Lexington Weather

Lexington, Massachusetts, USA

73°F

9/22/2021 1:50pm

Lexington Conditions: Overcast
Temperature: 73.0°F / 22.8°C
Dew Point: 68.5°F
Relative Humidity: 86%
Wind: SE 2 mph, 10-min avg: 3 mph, gust: 12 mph
Barometer: 30.10 in Steady
Visibility: 10 miles
Rain Today: 0.00 in

Area Forecast Discussion
for Boston, MA

        
000
FXUS61 KBOX 162317
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
717 PM EDT Thu Sep 16 2021

.SYNOPSIS...
A stalled cold front remains to our southeast overnight. Cloudy
and foggy conditions will bring us into the weekend before a 
coastal low tracks close to southern New England. Impacts will 
be low for our area, but the storm will generate building waves 
and surf and a modest increase in northeast winds Friday night 
into Saturday night for coastal New England. After the tropical 
system passes to our southeast, high pressure remains in control
with milder than average temperatures and several days of dry 
weather through midweek. Then a strong cold front brings the
region first real taste of autumn by late in the work week. 

&&

.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...

7 PM Update...

Forecast remains largely on track so only made minor changes to
bring it in line with obs. See discussion below. 

PREVIOUS DISCUSSION...

Low ceilings will be the continued story of the evening and 
overnight hours as showers continue to dissipate across the region. 
Dominant, light, northeast onshore flow, high dewpoints, low level 
moisture, and a cooling boundary layer are a recipe for fog 
overnight. The longer nights of late September will also aid in fog 
formation given more time will be spent during periods of low 
dewpoint depressions. Uncertainty lies in exactly where fog will 
form, as these same conditions typically generate a low stratus deck 
across eastern areas, such as Boston and Cape Ann. Some pockets of 
dense fog is possible in areas where cloud cover is not as thick, 
such as the Connecticut River Valley and central Connecticut. 

Dewpoints and significant cloud cover will be the dictating factors 
regarding overnight low temps. Dewpoints are expected to remain in 
the 60s west to near 70 across Cape Cod and the Islands overnight. 
Given poor radiational cooling conditions, we will see overnight 
lows in the 60s region wide. A few lingering showers may affect 
portions of the south coast given minor instability (100 J/kg 
MUCAPE) associated with the stalled cold front to our southeast, but 
most locations will remain dry through the morning; aided by a 
subsidence inversion.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/...
Friday...

A ridge axis continues to center itself over southern New England, 
which will aid in a mostly dry, but conditions remain largely the 
same as they will be overnight Thursday. Continued northeast flow 
and low level moisture will keep ceilings low across most of the 
area through Friday afternoon. Low clouds finally begin to loosen 
their grip on the region by Friday afternoon. 

Given that overnight lows only fall into the 60s, temperatures will 
not have too far to climb towards seasonable conditions. While there 
will be more clouds than sun, a few more breaks of compared to 
Thursday will allow many places to reach into the mid and upper 70s. 
Isolated showers are again possible, but blanket slight chance PoPs 
to account for the outside chance of precip, but most areas will 
again remain dry.   Friday Night...

Skies will finally begin to clear by daybreak on Saturday, but fog 
will again impact the region as northeast flow persists.
Given almost identical conditions to Thursday night, low 
temperatures will fall into the 60s once again. Dewpoints will be 
slightly higher across western MA, so even our coolest spots will 
only fall into the mid 60s. 

A tropical system being monitored currently monitored by the NHC is 
expected to pass to our southeast beginning Friday night. The 
impacts from this system are expected to be low in southern New 
England. The primary threat will be choppy surf and rip currents, 
primarily at our south facing beaches.

&&

.LONG TERM /SATURDAY THROUGH THURSDAY/...
Highlights: 

* Scattered showers with cool and breezy conditions on Saturday 
  before dry and seasonable conditions return on Sunday. 
  
* Dry weather and above normal temperatures likely heading into 
  middle of next week with high pressure in control. 

* Could have our first real taste of autumn late next week.

Details:

Saturday...

The potential tropical cyclone mentioned in the short-term 
discussion will track near or just south of the 70W/40N benchmark on 
Saturday. This would mean best chances of rain across southeast New 
England, though rains in most areas may prove more intermittent. 
Otherwise, with onshore flow, expect mostly cloudy, breezy and cool 
conditions with interval of rain showers along with increased swells 
and heightened risk of rip currents. While effects and impacts will 
ultimately hinge on the track and intensity of the cyclone, right 
now the impacts to Southern New England appear minimal. 

