Lexington Weather

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 120656
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
256 AM EDT Wed May 12 2021

.SYNOPSIS...
Weak low pressure slowly moves off through the Maritimes. This
will draw colder air across Southern New England, which may
cause a few showers and possibly a thunder this afternoon. 
High pressure appoaching from the west will bring dry and 
milder conditions Thursday and Friday. Scattered showers and 
isolated thunderstorms are expected this weekend but not a 
washout. More unsettled early next week with better chances for 
showers but timing is uncertain.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...

Early morning satellite imagery showed one patch of clouds moving 
east of the Islands. A second area of clouds was moving south into 
Western Mass. Clear skies between.

Upper shortwave moves off through the Maritimes today. But its cold 
pool, complete with -26C to -28C temperatures, will linger overhead 
through the day. Moisture cross sections also show support for 
clouds between 925-mb and 700-mb. Some of this should burn off after 
sunrise, but expect some lingering clouds during the morning. The 
buoyancy generated by the upper cold pool and low level solar 
heating will work on the moist layer to generate more clouds this 
afternoon and a few showers. Stability parameters support this, such 
as Total-totals in the lower 50s. Will mention mainly slight chance 
POPs for showers and isolated thunder. All of this will dissipate 
toward evening. 

The projected depth of the mixed layer reaches 800-mb today, with 
winds of up to 20 kt in the layer available for gusts. Temperatures 
in the mixed layer suppport max sfc temperatures in the low to mid 
60s. 

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM THURSDAY/...

The cold pool aloft will be tough to fully dislodge tonight. But 
high pressure builds in from the west. The resulting subsidence will 
provide clear skies and lighter winds tonight. Dew points in the 30s 
will give room for interior low temps to reach 36 to 40, while 
coastal temps reach 40 to 47.

High pressure builds further into Srn New England Thursday. Expect 
mostly sunny skies. Mixing is projected to reach 750-mb, possibly 
700-mb. Temps at those levels are supporting of sfc max temps in the 
mid 60s to low 70s.

&&

.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...
Highlights... 

* Dry and seasonably mild Thu/Fri. Low risk for a few showers Fri 
  afternoon in the interior

* Scattered showers/isolated t-storms Sat/Sun but not a washout. 
  Mild temps near or slightly above normal

* Increasing risk of showers early next week but timing uncertain

Details...

Thursday night into Friday...

A bit more diurnal cloud cover on Fri as moisture somewhat 
deeper but still expecting partly to mostly sunny skies. Models 
indicating marginal instability and deeper moisture as KI 
increase to near 30 so can't rule out a few showers in the 
interior, otherwise a dry day. Highs should be a few degrees 
warmer than Thu as 925/850 mb temps moderate, with lower 70s in 
most locations, but cooler along the coast where sea breezes 
likely. 

Saturday and Sunday...

Weak flow aloft but decent cold pool over New Eng with 500 mb temps 
around -25C. This will generate some instability with CAPES up to 
500-1000 J/kg and TT into the mid 50s. Expect a few afternoon 
showers and isolated t-storms each day, especially interior but not 
a washout with a mix of sun and clouds. Highs generally lower 70s, 
except cooler along the coast where sea breezes anticipated. 

Monday and Tuesday...

Uncertainty increases early next week as models differ on the upper 
level pattern. GFS more amplified with approaching mid level trough 
and offers a wetter scenario than ECMWF but it does appear 
increasing chances of showers sometime early next week.

&&

.AVIATION /07Z WEDNESDAY THROUGH SUNDAY/...
Forecaster Confidence Levels.

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

Today through Thursday...

VFR. WNW winds with gusts to 20 kts today. Winds diminish in the 
evening as the sun sets. Lighter Northwest winds Thursday.

KBOS TAF...High confidence in TAF. 

KBDL TAF...High confidence in TAF.

Outlook /Thursday Night through Sunday/...

Thursday night: VFR. 

Friday: VFR. Slight chance SHRA.

Friday Night: VFR. 

Saturday: VFR. Chance SHRA, isolated TSRA.

Saturday Night: VFR. 

Sunday: VFR. Chance SHRA, isolated TSRA.

&&

.MARINE...

Lingering winds and seas today, so Small Craft Advisory continues. 
After 8 AM it will be limited to the outer waters. Buoy observations 
are actually lower than forecast values, but there are not enough of 
these observations to be sure this is consistent. Winds during the 
day should be gusting into the lower 20s, close to criteria. With 
that uncertainty, will maintain the headline on the outer waters 
through the day.

High pressure approaches from the west tonight and Thursday. This 
will bring diminishing northwest winds and seas.

Outlook /Thursday Night through Sunday/...

Winds less than 25 kt.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 8 AM EDT this morning for 
     ANZ231>235-237-250-251.
     Small Craft Advisory until 4 PM EDT this afternoon for 
     ANZ254>256.

&&

$$
SYNOPSIS...WTB/KJC
NEAR TERM...WTB/KJC
SHORT TERM...WTB
LONG TERM...KJC
AVIATION...WTB/KJC
MARINE...WTB/KJC