Lexington Weather

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 182336
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
736 PM EDT Mon May 18 2026

.WHAT HAS CHANGED...
Heat Advisories issued for most of Southern New England Tuesday
and Wednesday with the first multi-day stretch of hot
temperatures and a moderate degree of humidity. Record high
temperatures remain possible.

&&

.KEY MESSAGES...

- First multi-day stretch of hot temperatures (in the mid to
  upper 90s away from the South Coast) with a moderate degree of
  humidity Tuesday and Wednesday. Near record heat possible.

- Isolated thunderstorms possible Tuesday north and west of 
  Interstate 95. Better chances for strong to severe 
  thunderstorms on Wednesday with strong straight line winds the
  main risk.

- Seasonable and dry Thursday and Friday. 

- Cooler and unsettled weather is possible Memorial Day 
  Weekend, but confidence remains low.

&&

.DISCUSSION...
KEY MESSAGE 1...First multi-day stretch of hot temperatures (in the 
mid to upper 90s away from the South Coast) with a moderate degree 
of humidity Tuesday and Wednesday. Near record heat possible.

A spell of anomalous heat (e.g. 850 mb temps running in the upper 
teens Celsius, which are 2-3 standard deviations above 
climatological normals for late May) will build into Southern New 
England on Tue, only slightly cooler on Wed. SW breezes should also 
mitigate the risk for cooling seabreezes but will keep the immediate 
South Coast, Cape and Islands considerably cooler than further 
inland (highs mid 70s/low 80s). Away from the South Coast, highs on 
Tuesday should reach the mid 90s in most areas with upper 90s 
possible due to westerly downsloping in the CT and Merrimack 
Valleys. With a southward-sagging frontal boundary nearby on Wed, 
highs project to be a few degrees cooler in the upper 80s/low 90s 
north of the Mass Pike and low-mid 90s in the CT-RI-SE MA corridor, 
with still cooler temps 70s/near 80 South Coast/Cape and Islands. 
Even though the NAM-based guidance shows dewpoints in the mid 60s to 
low 70s, it usually demonstrates a moist-bias in these strong-mixing 
days and values should be closer to the mid 50s to mid 60s Tue and 
Wed. That's also a factor which will be alluded to in the next Key 
Message to follow. 

We're looking at a spell of hot temperatures but a generally drier 
heat, not necessarily oppressively humid like we typically see with 
forecasted temperatures this hot. With dewpoints mid 50s to lower 
60s, it's more of "drier" heat and the difference between the air 
temperature and the apparent temperature (heat index) may not be 
that much different. Framing this setup a little more, if this type 
of setting were deeper in the summer months, it might not rise to 
the level of heat headlines. However NWS HeatRisk graphics 
outline portions of the CT and Merrimack Valleys in the Major 
HeatRisk classification. Given this datapoint, forecasted heat 
indices which are borderline (mid to upper 90s), and that there 
may be individuals who are not yet accustomed to this degree of 
heat as of yet, we opted to issue a Heat Advisory for Tuesday 
and Wednesday. 

KEY MESSAGE 2...Isolated thunderstorms possible Tuesday north and 
west of Interstate 95. Better chances for strong to severe 
thunderstorms on Wednesday with strong straight line winds the main 
risk.

The warm airmass Tuesday and Wednesday will create a marginally 
to moderately-unstable convective setting, with some risk for 
thunderstorms both days. I'm skeptical of the NAM's inflated 
instability parameters given the above moist-bias (upper 60s to 
low 70s) and the highest CAPE values stem from the NAM and its 
downscaled 3-km version, but even the drier GFS shows about 1000
J/kg on Tuesday and again on Wednesday. Soundings show a well-
mixed PBL favorable for downdraft and gust-front production if 
any showers or storms can pop up. 

What may trigger storms on Tuesday is debatable, as the best 
mid- level winds/stronger wind shear and strong cold front is to
our north and west. A passing sfc trough could be enough to 
generate isolated storms north and west of I-95; one or two 
could become strong if storms develop with strong winds the main
risk; the lack of sufficiently strong wind shear would lead to 
more pulse- type storms. Wednesday may offer a better chance at 
strong to severe storms with the approaching cold front 
interacting with moderate instability. The front's timing is in 
some question but somewhat better chances for thunderstorms near
to south of the Mass Pike. Both days feature a Level 1 of 5 
(Marginal Risk) for severe weather. 

KEY MESSAGE 3...Seasonable and dry Thursday and Friday. 

