Lexington Weather

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 171915
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
315 PM EDT Fri Jul 17 2026

.WHAT HAS CHANGED...

No significant changes made to the forecast...But we did issue small 
craft headlines Saturday into part of Sunday for all our coastal 
waters.

&&

.KEY MESSAGES...

- Showers and scattered thunderstorms with torrential rainfall 
  along with a localized flash flood threat Sat afternoon and night. 
  A few severe thunderstorms possible too with the greatest risk
  interior MA/CT and onto the south coast. Areas of smoke return
  late tonight into Sat but not as bad as what we had Wed. 

- Drying out Sunday into the start of next week, but more 
  active weather returns around midweek.

&&

.DISCUSSION...

KEY MESSAGE 1...Showers and scattered thunderstorms with torrential 
rainfall along with a localized flash flood threat Sat afternoon and 
night. A few severe thunderstorms possible too with the greatest 
risk interior MA/CT and onto the south coast. Areas of smoke return 
late tonight into Sat but not as bad as what we had Wed. 

The main concern with this forecast package will be a couple rounds 
of showers and scattered thunderstorms Sat afternoon and night. This 
activity will bring the potential for localized flash flooding as 
well a few severe thunderstorms as well. We will discuss this in 
more detail below.

High pressure slowly shifts east of our region tonight...But it will 
still remain in control of our weather. Diminishing wind and just 
some high clouds will result in another good night of radiational 
cooling. Low temps should be in the 50s across many low lying 
locations...while the urban heat island of Boston drops only into 
the middle to upper 60s. The high resolution model guidance also 
indicates the return of wildfire smoke overnight...But it does not 
look to be as significant as what we saw on Wed.

Otherwise...the focus is on the severe/localized flash flood 
potential for Sat afternoon and evening. This is a difficult 
forecast given a low instability/high shear environment. A vigorous 
shortwave for July standards will approach the region and induce an 
impressive southerly LLJ 3+ standard deviations above normal on the 
order of 45-50+ knots. This will combined with Pwats on the order of 
1.75 to 2.25 inches with the highest of those values near the south 
coast. These parameters are certainly favorable for pockets of 
torrential rainfall with any thunderstorms and a localized flash 
flood threat. This risk would increase if we are able to get any 
severe weather...which we will discuss below.

The greatest instability will be to our west and south where the 
highest severe weather probabilities will exist. 
Nonetheless...moisture return will allow for surface instability to 
reach near 1000 J/KG. While this is on the lower side...given the 
very impressive jet dynamics for July standard there will be the 
risk for a few severe thunderstorms into our region as well. 
Greatest risk will be across interior MA into CT and towards the 
south coast...but can not rule them out across the rest of the 
region. Appears the activity may come in two main rounds...the first 
being in the afternoon/early evening hours followed by a sound round 
later in the evening into the overnight hours with probably a bit of 
a lull in between. The main severe weather risk would be localized 
straight line wind damage. However...if a secondary meso-low 
pressure system is able to develop and enhance low level helicity 
there would be a low probability for a brief tornado. 

As for the localized flash flood threat...given the Pwat plume in 
place the potential exists for 2+ inches of rain within 30-45 
minutes. This would be problematic if it falls over a vulnerable 
urban center. That is why this risk will increase if we are able to 
get severe weather in more robust updrafts...which would favor 
interior MA into CT. However...flash flooding parameters are most 
favorable near the south coast where the nose of the strong low 
level jet impinges on 70+ dewpoints etc. Later shifts may need to 
consider a Flash Flood Watch...But still felt there was too much 
uncertainty to do it now. While the localized flash flood threat 
exists across our entire region...greatest risk is across interior 
MA/CT and onto the south coast. 

The last of the showers and thunderstorms should clear the 
Cape/Islands toward daybreak. It will be quite muggy Sat night with 
lows only dropping into the 60s to near 70 by daybreak Sun. 

KEY MESSAGE 2...Drying out Sunday into the start of next week, 
but more active weather returns around midweek.

The low pressure system and its fronts associated with Saturday's 
system shifts off to the northeast through the day Sunday as the 
post-cold frontal airmass takes hold over southern New England. 
Drier, less humid conditions prevail through much of the start of 
next week with seasonable temperatures. High pressure takes hold 
Monday and starts shifting more offshore Tuesday, which will lead to 
the return of more southerly flow. Moisture advected in by this 
southerly flow may increase dewpoints back into the upper 60s/low 
70s by Tuesday afternoon. NAEFS guidance also indicates anomalous 
PWATs for this time of year making a comeback over southern New 
England between the Tuesday/Wednesday period. These moist conditions 
combined with the arrival of another strong frontal system from the 
northern Plains will increase the chance for some downpours and 
overall widespread rain. As we are still far out from this system, 
details regarding timing, exact location, and severity/strength are 
still uncertain. But generally, guidance is in agreement regarding 
an unsettled midweek timeframe.

&&

.AVIATION /19Z FRIDAY THROUGH WEDNESDAY/...

Forecaster Confidence Levels...

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

18z TAF Update...

Tonight through Saturday night...High confidence in trends with 
moderate confidence in specifics and timing. 

Mainly VFR tonight into Sat morning despite the return of some smoke 
during the overnight hours. Conditions deteriorate Sat afternoon and 
night in round of showers and scattered t-storms. This will bring 
widespread MVFR conditions with localized IFR/LIFR conditions. A few 
strong to severe storms will be possible with localized brief strong 
wind gusts and torrential rainfall. Even outside of any 
thunderstorms...we expect SW wind to gust between 25 and 30 knots 
with a few gusts up to 35 knots Sat afternoon and evening. LLWS will 
be a concern as well. 

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.  

Outlook /Sunday through Wednesday/...

Sunday: VFR. Breezy. 

Sunday Night through Monday Night: VFR. 

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

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

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

&&

.MARINE...
Forecaster Confidence Levels...

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

Tonight through Saturday night...High confidence.

High pressure slowly shifts east of the waters tonight...but 
winds/seas remain below small craft advisory thresholds. Strong 
shortwave will induce an impressive southerly LLJ of 50+ knots Sat 
afternoon and night. Despite the inversion...we expect widespread 
southwest wind gusts of 25-30 knots with a few gusts near 35 knots. 
Seas will also respond and become quite rough across the waters. 
Therefore...have hoisted small craft headlines for all waters over 
this time. Scattered thunderstorms later Sat and Sat night may 
produce localized stronger wind gusts as well...which may require 
the issuance of some special marine warnings. 

Outlook /Sunday through Wednesday/...

Sunday: Winds less than 25 kt. Local rough seas. 

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

Monday: Winds less than 25 kt. Areas of seas 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, slight chance of thunderstorms. 

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

Wednesday: Low risk for gale force winds with gusts up to
35 kt. Rough seas up to 15 ft. Rain showers likely, slight
chance of thunderstorms. 

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory from 11 AM Saturday to noon EDT Sunday 
     for ANZ230>237-250-251.
     Small Craft Advisory from 11 AM Saturday to 5 PM EDT Sunday 
     for ANZ254>256.

&&

$$

DISCUSSION...Frank/Hrencecin
AVIATION...Frank/Hrencecin
MARINE...Frank/Hrencecin