Old Res Weather

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 181820
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
220 PM EDT Sat Jul 18 2026

.WHAT HAS CHANGED...
The risk for severe weather, including damaging winds and a 
tornado or two, has increased across portions of western and 
central Massachusetts and Connecticut.

&&

.KEY MESSAGES...
- Areas of smoke and haze over the region through the evening. 
  Tracking two rounds of showers and potential severe storms 
  this afternoon and evening. The first round impacts the area 
  between 3 and 8 pm and the second arrives after 9pm. Storms 
  primarily bring a damaging wind and flash flood threat with a 
  low chance of a tornado or two.

- Turning drier and seasonable Sunday and Monday. High risk for
  rip currents on south-facing beaches on Sunday.

- Dry weather continues into Monday Night, but more active
  weather returns around midweek. Dry weather expected late 
  next week.

&&

.DISCUSSION...
KEY MESSAGE 1...Areas of smoke and haze over the region through
the evening. Tracking two rounds of showers and potential severe 
storms this afternoon and evening. The first round impacts the
area between 3 and 8 pm and the second arrives after 9pm. Storms 
primarily bring a damaging wind and flash flood threat with a 
low chance of a tornado or two

Active afternoon and evening in store as a seasonably-strong
frontal system moves through Southern New England over the 
next 24 hours. First, on the returning smoke. Smoke progs and 
AQI readings show the thickest smoke over the region now through
23z before moving away from the region after 03z. Air Quality 
Alerts remain in effect for today for PM2.5, at request of our 
state environmental agency partners. 

In terms of the heavy rain/severe threat, we're still seeing a 
notably strong wind field developing this afternoon and evening 
with 800mb winds increasing to 45-50kts. The strong mid-level 
winds will help one to perhaps two rounds of severe weather form
this afternoon and overnight despite marginal instability. More
details below.

First round of precipitation arrives between 12 and 2pm as a
strong warm front moves in from the SW. The severe threat will
be limited with this feature as instability will be quite 
limited as it moves through. There may be a few elevated 
thunderstorms with CAMs such as the HRRR showing the greatest 
risk of that kind of activity across northern MA. The front will
serve to increase moisture and instability with PWATs surging 
to 2-2.25" by 18z.

Surface CAPE values won't be too impressive, with most CAMS 
showing values between 750-1000 J/kg, for the majority of our 
CWA. Greatest chance for >1000 J/kg lies across western and 
central Massachusetts and Connecticut with MUCAPE values perhaps
approaching 1500 J/kg in areas with localized clearing. While 
the absolute values of instability are marginal at best, the 
low-level, or 0-3 km CAPE values are noteworthy. The HRRR shows 
these values approaching 100-150 J for most of the CWA as well 
as a corridor of 150-200 J for NW Massachusetts. The importance 
of 0-3 km shear is that it provides enough vertical motion to 
help tilt any horizontally rotating updrafts in a highly sheared
environment.

Speaking of shear, guidance continues to show impressive values,
with bulk shear approaching 50-60kts after 18z. With the
aforementioned wind field, modeled hodographs are elongated and 
curved, especially west of the CT River Valley. HRRR BUFKIT 
soundings show 0-3 km SRH values >300 m2s2 and 0-1 km values 
between 150-250 m2s2! Looking at the parameters as a whole, the 
main threat with today's severe storms still appears to be areas
of straight line winds. Secondary threat is for a tornado or 
two, with the greatest threat across western and central 
Massachusetts and Connecticut.

Still have a high potential for areas of torrential rain with  
any storms that form, fortunately, fast, progressive storm 
motions will help to cap a more widespread flash flood risk. 
Best potential for a flash flood risk from high rain rates in 
urbanized areas this afternoon if and where any cell training 
occurs. Most of the CAMs pencil in areas south of the Mass Pike 
as having the most concentrated area of showers and storms, and 
especially the Hartford- Providence-Southeast MA corridor. Opted
for high Likely to Categorical PoP for these areas, then just a
generic Chance PoP north of here to make the distinction. The 
main risk with this first wave of storms is the risk for 
torrential downpours capable of street flooding. Continued QPF 
placement uncertainties precluded a Flood Watch with this 
issuance, as there are still a good number of guidance sources 
which shift the rain/storms into the southern waters. 

As this initial round of showers/storms shifts southward, the
next period of showers and thunderstorms arrives late in the day 
into tonight (between 6 to 11 PM) with the cold front. Most
models show a pretty extensive line of thunderstorms developing 
late this afternoon as the front moves southward across upstate 
NY and VT. We'll have to watch the northeast extent of this line
as it slips south-southeast into the Berkshires, northern CT 
and central MA, with less of a risk into eastern MA. Straight- 
line wind damage, frequent lightning and torrential downpours 
would be the main risks with this second round. The risk for 
late- evening strong storms is more uncertain the further south 
one goes, since these areas will probably have been worked over 
pretty well with the afternoon heavy downpours/storms, with 
weakening trends more likely. Front clears into the southern 
waters overnight, with northwesterly breezes and turning much 
less humid by sunrise. 


