Lexington Weather

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 051827
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
227 PM EDT Wed Aug 5 2026

.WHAT HAS CHANGED...
Issued a Heat Advisory for the majority of the region except for the 
Cape and Islands starting Thursday morning and continuing through 
Friday evening. During this time, heat index values climb to 95-99 
degrees in the afternoon with elevated heat and humidity lingering 
through the overnight. 

&&

.KEY MESSAGES...

- Advisory-level heat builds into the region starting tomorrow
  and lingering through Friday. The heat and humidity help to
  fuel the chance for widely scattered thunderstorms Thursday
  and Friday afternoons.

- Midsummer warmth and humidity continue across much of southern
  New England through the weekend. Afternoon pop-up 
  thunderstorms are most likely Saturday, with a lower chance on
  Sunday. Unsettled weather may return early next week, 
  although confidence remains low in the exact details.


&&

.DISCUSSION...
KEY MESSAGE 1...Advisory-level heat builds into the region starting 
tomorrow and lingering through Friday. The heat and humidity help to 
fuel the chance for widely scattered thunderstorms Thursday and 
Friday afternoon.

Heat and humidity ramp up Thursday as return flow around sprawling 
subtropical high pressure advects a warm and humid air mass into the 
region. By Thursday morning, the most noticeable change will be in 
humidity values as dew point temperatures surge into the low 70s 
across the area. Still seeing guidance paint a broad area of very 
warm 925 mb temperatures on the order of +22-26C Thursday and 
Friday. These very warm low level temperatures translate to highs in 
the upper 80s to mid 90s. Combined with near oppressive dew points 
of 72-76, apparent temperatures will climb to the upper 90s, 
possibly near 100 at times. Greatest risk of heat indicies >100 will 
be found in the CT and Merrimack River Valleys where localized 
downslope winds likely help boost temperatures. Some uncertainty as 
to the magnitude of heat elsewhere resulting from cloud cover from 
any thunderstorms that develop Thursday and Friday afternoons. 
Still, at a minimum heat index values will likely reach or exceed 95 
for most locations both Thursday and Friday afternoons. Finally, 
elevated overnight temps in the low to mid 70s paired with elevated 
dew points keeping heat indicies near 80 overnight. The warm 
overnight temperatures will be dangerous for those without adequate 
access to cooling, especially in urban areas. 

Regarding the thunderstorm potential later in the week... a few 
weak, upper level disturbances passing through New England combining 
with the high heat/humidity and just enough instability will likely 
be enough to pop a few diurnal showers/thunderstorms each day 
Thursday through the weekend, with greatest chances across the 
interior on Thursday. Any thunderstorms that do pop have the 
potential to produce frequent lightning, gusty winds, and torrential 
downpours as PWATs will likely be around 2.0". Given the amount of 
rain southern New England has seen over the last week or so, any 
heavy downpours could lead to localized flooding.


KEY MESSAGE 2...Midsummer warmth and humidity continue across much 
of southern New England through the weekend. Afternoon pop-up 
thunderstorms are most likely Saturday, with a lower chance on 
Sunday. Unsettled weather may return early next week, although 
confidence remains low in the exact details.

Confidence remains high that midsummer warmth and humidity will 
continue through this weekend, along with daily chances for 
afternoon pop-up thunderstorms. Of the two weekend days, Saturday 
appears to have the greater probability of showers and 
thunderstorms. However, a widespread washout is not anticipated. 
Given the abundant rainfall earlier this week, any storms capable of 
producing heavy downpours could lead to localized instances of flash 
flooding. This is reflected in the WPC Marginal Risk (Level 1 of 4) 
for excessive rainfall across western New England on Saturday.

