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762
FXUS61 KBOX 092349
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
749 PM EDT Thu May 9 2024

.SYNOPSIS...

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Some showers are possible this evening into early Friday as low
pressure tracks to the south, otherwise drier and cooler for
much of Friday as NE winds develop. Cooler than normal this
weekend with rain chances increasing Saturday night and Sunday,
then milder temperatures return early next week but an unsettled
pattern persists.
-- End Changed Discussion --

&&

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

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745 PM Update...

Caught under cyclonic flow with a shortwave trough lifting from
the OH Valley into southern New England late tonight. Low
pressure slides into the Mid Atlantic.

Could see isolated to scattered showers and perhaps brief
periods of steady rain to the north/northeast of the low sliding
south of our region. NBM probabilities appeared much too high,
especially compared to latest obs. Leaned heavily on latest HREF
guidance, but in particular the NSSL WRF. This is doing well
compared to obs so far. Anticipate showers spreading in from
west to east. Best shot for 0.10" or greater is across western
MA/CT. Elsewhere totals should remain below 0.10". The area of
highest precip also coincides with where PWATs approach 1 inch.

Prolonged onshore flow will bring us cooler temps tonight. Lows
will be in the 40s.
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&&

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

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745 PM Update...

Highlights

* Showers during the AM, especially for the MA-Pike south.
  Should gradually diminish as high pressure builds in.

* Dry and quiet weather Friday night, though a few spotty
  showers/drizzle possible over portions of the immediate south
  coast.

Still in cyclonic flow through this timeframe. A broad trough in
place over the central/eastern Great Lakes early on Fri. This
digs into the Mid Atlantic/Appalachians by late in the day.
Behind the trough a shortwave ridge builds into the eastern
Great Lakes. Low pressure remains south of our region on Fri and
slides further offshore on Fri as high pressure nudges in from
the north.

Main change in the latest update was to deviate from the NBM and
show decreasing chances of showers, especially across much of MA
as high pressure builds in. Consensus of guidance showing a dry
forecast especially with the upper/lower forcing more to our
S/SE. Showers could linger across portions of the south coast
late in the afternoon/Fri night, but wanted to show more of a
downward trend. Not out of the question there is some spotty
drizzle given the onshore flow into early Sat.

On top of the shower chances have also deviated from the NBM on
wind speeds/gusts. We`ve got a fairly decent 20-30 kt ENE/NE low
level jet developing near Long Island and perhaps into
Nantucket. Should not be terribly difficult to mix down per
Bufkit soundings, so have bumped up our wind speeds/gusts. Think
there could be some 20-25 mph gusts during Fri afternoon,
especially across RI and SE MA. Temperatures looked fine in
previous update, so have stuck with widespread 50s and perhaps
some low 60s in the CT River Valley.

Bit of a right turn on the forecast for Friday night. High
pressure building in, so have gone with a dry forecast. There
could be some spotty drizzle, but the NBM was way too high with
PoPs for eastern MA heading into early Sat. Should have a fair
amount of cloud cover until toward daybreak Sat. Lows in the
40s.
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&&

.LONG TERM /SATURDAY THROUGH THURSDAY/...

Highlights

* Dry for much of Sat, but there could be some spotty showers across
  the interior late.

* Hit or miss showers and perhaps a few rumbles of thunder on Sun.
  Could have some graupel/small hail with any stronger storms.

* Dry for much of Mon with a return of seasonable temps.

* Unsettled late Mon into midweek as a frontal boundary slides
  through. Though there is considerable uncertainty in how exactly
  things will evolve.

Saturday...

Stuck under cyclonic flow through this timeframe with a trough over
the central Great Lakes early on Sat and a shortwave ridge over the
eastern Great Lakes/Mid Atlantic. The shortwave ridge builds into
southern New England by the afternoon, while the trough digs into
the eastern Great Lakes/Mid Atlantic. The ridge axis slides offshore
Sat eve. A weak high nudges into our region through much of this
timeframe. A broad low slides into the eastern Great Lakes late in
the day.

Overall anticipating that the weather will be dry and quiet for much
of the day albeit still unseasonably cool due to onshore flow. The
onshore flow coupled with height falls later on Sat and diurnal
heating could bring some spotty showers during the afternoon. Not
super confident on this as forcing is quite weak and AOA measurable
24 hr QPF probs continue to diminish per EPS/GEFS and GEPS guidance
from run to run.

Should have fairly cool air in place with onshore flow. At this
point 925 hPa temps range from roughly 1-8 degrees Celsius with the
mildest temps across the Berkshires. The NBM temps actually seemed
pretty dialed in at this point, so haven`t made any changes. Highs
range from the low 50s along the east coast to the low 60s across
the CT River Valley.

Sunday...

The trough over the eastern Great Lakes late on Sat will lift into
and perhaps through portions of New England during this period.
There is potential that we get a cutoff beginning to set up over the
region. A broad low will be in place nearby or over southern New
England during this timeframe.

