458
FXUS61 KBOX 160150
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
950 PM EDT Wed May 15 2024

.SYNOPSIS...
Steady rains spread in tonight as low pressure moves south of
SNE. Rainfall could be heavy at times late tonight into early
Thursday across CT, RI and perhaps into SE MA with some embedded
thunder. Rain tapering off Thursday afternoon into the evening
gradually from north to south. High pressure nosing southward
from the Canadian Maritimes will result in mainly dry weather
Friday through the weekend with nothing more than a spot shower
or two. It will be cool along the immediate coast with onshore
flow through the weekend, but pleasant across the interior.
Warmer temperatures are expected for the first half of next week
with mainly dry weather continuing.

&&

.NEAR TERM /THROUGH THURSDAY/...

10 PM Update...

With light rain pushing into Worcester and Boston and moderate
to even heavy rain draped across RI and southern CT,
precipitation has extended farther north than depicted by much
of the hi-resolution guidance from earlier today. More recent
runs of the CAMs, like the 00Z HRRR, continue to struggle with
the placement of the axis of heaviest precipitation, but do show
the possibility  of brief lull in moderate-heavy rainfall
across SE MA/RI after 04Z, which would be beneficial in
alleviating the flooding potential in urban areas. Forecasters
are eagerly awaiting the 00Z SPC-HREF/QPF PMM guidance to better
grasp overnight precip potential.

As for rainfall totals, 6 hr QPF across southern RI has crept up
on the 3/4" mark with as much as 1/3" of rain falling in places
like Westerly over the last few hours. Fortunately, this amount
of rain has not yet translated to significant returns on CREST
unit streamflow.


Highlights...

650 PM Update...

* Steady rain spreads in late tonight into early Thu. Locally heavy
  rain possible across CT/RI and perhaps into southeast MA. Risk of
  poor drainage and nuisance flooding. Low prob of more significant
  urban/small stream flooding if higher rainfall amounts can be
  realized.

* Rain gradually tapering off on Thu from north to south as low
  pressure meanders further to the south/southeast

Previous forecast is on track. Showers...mainly light were
impacting parts of CT/RI and SE MA early this evening. The heavy
rain was now impacting Long Island and will slowly lift
northward impacting parts of CT/RI late this evening and
especially after midnight into Thu morning. Prior discussion
discusses this below.

Upper trough will lift toward southern New England overnight
and cut off just south of southern New England into early Thu.
The cutoff will gradually rotate south/southeastward later on
Thu as a ridge builds into the eastern Great Lakes/Mid Atlantic.
A frontal boundary is parked nearby our region while low
pressure meanders off the Mid Atlantic coast. The low will
gradually lift toward Long Island tonight before sliding further
offshore on Thu.

Guidance continues to handle the immediate near term quite poorly,
so confidence in the near term a bit lower than would like. Biggest
change to the forecast was to significantly increase QPF amounts and
have also increased PoPs. Main concern is the potential for poor
drainage/nuisance flooding across CT/RI and perhaps into SE MA late
tonight into early Thu. Though if higher rainfall totals are
realized than currently forecast and shown by CAMs then we could
have a more significant Urban/Small Stream flooding event occur.

Overall there aren`t any big glaring signals in the PWATs with both
the NAEFS/EPS guidance only 1.5-2 STD above model climo per the
situational awareness table. This translates to roughly 1-1.5 inch
PWATs, which is elevated but not overly impressive for this time of
year. Despite this our 925 hPa jet is anticipated to increase to
roughly 25-50 kt E to SE oriented LLJ. The GEFS is indicating a
roughly 3-4 STD E/W component of the wind, which impinges on the
south coast late tonight into early Thu. Will have this on top of a
relatively stagnant synoptic setup as a cutoff low at 700 hPa
developing just to our south tonight. During the winter time these
situations result in "mid level magic", which generates a heavier
band of precipitation. This time of year it can occur as well, but
the big question is the baroclinicity and where exactly does this
heavy band set up? At this point signs point to CT/ RI and perhaps
into southeastern MA per ensemble guidance. The HREF, NCAR MPAS and
C-SHiELD all show low to high probs (10-70+ percent) of total QPF
AOA 1 inch from roughly the MA Turnpike south with the highest probs
across the immediate south coast of RI/CT. These pieces of guidance
even are showing low to mod (10-50+ percent) of total QPF AOA 2
inches, again with the best shots across southern RI/CT. This even
matches up with the EPS and to some extent the GEFS/GEPS guidance
though the probs are much more muted. Based on these signals blended
the NERFC forecast with the latest NAM guidance, but if the LPMM per
the HREF is correct then these totals may need to be increased
further. For now am advertising 1-3 inches of rain across CT/RI with
the highest amounts across southern RI. Elsewhere anywhere from
roughly 0.25 to 1 inch is possible when precip is all said and done.

