Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 181125
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
725 AM EDT Mon Oct 18 2021

.SYNOPSIS...
Blustery and cool weather continues through Tuesday. Dry
weather persists Wednesday into Thursday with unseasonably mild
temperatures returning. Continued mild Friday but a few showers
are possible as a cold front moves into the region. Turning more
unsettled into the weekend with a possible weak coastal low
that may bring renewed rains by Saturday, followed by cooler
temperatures by next Sunday.

&&

.NEAR TERM /THROUGH TONIGHT/...
710 AM Update:

Overcast cloudiness across eastern MA, Cape Cod and the offshore
waters continues to shift eastward. Even a few showers showing
up over the waters in this cloudiness per regional radar
mosaic. Appears streets of ocean effect clouds should stay
mainly away from Cape Cod south of Martha`s Vineyard and
Nantucket. Should see a period of clearing in most areas by mid
to late morning with NW gusts then developing. Will start to
see increased cloud cover by afternoon as the next shortwave
disturbance, seen in water vapor imagery now roughly
pinwheeling through the Tug Hill region in NY, approaches from
the NW. May even see some spotty showers develop in the
prevailing NW low-level flow across interior MA/CT -
indications reflected in some convective-permitting model
solutions. However dry weather should prevail most of the time.

A cool start to the day with temperatures in the 40s to low 50s.
Highs top out in the mid 50s to lower 60s, about seasonable but
the breezes will make it feel a little on the cooler side.

Previous discussion from overnight:

Area of moisture in the 850-700mb layer rotating through the
region will move to the east by daybreak leading to mostly sunny
skies this morning. The only caveat is the potential for some
ocean effect clouds and perhaps a brief showers skirting the
outer Cape.

Additional moisture accompanying a shortwave rotating around
Maritimes upper low will move into the region from the west this
afternoon. This moisture under -25C cold pool at 500 mb will
result in sunshine giving way to increasing diurnal cu from
west to east this afternoon. A few diurnal showers are possible
again this afternoon with hi-res guidance focusing the shower
threat primarily across western MA and northern CT.

850 mb temps around 0 to -1C this afternoon will keep high temps
mostly in the 50s, but around 60 across eastern MA and RI.
Another breezy day with a deep and well mixed boundary layer
supporting NW gusts to 25 mph.

&&

.SHORT TERM /TUESDAY/...
Tonight...

Shortwave moves to the east tonight with some drying moving in
from the west. Expect partial clearing tonight but another area
of mid level clouds may rotate south across eastern New Eng
overnight assocd with another shortwave. Strong pressure
gradient persists tonight which should maintain a steady NW
wind, gusty at times along the coast. Core of the low level cold
air will settle over New Eng tonight with 925 mb temps down to 0
to 2C. Lows will range from upper 30s to mid 40s with enough
wind to prevent temps from falling further.

Tuesday...

Upper low remains over the Maritimes but temps aloft are warming
and there is less moisture in the column which will result in a
mostly sunny day, with perhaps a few extra clouds at times in
eastern MA as mid level moisture rotates south from ME and NH.
While low level temps are moderating this will be offset
somewhat by colder start to the day with net result being temps
near or slightly milder than today. Expect highs mid 50s to
around 60, perhaps some lower 60s in the lower CT valley and
near south coast. Another breezy day with strong gradient in
place and deep mixed layer supporting gusts to 25-30 mph at
times and perhaps a few 35 mph gusts, especially during the
afternoon.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...
Highlights...

* Above average temps return Wed into Thurs, highs low to mid 70s
  and dry weather.

* Unsettled period Fri-Sat. Frontal system early Fri could bring
  light showers but not a washout. Monitoring another possible low
  pressure lurking offshore later Fri into Sat.

* Unsettled and blustery Sunday into Monday with temperatures
  turning near to below average.

Details:

Tuesday Night:

Troughing aloft/cyclonic flow aloft gradually weakens thru Tuesday
night with surface ridge of high pressure nosing into western New
England. Partial cloudiness but dry weather Tuesday evening, with
modest WNW gradient winds 10 kt or less and 925 mb temps in the
lower teens Celsius supporting lows in the upper 40s to mid 50s.

Wednesday through Thursday:

Warming trend in this period with above normal temperatures, as we
get into developing 500 mb height rises and 925 mb temps in the mid
teens Celsius. Modest W winds on Wednesday gradually become SW with
modest SW gusts developing on Thursday, as high pressure over the
mid-Atl states builds offshore. This happens ahead of a frontal
system that arrives Thursday evening. Wednesday and much of Thursday
feature mostly sunny conditions for most of the period, though will
see lowering and thickening cloud cover late Thursday afternoon.
Both days should feature highs in the lower to mid 70s! Those who
prefer milder temperatures and dry weather will find these two days
to be quite pleasant.

These days likely to be the pick of the workweek, as our weather
pattern turns more unsettled late in the week and temperatures start
to take a tumble again for the weekend.

Thursday Night into Saturday:

Turning more unsettled in this period as a mid/upper level closed
low initially over central Canada translates eastward into northern
Quebec toward Saturday. There are a couple of rather potent
shortwave disturbances rounding the closed low.

