Old Res Weather

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 180824
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
424 AM EDT Fri Sep 18 2026

.WHAT HAS CHANGED...
Rip Statement issued for east facing beaches of MA for Saturday
due to increased risk for rip currents. 

&&

.KEY MESSAGES...

- Any lingering showers exit the Cape and Islands by late
  morning, giving way to a dry afternoon and evening. Fall-like
  temperatures expected Saturday with dry conditions. 

- Next chance for rain Sunday into Monday. There remains
  uncertainty in the details such as coverage, timing, and
  amounts. Showers may linger through early next week. 

&&

.DISCUSSION...

KEY MESSAGE 1...Any lingering showers exit the Cape and Islands by late
morning, giving way to a dry afternoon and evening. Fall-like 
temperatures expected Saturday with dry conditions. 

The remaining batch of showers continues to push east early this 
morning. Any remaining showers will likely exit the Cape and Islands 
by late morning. Behind the showers, a cold front moves across the 
region early this morning shifting winds to the NW. High pressure 
will settle across the region for today with subsident flow 
supporting dry conditions and clearing skies. NW winds with some 
gusts to 20 and up to 25 mph for the Cape/Islands. Cooler air begins 
to advect in from the north in the mid-levels later today, but 
likely won't have much impact on the high temperatures. Temperatures 
are likely to be in seasonable ranges in the 70s for most areas. 
Only exceptions may be upper 60s for the outer Cape and Nantucket 
where there is northerly onshore flow.

A cooler airmass pushes into the region later Friday into Saturday. 
Clear skies will optimize radiational cooling as well to make for a 
cool night. Temperatures likely fall into the upper 40s to low 50s 
for much of the region. Higher elevations, especially the northern 
Worcester Hills will lean cooler in the low 40s. Cooler airmass 
stays in place for Saturday with 850mb temperatures 6-9C. This will 
support Fall-like temperatures in the mid 60s for most. 
East/northeast flow along the eastern coastline may keep those 
immediate coastal areas cooler in the low 60s. Surface high pressure 
continues to dominate bringing dry conditions. 

Not exactly ideal beach weather with the cooler temperatures; 
however, there is an increased risk for rip currents for the east-
facing beaches. There will be an extended period of enhanced NNE 
winds tonight into Saturday which will help establish a swell with 
slightly longer wave periods. 


KEY MESSAGE 2...Next chance for rain Sunday into Monday. There 
remains uncertainty in the details such as coverage, timing, and
amounts. Showers may linger through early next week. 

The main surface high exits offshore Saturday night into early 
Sunday as the next system approaches from the west with our next 
chance of precipitation Sunday into Monday. The ingredients are 
generally in place for at at least a soaking rainfall across 
southern New England. A shortwave approaches from the northwest 
coupled with an 850mb jet positioning over southern New England. 
This provide ample moisture advecting and synoptic scale lift for 
precipitation. PWATS increase to 1.0-1.8" (150-180 percent of 
normal), which is anomalous compared to climo. A low pressure system 
tracks east from the Great Lakes region Sunday putting southern New 
England in the warm sector as its warm front shifts northward. With 
the ample moisture and additional forcing this raises some flags for 
heavy rain potential or at least embedded heavy showers.

There is still variance across guidance in the exact track of 
the system/low pressure which has resulted in lower confidence 
in the details such as timing, amounts, and the potential for 
heavy rain. There has also been lacking consistency run to run 
within individual guidance as well. There is at least a general 
consensus across guidance that the higher moisture and better 
forcing(warm front) will more likely be south. This would favor 
our more south, southwest areas for higher rain totals. GFS and 
Canadian solutions (deterministic & ensemble) seem to reflect 
that pattern with higher amounts south while the ECMWF has a 
wider footprint with generally higher totals. When it comes to 
amounts solutions vary from 0.30" to 1.0" across ensembles based
on the 25-50th percentiles. Deterministic guidance had 
displayed more "aggressive" solutions with swaths of higher 
amounts >1.0". However, these solutions have had the most 
variance from run to run which further adds to the uncertainty. 
The most likely case at this point will be anywhere from a few 
tenths to an inch. This seems reasonable given the mean track. 
There isn't much indication of instability which should limit 
the heavier rain rates. If the warm frontal forcing shifts north
enough, we could get embedded heavy rain supporting by the 
added lift and efficient rain processes. So overall, there is 
confidence that we will get at least a soaking rain with rain 
arriving through the day Sunday continuing into Monday. We'll 
need to continue monitoring for the isolated heavy rain 
potential as models come into better agreement with the track 
over the next day or so. 

