036
FXUS61 KBOX 180248
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
1048 PM EDT Sat Aug 17 2024

.SYNOPSIS...
Dangerous rip current continue due to distant Ernesto through
Sunday. Spotty showers possible across western/central MA and
CT tonight. Otherwise dry with low clouds and fog. Low clouds
slow to erode on Sunday due to persistent onshore flow. A slow
moving frontal system moving in from the Great Lakes will bring
several opportunities for showers and thunderstorms Sunday
evening through Tuesday. Locally heavy downpours are possible,
but severe weather potential is very low. Drier weather for the
middle portion of the week with cooler than normal temperatures.
Warming trend for Friday into the weekend but with continued
generally dry weather.

&&

.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
Stratus was located over the coastal waters east of MA late
this evening, and is expected to move onshore overnight.
Scattered showers over northern NJ and southeast NY state should
also arrive later tonight as well. The last few runs of the
HRRR seems to have the right idea, but perhaps is not perfect
with the timing. Generally liked the focus across portions of CT
and western and central MA, so used that idea to tweak rainfall
chances overnight.

Minor tweaks to temperatures to reflect observed trends.

730 PM Update:

Starting to see stratus re-develop and/or return back landward
from the coastal waters now that the sun is starting to set and
the boundary layer begins to cool. The trend will be for
continued SE to NW advance of stratus in general thru late
evening/midnight. Still an open question on the extent of fog,
but I think odds are against dense fog coverage. MOS products
seem to hit the potential the hardest, but recent GFS LAMP
output is starting to back off on the visbys. I`d feel somewhat
more confident about about larger dense fog coverage if the
near- surface lapse rates were more inverted/stable than what
the NAM/RAP/HRRR indicates in the lowest 2000 ft AGL. Nonetheless,
a pretty gray evening is expected with light southeast winds.
Though there are a few showers in the lower Hudson Valley,
they`re moving into the surface ridge axis and dissipating as
they do so, and I`d expect that to continue with dry weather by-
and- large.

Previous discussion:

Highlights

* Dangerous Rip Current Risk continues.

* Low clouds and stratus return. Spotty light showers across
  central/western MA and CT.

Cyclonic flow in place with a trough parked over the Great Lakes
region. A shortwave trough will lift from the eastern Great
Lakes/Mid Atlantic this evening into New England tonight. A
broad low is located over the central/eastern Great Lakes while
we remain under persistent onshore flow.

Anticipating dry and quiet weather for much of the region
tonight. Will gradually see PWATs increasing to roughly 1.5 to
1.75 inches across western/central areas tonight. This is also
where we will have a roughly 15-25 kt southerly low level jet
impinging on the region. Did adjust our precip chances based on
this setup, but think best shot for showers is across western
MA and CT. Main concern is we are quite removed from strong
forcing, which is why activity overnight may be spotty in
nature. Wondering if some guidance hinting more at a more
widespread drizzle vs showers. At this point given the moisture
available went with showers. Low temperatures in the 60s.

One thing we will need to keep a close eye on is the stratus
deck advecting onshore across eastern areas. Some pieces of
guidance, GLAMP/HRRR, show patchy dense fog moving in. Did lower
our visibilities a bit in the latest update, but did not go as
low as latest GLAMP/HRRR at this time. Will need to monitor as
night progresses.

&&

.SHORT TERM /6 AM SUNDAY MORNING THROUGH SUNDAY NIGHT/...
Highlights

* Dangerous Rip Current risk continues due to distant Ernesto.

* Spotty AM showers across western/central areas diminishing.
  Dry for much of Sun. Near to slightly cooler than seasonable
  temperatures.

* Showers along with isolated thunderstorms spread in from west
  to east Sun Night into Mon.

Cyclonic flow lingers across the Great Lakes and New England
through this timeframe. The trough over the central/eastern
Great Lakes early on Sun will slowly slide into the eastern
Great Lakes/Mid Atlantic by early Mon. Another shortwave lifts
from the Mid Atlantic late on Sun into New England Sun Night. A
slow moving frontal system will gradually slide into Upstate NY
on Sun/Sun Night.

Clouds slow to erode across southern New England on Sun due to
persistent onshore flow, but thinking improvement will be faster
than on Sat given there should be better mixing within the
boundary layer. In addition, think any lingering showers/drizzle
across western/central areas Sun AM will exit as the first
shortwave moves off to the northeast. High temps may be a few
degrees warmer than today given the better mixing and skies
clearing a tad bit faster. Highs range from the mid 70s to low
80s. Main concern along south facing beaches continues to be the
high risk for rip currents due to the increased swell from
distant Ernesto.

Return of more unsettled weather late Sun into Mon as another
shortwave lifts into the region. Will have a better shot for
more widespread showers as the broad low moves into Upstate NY.
Will see PWATs increasing to roughly 1.5-2 inches as the trough
approaches and southerly flow increases. There is a bit more
instability available with a few hundred J/kg of MUCAPE in
place. Not out of the question there are some heavy downpours
given the environment, but think risk greater during the day
Mon. See the Long term section for more of these details. Lows
in the 60s.

&&

.LONG TERM /MONDAY THROUGH SATURDAY/...
Highlights:

* Slow-moving frontal system brings mostly cloudy weather and
  several opportunities for showers/t-storms Mon and Tue.
  Severe weather potential is low, but localized downpours
  leading to isolated instances of flooding could be possible.
  Temps seasonable but humid.

* Drier weather with cooler than normal temps Wed and Thurs.

* Continues dry Fri into the weekend, but with a warming trend.