The big question will also be the convective influences. There isn`t
a whole lot of instability, but given the LLJ and forcing could see
some embedded thunder within the rain shield. Could also have some
efficient rain with warm cloud layer depths of 2.5-3.5 km. These
factors would increase the heavy rain and flood threat. Despite
these factors am not confident in the exact location and the
heaviest rains could be quite localized, so have held off on a Flood
Watch in the latest update. Though we will need to closely monitor
things tonight into early Thu.

Should see the precipitation gradually tapering off during the
afternoon as the low pulls away. So those precip chances will
diminish from north to south, but it may not be until Thu evening
where areas south of the MA Pike become dry. Given the stronger
E/ENE LLJ in the latest update have increased wind speeds/gusts, so
will be a bit breezy on Thu and feel quite cool given the onshore
flow.

&&

.SHORT TERM /THURSDAY NIGHT/...
Highlights

* Any lingering rain showers across the south coast push offshore.
  Cloud cover gradually decreasing with easing winds.

Ridge axis over the eastern Great Lakes/Mid Atlantic begins to build
into western New England by early Fri. Caught between high pressure
gradually building in from Upstate NY/northern New England and low
pressure meandering SE of our region.

Low pressure continues to move further offshore, which will allow
the high to gradually build in. This is anticipated to bring an end
to the shower activity across the south coast Thu evening. Should
see the cloud cover gradually diminishing, but there is some
uncertainty here. Onshore flow could bring in a marine stratus
layer, especially across eastern areas into early Fri. For now have
kept the forecast more optimistic, but may need to be fine tuned in
future updates especially if the high is weaker than currently
forecast and isn`t able to pump in the drier low level air.

Will still be a bit breezy during the evening, but should see those
wind speeds/gusts diminishing as high pressure relaxes the pressure
gradient.

&&

.LONG TERM /FRIDAY THROUGH WEDNESDAY/...
Key Points...

* Dry weather dominates Fri/Sat/Sun with perhaps a spot shower or two
* Highs Fri/Sat/Sun...55-65 immediate coast & 65-75 further inland
* Warmer the first half of next week with mainly dry weather

Details...

Fri/Sat/Sun...

High pressure over the Canadian Maritimes will nose southwestward
into our region Fri into the weekend. This will combined with a low
pressure system well to our south to generate onshore flow. This
should hold high temperatures mainly between 55 and 65 along much of
the immediate coast Fri and into the weekend. Temperatures inland
are more tricky and will be dependent on the amount of solar
insolation. There will be clouds at times with the onshore
flow...but also may see drier air circulating around the high
pressure system in the Maritimes. So away from the coast...current
thinking is highs will run between 65 and 75 and whether we end up
on the lower/higher side of those numbers will be dependent on the
amount of sunshine received.

There is not much synoptic scale forcing or deep moisture around to
support much precipitation Fri into the weekend. Perhaps just a spot
shower or two...but dry weather should dominate.

Mon/Tue/Wed...

It looks like the first half of next week should feature warmer
temperatures as some upper level ridging builds into the region. The
magnitude of the warmth remains uncertain as some shortwave energy
will be moving across the mid-Atlantic states which may act to
temper it. Nonetheless...temperatures probably average above normal
with highs in the 70s to perhaps near 80 at times at least away from
the immediate coast and any marine influences. Thinking dry weather
continues to dominate given upper level ridging and lack of synoptic
scale forcing.

