Old Res Weather

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 261927
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
327 PM EDT Sun Jul 26 2026

.WHAT HAS CHANGED...
Refined shower chances and flash flood potential through the
upcoming week. 

&&

.KEY MESSAGES...

- Low-impact weather through Monday night. A few thunderstorms
  are possible Monday afternoon across western portions of
  southern New England.

- Periodic chances of scattered showers and thunderstorms early
  to mid week next week, potentially lingering into Thursday. 
  Increasing potential for heavy rain late Tuesday through 
  Wednesday with at least localized flash flooding possible.

&&

.DISCUSSION...
KEY MESSAGE 1...Low-impact weather through Monday night. A few 
thunderstorms are possible Monday afternoon across western portions 
of southern New England.

A weak area of low pressure continues to spin southeast of 
Nantucket, maintaining low clouds and occasional light precipitation 
across the outer Cape and Islands. While not the best beach day 
there, a High Risk for rip currents continues through this evening 
for east-facing beaches of Cape Cod and Nantucket. Elsewhere, most 
of southern New England enjoys sunshine and comfortable conditions. 
The remainder of today through tonight will be relatively quiet. 
Lingering low-level moisture from northeasterly flow combined with 
light winds and mostly clear skies overnight may allow for patchy 
fog to develop in the typical low-lying and river valley locations 
of eastern Massachusetts. Overnight lows fall into the 50s inland 
and the lower 60s along the immediate coast, which is seasonably 
cool for late July.

Monday will feature seasonable temperatures, with highs in the lower 
to middle 80s inland and upper 70s along the coast. A shortwave 
trough dropping into the Northeast, along with an embedded 850-700mb 
disturbance, will provide enough forcing for isolated to scattered 
showers and thunderstorms during the mid to late afternoon, 
primarily west of the Connecticut River Valley. PWATs increase to 
around 1.3 inches, near to slightly above climatological normals but 
not especially anomalous. While a few thunderstorms may produce 
locally heavy downpours, rainfall amounts should generally remain 
modest, with isolated localized totals approaching one-half inch. 
Flash flooding is not anticipated, as 3-hour flash flood guidance 
remains around 2.5 to 3.0 inches. Instability increases to around 
800-1200 J/kg Monday afternoon, but weak deep-layer shear (generally 
less than 30 kt) should limit storm organization. As a result, 
convection is expected to remain isolated and pulse-like, with many 
locations remaining dry. The highest PoPs therefore remain focused 
west of the Connecticut River. CSU machine-learning guidance also 
keeps the better severe-weather signal west of eastern New York. 
While severe weather appears unlikely, the primary hazards with any 
storms will be cloud-to-ground lightning and brief, locally heavy 
rainfall.

Monday night, any lingering convection will diminish during the 
evening. Southwesterly flow aloft then increases moisture, with 
PWATs climbing toward 1.5 inches. Low clouds become more widespread 
overnight, and scattered light showers may begin to develop ahead of 
a more unsettled pattern on Tuesday. Overnight temperatures remain 
mild, holding in the 60s.

KEY MESSAGE 2...Periodic chances of scattered showers and 
thunderstorms early to mid week next week, potentially 
lingering into Thursday. Increasing potential for heavy rain 
late Tuesday through Wednesday with at least localized flash 
flooding possible.

A mid-level trough positions across the region through much of the 
week. A series of shortwaves embedded in the flow are expected to 
progress across the region through the week supporting periodic 
chances for showers and thunderstorms. 

Model guidance has remained generally consistent with a deepening 
trough into Tue/Wed with a strong signal for above normal moisture 
(120-170% of normal) advecting into the region by Tuesday. Most 
deterministic and ensemble solutions have now come on board with the 
development of a low pressure system. In many solutions now, the low 
pressure is quite anomalous in magnitude for late-July. Combined 
with the enhanced forcing from the low, this is a set-up for heavy 
rain potential. However, some model guidance has been more 
aggressive than others. The ECMWF and its ensemble members have 
shown the strongest signal for heavy rain/flash flooding potential 
with a stronger low pressure system tracking in the vicinity of Long 
Island, toward eastern MA/Cape Cod in many of its solutions. It also 
leans on a slower progression of the trough as well. Soundings 
exhibit the textbook environment for efficient rain processes with 
deep warm cloud depths aligned with instability. ECMWF ens probs 
show 40-75% probability for 1+ of rainfall later Tue into Wed and 10-
40% for 2"+ across a good portion of southern New England. The 
deterministic guidance over the last 24 hours of runs has showed a 
large footprint of 2"+ amounts across the region in that same 
timeframe. With weak steering flow this support slow storm motions 
of embedded heavy rain and thunderstorms. This combined with 
potential for training for training will increase the risk for flash 
flooding with more "scattered" occurrences as opposed to an 
isolated case or two. While the recent deterministic runs of the
Canadian and GFS have gone more on board with the heavy rain 
potential, their ensemble guidance still have shown solutions 
leaning toward a weaker low and differing track resulting 
leading to lesser rainfall amounts. There is still a signal for 
localized heavy rain/flash flooding within these solutions; 
however, they are more isolated or localized. There is still 
uncertainty; however, we are at least seeing increasing 
confidence for widespread rain with at least localized heavy 
downpours/flash flooding possible in spots, particularly later 
Tue through Wed. Depending on the track of the low, showers may 
linger through Thurs. 

