Lexington Weather

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 200610
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
210 AM EDT Sun Sep 20 2026

.WHAT HAS CHANGED...
No significant changes.

&&

.KEY MESSAGES...

- Quiet through the rest of today, cooler tonight, with clouds 
  returning before sunrise.

- Briefly dry early to mid morning, then beneficial rain 
  overspreads southern New England and continues through the 
  evening and overnight hours. Flooding concerns remain low. 

- High pressure Tuesday gives way to a coastal system for the 
  second half of the week

&&

.DISCUSSION...
KEY MESSAGE 1...Quiet through the rest of today, cooler 
tonight, with clouds returning before sunrise.

A 1020-1025 mb high pressure system responsible for today's 
gorgeous weather drifts offshore tonight ahead of our next 
system for Sunday. Before that arrives, continued fall-like 
weather is expected this afternoon and overnight. Winds decouple
overnight, becoming fairly light to calm in some areas. In 
addition, feel the cloud cover will be slower to move in. While 
clouds push in from the west overnight, thinking these will 
initially be mainly high clouds. Thus, should still see a period
of good radiational cooling during the first part of the night,
while eastern areas of southern New England may see a majority 
of the night before clouds fully arrive.

As is typical with this setup, the NBM appears too mild, so 
have blended in the cooler MOS/MET/MAV guidance. This really 
highlights the colder pockets along the I-495 corridor, the 
hollows of northern Massachusetts, and the Vineyard. All of 
these areas fall well into the lower 40s tonight, though it is 
not out of the realm of possibility to see an upper 30 sprinkled
in there. Otherwise, urban centers such as Boston, Providence, 
and Hartford remain in the upper 40s to lower 50s. Worcester 
stays relatively mild as well, given the city's elevation and 
location near the top of the nocturnal inversion.

KEY MESSAGE 2...Briefly dry early to mid morning, then 
beneficial rain overspreads southern New England and continues 
through the evening and overnight hours. Flooding concerns 
remain low.

As advertised, conditions Sunday aren't ideal for outdoor 
activities, with a raw, cool, and rainy day expected. Highs are 
mainly in the lower to middle 60s across the coastal plain, with
middle to upper 50s across the higher elevations of southern 
New England.

A subtle mid-level northern stream trough arrives from the west
early Sunday morning, accompanied by a modest 850 mb jet, 
though the jet weakens in magnitude as it shifts east. This 
should provide enough forcing to capitalize on PWATs ranging 
from 1.4 to 1.9 inches, or roughly 150-200 percent of normal. 
Despite the anomalous moisture, we aren't concerned about 
impacts similar to those experienced with last Sunday's 
heavy/torrential rainfall and flooding. Last Sunday's event was 
considerably more convective in nature, while Sunday's rainfall 
should be predominantly stratiform. This should favor lower 
rainfall rates and greatly reduce the potential for localized 
torrential downpours that contributed to flooding last weekend.

Timing this out, widespread rain develops during the mid to 
late morning and persists well into the evening. There is pretty
good agreement among the 19/12z suite of high-res guidance. By 
18z/2 PM, rain should be widespread across much of the region. 
The exception will be the outer Cape and Nantucket, which may 
remain dry for an additional hour or two. There is some, albeit 
limited, instability near the south coast, where we cannot rule 
out a rumble of thunder and briefly heavier rainfall. One other 
area to monitor is near the Mass Pike and the transition from 
the coastal plain into the higher terrain of central 
Massachusetts. High-res guidance shows a convergence boundary 
developing in this vicinity, which may lead to a corridor of 
enhanced rainfall.

Overall, rainfall amounts of 0.5-1.0 inch are expected, though 
some variability from town to town is likely, especially near 
any areas of enhanced rainfall. The HREF maximum does show 
localized amounts approaching 2 inches, representing the upper 
end of the guidance envelope, but confidence in amounts this 
high is quite low. Given the generally lower rainfall rates, the
risk for flooding remains low. Some minor ponding is possible 
where leaves or other debris clog storm drains, but otherwise 
this should be a beneficial rainfall for the region.

With plenty of cloud cover overnight and a breezy northerly 
wind, there are no concerns for a particularly chilly night. It 
will remain raw and wet, with lows in the upper 40s to lower 
50s. Rain shifts offshore with the front early Monday morning, 
coming to an end by or before daybreak.

