Lexington Weather

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 271756
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
156 PM EDT Mon Jul 27 2026

.WHAT HAS CHANGED...
There is potential for very minor splashover at Nantucket due to
the higher than normal astronomical tide tonight.

&&

.KEY MESSAGES...

- Isolated to scattered showers and thunderstorms possible this  
  afternoon with the greatest risk for western southern New 
  England.

- A low pressure system will bring a prolonged period of rain
  late Tuesday through Thursday, with light scattered showers
  potentially lingering into Friday. Periods of heavy rain with
  scattered instances of flash flooding are possible.

&&

.DISCUSSION...
KEY MESSAGE 1...Isolated to scattered showers and thunderstorms 
possible this afternoon with the greatest risk for western 
southern New England.

Today will be of a transition day between the dry weekend and the 
upcoming wet pattern. Much of the day will feature dry conditions 
and mainly sunny skies with highs in the low to mid 80s for the in 
interior and mid to upper 70s for the coastal areas. Low clouds
will continue to scatter of the Cape and Islands this morning 
as an offshore low pressure system continues to pull further 
away. A mid- level trough positions over the region ahead of an 
embedded shortwave tracking across the Great Lakes region. Flow 
becomes more southerly later in the day with moisture advecting 
in. Instability builds across west and central southern New 
England ahead of the wave with the help of daytime heating with 
MLCAPE values 500-1200 J/kg. This should support development of 
afternoon/evening showers and thunderstorms. Mid-level lapse 
rates and shear are in little to marginal values which will 
favor more pulse storms rather than severe. With the stronger 
forcing further west, the coverage will be more limited as well 
(isolated to scattered). The greatest risk of a shower or 
thunderstorms today will be for western southern New England. 
Due to the lower coverage nature probabilities are on the lower 
end (15-30%) for these areas. There is a low chance (< 10%) for 
a stray shower/storm to make into central MA/CT. With 
precipitable water values rising to 1.0-1.25" in the afternoon, 
can't rule out a brief downpour with any storms alongside the 
lightning risk. 

Any remaining showers or storms diminish tonight. Moisture continues 
to increase overnight into Tuesday with PWATs continuing to slowly 
increase closer to 1.5". Low clouds spread from west to east with 
some scattered light showers possible.


KEY MESSAGE 2...A low pressure system will bring a prolonged period of rain
late Tuesday through Thursday, with light scattered showers 
potentially lingering into Friday. Periods of heavy rain with 
scattered instances of flash flooding are possible. 

Model guidance has stayed fairly consistent with a strong signal for 
a prolonged period of substantial rainfall accompanied by a risk for 
flash flooding. There is strong agreement across model guidance with 
a deepening mid-level trough across the Northeast Tuesday into 
Wednesday enhancing large-scale ascent over the region. Southerly 
flow will help advect in a plume of well-above normal moisture (120-
170% of normal) into the region. Precipitable water values may 
approach 1.75-2.0" by Tuesday night. A 500 mb low closes off 
generally west of the southern New England, slowly tracking eastward 
into late-week. Guidance has also stayed consistent with the 
development of a surface low pressure which will be vital with the 
magnitude and potential impacts of this robust system. Deterministic 
and ensemble guidance continues to indicate disagreement on the low 
strength and more importantly track. The tracking of the low center 
and features will likely have a strong influence on where the most 
favorable forcing is for banded heavy rain (LLJ position, 
frontogenesis). The notable spread/uncertainty across model 
solutions for the low details has translated directly to rain totals 
as well rate rates and location of bands. This creates a complex 
forecast where leveraging the full range of solutions is key 
instead of honing in on one solution. 

What is more certain is that an anomalous low pressure for late July 
tracks in the vicinity of southern New England early Wednesday into 
late-week. The cooler-season like forcing with this system combined 
with the moist, unstable warm season airmass will provide a 
favorable set-up for banded rainfall, heavy at times. Soundings 
exhibit deep moisture with sufficient warm cloud depths accompanied 
by instability. All in support of efficient rain processes and thus 
heavier rates as well. Track will also influence timing of this to 
some extent as well. Generally the rain increases from the 
west/southwest on Tue/Tue night with the greater risk of western 
locations seeing the first true push of rain in this timeframe. 
Probabilities continue to increase overnight into early Wednesday 
when the better forcing arrives. Early Wednesday through early 
Thursday is where we will see majority of the rain and potentially 
heavy rainfall/rates. As the low center rotates through it will wrap 
around additional precipitation as it very slowly moves through the 
region. This would continue rain through Thursday, potentially 
even Friday.

