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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS61 KBOX 290533
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
133 AM EDT Thu Sep 29 2022

.SYNOPSIS...
Mainly dry and trending cooler tonight into Friday, as large
high pressure builds into New England. Still uncertain how far
north the rain from Ian gets this weekend, but the best chance
of showers is near the south coast Saturday night into Sunday.
The remnants of Ian are expected to move off the Mid Atlantic
coast early next week.

&&

.NEAR TERM /UNTIL 6 AM THIS MORNING/...
955 PM update...

Mid level trough axis moves into SNE overnight. Sct-bkn mid
level clouds across SNE this evening, with northern extent of
cirrus from upper jet skirting the south coast. 850-700 mb
moisture diminishes overnight which should lead to a clearing
trend and modest radiational cooling overnight, although high
clouds will persist near the south coast. Lows will be in the
40s, except low 50s in the urban areas and along the coast.
This is due to 1030+ mb over the Great Lakes advecting eastward,
precluding winds from decoupling, with winds becoming NNE.

&&

.SHORT TERM /6 AM THIS MORNING THROUGH 6 PM FRIDAY/...
330 PM update...

Thursday...

Mid/upper trough and cold pool overhead at 12z, but these
features ejecting to the northeast, resulting in warming temps
aloft and mid level flow shifting from cyclonic in the morning
to anticyclonic in the afternoon. This should yield less
clouds/more sun Thursday. Although cirrus shield may impact the
south coast from strong upper level jet streak along the coast
and offshore. Still, a very pleasant fall day tomorrow, with
temps just a few degs cooler than today, with highs Thu in the
mid to upper 60s. A few degs cooler than normal for late Sep.
Although pleasant given fairly light NNE winds, except up to
15-20 mph across Cape Cod and the Islands.

Thursday night...

1030+ mb high advects across New England, and accompanied by a
cool/dry fall airmass. Likely the coolest night of the week,
with lows in the upper 30s to mid 40s for most locations. Given
ideal radiational cooling, mid 30s possible across western MA.
Hence, areas of frost possible. Leaned toward the colder MOS
guidance to derive min temps given the synoptic setup.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
Highlights

* Dry and quiet weather expected on Friday across southern New
  England.

* High uncertainty in the forecast this weekend into early next week
  with a tight north/south precipitation gradient. Confidence higher
  in gusty winds and elevated seas especially late in the weekend
  into early next week.

Friday...

Shortwave ridge axis over New England on Friday flattens out and
moves offshore, while a subtle shortwave digs into northern New
England. Ian will lift into the Southeastern US. High pressure
remains overhead of New England Friday and Friday night.

Expecting dry and quiet weather through this period with high
pressure in control. Will see increasing high clouds from distant
Ian lifting into our region through the day Friday, but especially
Friday night. Main impact this brings to the forecast is some
uncertainty on exact temps on Friday and how strong the radiational
cooling is Friday night. Expecting N/NE flow with 850 hPa temps of 7-
10 degrees Celsius at 850 hPa on Friday and S/SE flow Friday night.
NBM looked reasonable for highs on Friday with readings in the 60s.
Did lower the temps to the 20th percentile of guidance. Thinking
that the thin high clouds won`t inhibit radiational cooling much
given the light winds. Lows generally in the 40s.

Saturday and Sunday...

Will be caught between cyclonic flow over Quebec/Ontario and the
remnants of Ian over the Southeastern US. Should have some ridging
across the Mid Atlantic/eastern Great Lakes during this timeframe as
well. Lots of uncertainty with how far north across the SE US the
remnants of Ian get. A pseudo warm front lifts toward southern New
England during this timeframe and could bring some spotty showers.

Lots of uncertainty through this timeframe with how far north
moisture will surge into our region. Given the uncertainty have
generally leaned on the NBM guidance. Cluster analysis shows that
all guidance except for 15 members indicate at least some
precipitation over southern New England Saturday. The dry solution
keeps a weaker and more compact upper level energy associated with
Ian over northern FL, whereas all other clusters feature a broader
and further north solution, around 85 members. Still indicative of a
tight precip gradient from north to south with the best shot for
showers along/south of the MA Turnpike on Saturday.

