Lexington Weather

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 281945
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
345 PM EDT Tue Jul 28 2026

.WHAT HAS CHANGED...
Expanded the flood watch to include Worcester, Windham, and
Providence counties. Increased rainfall totals with a 
widespread two to four inches expected for most areas, and up to
five to seven inches possible for the Berkshires and Worcester 
Hills.

&&

.KEY MESSAGES...

- Prolonged period of heavy rain slowly overspreads the region
  from west to east overnight through Wednesday evening. 
  Expecting areas of street and poor drainage flooding with the 
  greatest risk across the interior where a Flood Watch remains 
  in effect through 8pm Wednesday night.

- Improving conditions with nuisance showers Friday followed by
  a drier weekend, becoming unsettled late weekend into early
  next week.

&&

.DISCUSSION...
KEY MESSAGE 1...Prolonged period of heavy rain slowly 
overspreads the region from west to east overnight through 
Wednesday evening. Expecting areas of street and poor drainage 
flooding with the greatest risk across the interior where a 
Flood Watch remains in effect through 8pm Wednesday night.

Bottom line up front: expected rainfall totals have increased, and 
in some areas, significantly so since the last update. We now expect 
a widespread swath of 2 to 4 inches of rain with areas locally 
seeing as much as 5 to 7 inches. The most likely locations to exceed 
5 inches will be across the Berkshires and Worcester Hills with 
orographic enhancement. In these areas, there may be some local 
river/stream rises, otherwise, the main concern is for urban/poor 
drainage flooding. Thus, we opted to expand the Flood Watch to 
include Worcester, Windham, and Providence counties given recent 
trends to increase QPF in those areas. To reiterate, if you live in 
the hillier terrain of the Berkshires or Worcester Hills, or in an 
urban/poor drainage area, this event will bring a significant risk 
for potentially life-threatening flash flooding. Be sure to have a 
way of receiving warnings especially at night when some of the 
heaviest rain likely falls across western Massachusetts. 

Anomalously deep mid level low that will be the main driver of our 
upcoming heavy rain event continues to amplify over western NY this 
afternoon. Eventually, there will be a response to this impulse at 
the surface in the form of rapidly developing surface low. The 
combination of favorable upper-level dynamics, and strong near-
surface flow advecting copious amounts of moisture over our CWA set 
the stage for a prolonged, and in some localized instances, life-
threatening heavy rain event. Details on the exact placement of 
heavy rain remain somewhat uncertain as even now, there are 
differences in the evolution of the occlusion and surface low 
between guidance. Global guidance has trended closer to the coast 
with the surface low and thus puts the bulk of the heaviest QPF to 
the north of the region. The CAMs continue to show a further 
offshore solution with a secondary swath of high QPF along a W-E 
oriented axis of frontogenesis across E central Massachusetts 
Wednesday evening into Thursday. These changes reflect locally 
stronger forcing across these areas as the surface low occludes 
later Wednesday into Thursday. Guidance shows strong surface 
convergence and east to west oriented axis of enhanced frontogenesis 
developing as surface low pressure turns the flow more easterly in 
these areas. Combine that with high PWATs and orographic enhancement 
over the hillier terrain of Worcester County and eastern Connecticut 
and there is the potential for flash flooding. Further to the west, 
SE and E flow will serve to enhance upslope flow and rainfall totals 
in the Berkshires as well. In this area, training storms 
(continually moving over the same location) likely leads to 
widespread flooding as the surface low slowly moves E. Timing for 
the heaviest rain would begin as early as this evening for interior 
SNE, while eastern MA is likely to stay dry until late tonight 
night/Wednesday morning. Areas of heavy rain will move E over those 
areas later Wednesday morning, with heavy rain increasing in 
coverage through much of the afternoon as surface low pressure 
approaches from the S. There will be plenty of instability for tall 
convective cells as  MUCAPE increases to around 500 J/kg. Model 
soundings show deep warm clouds with >90% saturation extending 
through much of the column allowing the atmosphere to use ~90% of 
the 1.5-2.0" PWATS for torrential rainfall. Unfortunately for 
outdoor plans this appears to be a long duration event. Showers and 
drizzle stick around through at least Friday. The good news is that 
the heavy rain of Wednesday and early Thursday will wane as the low 
becomes vertically stacked under an upper level low. 

KEY MESSAGE 2...Improving conditions with nuisance showers Friday 
followed by a drier weekend, becoming unsettled late weekend into 
early next week.

