Small Craft Advisory 91, NWS Boston/Norton, MA

Area Forecast Discussion
National Weather Service Boston/Norton MA
137 PM EDT Fri Sep 22 2023

.SYNOPSIS...
High pressure will bring dry and seasonable conditions today.
Then a pattern change this weekend into Monday, with periods of
rain, gusty northeast winds and cooler than normal temperatures,
as a subtropical low tracks south of New England. A trend
toward drier weather should arrive later Monday into Tuesday and
then persist much of next week. Cooler than normal temperatures
linger into Monday and Tuesday, then modifying Wednesday and
Thursday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
1 PM Update...

See updated aviation discussion

10 AM Update...

No major changes to the forecast at this hour. Did increase
cloud cover the rest of the day with diurnal cumulus starting to
form in eastern MA and high cirrus clouds streaming in ahead of
our next weather system for this weekend. An area of dense fog
did form in Middlesex county and continues to move west. As the
daytime heating continues, that area of fog should burn off over
the next couple of hours.

715 AM Update...

No major adjustments in the latest update. Have brought things
in line with the latest observations. Forecast remains on track.

Previous discussion...

High pres shifts east of New Eng today which will provide one more
day of dry conditions and seasonably mild temps. Although less
sunshine today as high clouds overspread the region from the south
along with sct-bkn diurnal CU developing in the afternoon. East
winds and increased cloud cover will result in slightly cooler temps
than last few days, with highs upper 60s to lower 70s.

We have discontinued the rip current statement for today. Long
period SE swell has subsided across the waters which will limit the
risk for strong rip currents.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/...
Tonight...

Sub-tropical low lifts northward to the NC coast while high pres
persists from northern New Eng to Nova Scotia. Clouds will thicken
tonight but high pres to the north will provide sufficient low-mid
level dry air to keep rainfall mostly to the south through the
overnight period. However, the northern extent of the rain shield
may reach the Islands to perhaps the immediate south coast toward
daybreak with some light rain overspreading this area. Increasing E
winds expected overnight across the Islands as pressure gradient
increases, which leads to gusts to 20-25 mph. Lows upper 40s to mid
50s.

Saturday...

Sub-tropical low pres moves slowly north over the mid-Atlc coast.
Increasing PWAT plume lifts north into SNE which will bring rain
into SNE, but there remains uncertainty with rainfall amounts and
the northern extent of rainfall. Confluent flow across northern New
Eng will provide lots of low-mid level dry air to the north which
drains south into SNE. In fact at 12z Sat, there is considerable dry
air in the 850-700 mb layer north of the south coast. This
eventually erodes from south to north with increasing moisture
advection/transport but this will be a slow process, especially
north of the Pike. As a result, the steadiest rainfall Sat will be
confined to the south coast region into SE MA, and light rain may
not reach northern MA until the afternoon. It is important to note
that there is a sharp moisture gradient on the northern edge of the
rain shield so confidence is lower in northern MA as a slight shift
south would result in little rainfall here, while a northward shift
would bring steadier and more widespread rainfall. Recent trends in
ensemble guidance have shifted south.

Rainfall Saturday will be driven by baroclinic processes with
increasing low level WAA enhanced by an increasing low level jet
that lifts northward along the south coast. This will likely bring
periods of heavier rainfall near the south coast with rainfall
amounts dropping off quickly to the north.  In addition, there is a
low risk for isolated thunder over the Islands in the afternoon as
elevated instability moves north over the ocean. GEFS and EPS
ensembles only have moderate probs for greater than 1" rainfall near
the south coast through Sat which would represent the 75th
percentile of the ensemble distribution. Current thinking is
max rainfall up to an inch near the south coast on Saturday,
with worst case scenario of 1-2 inches if there is a northward
trend.

It will be a chilly day with NE winds and cloud cover/rainfall
keeping temps in the 50s to lower 60s. However, if southward trends
continue, temps would be milder, especially in northern MA. Becoming
breezy along the south coast and especially the Islands with
increasing NE winds, gusting up to 25-30 mph.

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
Highlights:

* Subtropical low brings periods of rain Sunday into early Monday.
  Rain may be moderate to heavy at times, but uncertain on rainfall
  totals given sharp north to south qpf gradient expected.

* Drying trend likely later Monday into Tuesday and possibly
  persisting much of next week.

* Temperatures cooler than normal this weekend, but coolest temps
  Monday night/Tuesday time frame, then modifying next Wed/Thu.

Details:

Saturday night...still some uncertainty on timing here, but
following ensemble mean suggest dry slot moves across the area,
bringing an end to the steady rains from Saturday, and replaced by
areas of mist and drizzle, in response to moist NE flow. Lows in the
50s.

