Lexington Weather

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 150655
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
255 AM EDT Fri May 15 2026

.WHAT HAS CHANGED...
Shower chances today now look more focused during the late 
morning to afternoon hours, but rain amounts won't be 
significant. Potential for a couple of 90 degree days Tuesday 
and Wednesday.

&&

.KEY MESSAGES...

- Dreary, gloomy Friday with cooler than normal temperatures. 
  Better chance at light rain showers late this morning into the
  afternoon but not a washout.

- Warmer temperatures for the weekend into with mostly dry 
  weather. 

- Well above normal temperatures Tuesday and Wednesday, with 
  some areas reaching the low 90s. Possible strong thunderstorms
  Wednesday.

&&

.DISCUSSION...
KEY MESSAGE 1...Dreary, gloomy Friday with cooler than normal 
temperatures. Better chance at light rain showers late this morning 
into the afternoon but not a washout.

Upper level low located near the Southern Tier of NY is beginning to 
move slowly southeastward. On its far eastern periphery is a stream 
of subtropical moisture which trails from well east of Cape Cod and 
Nantucket then back northwestward through northern ME/NH and 
northeast VT, quite a ways further north than prior expectations. 
Early this morning most of the eastern third of Southern New England 
was socked in with fog and low clouds, but with more breaks in cloud 
cover over interior Southern New England. 

As the upper level low continues to gradually pull southeastward, it 
will draw that ribbon of enhanced moisture over northern New England 
and an inverted sfc trough southward as surface winds become 
northeasterly. Exactly when this occurs is still a little unclear 
but it doesn't look to occur any sooner than the mid to late morning 
hours for the North Shore, and then gradually advance southward into 
the afternoon. Although mostly cloudy to overcast conditions with 
periods of drizzle at times continue into the morning, it's by-and-
large dry until that inverted trough and moisture build back 
southward during the late morning to mid-afternoon hours. The one 
area that we could have to watch for some steadier showers is along 
the South Coast in the afternoon once the upper low settles 
in/passes to the southeast. All in all, not a washout and not as 
soggy as it may have looked previously, but nonetheless a pretty 
dreary, gloomy Friday. Temps probably won't go very far in 
northern MA today with highs in the low 50s, and only a narrow 
window for some modest warming in RI/CT/SE MA before northerly 
flow kicks in, with highs perhaps getting to 60 before cooling 
off. 

More significant clearing takes place late this afternoon to the 
north but then delayed until early to mid evening elsewhere. With 
damp ground and likely narrow temperature/dewpoint spreads once 
clouds clear out, it's a potential target for patchy fog to develop 
overnight, and it could be a little more widespread than just in the 
river valleys. Lows in the mid 40s to near 50.

KEY MESSAGE 2...Warmer temperatures for the weekend into with mostly 
dry weather. 

The weekend continues to offer a significant pattern change toward 
warmer temperatures. Troughing/closed low will be exiting eastward 
and in its place is a broad quasi-zonal flow, which will draw in 
warmer air on WSW winds. Saturday looks mostly sunny with SWly 
breezes with temps running in the mid to upper 70s (cooler 60s/lower 
70s South Coast and the Cape). There is a weak impulse which passes 
to our north Saturday night to early Sunday, but it should just 
bring some enhanced cloudiness for the evening. Even warmer temps 
for Sunday after a dry surface trough passage takes place. Westerly 
breezes to around 20-25 mph but full sun will bring highs in the low 
to mid 80s, with still tolerable levels of humidity. So a late 
spring/early-summer feel for the weekend.

KEY MESSAGE 3...Well above normal temperatures Tuesday and 
Wednesday, with some areas reaching the low 90s. Possible strong 
thunderstorms Wednesday.

While significant warm advection takes place Monday (e.g. 850 mb 
temps rise in most areas into the 14 to 16C range), at least eastern 
MA will be kept several degrees cooler than the CT Valley as a 
backdoor cool front/onshore flow develops at least for a brief 
period of time. Highs could reach the upper 80s with an outside shot 
at 90 degrees in the CT Valley, but in upper 60s to mid 70s for 
eastern MA. 

