Small Craft Advisory 102, NWS Boston/Norton, MA

Area Forecast Discussion
National Weather Service Boston/Norton MA
146 PM EDT Tue Oct 31 2023

.SYNOPSIS...
Sunny, dry and cool across southern New England today. Rain and
a few isolated, very light snow showers possible tonight and
Wednesday. Appreciable rain most likely across the Cape and
Islands. Chilly temperatures peak Wednesday and Wednesday night
before high pressure builds in to wrap up this work week and
start this weekend. Temperatures to return to near or even above
normal level by Saturday.

&&

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

No changes needed to the forecast at this hour. Starting the day
off with mostly sunny skies across much of the region as clouds
remain contained to Cape Cod and the Islands. The area of
clouds will begin to expand north later today, but it will
remain dry this evening with shower chances increasing after
midnight for Cape Cod and the Islands. Temps this evening start
in the low to mid 40s around sunset and drop to the low to mid
30s my midnight. No frost or freeze headlines tonight as the
program ends for the season at midnight on Nov 1st.

7 AM Update...

No major changes to the forecast this morning.
Gusty winds towards SE MA and the adjacent coastal waters may
linger a bit longer into this afternoon. Otherwise, clouds and
spotty sprinkles will continue to move offshore this morning.
Still colder than we have been.

Previous Discussion...

Highlights

* Sunny with cooler than normal temperatures. Clouds increase
  across the south coast late in afternoon.

A shortwave ridge builds from the eastern Great Lakes early
today into northern New England/southern Quebec by late today.
A trough digs into the central Great Lakes late today. High
pressure builds over the eastern Great Lakes and into northern
New England as the day progresses.

Dry and quiet weather expected across southern New England with
high pressure in control. Temperatures will be roughly 10
degrees cooler than is typical for this time of year. This is a
result of northerly cold air advection. Will have 925 hPa temps
of +/- 1 degree Celsius. The result will be highs in the 40s
across the interior and around 50 along the immediate coastline.

&&

.SHORT TERM /WEDNESDAY/...
Highlights

* Rain for the immediate south coast, but especially the
  Cape/Islands as a coastal low passes by late tonight/Wed.

* The interior could see spotty rain showers and perhaps a few
  snowflakes mixing in on Wed.

A neutrally tilted trough will be over the central Great Lakes
Tue evening. The trough lifts over the eastern Great Lakes and
becomes slightly negatively tilted by early Wed. The trough
lifts into northern New England late Wed. A broad coastal low
will begin developing well to the SE of the Carolinas this
evening. The low lifts NNEward tonight and makes its closest
pass Wed AM before lifting further offshore later on Wed.

Increasing cloud cover across southern New England tonight as
the coastal low lifts toward us. Winds will be gradually picking
up as the pressure gradient tightens. Will be the coldest
airmass in place so far this season with lows bottoming in the
mid/upper 20s across the higher terrain and 30s elsewhere. Am
holding off on a Frost Adv at this point given the increasing
cloud cover and winds picking up.

Best shot for measurable precipitation is across the immediate
coastline, but especially the Cape/Islands. This is where PWATs
increase to 0.5 to 0.8 inches and where the coastal low makes
its closest pass to our area. At this point think the heaviest
precipitation would be toward daybreak. This is also when if
precip can spread further inland there may be snow showers
before temps warm up on Wed to change over to mostly rain, with
perhaps a few snowflakes mixing in especially across the higher
terrain. No snow accumulation is expected.

As for Wed afternoon am skeptical of precip across the interior
given the dry low/mid levels. This is despite what several CAMs
and deterministic guidance depict at this point. For now have
nudged down to slight chances to account for spotty showers,
whereas there still may be some scattered activity along the
immediate coast. Think the latest NSSL WRF has a pretty good
handle on how things will evolve.

Flow will be onshore aloft which will keep 0 to -2 degree
Celsius air overhead at 925 hPa. Highs in the 40s on Wed.

&&

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

* Widespread sub-freezing temperatures Wednesday night for most
  locations away from the coast

* Trending warmer Thursday through Sunday

* Dry/quiet weather through the first half this weekend.

* Low risk for some showers with a cold front Sunday into Monday.

