-- Discussion containing changed information from previous version are highlighted. --
493
FXUS61 KBOX 280546
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
146 AM EDT Tue May 28 2024

.SYNOPSIS...
Cold front brings heavy rain and thunderstorm chances to SNE
tonight. Warmer and brighter for Tuesday with a chance for an
isolated shower/t-storm across northern MA. Temperatures then
cool off to more seasonable levels into the mid to late week
period. Though dry weather should generally prevail, there are
better chances for rains develop on Wednesday night into
Thursday. Mostly sunny and dry weather conditions for the
weekend into early next week, with a warming trend to
temperatures.

&&

.NEAR TERM /UNTIL 6 AM THIS MORNING/...
Update: 1035pm

Widespread light to moderate rain continues to fall across much
of southern New England, a few pockets of heavy downpours too,
example; ASOS at Westerly, RI reports 0.26" of rain in less than
half an hour. Good news, these storms are moving fairly quickly,
based off KBOX storm motions are roughly 40 knots, thanks to a
strong LLJ. As mentioned, any thunder has been tough to come by
and the latest products show the core of the lightning southwest
of Block Island. These storms are moving from SSW to NNE, would
bring these heavier showers into southern RI and southeast MA
in the next couple of hours.

Thinking these showers and isolated rumbles of thunder will be
exiting west to east 07z-10z. Over all, no changes were made and
the prior forecast discussions remain valid.

See below for additional information.


735 PM Update:

Radar, lightning and METAR trends show a band of increasingly
steadier and moderate to bursts of heavier rains over Tolland
and Hartford Counties in CT and into adjacent sections of
central and western MA, although in an air mass of very weak
instability, little to no lightning has been observed. Ended up
backing off on the thunder risk in the zone forecast for most
locations tonight with this update and overnight as model
forecasts from the NAM and RAP are pretty sparse on most-
unstable-parcel CAPE. The possible exception comes in around
midnight and overnight into RI and adjacent portions of
southeast MA and the waters where the NAM and RAP show about
600-800 J/kg of most-unstable CAPE. HRRR and the NAM-3km also
show indications for thunder in this general area, and although
we`re not expecting storms to reach severe levels for those
areas, late- night boomers here could still be possible. The
rather humid air mass leading to high warm cloud depths seems
apt to favor downpours in any heavier showers and isolated hydro
issues could still result; the good news is that the showers
are moving along at a decent clip so think anything worse than
isolated instances of flooding are probably going to be hard to
come by, even in the more flood- prone areas. Given the strength
of the SE to S winds tonight, am becoming less convinced on
there being foggy areas for the overnight, and it could be quite
patchy if any develops, even over the waters.

Still going to be rather humid for most of the night with low
temps in the lower to mid 60s, even with the front nearing the
coastline by daybreak.

Previous discussion:

Low pressure continues to track to our north this evening into
southern Canada as associated cold front closes in on southern New
England from the west. Warm sector remains south and west of our
region, which will help mitigate the severe weather threat, but does
not totally limit the convective potential this evening.

Robust convective line has already developed in a NW to SE
orientation form the Syracuse to NYC area which is generally
well depicted in the 12Z NAM Nest, NSSL WRF, and ARW members of
the HREF. This line will continue to lift into the region over
the next several hours, with prefrontal downpours overspreading
SNE between 21-01Z. Convection will struggle to maintain
strength as it pushes east as we lack surface based instability
given our position in the cool sector and very very poor low
level lapse rates between 4.5- 5.5C/km, resulting in mainly
elevated convection that will capitalize on ~600J/kg MUCAPE.
This is good news, as storms will not be able to tap into low
LCLs, below 1000ft, and strong 0-1km shear present across
Upstate NY/PA, significantly curbing any tornado threat as the
front moves east.

Greatest threat with this cold front remains heavy rain with PWATs
climbing to around 2" and high warm cloud depths approaching
12,000ft. Aforementioned conditions combined with a southerly
oriented LLJ of ~40kt will lead to embedded tropical-like
downpours this evening and early overnight. Fortunately, this
front is quite progressive, looking to be offshore by ~07/08Z,
which will mitigate any significant flooding concerns. With that
said, 1-2" of rain over an urban center like Providence or
Springfield could lead to some isolated poor drainage flooding.
Moderately strong jet will be able to mix down in heaviest
downpours, yielding gusts in excess of 25kt along the frontal
boundary tonight.