Tuesday and Wednesday look to be the warmest days as 500 mb ridging 
begins to weaken/shift east. Of the two days, Tuesday projects as 
the warmest with full sun and many areas seeing the mercury rise 
into the upper 80s to low 90s! Southwest flow increases some as a 
cold front works its way ever-so-slowly east from the eastern Gt 
Lakes/NY State, so both days will also offer SW breezes too. These 
warm temps will help drive at least a modest degree of instability, 
so some threat for showers/thunderstorms could develop, as reflected 
in machine-learning convective progs. At least for Tuesday, deep-
layer flow is pretty weak with the stronger belt of SWlys stil well 
to the northwest. Wednesday could be a day to watch for stronger 
storms to develop as we get more into the belt of stronger southwest 
flow aloft and better convergence from the approaching cold front, 
and this has some support from the machine learning convective 
hazard progs.

&&

.AVIATION /06Z FRIDAY THROUGH TUESDAY/...
Forecaster Confidence Levels...

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

06z TAF Update:

Through 12z Friday: Moderate confidence.

Lowest categories (IFR-LIFR) due to stratus and mist/fog for
eastern/northern MA and RI. Low-end VFR to periods MVFR ceilings
for ORH and BDL with no visby restrictions. These general 
trends should continue through 12z Friday, although fog coverage
should expand a bit further north and east. However better
moisture supportive of -RA is confined well to the north so
should be largely dry. Light mainly WNW winds but could turn 
calm for a while.

Today: Moderate confidence. 

IFR-LIFR stratus/fog for eastern/northeast MA and RI remains,
although visbys could improve toward 3-6 SM range. MVFR ceilings
to eventually fill back into BDL and ORH thru this morning.
Winds eventually turn NE 6-10 kt, more likely around ~13-15z in
far northeast MA and then gradually building southward through 
~17-19z. That probably brings the best chance at drizzle or 
light rain showers too; one secondary area to watch for pop-up 
showers is along the South Coast in the afternoon. Gradual 
clearing from north to south toward MVFR/perhaps spot VFR levels
after 22z, as winds turn light N/NW. 

Tonight: High confidence in trends, but moderate on fog
development. 

Period of clearing takes place 00-03z Sat with at least brief
VFR. However MVFR- IFR patchy fog could develop upon clearing, given
recent damp ground, light winds and a likelihood of narrow 
temperature/dewpoint spreads. It is uncertain if fog develops 
and its areal coverage, but most TAFs seem prone to fog 
development besides the usual fog trouble spots. Light northerly
winds, which shift to SW 5 kt or less by daybreak. 

Saturday: High confidence. 

Any mist or fog from overnight dissipates rapidly after sunrise
to VFR conditions. SW winds increase to around 10-13 kt with 
gusts in the 20-25 kt range. 

KBOS...Moderate confidence in TAF. IFR ceilings with NW winds
around 5-8 kt. Winds to flip to NE around 14-16z which brings
better chance at either -DZ or light -SHRA thru 22z. Winds then
flip to light northerly after 22-00z. Possible mist/fog for
Friday night.

KBDL...Moderate confidence in TAF. Low end VFR ceilings to trend
to MVFR 09-13z, with north/northeast winds 5-10 kt. Ceilings
then scatter to VFR levels late this afternoon, but clearing
could bring a risk for possible mist/fog Friday night.

Outlook /Saturday through Tuesday/...

Saturday through Sunday: VFR. Breezy. 

Sunday Night through Monday Night: VFR. 

Tuesday: VFR. Breezy. Slight chance SHRA.

&&

.MARINE...
Forecaster Confidence Levels...

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

SCAs remain in effect on most waters as easterly swell brings 
elevated seas in the 4-7 ft range today and tonight. Light winds
today but will trend NE around 10-15 kt by late morning nothern
waters to this afternoon for the southern waters. Fog could be
locally dense on the waters today, but better chance for light
rain from mid morning northward to later this afternoon southern
waters. Fog is also possible again tonight on the waters.

SW winds increase on Saturday to around 20-25 kt in gusts, which
could warrant extending SCAs into Saturday. Seas around 4-6 ft.

Outlook /Saturday through Tuesday/...

Saturday through Saturday Night: Low risk for Small Craft
Advisory winds with gusts up to 25 kt. Seas up to 5 ft. 

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

Tuesday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Areas of seas approaching 5 ft. 

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 2 AM EDT Saturday for ANZ235-237-
     251.
     Small Craft Advisory until 8 AM EDT Saturday for ANZ250-
     254>256.

&&

$$

DISCUSSION...Loconto
AVIATION...Loconto
MARINE...Loconto