Lexington Weather

Area Forecast Discussion
for Boston, MA

        
000
FXUS61 KBOX 201349
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
949 AM EDT Mon Sep 20 2021

.SYNOPSIS...
Large high pressure remains over New England today and Tuesday,
bringing dry weather with mild days and cool nights. The high 
begins to drift offshore Wednesday, with increasing humidity and
scattered showers possible. A slow moving frontal system 
approaching from the west will be accompanied by widespread 
showers at times Thursday into Friday. Improving conditions 
expected Saturday but a few showers could linger in eastern 
Massachusetts and Rhode Island.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
950AM Update...

No changes made during this update to the forecast.

Temperature just before 10AM are upper-50s/upper-60s. The only 
minor adjustment made were to nudge the temps up by 1 or 2 
degrees as the 50th percentile of the NBM was a tad warmer than 
current forecast. This will yield temps in the upper-60s to 
low-70s at the coast and mid-70s across the interior. 

Honestly, we are splitting hairs on this "severe clear" kind of
day. Speaking of, we do not see much in the way of cloud cover,
outside an area of lower broken stratus clouds across the CT 
River Valley. Morning valley fog across northeastern Mass is 
diminishing as the sun rises higher and heats up the surface. 

Previous forecast discussion is below. 

Today...

* Spectacular weather continues, featuring abundant sunshine, 
  low humidity, mild days and cool nights

Subtropical ridge along the east coast amplifies to 593 dam heights 
at 500 mb over southern New England. Strong subsidence yields a 1030 
mb surface high from the Maritimes to New England. This synoptic 
setup will provide dry weather and abundant sunshine. Temps aloft 
(+14C at 925 mb and +10C at 850 mb), supports highs very similar to 
Sunday, low to mid 70s, slightly cooler along/near the coast. Just 
an absolute gem of a day along with light winds and low humidity 
(dew pts in the 50s). Enjoy.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/...
3 AM update...

Tonight...

Deep layer ridge remains over the area with 1030 mb surface high 
from the Maritimes to New England. Dry airmass, light winds and 
clear conditions will yield another round of radiational cooling. 
Thus, we will once again derive lows from the cooler MOS guidance. 
Another night of temps cooler than normal, with lows in the 40s and 
50s. Patchy fog is likely across the valley locations. 

Tuesday...

Not much change with deep layer ridge (591 dam heights) and 1030 mb 
high just offshore, supporting dry weather with cool nights and mild 
days. No change in airmass so expecting similar highs to Monday, low 
to mid 70s away from the coastline. Not quite as dry as today, as 
dew pts rise slightly thru the 50s to near 60 along the south coast, 
as winds become southeast. This slight increase in low level 
moisture may yield scattered to broken strato-cu clouds from time to 
time per model moisture profiles. Nevertheless, very pleasant weather 
given partial sunshine, mild temps and light winds.

&&

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

* Slow moving front brings scattered showers Thu/Fri
* Front may slow down and cause showers to linger Sat in east

Models continue to show strong mid level ridge shifting offshore Wed 
which may bring few showers across interior as moisture is drawn 
northward. Not expecting much in way of activity since any forcing 
is weak. 

Main time frame for scattered showers looks to be Thu into Fri as 
slow moving cold front approaches from Great Lakes. Still seeing 
timing differences among models/ensembles with GFS remaining most 
progressive and bringing front through Thu, while ECMWF and Canadian 
are slower and bring it through later Fri or early Sat. Have to 
think slower solution is more correct given high amplitude upper air 
pattern with deepening mid level trough over Ohio Valley. Still far 
enough out in time that we need to consider both options so NBM 
guidance offers a reasonable compromise. 

Looking further ahead to next weekend, it's possible front hangs up 
over eastern New England which would keep a few showers across 
eastern MA/RI. We may also see a few showers Sun as upper trough 
rotates through region, but overall confidence is very low given 
wide range of model solutions.

&&

.AVIATION /14Z MONDAY THROUGH FRIDAY/...
Forecaster Confidence Levels.

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

12z TAF Update: High confidence.

Lingering MVFR/IFR in early morning valley fog improves quickly
this morning but is expected to return later tonight. Otherwise
VFR. Light winds with sea breezes near coast.

KBOS Terminal...High confidence. VFR. 

KBDL Terminal...High confidence. VFR.

Outlook /Tuesday night through Thursday/

Tuesday: VFR. 

Tuesday Night: Mainly VFR, with local IFR possible. Slight
chance SHRA.

Wednesday: Mainly VFR, with local MVFR possible. Slight chance
SHRA.

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

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

Thursday Night through Friday: VFR. Breezy. Chance SHRA.

&&

.MARINE...
Forecaster Confidence Levels.

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

3 AM update...

Today/Tonight/Tuesday...

Tranquil boating weather with 1030 mb high stretching from the 
Maritimes to New England. Light NE winds this morning becoming light 
east this afternoon and tonight. Then becoming light SE Tue. Dry 
weather and good vsby thru the period. 

Outlook /Tuesday Night through Friday/...

Tuesday through Tuesday Night: Winds less than 25 kt. Seas
locally approaching 5 ft. 

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

Wednesday Night through Thursday Night: Winds less than 25 kt.
Seas up to 5 ft. Slight chance of rain showers. 

Friday: Winds less than 25 kt. Areas of rough seas. Slight
chance of rain showers.

&&

.CLIMATE...
The latest first sub-70 degree high heading into astronomical 
fall for BOS is September 20 1947. It is possible we may break 
the record by tomorrow. 

The latest first sub-70 degree high heading into astronomical
fall for PVD is September 26 2015.

Both BDL and ORH have already recorded sub-70 degree highs
earlier this month.

&&

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

&&

$$
SYNOPSIS...Nocera/JWD
NEAR TERM...Nocera/Gaucher/JWD
SHORT TERM...Nocera
LONG TERM...JWD
AVIATION...Nocera/JWD
MARINE...Nocera/JWD
CLIMATE...BOX Staff
      

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