Latest guidance suite in reasonable agreement with the synoptic
pattern through Thursday. After that time, the timing and amplitude
differences become more significant with time. The GFS remained less
amplified and progressive with a mid level trough than the ECMWF
camp late this week into this weekend.

At the surface, high pressure should move from the Gulf coast,
across the central Appalachians Thursday, then shift south of the
Maritimes towards Friday. This favors a cooldown the middle of this
week, with a warming trend Thursday into Sunday. In fact,
temperatures towards daybreak Thursday should be the lowest we have
seen so far this season. Much of southern New England away from the
immediate coast expected to be below freezing.

Looking rather dry until a frontal system approaches late this
weekend into early next week. Given the time range and the
differences in the guidance, kept precipitation chances low for now
until the details come into better focus.

&&

.AVIATION /18Z TUESDAY THROUGH SUNDAY/...
Forecast Confidence Levels:

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

Rest of Today...High confidence.

VFR with light NNE/E winds. Increasing mid to high clouds
across the south coast by late this afternoon.

Tonight...Moderate confidence.

VFR this evening before rain and MVFR ceilings move into Cape
Cod and the Islands after 06Z. Further inland, should stay dry
with only isolated showers. Winds turn north overnight.

Wednesday...Moderate confidence.

Rain continues for Cape Cod and the Islands through about 14z
before becoming more scattered showers. Further inland, dry air
likely keeps steady rain from making it to Boston, hence VCSH
was used. MVFR ceilings in the morning give away to VFR by early
afternoon. Winds turn NNW tomorrow afternoon and become gusty

Wednesday Night... High confidence

VFR with decreasing NW winds.

KBOS Terminal...High confidence. VFR with light NNE winds this
afternoon and evening. Scattered rain showers with marginal
MVFR ceilings late overnight into tomorrow morning. improving
back to VFR by late tomorrow morning

KBDL Terminal...High Confidence. VFR with light NNE winds this
afternoon and evening. marginal MVFR ceilings develop tomorrow
morning before lifting to VFR again tomorrow afternoon

Outlook ...

Thursday: VFR.

Thursday Night through Saturday: VFR. Breezy.

&&

.MARINE...
Forecaster Confidence Levels:

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

Adjusted the end timing for Small Craft Advisories across the
coastal waters south and east of Nantucket. Otherwise, no major
changes to the ongoing forecast. Still expecting gusts to
diminish across the nearshore waters by mid morning.

Today...High confidence.

Winds continue to decrease as high pressure nudges into our
region. Will see SCA conditions come to end this AM. Winds out
of the N at 10-20 kts shift to the NE/E this afternoon and
diminish to 5-10 kts. Seas diminishing to 2-4 ft.

Tonight...High confidence.

Increasing ENE to NE winds as a coastal low lifts toward the
region. Will have wind speeds of 15-20 kts late with gusts of
20-30 kts toward daybreak Wed. Will likely need to hoist a SCA.
Seas 2-4 ft.

Wednesday...High confidence.

Winds out of the NE shifting to the N late with speeds of 15-20
kts and gusts 20-30 kts. Seas 2-4 ft for the interior waters and
3-6 ft for outer.

.Outlook /Wednesday Night through Saturday/...

Wednesday Night: Winds less than 25 kt. Local rough seas.
Slight chance of rain.

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

Thursday Night: Winds less than 25 kt.

Friday: Moderate risk for Small Craft Advisory winds with gusts
up to 25 kt. Seas locally approaching 5 ft.

Friday Night: Moderate risk for Small Craft Advisory winds with
gusts up to 25 kt. Areas of seas approaching 5 ft.

Saturday: Winds less than 25 kt.

&&

.TIDES/COASTAL FLOODING...
A broad coastal low will lift northeast of our region on Wed.
High tide for Nantucket will be around 3.9 ft MLLW Wed
afternoon. This in combination with increased NNE to NE winds
and a surge around 1-1.5 ft could result in Minor coastal flood
per Stevens Institute guidance. Still not confident enough at
this point, but a Coastal Flood Statement or Advisory may be
needed in future updates.

&&

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

&&

SYNOPSIS...Belk/BL
NEAR TERM...BL/KP
SHORT TERM...BL
LONG TERM...Belk
AVIATION...KP
MARINE...Belk/BL
TIDES/COASTAL FLOODING...BL