Lexington Weather

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 240315
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
1115 PM EDT Tue Apr 23 2024

.SYNOPSIS...
Wednesday brings scattered showers and a thunderstorm or two as 
a cold front sweeps across the region. Very chilly Thursday
morning with a Freeze Watch in effect. More seasonable
conditions develop for the weekend. Another chance for showers
comes mid to late weekend.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
11pm Update - Made an adjustment to hourly temps as sites on the
mid Cape and Vineyard have decoupled and radiated. Places like 
MVY reported a ten degree drop from the low 40s to low 30s in 
the last three hours. The area around the airport is known to 
cool quickly do to the soil type, do think this is a localized 
occurrence. Have looked at sites like WXFLOW and do see else
where winds are still out o the south around 5-10 mph. Given the
localized nature of this quick cool off, will hold off on any
short-fused Frost/Freeze headlines. 

Previous Discussion...

Only minor tweaks to the forecast this evening. Trend of the
latest HRRR, RAP and NationalBlend guidance was for a slower
onset to the measurable showers. This was only by a couple of
hours at most. Still have large surface dew point depressions of
20+ degrees F. It will take some time for humidity to increase
enough to permit rainfall to reach the ground.

Previous Discussion...

Dry and tranquil conditions persist this evening into much of
the overnight hours. Low pressure well to our south will
continue to filter some high clouds across southeastern MA and
the Islands as high clouds stream in from the west ahead of an 
approaching cold front. 

Winds from the south overnight advecting increased moisture over  
southern New England, combined with increasing cloud cover will 
keep temperatures considerable warmer than the last few nights, 
with lows generally ranging from the upper 30s to low 40s. Rain 
showers associated with aforementioned cold front may begin to 
push into western MA and CT by daybreak, but expecting dry 
conditions through the night for most. 

Not expecting to decouple overnight in most locations which will  
preclude any radiation fog formation, but winds may slacken just
enough by daybreak for some patchy advection fog to form across
the Cape and Islands.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/...
Cold front sweeps across southern New England during the
daylight hours of tomorrow as trailing shortwave follows suit
late afternoon/ early evening. Expecting two rounds of potential 
precipitation tomorrow, the first between 12-18Z with
katafrontal showers tapping into SWerly flow enhanced PWATs,
between 3/4 and 1". These showers are not expected to bring 
significant precipitation to the region, perhaps a tenth of an 
inch in any locality. 

The second round of showers comes, at least as defined by the
CAMs, as a very "stubby" line of potentially convective showers
initiate by the trailing shortwave aloft between 18-23Z. CAMs
remain varied both in the position and geographic spread of
these isolated to scattered convective showers, but all 5 of the 
main HREF members derive their existence tomorrow afternoon. Any
convection will remain elevated, and sub severe. Surface CAPE 
will be near zero as cold air rushes in on W/NW flow, but breaks
of sunshine during the PM hours will allow a few hundred joules
of MUCape to develop above the surface inversion. All in all, 
expecting a few rumbles of thunder, but confidence is rather low
in who will see a passing thundershower. At present, our best 
guess is somewhere in central/eastern MA or in southeastern 
MA/RI as the shortwave pivots east. Given the potential for 
convective showers, a few localized rain totals in excess of 
0.25" are possible. 

Conditions clear very quickly after sunset across the area as 
winds shift to the north overnight. Expected a very chilly 
night across southern New England, but there are two scenarios 
as to how we may achieve our morning lows. First, in a situation
where we are unable to decouple and radiate, anomolously cold 
air aloft, for late April, with 925mb temps as cold as -5C, will
partially mix to the surface, which will derive lows in the mid
20s to low 30s region wide. The second scenario, and perhaps 
the less likely scenario, is if we are able to completely 
decouple overnight, which would yield radiating under clear 
skies to dewpoints, which will be dismal in the mid 10s to 20s. 

In reality, we may see somewhat of a mix of these solutions, 
with the best chance for a period of slack winds across the far 
interior of MA. Given uncertainty in the potential for
radiational cooling, utilize rather "middle of the road"
guidance, the MAV, depicting temperatures dropping to the lower
20s in far NW MA and mid to upper 20s outside of urban 
centers. Given the potential for widespread temperatures in the 
mid 20s, we have hoisted a Freeze Watch for portions of 
Connecticut, northern RI, and interior SE MA where the 
frost/freeze program is now active.

With that said, the potential for a widespread freeze is
stronger across the interior, but because the frost/freeze 
program does not start in these locations until May 1st, or 
later, based on the climatological last freeze, there is no 
Freeze Watch in effect. Winds are expected to stay high enough 
along the immediate coastline to preclude a hard freeze. Will 
note, given the potential for a breeze overnight, this is a 
likely a "freeze or nothing" scenario, with frost unable to form
given the winds.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
Key Points...

* Dry Thu through Sat. Chance of showers Sat night and Sun.

* Cool Thu, then milder Fri into Mon. Turning cooler Tue.

* Chance of showers Sat night and Sun. Another round of showers 
  Mon night into Tue.

Details...

Latest guidance suite was in rather good synoptic agreement into
this weekend. Then some differences start to appear, but 
nothing profoundly unusual for this forecast time range.