Details:

Monday and Tuesday:

This is the more active period of weather in the long-term forecast
period. Mid/upper level trough and its associated surface frontal
system initially over NY/PA to slowly progress eastward and offshore
through Tue. Although the forecast PoPs (Likely/Categorical Mon,
then solid Chance for Tue) are pretty broad-brushed, guidance seems
to convey some agreement in there being multiple opportunities for
showers and embedded t-storms in this timeframe. One is with an
approaching warm front which moves in Mon AM to early aftn with a
related rise in deep moisture/PWAT values (to around 1.5 to 1.7
inches). We then should see more broader warm-sector-airmass showers
and embedded t-storms for Mon aftn to evening, and while not
everyone will see rains in this general timeframe, there will be
showers and storms around. The final period of showers and storms,
perhaps energized by disorganized trough energy over the lower mid-
Atlantic region, comes in on Tue before the cold front slips
offshore later Tue aftn or early night.

Convective instability for both days looks to be tempered by reduced
diurnal heating from lots of cloud cover, and wind shear also isn`t
supportive of organized severe weather. However there should be
enough instability for lightning, with forecast MUCAPEs in the 500-
1000 J/kg range. With several opportunities for showers and t-storms
in this period, infused by elevated PWATs and warm cloud depths
around 11-12 kft, localized instances of flooding could need to
be monitored in this period; but with that said, models show
varied placement of QPF bullseyes and that makes it difficult to
provide any location or timing specifics at this juncture.
Particularly for Tue, consideration will also have to be given
to where prior heavy rain footprints took occur/where FFG is
locally reduced, which are obviously unknown at this time.
Trained machine- learning progs also indicate limited/marginal
potential for excessive rainfall.

Temperatures in this period should run near to slightly cooler than
normal (could see highs only in the 70s, with upper 60s/low 70s
Tue), but it won`t feel that way with elevated humidity levels until
the cold front clears us out by Tue evening.

Wednesday and Thursday:

Cyclonic flow aloft associated with a shot of anomalously cool 925-
850 mb temps then settles in over the Northeast states, with surface
high pressure supplying MUCH drier conditions. Overall a pretty
pleasant stretch of weather that may feel more like a typical day in
mid-September than mid-August. Will have to watch for
nighttime/early morning fog potential with a pattern of clear skies,
light winds favored each night and wetting rains from early in the
week. Highs mainly in the 70s both Wed and Thurs, with lows in the
50s, perhaps some mid/upper 40s NW MA and the sites which typically
radiate well at night under clear/calm conditions.

Friday into Next Weekend:

Drier stretch of weather continues into this forecast period as
broad high pressure shifts seaward over the mid-Atlc waters. Warming
trend closer to above normal temps then becomes more favored by late
in the week into the weekend.

&&

.AVIATION /03Z SUNDAY THROUGH THURSDAY/...
Forecast Confidence Levels:

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

Tonight: High confidence in trends, moderate in timing.

Gradual deterioration toward IFR/LIFR conditions (occurring
soonest with the worst categories S/E of I-95), mainly in
stratus. Development of fog is still a low-confidence prospect
and with GLAMP now starting to back off on reduced visbys, its
coverage may end up being more sparse than last night/this
morning. Light E/SE winds.

Sunday...Moderate confidence.

IFR stratus to start will slowly lift to MVFR with some spotty
VFR as onshore flow continues. Though we may mix enough in
eastern areas where we lift those lower ceilings more firmly to
VFR levels. Spotty AM showers out in western areas. SE winds
5-15 kts.

Sunday Night...Moderate confidence.

Return of IFR to LIFR stratus and fog with persistent onshore
flow. Showers and isolated thunderstorms spreading in from west
to east as the night progresses. E to SE winds around 5-10 kts
or less.

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

Gradual deterioration to MVFR stratus decks, then a return to
IFR decks later tonight which continue into the early Sunday
morning hrs. Slow improvement to MVFR on Sun due to persistent
onshore flow. SE winds 5-8 kt.

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

Borderline MVFR ceilings linger heading into this evening.
Spotty rain showers along with ceilings lowering to MVFR/IFR
conditions roughly through 05Z. Slow improvement to higher MVFR
levels on Sun due to prolonged southeasterly flow.

Outlook /Monday through Thursday/...

Monday: Mainly MVFR, with local IFR possible. SHRA, scattered
TSRA.

Monday Night through Tuesday: Mainly VFR, with local IFR
possible. Chance SHRA, isolated TSRA.

Tuesday Night: VFR. Chance SHRA.

Wednesday through Thursday: VFR.

&&

.MARINE...
Forecaster Confidence Levels:

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

Tonight through Sunday Night...High confidence.

Expecting persistent SE flow at 5-10 kts through the vast
majority of the period. Wind gusts remain generally below 20 kts
as well. Seas remain elevated due to the powerful swell
associated with Ernesto. This keeps 5-7 ft seas across the outer
waters through Mon, so have extended the Small Craft Advisory.
This is also the case for the nearshore waters across coastal
Rhode Islands where 3-5 ft seas linger. Rain showers along with
thunderstorms could spread into the waters Sun Night.

Outlook /Monday through Thursday/...

Monday: Winds less than 25 kt. Areas of rough seas. Chance of
rain showers, isolated thunderstorms.

Monday Night through Tuesday: Winds less than 25 kt. Seas up to
5 ft. Chance of rain showers.

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

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

Wednesday Night through Thursday: Winds less than 25 kt.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...High Risk for Rip Currents from 6 AM EDT Sunday through Sunday
     evening for MAZ007.
     High Risk for Rip Currents through Sunday evening for MAZ022>024.
RI...High Risk for Rip Currents through Sunday evening for RIZ006>008.
MARINE...Small Craft Advisory until 8 PM EDT Monday for ANZ235-237.
     Small Craft Advisory until 8 AM EDT Tuesday for ANZ250-254>256.

&&

$$

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