Think this is our next opportunity for hit/miss showers and perhaps
a few thunderstorms. We really do not have a whole lot of moisture
to work with as EPS/NAEFS situational awareness tables
indicating that PWATs are around -0.5 to -1 STD below model
climo. This translates to PWATs roughly around 0.5". Though we
do have anomalously low 500 hPa height overhead (-2 STD per both
SATA tables). The 500 hPa trough/cutoff overhead is also quite
cold with temps of -20 to -25 degrees Celsius. This in
combination with diurnal heating should result in hit/miss
shower and thunderstorm activity. Given the cold core in place
do think there could be some graupel/small hail if any more
vigorous showers/storms can develop. Should note that mid level
lapse rates are around 5-7 degrees Celsius with low level lapse
rates around 5-8 degrees Celsius. Though these lapse rates
really depend on the deterministic model being viewed, so there
is considerable uncertainty given the weak forcing. Deep layer
shear in the 0-6 km layer is quite low at this point, so not
anticipating any strong/severe storms. Though given the
environment think there could be some small hail pending how
much instability is in place.

The longer term convective allowing guidance, ie the NCAR MPAS and C-
SHiELD do show max vertically integrated graupel spottiness
over the Northeast of 0.1 to roughly 0.5 inches. Though again
appears very hit/miss in nature. Could have a bit of MUCAPE in
place, but guidance all over the place. Most robust is with the
NCAR MPAS with a few hundred J/kg over western MA/CT. Stay tuned
for future updates.

Monday through Wednesday...

Still caught under cyclonic flow through this timeframe with a
cutoff in place over Quebec/Ontario/Hudson Bay. A ridge axis
flattens out as it is building into the Mid Atlantic on Mon. A
shortwave begins lifting in on Tue and behind it a deeper trough may
slide in for Wed. A weak high nudges in for a good portion of Monday
before a frontal boundary slides in late in the day. The front may
still be sliding in on Tue or overhead and into Wed.

Dry and quiet weather anticipated for much of Mon with high pressure
nudging into our region from offshore of the Mid Atlantic. Late in
the day some guidance begins sliding a front in, but lots of
discrepancy here. So, have just stuck with the NBM at this point in
time. We should rebound temp wise quite nicely to more seasonable
values, ie readings in the 60s to low 70s.

The front will still be overhead and perhaps working its way through
on Tue/Wed. Though there is a lot of uncertainty in how things
evolve as a deeper low could ride along the front bringing us
heavier rains or kicking the front offshore. Ensemble guidance is
also spread with the EPS a bit more amped up (like the ECWMF) with
moderate/high probs (30-80 percent) of 24 hr QPF AOA 0.5 inches into
Wed. The GEPS/GEFS show low probs (10-30 percent) of the same QPF
binning. The EPS even showing some low probs of 24 hr QPF AOA 1
inch. Something to keep an eye on as we get closer, but for now
think that the NBM suffices. Temperatures trending near to above
normal through this period.

&&

.AVIATION /00Z FRIDAY THROUGH TUESDAY/...

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Forecast Confidence Levels:

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

Tonight...High confidence.

VFR to start across all terminals. Will see MVFR ceilings
spreading in from west to east as scattered showers/rain spreads
in. Timing wise is generally looking like 02-04Z, but a few
spots may take a bit longer. Could see some IFR conditions as
well, but think this risk is highest across ACK and the eastern
slopes of the Berkshires. Winds out of the ENE/NE at 5-10 kts.

Friday...High confidence in trends, moderate in timing.

MVFR to start for most terminals, but will gradually trend to
VFR from NE to SW as drier air pushes in. Could have some
lingering showers for the MA-Pike South into the afternoon. Will
take the longest for showers to clear along the south coast.
Winds ENE/NE at 10-15 kts. Could see some 15-25 kt gusts along
the immediate south coast.

Friday Night...High confidence.

VFR everywhere except for the Cape/Islands where MVFR ceilings
may linger. There could also still be some spotty
showers/drizzle over the Cape/Islands to start, but the trend
should be downward as the night progresses. Will have NE winds
diminishing.

KBOS TAF...High confidence in TAF.

KBDL TAF...High confidence in TAF.

Outlook /Saturday through Tuesday/...

Saturday: VFR. Breezy. Slight chance SHRA.

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

Sunday: Mainly VFR, with local MVFR possible. Slight chance
SHRA, isolated TSRA.

Sunday Night: VFR.

Monday through Monday Night: VFR. Breezy. Slight chance SHRA.

Tuesday: VFR. Breezy. Chance SHRA, isolated TSRA.
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&&

.MARINE...

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Forecaster Confidence Levels:

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

Through Friday night...

Northeast winds below 20 kt may increase with gusts 20-25 kt
tonight into Fri, especially over south coastal waters as low
pres tracks to the south. Seas build to 5 ft over the southern
and eastern outer waters Friday AM into Friday night. Due to
this have hoisted a SCA in the latest update.

Outlook /Saturday through Tuesday/...

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

Sunday Night through Monday: Winds less than 25 kt. Areas of
seas approaching 5 ft.

Monday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Seas up to 5 ft. Slight chance of rain
showers.

Tuesday: Moderate risk for Small Craft Advisory winds with
gusts up to 25 kt. Seas up to 5 ft. Slight chance of rain
showers, isolated thunderstorms.
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&&

.TIDES/COASTAL FLOODING...

High astronomical tides coupled with onshore flow may result in some
splashover/very minor coastal flooding during the overnight high
tide cycle tonight. However, wind/waves do not appear high enough to
result in a significant issue. The daytime high tides are lower and
do not pose a threat for coastal flooding.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory from 8 AM Friday to 8 AM EDT Saturday for
     ANZ254>256.
-- End Changed Discussion --

&&

$$

SYNOPSIS...BW/BL
NEAR TERM...BW/BL
SHORT TERM...BW
LONG TERM...BL
AVIATION...BW/BL
MARINE...BW/BL
TIDES/COASTAL FLOODING...