562
FXUS61 KBOX 271746
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
146 PM EDT Mon May 27 2024

.SYNOPSIS...
Increasing chances for showers and thunderstorms ahead of an
approaching cold front, mainly late this afternoon into this
evening. This cold front moves offshore early Tuesday, with
decreasing clouds and drier weather conditions. Generally dry
from mid week into the weekend with more seasonable
temperatures. Cannot rule out a few hit or miss showers later
on Wednesday into Thursday.

&&

.NEAR TERM /THROUGH TONIGHT/...
9 AM Update...

Fog has begun to dissipate across most interior zones and
portions of the Cape but continues to hang tough along the
immediate coastline as well as along Narragansetts Bay. With
1/4SM vsbys still observed at many coastal observation sites,
did extend the dense fog advisory until noon today. Further
extensions of the advisory are possible as some hi-res guidance,
like the HRRR, show fog will hang tough though 21Z and will
again redevelop closer to sunset this evening. Overall, not a
particularly stunning Memorial Day for beachgoers.

Aside from the fog, we have seen some light showers tracking
across central/wester MA and CT this morning. A more robust
shield of rain is currently making it`s way into western
Connecticut, but with the LLJ oriented S to N, should mainly
track north into the Litchfield Hills/Berkshires rather than
push east. Still, Hartford and the CT River Valley may see a
period of more widespread rainfall in the next two hours before
we transition back to a more showery/drizzly mode ahead of
approaching cold front this evening.

Previous update...


Regional radar data showed that the last few runs of the HRRR
were too aggressive in triggering convection across our region
early this morning. Based the forecast for next few hours off
the 00Z HREF, as well as the latest RAP and NationalBlend runs.
Thinking there is a risk for at last some showers as a decaying
convective band approaches from eastern PA towards daybreak.
Even lower risk for thunderstorms, but not impossible.

Issued a Dense Fog Advisory for RI and the south coast of MA.

Low pressure is expected to move across the Great Lakes into
southern Canada today. The warm front associated with this low
will struggle to move much north of our region. Noticeably
cooler today, but still just as humid. Most of southern New
England should see less than one quarter inch of rainfall today,
where it rains at all. There is a risk for locally heavier
rainfall from the stronger showers and less frequent
thunderstorms.

Instability will be tough to generate today due to the stratus
and fog which will take time and energy to dissipate. Overall
MUCAPE values should remain less than 500-700 J/kg today,
meaning there is not much of a severe weather threat, other than
locally heavy rainfall.

&&

.SHORT TERM /TUESDAY/...
The cold front associated with a low pressure in southern Canada
should move into our region some time tonight into Tuesday.
Timing this front has been a challenge, as many different
possibilities have been presented over the past few days.
Consensus timing now has this front arriving after midnight
tonight, but still somewhere just east of southern New England
by Tuesday evening. We should not see the humidity diminish
until this front passes completely by our region. Looking at
another round of stratus and fog tonight.

The heaviest and most widespread rainfall is expected this
evening into tonight. Not as much forcing for Tuesday, so
thinking we can get through most of the day without rainfall.

Above normal temperatures continue.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...
Highlights

* Dry for much of Wed with more seasonable temps. Could have
  spotty showers across the interior during the
  afternoon/evening.

* Shot for more widespread rain Wed Night into Thu, but there is
  a lot of uncertainty at this point.

* Overall looking pleasant Fri through the weekend with dry
  conditions. Temps near seasonable levels.

Tuesday Night through Wednesday...

Stuck in blocky cyclonic flow through this period. Will have a
trough over Upstate NY and a shortwave over the western Great
Lakes to start. The first wave lifts northeastward, while the
shortwave digs into the OH Valley. The shortwave begins
interacting with the larger trough over the eastern Great
Lakes/Mid Atlantic by late in the day. A cold front slides
through Tue Night into early Wed. Could see a secondary cold
front or surface trough sliding in late in the day.

Anticipating dry and quiet weather through the vast majority of
this period. Does appear that there will be an opportunity for
spotty showers Wed afternoon into the evening as the surface
trough/secondary cold front is sliding in. Some uncertainty on
this as winds will be W to WNW through the period. Should have a
roughly 15-30 kt LLJ sliding through. This should aid in
keeping us well mixed and drying things out as seen in the
1000-850 hPa RH. Moisture values really not that impressive with
meh PWATs at +/- 1 STD above/below NAEFS/EPS model climo. The
result are PWATs roughly of 0.75 to 0.85 inches. Dialed back our
precip chances to slight for much of the period, though western
areas could see some scattered activity push in toward the
evening.

Only other adjustment made during this timeframe was to
increase temperatures given the W/WNW flow. This should result
in downsloping with 925 hPa temps of 9-15 degrees Celsius. Opted
to bump us up to the 75th percentile of guidance. Highs will
generally be in the 70s.

Wednesday Night through Thursday...

Still in cyclonic flow. Will have that second shortwave/trough
over the Mid Atlantic lifting toward/through southern New
England by early Thu. It continues lifting toward Nova Scotia by
late Thu, though could pivot across the Cape on Thu. A cold
front will be lingering nearby. We could have an area of low
pressure ride along the front, though there is a fair amount of
uncertainty.

Big question mark during this period appears to be when the
trough lifts toward us late Wed and how negatively tilted it
becomes. Guidance is split with the GFS lifting the trough into
the Gulf of ME before it becomes negatively tilted, while the
international guidance as a whole does it as it lifts into/just
south of Long Island. This also results in the trough continuing
to slowly lift through on Thu vs the more progressive GFS. The
international guidance solutions put a baggy trough or perhaps
cutoff nearby southern New England heading into Thu at 700 hPa,
whereas the GFS does not. This cutoff solution would bring the
risk for heavier precip.

