❰ Current Conditions

NWS Area Forecast Discussion
Area Forecast Discussion
National Weather Service Boston/Norton MA
954 PM EDT Wed Sep 22 2021

.SYNOPSIS...
During tonight through Thursday our area remains in between 
high pressure to our east, and a slow moving cold front to our
west. For southern New England, this will result in considerable 
cloudiness and scattered showers. The slow-moving front will
bring showers and thunderstorms into western sections late
Thursday night, moving eastward across the region during Friday. 
Showers/thunder then linger on the Cape and Islands Saturday,
otherwise improving weather. More showers possible Sunday and
Tuesday. No big warm ups or cool downs are expected.

&&

.NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
No major changes to the forecast this evening. Will be
contending with periods of clouds and scattered showers
overnight, mainly across the western half of southern New
England. Cape and islands may actually remain mostly clear
overnight. Tweaked temperatures to account for observed trends.

7 PM Update...

Narrow band of showers through NE CT/NW RI/Central MA moving
north. The south end of this line is moving north across the
region and should mark the end of precip from this feature.
Meanwhile widely scattered showers over Central CT and Western
MA are also moving north bringing some light precipitation
amounts. 

PW values diminish a little during the early night. Values
around 1.5 inches during the day will diminish to less than an
inch in Eastern MA and to 1.2 inches in the CT Valley. Even so,
low level moisture should linger from the showers of the day and
evening. Dew points have been in the upper 60s and lower 70s
leading into the evening. Even with the deep layer moisture
diminishing a little, expect dew points of 65 to 70 overnight.
With this moisture, expect clouds and fog all areas. Widely
scattered showers will linger over western parts of CT and MA.

Based on observed dew points, forecast min temps have been
bumped up 1-2F degrees.

Previous Discussion...

This evening/Tonight...

Our area managed to get some marginal instability which helped 
to enhance the amount of showers crossing the area. Continued 
chance to likely POPs for much of the area through 8 pm. 
Instability should diminish after sunset, reducing the number of
showers. 

Then for tonight, little change in airmass tonight as mid level
ridge remains centered to our east, and deepening cut off low 
remains over the Ohio Valley and Great Lakes. Southerly flow 
brings a continued feed of low level moisture, bringing a 
continued risk for isolated to widely scattered showers. Without
a significant front to provide a focus, any rainfall amounts 
will be light. As with last night, we have ample low level 
moisture to allow for areas of low clouds and fog, especially 
later tonight into early Thursday. Quite muggy with lows in the 
mid and upper 60s.

&&

.SHORT TERM /6 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/...
Thursday...

Our area remains in between a deep cutoff low over the Great 
Lakes Region, and a mid level ridge centered offshore. A cold 
front is expected to slowly approach our area from the west, 
tracking from Lake Ontario/central PA, east to central NY State 
and the Mid Atlantic coastline by evening. Models show the 
deeper moisture remaining close to the front, with moisture in 
southern New England limited to 800 mb and lower. We'll have 
continued isolated to scattered showers, with considerable 
cloudiness. The Cape and Islands will likely have some sunshine 
though. Another warm day with highs in the 70s. 

Thursday Night...

The cold front to our west slowly approaches the NY/New England
border during Thursday night into early Friday. Cross sections 
continue to show limited moisture for much of the night, but 
moist layer deepens over the far interior late Thu night. Expect
to have an increase in shower activity, especially over the 
interior, with likely POPs there mainly after midnight. Model 
soundings show marginal instability in the vicinity of the I-91 
corridor- could have a few rumbles of thunder there before 
daybreak. QPF amounts should be limited to up to a quarter of an
inch, however if model timing of front is too slow, higher 
amounts could be seen in the east slopes of the Berkshires and 
Hartford County. Very muggy by mid- September standards with 
lows in the 60s to near 70.

&&

.LONG TERM /FRIDAY THROUGH WEDNESDAY/...
Big Picture...

Unsettled pattern for the long term period. Closed low over the 
Great Lakes Friday lifts north to Hudsons Bay. This gradually turns 
the upper flow over New England from south to southwest. A second 
upper low drops down from the Canadian Prairies, moving across New 
England Sunday night or Monday. Two shortwaves then come together as 
they move around the departing second low, crossing New England 
later Tuesday or Wednesday.

The upper flow over North America remains meridional Friday, 
flattens to near zonal over the weekend, then trends meridional 
again by the middle of next week. As per the upper height contours, 
the deep layer temps drop below climatology with the passage of each 
closed low as noted above. But combined with surface level 
features...expect temperatures within a few degrees of normal.

Mass fields are similar through Friday, then greatly diverge over 
the weekend with highs and lows in different places the first half 
of next week. Confidence is moderate-high Friday, trending to low by 
Monday.

Details...

Friday...Southerly upper flow starts the day. Low pressure surface 
and aloft both lift north in this flow...through Canada to Hudsons 
Bay. Upper jet supporting the weather system starts the day over 
Eastern or Central NY and lifts north, clipping VT/NH. 