KEY MESSAGE 2...Turning drier and seasonable Sunday and Monday.
High risk for rip currents on south-facing beaches on Sunday.

The cold front brings a stretch of pleasant summer weather Sunday 
and Monday as PWATs behind the front fall to >0.5". The drier air 
mass will bring significantly lower humidities and temperatures in 
the 70s to mid 80s.. At the beaches though, swell directions Sunday 
are still oriented from SW to NE from today's storm system, with 
seas in the 4 to 6 ft range. This will favor a high risk for 
dangerous rip currents. With Sunday being a better day to go to the 
beach than today, issued a rip current statement for the southern 
beaches.


KEY MESSAGE 3...Dry weather continues into Monday Night, but
more active weather returns around midweek. Dry weather 
expected late next week.

Latest guidance suite was in reasonable agreement with the
evolution of the overall synoptic pattern next week. That 
said, the usual timing and magnitude differences are present, 
leading to an average confidence in the forecast details.

Monday should be dry with slightly below normal temperatures.
Then we enter a period of more unsettled weather between Tuesday 
and Wednesday Night. This is when a low pressure should move 
across eastern Canada and sweep a series of fronts through
southern New England. It is not anticipated to be raining this 
entire time, but the risk for showers and a few thunderstorms 
will be there. The most likely time period is currently Tuesday 
night into Wednesday, but this timing could change with later 
forecasts.

A large high pressure is anticipated to arrive late Thursday
into Friday with drier weather.

&&

.AVIATION /18Z SATURDAY THROUGH THURSDAY/...
Forecaster Confidence Levels...

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

Through Tonight: Moderate confidence.

VFR CIGS with MVFR visibility this afternoon and evening.
Widespread showers and areas of smoke/haze. TSRA more isolated
for the moment. A cold front approaching from the N will lead to
a period of more widespread IFR/MVFR until this front passes by.
Thinking VFR develops just before the Sunday morning push at
most locations.

Sunday and Sunday Night: High confidence. 

VFR. NW winds around 10-15 kt, diminishing Sunday Night.

KBOS Terminal...Moderate confidence in TAF due to timing issues.
Mentioned prevailing SHRA, but not thinking it will be raining 
this entire time. Greatest risk for any TSRA between
18/21-19/03Z.

KBDL Terminal...Moderate confidence in TAF due to timing issues.
Likely to see a lull in TSRA 18/20-22Z, but SHRA still around.

Outlook /Monday through Thursday/...

Monday through Monday Night: VFR. 

Tuesday: VFR. Breezy. Chance SHRA, chance TSRA.

Tuesday Night: Mainly MVFR, with local IFR possible. Windy with
local gusts up to 30 kt. SHRA, slight chance TSRA.

Wednesday: Mainly VFR, with areas MVFR possible. Windy with
gusts up to 35 kt. SHRA likely, chance TSRA.

Wednesday Night: VFR. Windy with local gusts up to 35 kt.
Chance SHRA.

Thursday: VFR. Breezy. Slight chance SHRA.

&&

.MARINE...
Forecaster Confidence Levels...

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

Through Tonight: High confidence.

Small Craft Advisories continue. Southwest wind gusts 25-30 kt  
are expected with seas becoming rough. Scattered thunderstorms 
may produce localized stronger wind gusts as well, which may
require the issuance of some Special Marine Warnings.

Sunday: High confidence. 

NW winds around 15-20 kt, with seas around 4-6 ft. High risk for
rip currents at the beaches/surf zone.

Sunday Night: High Confidence.

Winds and seas diminish.

Outlook /Monday through Thursday/...

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

Monday Night: Winds less than 25 kt. 

Tuesday: Winds less than 25 kt. Seas locally approaching 5 ft.
Chance of rain showers. 

Tuesday Night: Moderate risk for Small Craft Advisory winds
with gusts up to 30 kt. Areas of rough seas. Rain showers,
chance of thunderstorms. Local visibility 1 to 3 nm.

Wednesday: Moderate risk for gale force winds with gusts up to
40 kt. Rough seas up to 13 ft. Rain showers likely, slight
chance of thunderstorms. Local visibility 1 to 3 nm.

Wednesday Night: Low risk for gale force winds with gusts up to
40 kt. Rough seas up to 14 ft. Chance of rain showers, slight
chance of thunderstorms. 

Thursday: Winds less than 25 kt. Rough seas up to 11 ft. Chance
of rain showers. 

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Air Quality Alert until midnight EDT tonight for CTZ002>004.
MA...Air Quality Alert until midnight EDT tonight for MAZ002>024-
     026.
     High Risk for Rip Currents from Sunday morning through Sunday 
     evening for MAZ020-022>024.
RI...High Risk for Rip Currents from Sunday morning through Sunday 
     evening for RIZ006>008.
     Air Quality Alert until 11 PM EDT this evening for RIZ001>008.
MARINE...Small Craft Advisory until noon EDT Sunday for ANZ230>237-250-
     251.
     Small Craft Advisory until 8 PM EDT Sunday for ANZ254>256.

&&

$$

DISCUSSION...Belk/FT
AVIATION...Belk
MARINE...Belk/FT