&&

.AVIATION /02Z THURSDAY THROUGH MONDAY/...
Forecast Confidence Levels:

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

00z TAF Update...

Tonight...Moderate confidence.

Conditions deteriorate to MVFR/IFR from south to north and
perhaps even some localized LIFR possible after 06Z as steadier
rains push in from the south. Could see an embedded t-storm or
two overnight into Thu am push for portions of the south coast,
but given the limited instability the risk was too low to
include in the TAF forecast. Heavier rains possible across CT,
RI and perhaps into SE MA. Winds shift to the E and increase to
around 10-15 kts. Will have some 20-25 kt gusts along the
immediate south coast late.

Thursday...Moderate confidence.

Expecting to start the day with MVFR to IFR conditions and
perhaps some localized LIFR with a steady rain. Should see this
precip shield gradually collapsing as the day progresses, but
still could have some spotty showers linger into the afternoon.
Ceilings will gradually improve to MVFR/VFR by the afternoon
from north to south, but it will be a slow process given the
prolonged onshore E to ENE winds. Will have speeds of 10-15 kts
with gusts of 20-25 kts and the highest speeds along the
immediate south coast.

Thursday Night...Moderate confidence.

Should see conditions improve to VFR for most. Any lingering
showers across the south coast should push offshore before
midnight. Given the prolonged onshore flow there is potential
that eastern MA could see some MVFR/IFR stratus/fog. For now
have kept in VFR range, but something we will need to hone in on
as we get closer as it would likely impact the Fri AM push.
Winds ENE to NE at 5-10 kts.

KBOS TAF...Moderate confidence in TAF.

Conditions lowering to MVFR/IFR tonight as steadier rains move
in and will impact the Thu AM push. Winds shift to the E/ENE
tonight. Winds remaining out of the ENE on Thu with rain showers
gradually tapering off and ceilings slowly improving to
MVFR/VFR.

KBDL TAF...Moderate confidence in TAF.

MVFR conditions will gradually lowering to IFR tonight and
winds shift from the SE to the E/ENE. Winds becoming gusty Thu
morning through the afternoon, but showers gradually tapering
off and slowly improving from IFR to MVFR/VFR.

Outlook /Friday through Monday/...

Friday: VFR. Breezy.

Friday Night through Saturday: VFR.

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

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

Sunday Night: Mainly VFR, with local IFR possible. Breezy.
Slight chance SHRA.

Monday: VFR.

&&

.MARINE...
Forecaster Confidence Levels:

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

Have expanded the SCA to include Buzzards Bay and Vineyard Sound
due to the strong E/ENE winds/gusts.

Tonight...High confidence.

Increasing E/ENE winds 15-25 kt toward Thu morning. Winds
gusting 15-30 kts with the highest gusts across the southern
waters. Could even have some brief gale force gusts across the
southern outer waters into Thu AM. Could also see some embedded
thunderstorms across the southern waters. Vsby lowers to 1-3
miles in areas of fog and rain, especially southern MA and RI
waters.

Thursday...High confidence.

E/ENE winds 15-25 kts, gusts 20-30 kts possible. Areas of rough
seas with rain. Could still see some embedded thunderstorms
across the southern waters through the first half of the day.
Rain tapering off from north to south across the eastern waters
during the afternoon.

Thursday Night...High confidence.

Rain tapering off across the southern waters north to south as
the night progresses. Winds ENE to NE at 10-20 kts with gusts
15-20 kts gradually easing as the night progresses.

Outlook /Friday through Monday/...

Friday through Saturday: Winds less than 25 kt. Seas up to
5 ft.

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

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

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory from 2 AM to 8 PM EDT Thursday for
     ANZ231>234-236.
     Small Craft Advisory from 2 AM to 11 PM EDT Thursday for
     ANZ235-237-254.
     Small Craft Advisory until 11 PM EDT Thursday for ANZ255-256.

&&

$$

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