Temperatures lean seasonably cool to slightly below normal at times 
through the week ranging in the mid 70s to low 80s.

&&

.AVIATION /19Z SUNDAY THROUGH FRIDAY/...
Forecaster Confidence Levels...

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

18z TAF Update:

This Afternoon & Tonight...High confidence, except moderate 
confidence across Cape Cod and the Islands.

Mainly VFR conditions with ENE to NE winds, becoming light and 
variable or northerly overnight. The exception is across Cape Cod 
and the Islands, where a distant offshore low continues to advect 
MVFR stratus and reduced visibilities into the region. There remains 
a low to moderate potential for this stratus to expand north and 
northwest toward KPVD and KOWD after 08z before retreating back 
toward the coast around sunrise.

Monday...High confidence, except moderate confidence across Cape Cod 
and the Islands.

Predominantly VFR. Areas of MVFR stratus may linger across Cape Cod 
and the Islands through around 15z before improving to VFR by 18z. E 
to ESE winds remain below 10 kt. A weak front approaches western 
southern New England after 18z, bringing a slight chance of isolated 
showers or thunderstorms to western terminals, including KBDL. 
Confidence remains too low to include in the current TAF package, 
though later forecasts may need to introduce VCSH or VCTS if 
confidence increases.

Monday Night...Moderate confidence.

VFR conditions at the start of the period should gradually 
deteriorate to MVFR, with localized IFR/LIFR ceilings developing 
overnight. Confidence in the timing of these restrictions remains 
lower. Scattered showers are also possible overnight, with more 
widespread precipitation expected by Tuesday. Light S winds.

KBOS Terminal...High confidence.

KBDL Terminal...High confidence.

Outlook /Tuesday through Friday/...

Tuesday: Mainly VFR, with areas MVFR possible. Breezy. Chance
SHRA, slight chance TSRA, patchy BR.

Tuesday Night: Mainly MVFR, with local IFR possible. Breezy.
SHRA likely, slight chance TSRA, patchy BR.

Wednesday: Mainly MVFR, with areas IFR possible. Breezy. SHRA,
slight chance TSRA, patchy BR.

Wednesday Night: Mainly VFR, with areas MVFR possible. Breezy.
SHRA likely, chance TSRA.

Thursday: VFR. Breezy. Chance SHRA, slight chance TSRA.

Thursday Night: VFR. Chance SHRA, slight chance TSRA.

Friday: VFR. Slight chance SHRA, slight chance TSRA.

&&

.MARINE...
Forecaster Confidence Levels...

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

Through Tuesday...High confidence.

Light winds and seas 1-4 ft through Monday. Increasing winds 
and seas for Sunday; however, below SCA. NE winds 10-15 kts 
with gusts to 20 kts. Light NE to SE winds Monday. 

Seas increase late Tuesday into Wednesday with SCA conditions 
becoming more likely from winds and increased seas. 

Increased risk for rip currents today with the greatest risk 
for the east-facing beaches/coastline.

Outlook /Tuesday through Friday/...

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

Tuesday Night: Winds less than 25 kt. Areas of seas approaching
5 ft. Chance of rain showers, slight chance of thunderstorms,
patchy fog. 

Wednesday: Moderate risk for Small Craft Advisory winds with
gusts up to 30 kt. Seas up to 5 ft. Rain showers likely, slight
chance of thunderstorms, patchy fog. 

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

Thursday: Winds less than 25 kt. Areas of rough seas. Chance of
rain showers, slight chance of thunderstorms. 

Thursday Night: Winds less than 25 kt. Areas of rough seas.
Slight chance of rain showers, slight chance of thunderstorms. 

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

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...High Risk for Rip Currents until 8 PM EDT this evening for MAZ022-
     024.
RI...None.
MARINE...None.

&&

$$

DISCUSSION...Dooley/Mensch
AVIATION...Dooley/McMinn
MARINE...Dooley/McMinn