As for Sunday, a similar situation. All clusters feature at least
some precipitation over portions of southern New England. Still need
to hone in on the exact track of the upper energy associated with
Ian. A further southerly solution keeps much of the region out of
showers, whereas further north/east brings us a higher risk for
showers. At this point appears any activity would be relatively
light given the 20-80 percent probabilities of 24 hr QPF AOA 0.1
inches per EPS guidance. The GEPS is more of a compromise than the
very dry GEFS guidance. All leading to the idea there is a lot of
uncertainty at this point in time. For now have chances of
precipitation along/south of the MA Turnpike through the weekend and
slight chances further north. High temps in the 60s through the
weekend, but will be in the low/mid 60s on Sunday due to
northeasterly flow.

Confidence higher in increasing wind speeds/gusts especially for
Sunday as the pressure gradient tightens over our region. The NBM at
this point looked reasonable, but may need to be increased in future
updates with a 30-40 kt 925 hPa northeasterly jet pushing in.

Monday and Tuesday...

A lot of uncertainty on what happens with the upper energy
associated with Ian early next week. Still think that the NBM slight
chance to chances of precip is reasonable given the uncertainty.
Still most confident in gusty winds due to a tightened pressure
gradient along the south coast. Temperatures will be below normal
with highs ranging from the 50s to the mid 60s. Should see seas
remaining elevated. This in combination with an extended period of
gusty NE winds will lead to rough seas and potential for some minor
coastal erosion issues for the afternoon high tides; fortunately
astronomical tides will be on their way down, but something to keep
an eye on.

&&

.AVIATION /06Z THURSDAY THROUGH MONDAY/...
-- Changed Discussion --
Forecaster Confidence Levels:

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

Today...High confidence. VFR. Sct CU developing. N/NNE wind
10-15 kt with gusts to 20 kt Cape/Islands.

Tonight and Friday...High confidence. VFR with high clouds.
N/NE wind 5-10 kt. Sea breezes may develop along eastern MA
coast Fri.

KBOS Terminal...High Confidence.

KBDL Terminal...High Confidence.

Outlook /Friday Night through Monday/...

Friday through Friday Night: VFR.

Saturday: VFR. Slight chance SHRA.

Saturday Night: VFR. Breezy. Slight chance SHRA.

Sunday: VFR. Windy with gusts up to 30 kt. Slight chance SHRA.

Sunday Night: VFR. Windy with areas of gusts up to 30 kt.
Slight chance SHRA.

Monday: VFR. Windy with local gusts up to 30 kt. Slight chance
SHRA.
-- End Changed Discussion --

&&

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

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

330 PM update...

VFR cloud bases thru the period. Dry weather prevails, just a
low risk for an isolated brief light rain shower until 00z or so
across western MA. WNW winds 10-15 kt late day Wed, becomes
north tonight, then NNE Thu before becoming light and variable
Thu night.

Outlook /Friday Night through Monday/...

Friday through Friday Night: Winds less than 25 kt.

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

Saturday Night: Winds less than 25 kt. Areas of seas
approaching 5 ft. Chance of rain showers.

Sunday: Strong winds with gusts up to 30 kt. Rough seas up to
9 ft. Chance of rain showers.

Sunday Night: Strong winds with gusts up to 30 kt. Rough seas
up to 10 ft. Slight chance of rain showers.

Monday: Moderate risk for Small Craft Advisory winds with gusts
up to 30 kt. Rough seas up to 10 ft. Slight chance of rain
showers.
-- End Changed Discussion --

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...None.

&&

$$
SYNOPSIS...Nocera/BL
NEAR TERM...KJC/Nocera
SHORT TERM...Nocera
LONG TERM...BL
AVIATION...KJC/BL
MARINE...Nocera/BL