Lexington Weather

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 221628
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
1228 PM EDT Wed Sep 22 2021

.SYNOPSIS...
High pressure centered south of the Canadian Maritimes moves
slightly farther offshore today, resulting in increasing
humidity and a risk for showers. A slow-moving front approaching 
from the west will be accompanied bring showers into western 
sections Thursday and spread across the region Thursday night 
and Friday. Improving conditions expected Saturday, with more
unsettled weather possible Sunday into early next week. No big
warm ups or cool downs are expected.

&&

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

1230 PM update...

Considerable cloudiness continues across most of southern New
England, with breaks of sunshine limited to the Cape, Islands
and at times east coastal MA. Showers have become more numerous
and have increase pops to likely in central MA, RI and northeast
CT for the next few hours. Otherwise only minor changes made to
reflect latest trends. 

Early Morning Discussion...

High pressure remains centered south of the Maritimes. 
Meanwhile, still thinking a deepening mid level trough should 
arrive in the upper Ohio river valley by this evening. The 
combination of weakening subsidence and increasing humidity will
mean clouds and scattered showers across our region at times. 
Moisture is not that persistent or deep, so no looking like a 
washout of a day. With a decent southerly low level flow, 
thinking above normal temperatures despite the clouds.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM THURSDAY/...
22/00Z guidance continued to show a stagnant mid level pattern,
with a ridge holding over the North Atlantic, and a cutoff
lingering over the Ohio Valley and Great Lakes. This pattern
keeps a risk of showers, mainly across the western half of
southern New England during this time. With no significant
fronts nearby to provide a focus, rainfall amounts should be on
the lighter side. Persistent southeast flow will be enough to
maintain above normal temperatures and humidity. Expecting 
rounds of lower clouds and fog, especially late tonight into
early Thursday.

&&

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

* Humidity holds on through Friday/Saturday before drier air
  moves in 

* Series of low-pressure systems bring off and on periods of 
  unsettled weather through the weekend and into next week

Details...

Thursday night through Friday...

High pressure remains anchored over the west Atlantic through 
the end of the week as a surface low pressure lifts north 
through the eastern Great Lakes into southern Ontario. This 
places southern New England under continued cool and moist 
southeast flow keeping things cloudy and wet ahead of an 
approaching cold front. Cross sections show a lack of deep layer
moisture, with most concentrated in the low levels below 850 mb
Thursday night into early Friday. This portends more of a 
drizzly regime than substantial rainfall. Deeper moisture moves 
in Friday morning/afternoon from west to east ahead of the 
surface cold front. While there remains uncertainty as to the 
timing of the front, it should take its time moving from west to
east during the day, Friday perhaps even lingering across 
eastern MA into Saturday morning. The additional convergence 
will supplement the more synoptic scale forcing from WAA and the
300 mb/850 mb jets meaning more significant rainfall Friday as 
well as a few thunderstorms. Instability increases Friday 
afternoon in the warm sector (CAPE values ~1000 J/kg) and 
though 0-6 km bulk shear is marginal, low level shear is more
interesting (on the order of 28 kts 0-1 km shear and 200 ms2 of
0-1 km SRH); this raises the possibility of some rotating 
storms. 

Saturday and Sunday...

Over the last 24 hours guidance has continued to slow the 
progression of Friday's cold front, some suggesting that it may 
linger over eastern MA into Saturday morning. For this reason, 
POPs have been increased on Saturday, especially for eastern 
MA/RI. Either way the front lingers in the vicinity over the 
weekend, increasing chances of some showers Sunday too as a 
second shortwave swings through. This with instability driven by
a cold pool aloft will keep clouds and shower chances around. 
High temperatures will be similar to the previous week, in the 
70s. Lows, however, will be cooler thanks to the drier post 
frontal air; in the 50s away from the coasts. 

Monday and Tuesday...

No extended break in the unsettled weather pattern early next 
week as the broad troughing pattern continues over the eastern 
U.S. Too soon to time out any specific rain makers, but 
temperatures are trending cooler toward mid week.

&&

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

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

14Z update...

Today...IFR/MVFR CIGs improving to VFR, except for far interior
terminals, where CIGs only improve to MVFR. Patchy fog lingers 
until 14-15Z. Scattered -SHRA, mainly over the interior. SE 
wind 5-10 kts with gusts to 20 kts possible south coast, Cape 
and Islands.

Tonight...Conditions once again lower to MVFR/IFR across most 
terminals, with low clouds and areas of fog. Slight chance -SHRA.

Thursday...Mainly MVFR, with areas VFR possible. Breezy. Chance
SHRA. Isolated TSRA possible late in the day along and west of
the CT River.

KBOS Terminal...High confidence in trends. Moderate confidence
in timing.

KBDL Terminal...High confidence in trends. Moderate confidence
in timing.

Outlook /Thursday Night through Sunday/...

Thursday Night: Mainly VFR, with local IFR possible. Breezy.
Chance SHRA, slight chance TSRA.

Friday: Mainly VFR, with local MVFR possible. Breezy. SHRA
likely, chance TSRA.

Friday Night: VFR. Breezy. Chance SHRA, slight chance TSRA.

Saturday: VFR. Slight chance SHRA, isolated TSRA.

Saturday Night: VFR. Slight chance SHRA.

Sunday: VFR. Breezy. Slight chance SHRA.

&&

.MARINE...
Forecaster Confidence Levels.

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

Relatively light winds and seas into tonight. Persistent SE flow
will likely increase late tonight into Thursday. While winds
generally expected to remain less than 20 kt, seas could build 
to around 5 feet across the southern coastal waters late tonight
into Thursday. A few gusts up to 25 kt possible late Thursday.
Patchy showers/drizzle and fog possible across the waters.
Reduced visibility possible in the fog, but not quite as much
within precipitation.

Outlook /Thursday Night through Sunday/...

Thursday Night through Friday: Low risk for Small Craft
Advisory winds with gusts up to 25 kt. Seas up to 5 ft. Chance
of rain showers, slight chance of thunderstorms. 

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

Saturday: Winds less than 25 kt. Seas up to 5 ft. Slight chance
of rain showers, slight chance of thunderstorms. 

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

Sunday: 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...Small Craft Advisory from 7 AM to 9 PM EDT Thursday for ANZ255.
     Small Craft Advisory from noon to 9 PM EDT Thursday for ANZ235-
     237.
     Small Craft Advisory from midnight tonight to 9 PM EDT 
     Thursday for ANZ256.

&&

$$
SYNOPSIS...Belk/BW
NEAR TERM...Belk/BW/NMB
SHORT TERM...Belk
LONG TERM...BW
AVIATION...Belk/BW/NMB
MARINE...Belk/BW