Lexington Weather

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 192341
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
741 PM EDT Tue May 19 2026

.WHAT HAS CHANGED...
No changes were made to this forecast package.

&&

.KEY MESSAGES...

- Lingering thunderstorm activity quickly winds down with the loss 
of daytime heating. Expecting very warm minimum temps tonight with 
lows in the 60s and low to mid 70s in more urban areas. 

- Final day of record warmth Wednesday with highs in the 90s away 
from the immediate coast. Chance for strong to severe thunderstorms 
by the afternoon with a risk for damaging winds and small hail. 

- Cooler temperatures through Memorial Day Weekend.
Unsettled weather returns for the weekend with periods of rain 
possible, but confidence is low.

&&

.DISCUSSION...
KEY MESSAGE 1...Lingering thunderstorm activity quickly winds down 
with the loss of daytime heating. Expecting very warm minimum temps 
tonight with lows in the 60s and low to mid 70s in more urban areas. 

Showers and storms are expected to develop across western portions 
of our CWA between 3 and 5pm and then quickly move east. Main but 
guidance is showing the potential for storms to initiate over the 
Southern Adirondacks and Green Mountains and then gradually coalesce 
into a more organized line or cluster that generally moves west to 
east through the afternoon. Main threats with any organized storms 
will be strong to damaging straight-line winds given drier air in 
the low to mid-levels resulting in inverted-V soundings. Somewhat 
notably, the 12z ALB and OKX soundings featured a modified EML 
between 700 and 500mb with mid level lapse rates between 6.5 and 7.5 
C/km. Thankfully RAP mesoanalysis indicates limited bulk shear 
around 25 kt this afternoon so we're not looking at a widespread 
severe weather episode. Still, with CAPE values between 1200 and 
1500 J/kg, there could be a severe storm or two north of the Mass 
Pike. Activity quickly wanes with the loss of daytime heating this 
evening. 

Bigger story tonight will be continued heat in humidly. Overnight 
lows remain in the upper 60s for much of the region and closer to 
the lower and even middle 70s across urban areas. These 
temperatures, paired with dewpoints in the 60s will keep apparent 
temperatures in the upper 70s to low 80s even well into the night. 
Those without adequate access to cooling and air conditioning are at 
greatest risk with the elevated overnight lows. There won't be much 
in the way of relief from winds as the boundary layer decouples and 
winds calm to less than 5 mph for much of the CWA.  


KEY MESSAGE 2... Final day of record warmth Wednesday with highs in 
the 90s away from the immediate coast. Chance for strong to severe 
thunderstorms by the afternoon with a risk for damaging winds and 
small hail. 

Upper level ridge begins to break down on Wednesday ahead of an 
approaching trough. Despite the changes to the upper pattern, 850 
temps will remain above +15C S of the Mass Pike and slightly cooler 
to the N. Guidance has somewhat increased the speed of the cold 
front resulting in increased cloud cover over much of northern 
Massachusetts where temperatures have trended a few degrees cooler. 
Even in these areas, still think that valley locations see highs in 
the low 90s with downsloping from west winds. Further south, there 
is higher confidence in more widespread readings in the low to mid 
90s away from the water. Greatest chance will be in the HFD PVD and 
BOS urban corridor. Dewpoints in the low to mid 60s (discounting the 
outlier NAM) will make it feel humid but not tropical by any means. 

An approaching cold front provides a much more robust lifting 
mechanism on Wednesday. With temperatures quickly warming into the 
90s, expecting surface CAPE values to rise to 1000-1500 J/kg. 0-6km 
bulk shear values, while fine (25-30 kts), are less favorable for 
organized severe weather than they are further north where the 
stronger mid level flow resides. So, the ingredients are there for 
some strong to severe storms. Wednesday the cold front will serve as 
a better lifting mechanism, timed coincident with peak diurnal 
heating so would expect more coverage of storms and any storms that 
do form have the chance to produce strong to damaging winds 
primarily given inverted-v soundings. Greatest threat for any severe 
weather will likely be S of the Mass Pike where higher instability 
exists. 

KEY MESSAGE 3...Cooler and unsettled weather is possible Memorial 
Day Weekend, but confidence remains low.

