds; light through mid day Thursday before
increasing to around 10kt. Diurnal cumulus dissipates around 23Z
tonight.

VFR. Light west winds.

Outlook /Friday through Monday/...

Friday: Mainly VFR, with local IFR possible. Breezy. Chance
SHRA, chance TSRA.

Friday Night: Mainly VFR, with local IFR possible. Chance SHRA,
slight chance TSRA.

Saturday through Monday: VFR.

&&

.MARINE...
Forecaster Confidence Levels:

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


Tonight:

Calm conditions with continued WSW winds near 10 knots at seas
1-2 feet.

Tomorrow:

Increasing SW winds gusting to 20 knots. Seas increase to 2-4
feet.

Friday:

Cold front approaches the waters Friday afternoon and early
evening. Low risk for small craft conditions across the
northeast waters with gusts to 25kt possible. Seas approaching
but generally less than 5 ft.

Outlook /Friday night through Monday/...

Friday Night: Winds less than 25 kt. Chance of rain showers.
Wind shift from the SW to the NW expected behind passing cold
front.

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

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

Sunday through Monday: Winds less than 25 kt.

&&

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

&&

$$

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