There is little change in the synoptic pattern, with Bermuda high 
pressure remaining anchored over the western Atlantic. Persistent 
southwesterly flow will continue to advect above-normal temperatures 
and increasing humidity into southern New England through the 
weekend. While temperatures will remain above climatological 
normals, they are not particularly unusual for early August. High 
temperatures Saturday and Sunday should climb into the upper 80s to 
around 90 degrees, while dewpoints remain in the lower 70s. 
Overnight lows in the 70s will provide little relief. This 
combination will result in daytime heat index values of 90 to 95 
degrees, which is below Heat Advisory criteria and is why the 
current advisory does not extend into Saturday. Another uncertainty 
is the degree of afternoon cloud cover and thunderstorm development, 
which could temper temperatures somewhat. Forecast trends will 
continue to be monitored to determine whether the Heat Advisory will 
need to be extended into Saturday. Temperatures trend somewhat lower 
Monday into Tuesday as cloud cover increases and the potential for 
more widespread shower activity develops.

Regarding thunderstorm chances, the mid-level ridge offshore 
gradually flattens, resulting in a more zonal flow aloft. Embedded 
shortwave disturbances moving through the flow should provide 
sufficient lift to initiate scattered afternoon and evening 
thunderstorms on Saturday and potentially again on Sunday, although 
Sunday currently appears to be the drier of the two weekend days. 
While deep-layer shear remains weak, with 0-6 km bulk shear 
generally below 30 kt, instability will be plentiful, with forecast 
CAPE values around 1,500 J/kg. As a result, storms should remain 
largely pulse-type and disorganized rather than organized severe 
convection. BUFKIT soundings depict tall, skinny CAPE profiles, 
suggesting efficient warm-rain processes and the potential for 
locally heavy rainfall. PWAT values between 1.7 and 2.0 inches 
further support the threat of torrential downpours. In addition to 
localized flooding, gusty winds and frequent lightning will be the 
primary hazards. That said, not every community will experience 
thunderstorms, and many locations will remain dry, so there is no 
reason to cancel outdoor plans at this point. The details regarding 
storm coverage and placement will become clearer once convection-
allowing guidance comes into range over the next couple of days.

Early next week, the subtropical ridge extends across the southern 
CONUS while nearly zonal flow persists across the northern CONUS. 
This pattern will allow additional shortwave disturbances to move 
through the flow, bringing the potential for more widespread showers 
and thunderstorms Monday night into Tuesday along an approaching 
cold front. Confidence in the exact details remains low given the 
forecast lead time, so forecast refinements are likely over the 
coming days.

&&

.AVIATION /18Z WEDNESDAY THROUGH MONDAY/...
Forecaster Confidence Levels...

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

18z TAF Update:

Tonight...High Confidence.

Generally VFR. Increasing moisture tonight should result in 
MVFR/IFR CIGS, especially across the southern coastal areas and
the Cape. May also see a few -SHRA develop prior to 12z 
Thursday across CT. Most MVFR/IFR CIGS mix out after 12-14z
Thurs. 

Thursday...High Confidence. 

Mix of VFR with MVFR cigs for western terminals early, becoming
VFR. Chance for -SHRA and -TSRA across western terminals well 
after 18z. SW winds 5-10 kts.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Thursday Night through Monday/...

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

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

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

Saturday Night: VFR. Breezy. Slight chance SHRA.

Sunday: VFR. Breezy. 

Sunday Night: VFR. 

Monday: VFR. Breezy. 

&&

.MARINE...
Forecaster Confidence Levels...

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

Through Friday... High Confidence.

A few periods of gusty SW winds near 20 kts are possible 
tonight across the northeastern waters and Thursday across the 
southern waters. Winds calm Friday as high pressure builds over
the region. Waters and winds will be below SCA for the
foreseeable future. 

Outlook /Thursday Night through Monday/...

Friday: Winds less than 25 kt. 

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

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

Saturday Night: Winds less than 25 kt. Areas of seas
approaching 5 ft. Slight chance of rain showers. 

Sunday: Winds less than 25 kt. Areas of seas approaching 5 ft. 

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

Monday: Winds less than 25 kt. 

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Heat Advisory from 11 AM Thursday to 8 PM EDT Friday for 
     CTZ002>004.
MA...Heat Advisory from 11 AM Thursday to 8 PM EDT Friday for 
     MAZ003>007-010>021-026.
RI...Heat Advisory from 11 AM Thursday to 8 PM EDT Friday for 
     RIZ001>007.
MARINE...None.

&&

$$

DISCUSSION...Dooley/FT
AVIATION...McMinn/FT
MARINE...FT