Residual moisture, with dewpoints still in the upper 50s/low 60s,
and flow veering to the SW behind the front will lead to marine fog
development again overnight. Strong southerly flow, in excess of
10kt, will mitigate the fog threat across the interior, but do
expect some dense fog development overnight along the south coast
and Cape.

&&

.SHORT TERM /6 AM THIS MORNING THROUGH 6 PM WEDNESDAY/...
While some stratus and fog will hang on for the first part of the
morning, southerly flow and mid level dry air will begin to scour
out the clouds between 12-15Z, leading to a brighter and warmer day
for much of southern New England! Overnight`s cold front will stall
offshore, and given proximity to the Cape and Islands results in
cloud cover sticking around until at least the early afternoon,
though, not too optimistic that Nantucket will break into sunshine
at all. With SW surface flow and WAA, will see temperatures climb
into the upper 70s and low 80s away from the coast, and with SSTs
approaching 60F, even the south coast looks mild tomorrow, in the
upper 60s!

Trough begins to sag south from Canada tomorrow afternoon which may
initiate some spotty shower and thunderstorms across far northern MA
tomorrow afternoon and evening with ~600J/kg MUCape and low level
lapse rates approaching 8C/km. Shear will definitely be present,
with 0-6km bulk shear exceeding 50KT between 18-00Z, but mid level
lapse rates are quite poor, less that 6C/km as dry air continues to
work into the mid levels. Given this limiting factor, expecting low
topped convection at best that may result in isolated showers and a
few rumbles of thunder. Will note that instability is much higher
across SE MA/RI where temps/dew points will be a few degrees warmer,
with MUCAPE growing in excess of 1000J/kg, but shear and lapse rates
are not colocated with that axis of greatest CAPE. If showers are
even able to develop, they will be mainly north and west of
Worcester/I-495.

Winds shift to the W/NW tomorrow night which will significantly curb
the threat for widespread fog, even with dewpoints in the upper 50s
to near 60F. Cant rule out some Marine fog for the Cape and Islands,
but certainly expect it to be much less widespread than the past few
nights.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/...
Highlights:

* Temps cool off toward more seasonable levels with comfortable
  humidity levels Wed thru Fri.

* Stronger indications for rains on Wed night into Thurs, although
  soaking rains aren`t expected. Dry weather prevails otherwise.

* Remains dry this weekend into early next week, but with steadily
  warming temperatures toward above normal levels.

Details:

The mid to late week period will be governed by a deep upper level
trough associated with a pocket of cooler air aloft (e.g. 850 mb
temps in the mid to upper single digits C range). Most of the first
part of Wednesday looks dry, although rain chances start to increase
later in the day associated with a cold front aloft. Really can`t
see too much in the way of QPF with this late-day activity
(indicated about 20-25% PoP), but the rain risk increases Wed night
into Thurs associated with shortwave trough energy rounding the
periphery of the upper longwave trough. While the Canadian GEM is
still a suppressed/southern outlier, the GFS has now depicted
showers for Wed night into Thurs, as indicated previously by the
ECMWF and its ensembles. There is more uncertainty as it comes to
QPF amts, with the ECMWF remaining quite bullish and the GFS being
lighter. Potential is increasing for at least a steadier light rain
event, but probably not looking at a soaking rain situation. Thus
brought PoPs up into the 40-55% range for now for Wed night into
Thurs. This could still be a bit conservative and could need
adjusting upward in future updates. Otherwise, dry weather prevails
into Friday. Temps in this period cool off to more seasonable levels
with no significant temperature anomalies one way or the other. Wed
looks to be the warmest in the 70s to a few low 80s, with a cooling
trend into the lower to mid 70s for Thurs and Fri, with upper 60s
near the coasts with better chances for seabreezes/onshore flow on
Thurs and Fri.