As heights rise behind the exiting cyclone, there should be 
significant improvement on Sunday. Sunshine may even return on 
Saturday afternoon if the low pressure system is faster to exit. Dew 
points fall from the 60s into the 40s and with sunshine and 
seasonable conditions returning, it will feel very comfortable for 
outdoor activities on Sunday. For reference, normal daytime highs 
are in the low to mid 70s. 

Monday into Wednesday...

Monday looks to feature mostly dry and seasonable conditions with 
high pressure in control. Then mid-level ridging amplifies over the 
eastern US, with global guidance ensembles showing 588+ Dm ridge 
building into Southern New England by middle of next week. With mean 
850mb temperatures in the low to mid teens, expect daytime highs to 
reach the upper 70s to lower 80s, which would be 5 to 10 degrees 
above climatological normal. Have gone with NBM guidance for now 
given we are so far out but it is possible that daytime highs will 
be increased in future updates with southwest flow aloft boosting 
downsloping adiabatic warming. In fact, the Climate Prediction 
Center 6 to 10 day outlook features above 80 to 90 percent 
probability of above normal temperatures. So even as we head into 
astronomical fall for the Northern Hemisphere (beginning September 
22), it will feel more like late August. PWATs look to be slightly 
below to near normal so expect generally mild days and cool nights 
given the dry air mass.

Thursday...

There are increasing signs that we could have our first real taste 
of fall heading into the latter part of the upcoming work week. Some 
deterministic guidance are showing 850mb temperatures in Celsius 
crashing from the mid teens to the low single digits, with sub-
zero temperatures just north of our area. Trough digging into 
the Great Lakes will eventually erode the ridge over Eastern US 
by Wednesday/Thursday time frame. Models still differ on the 
timing of the front and strength of the dynamics. But it is 
reasonable to expect a line of showers and thunderstorms out 
ahead of the advancing cold front. This far out, it is too 
premature to speculate on the potential for severe weather but 
it looks increasingly likely that the stretch of above normal 
temperatures will come to an abrupt end by late in the work 
week. &&

.AVIATION /20Z THURSDAY THROUGH TUESDAY/...
Forecaster Confidence Levels.

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

Tonight...Moderate confidence.

Trending down to IFR/LIFR along with patchy fog. Mainly dry 
weather. Light ENE winds. 

Friday...Moderate confidence. 

IFR/LIFR to begin the day, but cigs lifting to MVFR/VFR during
the afternoon. Mainly dry weather with modest ENE winds. 

KBOS Terminal...Moderate confidence. NE flow combined with high
humidity will encourage IFR and possibly LIFR ceilings to
develop overnight through Friday morning. 

KBDL Terminal...Moderate confidence. Breaks of sun and MVFR
conditions during the early afternoon will transition to VFR
overnight. Fog develops overnight. 

Outlook /Friday night through Sunday/

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

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

Sunday: VFR. Breezy. 

Sunday Night through Tuesday: VFR.

&&

.AVIATION /00Z FRIDAY THROUGH TUESDAY/...

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

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

Sunday: VFR. Breezy. 

Sunday Night through Tuesday: VFR. 

&&

.MARINE...
Forecaster Confidence Levels.

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

Tonight...ridge of high pressure offshore builds across the waters. 
Patchy dense fog will yield poor vsby at times. Light ENE winds.
A spot shower is possible across the southern waters. 

Friday...ridge of high pressure remains across the waters, yielding 
light ENE winds and mainly dry weather, a spot shower is again 
possible. By 12Z, waves begin to build as a tropical system 
approaches from out south. A small craft advisory has been hoisted 
starting at this time. 

Outlook /Saturday through Tuesday/...

Saturday: Winds less than 25 kt. Local rough seas. Slight chance of 
rain showers. A small craft advisory will be in place for our 
southern waters. 

Saturday Night: Winds less than 25 kt. Seas up to 5 ft. Slight 
chance of rain showers. A small craft advisory will need to be 
considered to cover this period. 

Sunday through Monday: Winds less than 25 kt. Seas up to 5 ft. A 
small craft may need to be considered for this period.  

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

Tuesday: Winds less than 25 kt. Seas up to 5 ft.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory from 8 AM Friday to 8 PM EDT Saturday for 
     ANZ235-237-254>256.

&&

$$
SYNOPSIS...Chai/KS
NEAR TERM...Chai/KS
SHORT TERM...KS
LONG TERM...Chai
AVIATION...Chai/KS
MARINE...Chai/KS