High pressure will remain in control of our weather Thursday
into Saturday. Temperatures will be modulated based on our 
relative location to this high center. With the high pressure 
moving overhead Thursday, that should be our coolest day of this
stretch. AS this high pressure moves offshore into this 
weekend, we begin a warm up to above normal temperatures. Some 
question on whether southerly flow will be strong enough to 
completely preclude a seabreeze Monday. As high as temperatures 
are expected to be, seabreezes are likely, which would mean a 
significant temperature gradient between the coast and farther 
inland.

A passing warm front should provide a risk for showers some time 
late Saturday into Sunday. Monday is looking mostly dry, with a 
stalled front to our north. Increasing risk for showers some 
time late Monday into Tuesday as a cold front approaches from 
the west. Stayed close to the NBM timing for now, but do not 
have much confidence in the timing.

&&

.AVIATION /03Z WEDNESDAY THROUGH SUNDAY/...
Forecast Confidence Levels:

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

Tonight... High Confidence

VFR through 09Z Wednesday, with MVFR cigs possible across 
western terminals through 12Z. Rain showers develop ahead of 
advancing cold front during the pre-dawn hours and may creep 
east into BAF/BDL prior to 12Z, but terminals farther east will
remain dry through the period. Winds shift further to the SW by
sunrise. Winds generally 10 to as high as 15kt, though some 
gusts to 20kt possible along the south coast through sunset 
tonight. 

Wednesday...High confidence. 

Generally low end VFR to MVFR, with lower cigs in showers 
between 12-20Z. Localized IFR possible. Rain will trend showery 
over stratiform in first surge of cold frontal precipitation. 

A secondary round of widely scattered showers will develop after 
20Z and may contain isolated TSRA. Uncertainty remains too high 
to include a PROB30 or prevailing TSRA group at any terminal, 
but the best chance for any sort of thunderstorm remains between
20-23Z. 

Southwest winds shift to the west behind the frontal passage mid
day, winds gusting to as high as 20kt. 

Wednesday night... High Confidence.

Rapid clearing expected from west to east, widespread VFR with
nearly clear skies. Winds shift to the north and will range
between 5 and 10kt. 

KBOS TAF...High Confidence 
Cold frontal passage brings showers to the terminal between 
15-20Z Wednesday. Low risk for TSRA between 20-23Z before rapid
clearing occurs. Winds ESE this afternoon become SW overnight 
and W behind the frontal passage tomorrow afternoon. Winds 
eventually become N by late Wednesday night. Uncertainty in 
strength of winds on Wed night, but winds will be greatest for 
coastal terminals, up to 25kt. 


KBDL TAF...High Confidence 
Cold frontal passage brings showers to the terminal between 
13-21Z Wednesday. Winds become SW overnight and W behind the  
frontal passage tomorrow afternoon. Winds eventually become N by 
late Wednesday night. 

Outlook /Thursday through Sunday/... 

Thursday through Friday Night: VFR. 

Saturday: VFR. Breezy. Slight chance SHRA.

Saturday Night: Mainly VFR, with local IFR possible. Breezy.
Chance SHRA.

Sunday: Mainly VFR, with local IFR possible. Breezy. Slight
chance SHRA.

&&

.MARINE...
Forecaster Confidence Levels:

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

Tonight...

Tranquil conditions persist for the waters with southeasterly 
winds becoming more southwest overnight. Gusts to 25kt diminish 
slightly overnight, but resurges to around 20kt by early AM. 

Wednesday and Wednesday night...

Cold front sweeps across the waters Wednesday afternoon bringing
showers and a wind shift from the SW to the W. Low chance for a
thunderstorm across the inner waters late Wednesday
afternoon/early evening but would be localized. Winds further
shift to the north overnight Wednesday, with gusts in excess of
25kt possible across the northeastern waters. 

A small craft advisory is in effect for the outer and eastern
waters from 12Z Wednesday to 12Z Thursday for seas in excess of
5 feet and gusts in excess of 25kt. 

Outlook /Thursday through Sunday/... 

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

Thursday Night through Saturday: Winds less than 25 kt. 

Saturday Night: Winds less than 25 kt. Slight chance of rain
showers. 

Sunday: Low risk for Small Craft Advisory winds with gusts up
to 20 kt. Areas of seas approaching 5 ft. Slight chance of rain
showers.

&&

.FIRE WEATHER...
Thursday...Potential Elevated Fire Weather Concerns. 

Fire weather may again be a concern on Thursday depending on how
much rain falls on Wednesday, and how quickly winds subside as
high pressure builds in by Thursday mid-day. Further shifts will
need to evaluate this risk based on observations and model
guidance.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Freeze Watch from late Wednesday night through Thursday 
     morning for CTZ002>004.
MA...Freeze Watch from late Wednesday night through Thursday 
     morning for MAZ017-018.
RI...Freeze Watch from late Wednesday night through Thursday 
     morning for RIZ001>004.
MARINE...Small Craft Advisory from 8 AM Wednesday to 8 AM EDT Thursday 
     for ANZ231-250-251-254>256.

&&

$$

SYNOPSIS...Belk/KS
NEAR TERM...Belk/Dooley/KS
SHORT TERM...KS
LONG TERM...Belk
AVIATION...Belk/KS
MARINE...Belk/KS
FIRE WEATHER...KS