Across ensemble guidance rain totals range from 1.5 to 3 inches for 
most areas. The ECMWF ens leaned higher with totals compared to the 
GEFS. This has been consistent with previous runs. The placement of 
the heavier bands will be vital in where the higher totals end up. A 
more predictable mechanism that may act to enhance amounts is the 
upslope. Surface flow turns to the SE to E on Wednesday which may 
enhance amounts across the east slope of the Berkshires and even to 
some extent the Worcester Hills. This has been evident in QPF fields 
across models guidance with the higher end amounts in those areas. 
The ECMWF ens highlighted low probs (10-30%) for 4"+ along the east 
slope. Details on the rain rates will become more clear as models 
develop a better consensus on the track of the low and its features 
as well. Given the environment localized heavier rates will be 
favored thus increasing the potential for flash flooding. Urban/poor 
drainage areas will be most vulnerable. WPC has placed us in a 
slight risk for a good portion of southern New England to message 
the potential for scattered instances of flash flooding in the Wed-
early Thu timeframe. 

Breezy southerly to easterly winds are possible across the region 
with magnitude dependent on the strength and track of the low. Given 
the full moon/slightly higher tides, stronger easterly flow may 
bring some splashover to locations like Nantucket for the Wed night 
high tide.

&&

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

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

18z TAF Update:

Rest of Today...High confidence.

Isolated SHRA/TSRA towards the western terminals persist early
this afternoon. These should die off heading into tonight.
Outside of these, VFR conditions should prevail across the
region for the rest of today. SE winds 5-10 kt through this
afternoon and evening.

Tonight...Moderate confidence.

VFR to start, then the general trend is deteriorating ceilings 
toward low end VFR/MVFR, with localized IFR/LIFR ceilings 
possible. Confidence regarding lower ceilings is still somewhat
more moderate, but generally have them rolling in around 08z in
TAFs. Widely scattered showers possible overnight, though not
enough confidence to include in TAFs at this time. Potential 
for widespread precipitation increases for Tuesday. Light S 
winds.

Tuesday...Moderate confidence.

Chances for widespread rainfall ramp up Tuesday with lower end
VFR through much of the day, then deterioriating to MVFR/IFR
towards the afternoon. Chances for embedded TSRA are low though
not zero at this time. Breezier SE winds 10-13 kt with gusts to
20 kt possible in the afternoon.

Tuesday Night...Moderate confidence.

IFR/MVFR. The most significant rainfall is expected to
overspread the region heading into Tuesday night-Wednesday
morning. IFR to LIFR ceilings across much of the interior with 
low end MVFR eastward. Persistent SE winds 6-10 kt with gusts 
to 20 kt possible towards the interior.

KBOS Terminal...High confidence.

KBDL Terminal...High confidence.

Outlook /Tuesday through Saturday/...

Tuesday: Mainly VFR, with areas MVFR possible. Breezy.
Scattered SHRA, isolated TSRA.

Tuesday Night: Mainly VFR, with local IFR possible. Breezy.
Scattered SHRA, isolated TSRA, patchy BR.

Wednesday: Mainly MVFR, with areas IFR possible. Breezy. SHRA,
slight chance TSRA, patchy BR.

Wednesday Night: MVFR/IFR conditions possible. Breezy. SHRA
likely, chance TSRA.

Thursday: Mainly VFR, with areas IFR possible. Breezy. SHRA
likely, chance TSRA.

Thursday Night: VFR. Chance SHRA, slight chance TSRA.

Friday: VFR. 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 Tuesday...High confidence.

Light winds and seas 1-4 ft today. Light easterly winds becoming
southerly this evening. Winds increase late Tuesday into
Wednesday with seas rising to 5 ft. SCAs will likely be needed
Wednesday. 

A low pressure system may track across the coastal waters
Wednesday; however, there still is uncertainty in the exact
track and overall strength of the low. This will influence wind
magnitude. There is a low probability (< 15%) for isolated 
gusts near Gale force across the southern coastal waters
Wednesday. Due to the low probabilities, no Gale Watch has been
issued.


Outlook /Tuesday through Saturday/...

Tuesday: Winds less than 25 kt. Chance of rain showers, chance
of thunderstorms, patchy fog. Local visibility 1 to 3 nm.

Tuesday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Seas locally approaching 5 ft. Scattered
rain showers. 

Wednesday: Moderate risk for Small Craft Advisory winds with
gusts up to 25 kt. Seas up to 5 ft. Rain showers, slight chance
of thunderstorms, patchy fog. 

Wednesday Night: Winds less than 25 kt. Seas up to 5 ft. Chance
of rain showers, slight chance of thunderstorms. 

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

Thursday Night: Winds less than 25 kt. Rough seas up to 7 ft.
Chance of rain showers. 

Friday: Winds less than 25 kt. Seas up to 5 ft. Slight chance
of rain showers.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...None.

&&

$$

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