First lead shortwave moves into northern New England late Thursday
night into Friday, a feature that swings a surface frontal system
through New England Thursday after midnight into early on Friday.
Best dynamic forcing is across northern New England where better QPF
looks to lie; more uncertainty across Southern New England on QPF
with frontal passage here on Friday. Kept PoP in the Chance range
but it is possible that our area gets robbed of deeper moisture and
little if any rain ensues. Timing of the frontal passage suggests
eastern areas may be milder (upper 60s-near 70) than the interior
(upper 50s to mid 60s). 925-850 mb temps start to fall Friday
afternoon/Friday evening with baroclinic zone lying over or just
south of our area.

Next shortwave disturbance moves rather quickly through the central
Appalachians and mid-Atlantic states later Friday night into
Saturday. This brings about at least an increase in moisture along
the baroclinic zone or a weak low pressure offshore the mid-
Atlantic. Several sources of uncertainty in the 00z deterministic
models, more specifically on the amplitude of this shortwave
disturbance, how strong a surface low reflection might become and
the extent to which related rains tied to the low affect our area.
GFS is considerably weaker with the shortwave, which leads to little
if any surface low development and rains remaining well offshore.
The international suite of guidance is stronger; with the ECMWF
being the most robust which brings a 1000 mb low across Cape Cod and
leads to a cloudy and rainy Saturday for most of our area. Opted to
go with slight (15%) PoP in the interior and most of the coastal
plain, and lower chance (25-30%) across south coastal MA/RI, Cape
and Islands. Kept temps around NBM values in the mid 50s to lower
60s, still fairly seasonable.

Sunday and Monday:

EPS/GEFS means point to longwave troughing and lower than normal 500
mb heights establishing itself across Southern New England in this
period. This should bring increasingly cooler temperatures and
generally unsettled conditions driven by the cold pool aloft; though
the magnitude of the cooler air is still unclear. Earlier cycles of
the GFS brought rather notably colder air (850 mb temps -6 to -9C)
in this period but has since backed off to values around -2 to -4C;
an illustration of its cold bias in the extended range. ECMWF is
similar at -1 to -3C at 850 mb by Monday. Rather strong northerly
pressure gradient develops between coastal/offshore wave low and a
large 1030 mb high pressure area, suggesting rather blustery
northerly winds and also rather chilly apparent temps. Some of those
apparent temps could be in the sub-freezing range during the
nighttime hours. Definitely not uncommon to see sub-freezing
apparent temperatures in the 3rd week of October but would likely be
the first occurrence of sub-freezing apparent temps in some time.
Something to monitor.

While the extent of the colder air is still less clear, idea for
below-normal temperatures looks on track for this period. Highs
upper 40s to mid 50s, lows in the 30s.

&&

.AVIATION /11Z MONDAY THROUGH FRIDAY/...
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.

12z TAF Update:

Today: High confidence.

VFR with broken cigs 4-5k ft developing from west to east late
morning into the afternoon. Could see periodic MVFR bases for
western airports. A brief afternoon shower possible across
western MA and northern CT. NW gusts to 20-25 kt.

Tonight...High confidence.
VFR. NW wind 5-15 kt, gusts to 20 kt along the immediate coast.

Tuesday...High confidence. VFR. NW gusts to 25 kt.

KBOS Terminal...High confidence.

KBDL Terminal...High confidence. Possible MVFR bases after 16z.

Outlook /Tuesday Night through Friday/...

Tuesday Night: VFR. Windy with local gusts up to 30 kt.

Wednesday: VFR. Breezy.

Wednesday Night: VFR.

Thursday: VFR. Breezy.

Thursday Night: VFR. Breezy. Chance SHRA.

Friday: VFR. Breezy. Slight chance SHRA.

&&

.MARINE...
Forecaster Confidence Levels.

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

Persistent NW flow with strong pressure gradient in place will
support gusts 20-25 kt at times. Winds will increase late
tonight and especially Tue as low level wind field increases.
Expect gusts to approach 30 kt by Tue afternoon, but should fall
short of gales so we will drop the gale watch and extend the SCA
through Tue.

Outlook /Tuesday Night through Friday/...

Tuesday Night: Low risk for Small Craft Advisory winds with
gusts up to 30 kt. Local rough seas.

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

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

Thursday Night: Moderate risk for Small Craft Advisory winds
with gusts up to 25 kt. Local rough seas. Chance of rain
showers.

Friday: Winds less than 25 kt. Areas of rough seas. Slight
chance of rain showers.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory from 8 PM this evening to 8 PM EDT
     Tuesday for ANZ231>235-237-251.
     Small Craft Advisory from 2 AM to 8 PM EDT Tuesday for ANZ230-
     236.
     Small Craft Advisory from 5 PM this afternoon to 8 PM EDT
     Tuesday for ANZ250-254.
     Small Craft Advisory until 8 PM EDT Tuesday for ANZ255-256.

&&

$$
SYNOPSIS...KJC/Loconto
NEAR TERM...KJC/Loconto
SHORT TERM...KJC
LONG TERM...Loconto
AVIATION...KJC/Loconto
MARINE...KJC/Loconto


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