Thursday and Friday are fairly straightforward, with a 1030mb high 
pressure system across the Great Lakes on Thursday before shifting 
east on Friday, leading to dry weather. What is most noticeable is 
the cooler air mass. 850mb temperatures lower to +2C to +5C on 
Thursday, a far cry from the +18C to +20C observed just a few days 
prior. With a dry and well-mixed boundary layer, mixing up to 850mb 
seems likely, tapping into those cooler temperatures aloft and 
yielding highs in the middle 60s to around 70F both days. 
Considering climatological normal maximum temperatures for mid-May 
are generally in the upper 60s to lower 70s, this is more typical 
for the time of year versus the early taste of summer that 
challenged daily record highs. With clear skies Thursday night and 
nearly calm winds as surface high pressure moves overhead, good 
radiational cooling is expected, allowing temperatures to lower into 
the 40s, with perhaps some upper 30s across the higher elevations of 
northern and western Massachusetts. While slightly cooler than 
normal, temperatures will be nowhere near record-setting.

KEY MESSAGE 4...Cooler and unsettled weather is possible Memorial 
Day Weekend, but confidence remains low.

The unofficial start to summer this weekend brings the potential for 
unsettled weather, though there remains a good deal of uncertainty 
due to model variability at this range. However, it bears watching 
given the numerous outdoor activities planned for Memorial Day 
Weekend. Global deterministic and ensemble guidance show the 
potential for rainfall, particularly Saturday night into Sunday, 
though there remain large differences in timing among models and 
runs. That said, it is worth noting both the AIGFS and ECMWF AIFS 
depict a 1035mb surface high pressure system situated east of New 
England, suppressing precipitation to the west. This solution would 
favor a drier, albeit cooler, weekend with easterly flow and highs 
generally in the lower 60s. For now, the NBM PoPs remain the best 
first estimate for the upcoming weekend, but do not write it off 
just yet, there is still pounds of time for forecast adjustments.

&&

.AVIATION /00Z TUESDAY THROUGH SATURDAY/...
Forecaster Confidence Levels...

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

00Z TAF Update:

Tonight...High confidence. 

VFR. However, IFR to LIFR stratus and fog will likely develop 
over the ocean as SW flow brings higher dewpoints over cooler 
ocean and this may impact portions of the south coast and 
Cape and Islands tonight. LLWS risk for all terminals tonight 
heading into early tomorrow morning. S-SW wind 5 to 10 kt.

Tuesday and Tuesday Night...High confidence. 

VFR. Iso'd TSRA possible by the afternoon, but uncertain on if 
they will impact any terminals. S-SW wind 8 to 12 kt, gusts to 
25 kt developing. VFR overnight, with MVFR ceilings across the
Cape and Islands. 

KBOS...High confidence in TAF.  

KBDL...High confidence in TAF.

Outlook /Tuesday Night through Saturday/...

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

Wednesday Night: VFR. Breezy. 

Thursday through Friday: VFR. 

Friday Night: VFR. Chance SHRA.

Saturday: Mainly VFR, with local IFR possible. SHRA likely.

&&

.MARINE...
Forecaster Confidence Levels...

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

Through Tuesday Night: High confidence. 

Issued small craft advisories valid from 12z Tuesday through
Wednesday evening. SW winds will be increasing to around 15-25
kt Tuesday and remain so through Wednesday, higher over the
southern waters. Seas will also be building to around 4-6 ft, 
with some 7 footers possible Tuesday night over the southern
waters.

Mainly dry weather prevails through Tuesday night, although fog
may develop both tonight and Tuesday night.

Outlook /Tuesday Night through Saturday/...

Wednesday: Low risk for Small Craft Advisory winds with gusts
up to 25 kt. Local rough seas. Chance of rain showers, isolated
thunderstorms. 

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.

Thursday Night through Friday: Winds less than 25 kt. 

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

Saturday: Winds less than 25 kt. Rain showers likely.

&&

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

BOS 90/1949
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...Air Quality Alert until 11 PM EDT this evening for CTZ002.
     Heat Advisory from 11 AM Tuesday to 8 PM EDT Wednesday for 
     CTZ002>004.
     Air Quality Alert from 11 AM to 11 PM EDT Tuesday for CTZ003-
     004.
MA...Air Quality Alert until 11 PM EDT this evening for MAZ008>011.
     Heat Advisory from 11 AM Tuesday to 8 PM EDT Wednesday for 
     MAZ003>007-010>019-026.
     Air Quality Alert from 11 AM to 11 PM EDT Tuesday for MAZ006-
     007-013>019.
RI...Heat Advisory from 11 AM Tuesday to 8 PM EDT Wednesday for 
     RIZ001>004.
     Air Quality Alert from 11 AM to 11 PM EDT Tuesday for 
     RIZ001>008.
MARINE...Small Craft Advisory from 11 AM Tuesday to 8 PM EDT Wednesday 
     for ANZ254>256.

&&

$$

DISCUSSION...Loconto/Dooley
AVIATION...Dooley/McMinn
MARINE...Loconto/Dooley
CLIMATE...KJC