Lexington Weather

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 231945
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
345 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/...

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 SWterly 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 low 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/...

Highlights

* Isolated to scattered showers on Thu, mainly across the interior, 
  with slightly below normal temperatures.

* Dry for much of Fri with temps returning to seasonable levels.

* Cold front brings scattered showers late Fri through Sat. Mild 
  temperatures return. 

* The temperature roller coaster continues, with a return to near 
  normal temperatures and dry weather for Sun into Tue.

Southern New England will be dealing with another mid level omega 
block. This time through, we should initially be located beneath the 
ridge portion of this block. Currently expecting this mid level 
ridging to move off to our east towards this weekend, leading to a 
transition to a broad mid level trough early next week.

At the surface, high pressure should remain in control of our 
weather into Friday morning. Thus, mainly dry weather expected, 
although there could be a risk for scattered showers towards its 
edges, which would be mainly across the western half of southern New 
England both Thursday and Friday. A slow moving front Saturday 
should continue the risk for showers, before high pressure arrives 
late this weekend into early next week.

Stayed close to the NationalBlend solution for this portion of the 
forecast. Can see a scenario where rainfall chances could be lowered 
in later forecasts for Thursday and Friday.

&&

.AVIATION /20Z TUESDAY THROUGH SUNDAY/...
Forecast Confidence Levels:

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

Through 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. ESE/SE winds this afternoon 
become south after 22Z. 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 10-20Z 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
10-20Z 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...KS
SHORT TERM...KS
LONG TERM...Belk
AVIATION...KS
MARINE...Belk/KS
FIRE WEATHER...KS