It is also possible showers may linger across the south coast and 
Cape and Islands Monday into Tuesday. This will depend on if the low 
or a boundary stalls across the waters. Wednesday leans mainly dry 
with potential for another system to arrive late-week with rain 
possible. 

&&

.AVIATION /08Z FRIDAY THROUGH TUESDAY/...
Forecaster Confidence Levels...

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

06Z TAF Update: 

Through 12Z...Moderate confidence.

MVFR with patchy areas of IFR and spotty VFR ceilings. Showers
push across Cape and Islands, exiting by 13Z. SW winds 6-12 kts
with a few occasional gusts to 18 kts in spots. Winds shift to 
NW 06-12z with gradually improving ceilings. Moderate confidence
is with respect to the ceilings. It is possible ceilings scatter
more quickly behind the frontal passage. Current TAF keeps
lingering MVFR through 13-14Z before scattering out. 

Friday and Friday Night...High confidence.

Areas of MVFR linger across Cape Cod and the Islands through
15z, otherwise VFR. NNW winds 8-12 kt with an occasional gust 
up to 20 kts. There may be an additional period of gusts around
20 kts in the evening (after 00Z).

KBOS Terminal...Moderate confidence in TAF.

Cold front moves through between 07-10Z shifting winds to the
NW. Ceilings this morning bounce between low end VFR and MVFR
with improvements to solid VFR once the front pushes through.
Lower confidence on exact timing, but should be likely be VFR by
13Z. Winds increase after 15Z to 8-12 kts with a few gusts
around 18 kts today. Very low probability for a sea breeze to
sneak in before 15Z before the winds increase. The current pattern
should make this unlikely. 

KBDL Terminal...Moderate confidence in TAF.

Outlook /Saturday through Tuesday/...

Saturday through Saturday Night: VFR. 

Sunday: Mainly VFR, with local MVFR possible. Chance SHRA,
patchy BR.

Sunday Night: MVFR/IFR conditions possible. Breezy. SHRA
likely, isolated TSRA, patchy BR.

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

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

Tuesday: VFR. Windy with areas of gusts up to 30 kt. Chance
SHRA.

&&

.MARINE...
Forecaster Confidence Levels...

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

Through Saturday...High confidence.

Showers exit the coastal waters by the afternoon. Seas remain 
2-3 ft increasing to 3-4 ft early Saturday morning up to 5 ft 
across the outermost waters. Winds shift to the NW this morning
with speeds 8-15 kts and gusts to 20 kts this afternoon. 
Briefly Friday night into early Saturday, gusts may approach 
Small Craft Advisory criteria briefly. Gusts 20-25 kts are
possible generally between 03Z and 09Z. Due to it being very 
borderline and in brief duration, no SCA has been issued. 


Outlook /Saturday through Tuesday/...

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

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

Sunday: Winds less than 25 kt. Chance of rain showers. 

Sunday Night: Winds less than 25 kt. Seas locally approaching
5 ft. Rain showers likely, isolated thunderstorms, patchy fog. 

Monday: Low risk for Small Craft Advisory winds with gusts up
to 30 kt. Seas up to 5 ft. Chance of rain showers, slight
chance of thunderstorms. 

Monday Night: Moderate risk for Small Craft Advisory winds with
gusts up to 30 kt. Seas up to 5 ft. Chance of rain showers. 

Tuesday: Strong winds with gusts up to 40 kt. Seas up to 5 ft.
Chance of rain showers. 

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...High Risk for Rip Currents from Saturday morning through Saturday 
     evening for MAZ007-015-016-019-022.
RI...None.
MARINE...None.

&&

$$

DISCUSSION...Mensch
AVIATION...Mensch
MARINE...Mensch