The deep upper-level trough then moves offshore as we move into the
weekend into early next week, allowing for 500 mb heights to rise
along with warming 925-850 mb temps. Expect generally dry weather in
this forecast period as high pressure over the Gt Lakes moves ESE
into the mid-Atlc waters through the weekend and remain in place
into next Mon. As 500 mb heights start to rise and low-level temps
begin a steady warming trend, expect temps to begin to rise back to
slightly above normal levels with highs in the mid 70s to lower 80s
with tolerable humidity levels (dewpoints in the lower 50s). Overall
outlook for the first weekend in June seems pretty favorable for
outdoor plans with dry weather, seasonable to slightly milder than
normal temps and low humidity levels.

&&

.AVIATION /06Z TUESDAY THROUGH SATURDAY/...
-- Changed Discussion --
Forecast Confidence Levels:

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

06z TAF Update:

Through 12z... Moderate confidence.

Moderate to heavy rain continues along and east of I-95 and
thunder primarily over the southern waters, Cape Cod, and the
islands. Rain generally diminishes in most areas between
09-12z, though may linger into Tue AM across a portion of SE MA.
SE to S winds around 10-13 kt with gusts 20-25 kt, and that
should be enough to preclude widespread fog development, though
isolated areas of dense fog does remain possible.

Today... High to moderate confidence.

Cold front stalls across southern New England, generally across
the eastern third, yielding VFR conditions after 15Z, exception
being the Cape and Islands, especially ACK, which will struggle
to shake IFR. Will say, have lower confidence in timing of
improvement of CIGs for Cape and Islands. Brisk WSW flow of up
to 20kt. Widely isolated low topped thunderstorms are possible
in northern MA after 21Z.

Tonight... High confidence.

Generally VFR, the outer Cape and Islands may return to MVFR,
there confidence is not as high. In addition, can not rule out
some marine stratus and fog. Breezy WSW flow continues.

Wednesday... High to moderate confidence.

Generally VFR, CIGs 040-060 ft with chance SHRA or an isolated
TSRA by early afternoon. Winds are W to NW across the interior
and SW to W across the coast.

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

IFR, rain continues through about 07z-09z and shifts offshore;
the IFR cigs likely to continue until ~13z with a more rapid
return to VFR. S winds around 10-13 kt before shifting to SW by
early AM. Wind gusts increase to 18-20 kts after 16z, low chance
for a widely isolated thunderstorm after 21z, confidence remains
too low to include, even in a PROB30.

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

IFR, cigs to trend MVFR by 09-12z with more rapid improvement
to VFR by mid morning. S winds around 10-12 kt before becoming
SW. Wind gusts increase to 18-20 kts after 15z, low chance
for a widely isolated thunderstorm after 21z, confidence remains
too low to include, even in a PROB30.

Outlook /Wednesday Night through Saturday/...

Wednesday through Thursday: VFR. Chance SHRA.

Thursday Night: VFR. Slight chance SHRA.

Friday through Saturday: VFR.
-- End Changed Discussion --

&&

.MARINE...
-- Changed Discussion --
Forecaster Confidence Levels:

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

310 PM Update:

Small craft advisories continue on most waters tonight into Tue,
with SE winds near 25 kt and seas building to around 5 ft.

For tonight, SE winds increase to around 20-25 kt with seas 4-5 ft
offshore and around 2-3 ft nearshore. Winds will shift to SW later
in the overnight as a cold front passes over the waters. Low clouds
and fog could restrict visibility through later this evening. There
is also a risk for thunderstorms capable of lightning and brief
downpours, although severe weather is not expected.

For Tue into Tue night, SW to WSW winds around 15-20 kt are expected
with seas rising to around 5-6 ft on the southern and eastern outer
waters and around 3 ft or less nearshore. Dry weather with good
visibility

Outlook /Wednesday Night through Saturday/...

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

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

Thursday Night through Saturday: Winds less than 25 kt.
-- End Changed Discussion --

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
-- Changed Discussion --
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 11 AM EDT this morning for ANZ232-
     233-235-237.
     Small Craft Advisory until 5 AM EDT Wednesday for ANZ250-
     254>256.
-- End Changed Discussion --

&&

$$

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