Despite these details am not overly amped up at this point
about the rain risk late Wed into Thu. NAEFS/EPS guidance show
PWATs of 1 STD below model climo. Values anticipated to be
generally around 0.75 to perhaps 1 inch. So, not a whole lot of
moisture to work with. Though if we can get the trough nearby
southern New England we could really squeeze out the moisture
available. Should also have some roughly 1.5-2.5 km warm cloud
layer depths. Despite not being amped up about it there is some
ensemble hints at more widespread QPF, especially across the
south coast. The GEFS like the GFS is the least amped up with
only low probs (10 percent) of 24 hr QPF AOA 0.5 inches. The
GEPS/EPS show low to moderate probs (10-50 percent) of 24 hr QPF
AOA 0.5 inches. Both the GEPS/EPS also showing some low probs
of 24 hr QPF AOA 1 inch. So will be something to keep an eye on.
Though at this point not anticipating to have much instability.
High temperatures on Thursday will be right around seasonable
levels.

Friday through the weekend...

Cyclonic flow continues through much of the period. A cutoff
swings through much of New England on Fri as a ridge builds into
the western/central Great Lakes. The ridge axis builds toward
the eastern Great Lakes by late Sat and Sun. High pressure
begins nudging in on Fri, but should build in for the weekend.

Generally am anticipating dry and quiet weather late in the
week and through the weekend with high pressure in control. Do
wonder if the cutoff will be close enough on Fri that we still
can see some spotty showers, but for now think NBM slight
chances suffice. Dry through the weekend with temperatures still
around seasonable levels.

&&

.AVIATION /18Z MONDAY THROUGH FRIDAY/...
Forecast Confidence Levels:

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

Through Tonight...Moderate Confidence.

Scattered moderate to heavy rain showers through ~21Z before
more stratiform rain moves into western MA. Rain will continue
to push east through ~02Z, and 05Z for the outer Cape and
Islands. Approaching cold front will generate a strong line of
thunderstorms capable of producing heavy rainfall and frequent
lightning generally between 01-07Z tonight, with TEMPOs for TSRA
lasting 2-3hours for any given terminal. Rain wraps up quickly
behind the front but its likely the fog will form again across
the terrain and south coast. IFR becoming MVFR by daybreak with
localized VFR possible except in fog where VLIFR will persist.
Winds shift from the SE to the SW behind the front and may gust
to 25kt.

Tuesday... High Confidence. Cold front stalls offshore but will
generally yield VFR conditions after 15Z tomorrow, the
exception being the Cape and Islands, especially ACK, which will
struggle to shake IFR. Brisk SW flow of up to 20kt. Some
isolated low topped thunderstorms possible in northern MA after
21Z tomorrow afternoon.

Tomorrow Night...
Generally VFR, though again cant rule out some marine stratus
and fog. Breezy SW flow continues.

KBOS TAF...High confidence in
TAF trends, moderate in timing.

Fog was very slow to burn off but IFR/VLIFR conditions prevail
given cigs around 003ft. Shower activity increases after 23Z
tonight with a strong line of thunderstorms brining heavy rain
and lightning between 03-06Z. Wind shift from the SW to the SE
expected behind the cold front, gusts to 25kt possible.
Improvement to MVFR and eventually VFR tomorrow.

KBDL TAF...High confidence in TAF trends, moderate in timing.

MVFR deteriorating to IFR tonight as shower activity ramps up
after 21Z. Low chance for afternoon thunder between 21-23Z. Cold
front brings a renewed, greater chance for TSRA between 01-04Z.
Conditions improve to MVFR and eventually VFR behind the front
tomorrow as winds shift to the WSW gusting to 20kt.

Outlook /Wednesday through Friday/...

Wednesday: VFR. Slight chance SHRA.

Wednesday Night through Thursday: VFR. Chance SHRA.

Thursday Night through Friday: VFR. Slight chance SHRA.

&&

.MARINE...
Forecaster Confidence Levels:

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

Through Tuesday...Moderate Confidence.

Dense marine fog expected as winds prevail from the east.
Increased shower activity late today before a cold front brings
the chance for more potent rain and embedded thunderstorms to
the waters late this evening and early Tuesday morning. Winds
increase, gusting between 25-30kt from the east before turning
south behind this front. Waves climb in excess of 5 feet across
the outer waters. Small Craft Advisories remain in effect for
the eastern outer, and southern waters, including Nantucket
Sound late today into tonight.

Outlook /Tuesday Night through Friday/...

Tuesday: Winds less than 25 kt. Seas up to 5 ft. Patchy fog.

Tuesday Night: Winds less than 25 kt. Seas up to 5 ft.

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

Wednesday Night: Winds less than 25 kt. Seas locally
approaching 5 ft. Chance of rain showers.

Thursday: Winds less than 25 kt. Chance of rain showers.

Thursday Night through Friday: Winds less than 25 kt. Slight
chance of rain showers.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory from 9 PM this evening to 11 AM EDT
     Tuesday for ANZ232-233-235-237.
     Small Craft Advisory from 6 PM this evening to 5 AM EDT
     Wednesday for ANZ250-254.
     Small Craft Advisory from 9 PM this evening to 5 AM EDT
     Wednesday for ANZ255-256.

&&

$$

SYNOPSIS...Belk/BL
NEAR TERM...Belk/BL/KS
SHORT TERM...Belk
LONG TERM...BL
AVIATION...Belk/BL/KS
MARINE...Belk/BL