KEY MESSAGE 3...High pressure Tuesday gives way to a coastal 
system for the second half of the week. 

Ensemble guidance remains in good agreement that an anomolously
strong high pressure system will develop over Quebec Tuesday 
and linger into Wed/Thurs. This system likely keeps temperatures
across New England cooler than normal, in the low to mid 60s.
 
Both deterministic and ensemble guidance continue to indicate 
potential for a coastal low to form sometime Thursday into 
Friday. While there is still a large amount of uncertainty in 
the forecast this time, the anomolously strong high to our north
and a low offshore will certainly lead to a tight gradient and 
strong winds throughout the week, especially over the south 
coast, Cape/Islands, and waters. Depending on how close the low 
tracks, there is also potential for a few rounds of rain.

&&

.AVIATION /06Z SUNDAY THROUGH THURSDAY/...
Forecaster Confidence Levels...

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

06Z TAF Update:

Through 12Z...High confidence.

VFR with increasing mid-level clouds. Light ESE wind with some 
areas light and variable. 

Sunday...High confidence in trends, moderate confidence in 
timing

VFR to start, trending downward to MVFR from west to east as 
rain overspreads the region 13-17Z. Potential for IFR ceilings
to develop this afternoon-evening although coverage is lower 
confidence. The threat for LLWS has lessened, although can't be
ruled out as a weaker low level jet shifts across southern New
England between 18Z-06Z. Otherwise winds generally SE to ESE at
5-10 kt. Chances for embedded thunderstorms are very low to 
near zero. 

Sunday Night...High confidence in trends, moderate confidence 
in timing and ceilings. 

IFR ceilings with LIFR possible in areas. Visibilities 2-5SM
with -RA continuing overnight. -RA ends from west to east
mainly between 06Z-12Z. Winds shift to the N at 8-12 kt.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Monday through Thursday/...

Monday: Mainly VFR, with local MVFR possible. Breezy. 

Monday Night through Tuesday: VFR. Breezy. 

Tuesday Night: VFR. Windy with local gusts up to 30 kt. 

Wednesday: VFR. Windy with gusts up to 35 kt. 

Wednesday Night: VFR. Strong winds with local gusts up to
40 kt. Slight chance SHRA.

Thursday: Mainly VFR, with local IFR possible. Strong winds
with local gusts up to 40 kt. Slight chance SHRA.

&&

.MARINE...
Forecaster Confidence Levels...

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

Through Sunday Night...High Confidence.

An area of high pressure moves east of the coastal waters 
overnight and toward the Canadian Maritimes by Sunday night. 
With the departure of the high, a low pressure system riding 
along a frontal boundary ushers in unsettled weather by late 
Sunday morning, with rain becoming widespread Sunday afternoon. 
There is a low chance for a few rumbles of thunder, primarily 
across the southern waters. Rain ends early Monday morning, 
though a few showers could linger across the southern outer 
waters as the front sags to the south. Seas throughout this 
period remain between 2-4 ft, with the greatest wave heights 
across the outer waters. Winds are northeast through this 
evening, then become southeast on Sunday. As the front pushes 
south, winds shift to the north/northeast. Gusts begin to 
increase with cold air advection Monday morning, which may 
require Small Craft headlines.

Outlook /Monday through Thursday/...

Monday: Low risk for Small Craft Advisory winds with gusts up
to 30 kt. Areas of seas approaching 5 ft. Slight chance of rain
showers. 

Monday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Local rough seas. 

Tuesday: Moderate risk for Small Craft Advisory winds with
gusts up to 30 kt. Rough seas up to 10 ft. Slight chance of
rain showers. 

Tuesday Night: Moderate risk for Small Craft Advisory winds
with gusts up to 30 kt. Rough seas up to 11 ft. Slight chance
of rain showers. 

Wednesday: Strong winds with gusts up to 40 kt. Rough seas up
to 13 ft. Slight chance of rain showers. 

Wednesday Night: Strong winds with gusts up to 40 kt. Rough
seas up to 16 ft. Slight chance of rain showers. 

Thursday: Moderate risk for gale force winds with gusts up to
45 kt. Rough seas up to 12 ft. Chance of rain showers. 

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...None.

&&

$$

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