Area Forecast Discussion
National Weather Service Boston/Norton MA
723 AM 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/...

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 /11Z FRIDAY THROUGH TUESDAY/...

Forecast Confidence Levels:

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

Today...High confidence.

Any MVFR to IFR fog/stratus burns off/lifts by 13-14Z. Otherwise
will be VFR through the forecast with ENE to E winds at 5-10
kts. Increasing mid/high clouds.

Tonight...Moderate confidence.

VFR for much of the night, but patchy MVFR cigs may develop
after 06Z east of CT valley. Light rain gradually spreads N 
over the Islands and immediate south coast 09-12Z. E wind gusts
to 20 kt developing over the Islands. 

Saturday...Moderate confidence.

MVFR conditions with patchy IFR south of the Pike in the 
afternoon. Rain overspreads the region from S to N, but may take
until the afternoon to reach northern MA. Increasing NE winds 
with gusts to 25-30 kt developing over Islands to south coast 
during the afternoon. 

KBOS TAF...High confidence in TAF with onshore winds today.

KBDL TAF...High confidence in TAF. 

Outlook /Saturday Night through Tuesday/... 

Saturday Night: MVFR/IFR conditions possible. Windy with local
gusts up to 30 kt. Chance SHRA.

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.

&&