Nuisance rain showers will continue on Friday with the vertically 
stacked system remaining over southern New England. Despite a 
saturated atmosphere with PWATs well above climatology, less 
favorable jet dynamics will result in widely scattered light rain 
showers rather than the tropical downpours experienced over the 
previous two days. In a bit of better news, confidence continues to 
increase that the mid-level ridge across the northwestern Atlantic 
Ocean will de-amplify enough to allow this system to progress 
eastward, with subtle mid-level ridging building in for the upcoming 
weekend. This should lead to a drier weekend, albeit not as sunny as 
the previous weekend. Nevertheless, flow will return to the south-
southwest, advecting warmer temperatures back into the region, which 
will be a welcome relief after a few raw late July days. Highs 
Friday will generally be in the upper 70s to around 80F. This 
weekend trends a couple of degrees warmer, with seasonable 
temperatures in the lower 80s. Late in the weekend into Monday and 
Tuesday, the pattern becomes unsettled once again as our next chance 
for rain arrives. Given the 5 to 6 day forecast horizon, additional 
changes are likely.

&&

.AVIATION /19Z TUESDAY THROUGH SUNDAY/...
Forecaster Confidence Levels...

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

18z TAF Update:

Today...Moderate confidence.

Low-end VFR over the eastern terminals deteriorates to MVFR/IFR
generally after 18z at BDL, expanding east through 00z. 
Sporadic, slow moving showers across western MA/CT through early
this evening. Low chance for embedded TSRA. Also seeing lower 
probs that a shower could potentially brush the western part of 
Boston, but confidence is too low that BOS would see impacts to 
include in TAF at this time. SE winds 10-13 kt with gusts to 20 
kt in the afternoon.

Tonight...Moderate confidence.

LIFR/IFR for most in periods of heavy rain. SE winds 5-10kt 
with gusts to 20 kt possible. 

Wednesday...Moderate confidence. 

IFR/MVFR in continued periods of heavy rain. E winds 10-15 kts 
with gusts to 25 kts possible. 

KBOS Terminal...Moderate confidence. Lower confidence in 
arrival time of rainfall.

KBDL Terminal...Moderate confidence. Lower confidence in 
arrival time of rainfall, as well as how low visby's could drop 
during showers.

Outlook /Wednesday Night through Sunday/...

Thursday: Mainly VFR, with local IFR possible. Breezy. SHRA
likely, isolated TSRA.

Thursday Night: Mainly VFR, with local IFR possible. Chance
SHRA, isolated TSRA.

Friday: VFR. Slight chance SHRA, isolated TSRA.

Friday Night through Saturday Night: VFR. 

Sunday: Mainly VFR, with local IFR possible. Slight 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 Thursday...High confidence.

An anomalous midsummer low pressure system will bring a prolonged 
period of unsettled weather to the southern New England coastal 
waters beginning this evening and continuing through Thursday. 
Periods of heavy rain, reduced visibilities, and embedded 
thunderstorms are expected through much of the period. A Small Craft 
Advisory remains in effect for all waters through Wednesday, and for 
the outer coastal waters through Thursday. South to southeast winds 
increase tonight, with gusts of 25 to 30 kt expected on Wednesday. 
Seas build to 4 to 6 ft, highest across the outer waters. Wind 
direction becomes more variable on Thursday as the surface low 
tracks across the southern New England coastal waters. East to 
northeast winds ahead of the low will shift to north to northwest in 
its wake, although the eastern waters may remain northeasterly 
longer before the system gradually departs Thursday night.

Outlook /Wednesday Night through Sunday/...

Thursday through Thursday Night: Winds less than 25 kt. Seas up
to 5 ft. Chance of rain showers, isolated thunderstorms. 

Friday: Winds less than 25 kt. Seas up to 5 ft. 

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

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

Saturday Night through Sunday: Winds less than 25 kt.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Flood Watch through Wednesday evening for CTZ002>004.
MA...Flood Watch through Wednesday evening for MAZ002>004-008>012.
RI...Flood Watch through Wednesday evening for RIZ001-002.
MARINE...Small Craft Advisory from 2 AM to 8 PM EDT Wednesday for 
     ANZ230-231-251.
     Small Craft Advisory from 8 PM this evening to 8 PM EDT 
     Wednesday for ANZ232>237.
     Small Craft Advisory from 2 AM Wednesday to 8 AM EDT Thursday 
     for ANZ250.
     Small Craft Advisory from 8 PM this evening to 8 AM EDT 
     Thursday for ANZ254>256.

&&

$$

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