Southern New England is at the southern edge of the right entrance 
region of the jet, so that may be enough to aid in convection. PW 
values at 1.5 to 1.75 inches are well above seasonal normals. CAPE 
values of 500-1000 J/Kg are sufficient for some convection, and 
Total-Totals show upper 40s during the day, also supportive, and 
Lifted Index is forecast at minus 2 to minus 5. So the potential is 
there for showers and scattered thunder. Will it be more? Note that 
SPC has our area in a Marginal Risk for severe storms. Their focus 
is on the low-level wind field. Note the winds forecast at 850-mb 
are 30 kt, and at 500-mb 30-40 kt. 850-mb is fine, 500-mb is a 
little light. Better winds in NY and VT. For now our forecast will 
focus on showers and scattered thunder, with the most significant 
action along/just ahead of the cold front. The south-north flow will 
mean the front moves slowly across our region.  The high PW values 
suggest local downpours, possibly urban/poor drainage flooding in 
spots.

Saturday-Sunday...

The cold front slowly moves offshore, but lingers there long enough 
for additional showers, possibly in the east and primarily on the 
Cape and Islands. Moisture cross sections show mainly dry air over 
Worcester Friday night and Saturday, just a little moisture at 850-
mb that may be good enough for a few diurnal clouds. This moisture 
increases over Easton/Eastern MA and suggests a cloudier day.

Sunday may start drier, but the next upper low/shortwave approaches 
by afternoon accompanied by an impressive cold pool, minus 20 at 500-
mb. The additional moisture and the cold air advection may generate 
a few showers on Sunday.

Mixing to 900-mb Saturday and 850-mb Sunday taps 850-equiv temps of 
8-10C, supporting max sfc temps in the low to mid 70s. Dew points 
slowly lower through the weekend, in the mid 50s to mid 60s Saturday 
and 50s to low 60s Sunday. Expect min sfc temps just a couple of 
degrees higher than the dew points.

Monday through Wednesday...

The upper low lifts out through the Maritimes Monday. Another trough 
digs over New England Tuesday-Wednesday. This will mean another 
period of showers Tuesday-Wednesday. Temps trend a little cooler, 
but still upper 60s/70s days and upper 40s/50s nights.

&&

.AVIATION /02Z THURSDAY THROUGH MONDAY/...
Forecaster Confidence Levels.

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

VFR CLR over the Cape and Islands, and VFR clouds over the
coastal plain of Eastern MA/RI. Mixed MVFR/IFR across the
interior. Expect the moist air and light south flow to support
lowering ceilings and areas of fog formation in most of Srn New
England. Widely scattered showers overnight, mainly CT Valley 
and East Slopes of the Berkshires.

Thursday...IFR/MVFR to start the day in low clouds and areas of
fog. then improving to mainly MVFR, with areas VFR eastern MA 
possible. Chance SHRA.

Thursday Night...CIGs/VSBYs lower to MVFR/IFR in low clouds and
areas of fog. SCT SHRA to start, becoming more numerous
overnight with TSRA possible after 10Z BDL-BAF corridor.  

KBOS Terminal...High confidence in trends. Moderate confidence 
in timing. VFR with clouds lowering to MVFR cigs after 06Z, then
lifting to VFR 13Z to 14Z.

KBDL Terminal...High confidence in trends. Moderate confidence
in timing. MVFR ceilings and vsbys in BR. MVFR cigs continue
after 13Z to 14Z.

Outlook /Friday through Monday/...

Friday: Mainly MVFR, with local IFR possible. Breezy. SHRA
likely, chance TSRA.

Friday Night: MVFR/IFR conditions possible. Chance SHRA, slight
chance TSRA, patchy BR.

Saturday: Mainly VFR, with local IFR possible. 

Saturday Night: Mainly VFR, with areas IFR possible. Patchy BR.

Sunday: Mainly VFR, with local MVFR possible. Slight chance
SHRA.

Sunday Night: VFR. 

Monday: VFR. Breezy.

&&

.MARINE...
Forecaster Confidence Levels.

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

Tonight through Thursday Night...

Southeast winds of 10 to 15 kts persist through the period, 
with gusts around 20 kts at times. With the persistent SE flow,
expecting seas to build to 4-6 feet on the southern coastal 
waters. SCA headlines are posted for tonight into Thursday night
for the southern outer coastal waters, and SCA for Block Island
and RI Sounds for Thu-Thu night. 

Patchy showers/drizzle and fog possible across the waters during
the period. Reduced visibility possible in the fog, but not 
quite as much within precipitation.

Outlook /Friday through Monday/...

Friday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Seas up to 5 ft. Rain showers likely, chance of
thunderstorms. 

Friday Night: Winds less than 25 kt. Areas of seas approaching
5 ft. Rain showers likely, slight chance of thunderstorms. 

Saturday: Winds less than 25 kt. Seas locally approaching 5 ft.
Chance of rain showers, slight chance of thunderstorms. 

Saturday Night: Winds less than 25 kt. Seas locally approaching
5 ft. Slight chance of rain showers, slight chance of
thunderstorms. 

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

Sunday Night: Winds less than 25 kt. Seas up to 5 ft. 

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

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory from 7 AM Thursday to 8 AM EDT Friday for 
     ANZ255.
     Small Craft Advisory from 7 PM Thursday to 8 AM EDT Friday for 
     ANZ254.
     Small Craft Advisory from noon Thursday to 8 AM EDT Friday for 
     ANZ235-237.
     Small Craft Advisory until 8 AM EDT Friday for ANZ256.

&&