High pressure builds and mid-level ridging building in behind 
Wednesday's system bringing near-seasonable temperatures and dry 
conditions. 

For Memorial Day weekend there is a signal for the pattern to 
become more unsettled with a few disturbances moving through 
bringing periods of rain. However, there is still a sufficient 
amount uncertainty in the details of the track and timing of the
waves which will influence rain chances, timing, and amounts 
this holiday weekend. Across deterministic guidance, there is 
plenty of spread in the track and details of a system that moves
in from the Ohio Valley. This has led to a range of scenarios 
from a drier weekend with seasonable temperatures to a weekend 
with periods of rain and cooler temperatures. Whats adds to the 
uncertainty is the lack of consistency of models from run to run
as well. From a probabilistic view, ensemble means are keying 
in on moderate probabilities for rain across their respective 
members at this time, so the risk is there for periods of rain 
over the weekend into the holiday. We'll have to keep monitoring
as well get closer and model guidance comes into better 
agreement.

&&

.AVIATION /00Z WEDNESDAY THROUGH SUNDAY/...
Forecaster Confidence Levels...

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

00Z TAF Update:

Tonight...High confidence. Low to moderate confidence for ACK
CIGS

VFR overnight for most of the terminals. Possible IFR at ACK 
with areas of fog (lower confidence). SW winds 8-15 kts. LLWS
possible for east and southeast terminals overnight. 

Wednesday...Moderate confidence (TSRA chances).  

VFR. Scattered TSRA along and ahead of a cold front in the
afternoon and evening. Given widely scattered nature and
uncertainty in location of TSRA, have not included in TAF (lower
confidence). WSW winds 8-12 kts with gusts 20-25 kts turning NW
behind the cold front between 16-00Z. Brief strong gusts 
possible in any thunderstorms.

KBOS...High confidence in TAF.

KBDL...High confidence in TAF.

Outlook /Wednesday through Sunday/...

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

Wednesday Night: VFR. Breezy. 

Thursday through Friday: VFR. 

Friday Night through Saturday Night: VFR. Chance SHRA.

Sunday: VFR. Breezy. 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 Wednesday: High confidence. 

Small craft advisories remain in effect through 06z Thurs. SW 
winds of 15-25 kt continues this afternoon and overnight 
Seas will also be building to around 4-6 ft, with some 7 
footers possible tonight over the southern waters.

Mainly dry weather prevails through tonight with a few showers
or thunderstorms possible for the northern waters., Some fog may
persist and expand across the southern waters tonight on
increasing southwest winds.

Outlook /Wednesday through Sunday/...

Wednesday: Low risk for Small Craft Advisory winds with gusts
up to 25 kt. Local rough seas. Slight chance of rain showers. 

Wednesday Night: Winds less than 25 kt. Areas of seas
approaching 5 ft. Chance of rain showers. 

Thursday: Winds less than 25 kt. Seas locally approaching 5 ft.
Slight chance of rain showers. 

Thursday Night through Friday: Winds less than 25 kt. 

Friday Night: Winds less than 25 kt. Chance of rain showers. 

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

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

&&

.CLIMATE...
Record Highs for Tue May 19...

BOS 90/2017
BDL 94/1962 
PVD 91/2017 
ORH 92/1962

Record Highs for Wed May 20...

BOS 91/1996
BDL 99/1996
PVD 95/1996
ORH 91/1903

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Heat Advisory until 8 PM EDT Wednesday for CTZ002>004.
     Air Quality Alert until 11 PM EDT this evening for CTZ003-004.
MA...Heat Advisory until 8 PM EDT Wednesday for MAZ003>007-010>019-
     026.
     Air Quality Alert until 11 PM EDT this evening for MAZ006-007-
     013>019.
RI...Heat Advisory until 8 PM EDT Wednesday for RIZ001>004.
     Air Quality Alert until 11 PM EDT this evening for RIZ001>008.
MARINE...Small Craft Advisory until 2 AM EDT Thursday for ANZ254>256.

&&

$$

DISCUSSION...Mensch/FT
AVIATION...Mensch/FT
MARINE...BW
CLIMATE...BW/Hrencecin