Sunday...still lots of uncertainty on the exact track of potential
tropical cyclone 16 and its associated rainfall. Strong positive
height anomaly over Ontario and Hudson Bay this period, yielding a
fairly amplified downstream trough over eastern Quebec. This places
confluent zone across Maine, supporting a sharp qpf gradient from
north to south across SNE. Consensus forecast is for the subtropical
low to track near/along 40N latitude. If this track verifies, it
will be close enough to SNE for a wet Sunday with periods of rain,
moderate to heavy at times in response to strong synoptic scale lift
acting on tropical moisture, boosting PWATs up to 150% of normal.
Chilly and a raw feel to the airmass, given gusty NE winds and dew
pts in the 50s providing chilly wet bulb temps.

Monday...ensembles indicate remnant subtropical low south of New
England early Monday, but progressive with low then exiting eastward
across Georges Bank. So some improvement possible later Monday from
west to east, with steady rains exiting. However, it will remain
cool with NE flow increasing behind departing low. Could become
windy as low pulls away and low level NE jet develops.

Tue/Wed/Thu...chilly airmass Monday night into Tue on gusty NE flow,
feeling more like October than September, with highs in the 60s and
lows in the 40s. Airmass slowly modifies Wed, then near normal temps
by Thursday, highs 65-70. PWATs below normal this period per
ensembles, so expecting a dry stretch of weather this period.

&&

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

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

Rest of Today...High confidence.

Mainly VFR this afternoon with scattered pockets of MVFR due to
a diurnal cumulus deck. Diurnal cumulus will quickly dissipate
this evening. Winds out of the east at 5 to 10 knots inland and
10 to 15 knots near the coasts.

Tonight...Moderate confidence.

VFR for the majority of the night before MVFR ceilings move
from south to north towards day break reaching Cape Cod and the
Islands first around 09z to 10z. Rain should hold off much of
the night with showers just reaching Cape Cod and the Island
near day break. Winds stay out of the east at 5 to 10 knots.

Saturday...Moderate confidence.

MVFR with IFR for SE MA, RI, along with Cape Cod and the
Islands. Rain begins spreading from south to north, however the
extent north remains uncertain at this time. Most terminals
should see light rain/showers with moderate to heavy rain for
Cape Cod and the Islands. Winds remain out of the east at 10 to
15 knots gusting to 20 knots in the afternoon.

Saturday Night...Moderate confidence

MVFR with localized IFR possible with continued rain/showers.
Rain begins to dissipate towards daybreak Sunday. Winds turn
ENE at 10 to 15 knots gusting 20 knots.

KBOS TAF...Moderate Confidence in TAF. VFR today with east winds
at 10 to 15 knots. MVFR ceilings move in around 14z tomorrow.
rain/showers should hold off until 18 to 19z tomorrow. Winds
remains easterly at 10 to 15 knots.

KBDL TAF...High confidence in TAF. VFR today with MVFR working
in around 11z tomorrow morning. Rain should move in between 15
and 16z.

Outlook /Saturday Night through Tuesday/...

Sunday: MVFR/IFR conditions possible. Breezy. Chance SHRA,
isolated TSRA.

Sunday Night: Mainly MVFR, with local IFR possible. Windy with
local gusts up to 30 kt. SHRA likely, isolated TSRA.

Monday: Mainly VFR, with local MVFR possible. Windy with gusts
up to 30 kt. Chance SHRA.

Monday Night: VFR. Windy with gusts up to 30 kt. Chance SHRA.

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

&&

.MARINE...
Forecaster Confidence Levels:

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

Today and tonight...High confidence. Increasing E winds through the
period, 15-20 kt over the southern waters this afternoon, then 20-25
kt tonight, with less wind further north. Seas building to 5-6 ft
over southern waters tonight.

Saturday...Moderate confidence. E/NE wind gusts to 30-35 kt
developing over southern waters, strongest over RI coastal waters.
Gale watch for RI nearshore waters and outer southern waters with
SCA for rest of southern waters. Building seas to 10+ feet over
southern outer waters.

Outlook /Saturday Night through Tuesday/...

Saturday Night: Moderate risk for Small Craft Advisory winds
with local gusts up to 30 kt. Rough seas up to 12 ft. Rain
showers likely, isolated thunderstorms.

Sunday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Rough seas up to 10 ft. Chance of rain showers,
isolated thunderstorms.

Sunday Night: Low risk for Small Craft Advisory winds with
gusts up to 30 kt. Rough seas up to 8 ft. Chance of rain
showers, isolated thunderstorms.

Monday: Moderate risk for Small Craft Advisory winds with gusts
up to 30 kt. Areas of rough seas. Chance of rain showers.

Monday Night: Moderate risk for Small Craft Advisory winds with
areas of gusts up to 30 kt. Rough seas up to 8 ft. Slight
chance of rain showers.

Tuesday: Moderate risk for Small Craft Advisory winds with
gusts up to 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 AM Saturday to 8 AM EDT Sunday for
     ANZ232>234-236-254.
     Small Craft Advisory from 2 AM to 2 PM EDT Saturday for ANZ235-
     237.
     Gale Watch from Saturday afternoon through Saturday evening
     for ANZ235-237.
     Small Craft Advisory from 8 PM this evening to 11 AM EDT
     Saturday for ANZ255-256.
     Gale Watch from Saturday morning through Saturday evening for
     ANZ255-256.

&&

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