Lexington Weather

Lexington, Massachusetts, USA

75°F
9/22/2021 1:08pm 
  • Lexington Conditions: Overcast
  • Temperature: 74.8°F / 23.8°C 
  • Dew Point: 68.2°FDecreased 0.7°Fsince last hour.
  • Relative Humidity: 80%Decreased 2.0% since last hour.
  • Wind: Wind from NW NW 2 mph, 10-min avg: 3 mph, gust: 11 mph
  • Barometer: 30.10 inFalling 0.02  inHg/hr Falling Slowly
  • Visibility: 10 miles
  • Rain Today: 0.00 in
  •   

Area Forecast Discussion
for Boston, MA

        
000
FXUS61 KBOX 180234
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
1034 PM EDT Fri Sep 17 2021

.SYNOPSIS...
Tropical Storm Odette will pass far southeast of southern New
England on Saturday. However it will give the Cape and the islands
a chance of showers late tonight and Saturday morning and 
generate some surf and a higher chance of rip currents. As 
Odette moves out to sea, high pressure builds in and will bring 
sunny, dry and warm conditions from later in the weekend into 
mid week. A cold front Thursday ushers in a cooler autumn-like 
air mass with below normal temperatures to end the week.

&&

.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
1030 PM update...

Forecast is on track. The Dense Fog Advisory remains in effect 
for much of the central and eastern portion of southern New 
England. Light northeasterly flow was keeping lots of low level 
moisture in place. Several spots were reporting visibility as 
low as 1/4 to 1/2 mile, along with some drizzle...and that 
should continue overnight. Temperatures were mainly in the 60s 
as of 1030 AM, but Martha's Vineyard and Nantucket were still 
near 70.

Have updated grids to shift winds a bit more northerly along the
northeast MA coast and more ENE over Cape Cod and the islands.

Tropical Storm Odette was approximately 300 miles south of
Nantucket. Winds may increase a bit over the Cape and islands
and the southern RI coast late tonight and Saturday morning 
with gusts to 20-30 mph possible. Otherwise te main impacts will
be increased swells and an increased rip current risk.

&&

.SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/...
305 pm update...

The potential tropical/subtropical low pressure system will be
moving well out to sea during the day. As it does so, the flow
in the lower to mid levels will start to turn more northerly and
northwest. With no more onshore flow, we should start to see the
clouds break up with increasing sunshine as the day wears on.
There is a subtle mid/upper level trough that will approach the
region late in the day. Most of the hi-res convective allowing
models do generate a few showers to our northwest during the
afternoon, and reaching into northeast MA by late afternoon.
Model soundings suggest some surface based CAPE, so can't rule
out an isolated thunderstorm in the mix. Not expecting anything
strong, nor even much coverage. Leaned on a mix of guidance and
still only came up with PoPs topping out in the 15-25% range
for areas north and west of a Boston-Hartford line. Although
temperatures aloft will be rather warm (eg: 925mb temperatures
around 19C) which would support mid 80s for highs, clouds in 
the morning and only moderate mixing of the boundary layer 
should mean we will fall short of the maximum potential. Still 
highs in the upper 70s to lower 80s are expected -- well above
normal for mid-September.

On a last note, given the expected uptick in waves and swell,
especially on southeast facing shores, due to the low pressure
passing by, there is an increased chance of rip currents on
those southeast facing beaches. Will issue a Rip Current
Statement just to highlight that risk. Don't want anyone to be
unaware and getting themselves into trouble. 

Saturday night: looking to be rather quiet. With light northwest
flow, we should see dewpoints starting to come down. That, along
with more clear skies, will allow temperatures to fall into the
50s across the interior to lower 60s southeast/Cape/Islands.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...
305 pm update...

Highlights: 

* Quiet stretch of weather through middle of next week with above 
  normal temperatures with highs reaching upper 70s to low 80s. 
  
* First strong fall cold front arrives sometime on Thursday with 
  line of showers and thunderstorms out ahead of it. Turning much 
  colder on Friday with highs only reaching the 60s. 

Details:

Sunday...

As heights rise behind the aforementioned exiting low pressure 
system, there should be significant improvement on Sunday. 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. Only concern would be the lingering 
swells and elevated rip current risk from the distant coastal low. 

Monday into Wednesday...

Monday looks to feature mostly dry and seasonable conditions with 
surface high overhead. Then mid-level ridging amplifies over the 
eastern US, with global guidance ensembles showing 590+ 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. Forecast wise, have gone with the 75th 
percentile NBM for Monday and 90th percentile NBM guidance for 
Tuesday and Wednesday with southwest flow aloft boosting downsloping 
adiabatic warming. Note that Monday's high temperatures may 
underperform a little if northeast flow hangs on a little longer 
before switching to the south and southwest as high pressure moves 
offshore. 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.

Wednesday night into Friday...

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 by Friday morning. Trough 
digging into the Great Lakes will eventually erode the ridge over 
Eastern US by late Wednesday into Thursday time frame. While there 
are still differences in timing amongst the models, siding with a 
blend of EC and Canadian over the GFS (which has a tendency to be a 
tad too progressive at this time frame) suggests the cold front 
crossing the area on Thursday. In addition, given the strong 
dynamics involved with impressive pressure rise/fall couplet, expect 
a line of showers and thunderstorms out ahead of the front with 
strong W/NW winds behind it and associated gale force conditions 
over the waters given steep delta T between 850mb and SSTs. In 
addition, with PWATs rising to over 1.75 inches (90th percentile 
according to NAEFS), locally heavy downpours are possible late 
Wednesday into Thursday. With 850mb temperatures falling into the 
single digits post frontal, daytime highs on Friday could only get 
into the 60s, with 50s possible in the high terrain. Overnight lows 
fall into the 40s, with some mid to upper 30s possible in the usual 
cold spots.

&&

.AVIATION /02Z SATURDAY THROUGH WEDNESDAY/...
Forecaster Confidence Levels.

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

0230Z update:

Overall moderate confidence in the forecast.

Overnight...

Widespread LIFR in central and eastern portions of the region
overnight. Visibilities 1 mile or less with several areas down
to 1/4 to 1/2 mile. In western MA and northwestern CT, IFR
ceilings but visibilities may not get below 2 miles overnight.
A few light rain showers possible along the south coast.

Saturday...

Ceilings will be lifting with VFR conditions expected in most 
areas after 16z. Outside chance of showers late in the day, 
primarily across the interior. Probabilities are so low that 
they don't warrant even a "VCSH" in the TAFs at this point.

KBOS Terminal...moderate confidence. LIFR overnight with vsby
1/2 mile or less. MVFR conditions expected to develop after 
15z with VFR briefly possible after 18Z.

KBDL Terminal...Moderate confidence.

Outlook /Saturday night through Sunday/

Saturday night: VFR except for MVFR on Cape terminals. Breezy. 

Sunday: VFR. Breezy. 

Sunday Night through Tuesday: VFR. 

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

Wednesday: Mainly MVFR, with areas VFR possible. Breezy. Chance
SHRA.

&&

.MARINE...
Forecaster Confidence Levels.

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

1030 PM update...

Generally high confidence in the forecast. Primary driver of the
forecast will be Tropical Storm Odette passing well to our 
south late tonight and then far offshore to the southeast on 
Saturday. It is expected to generate enough of a swell, combined
with some winds touching 25 knots (especially near the Cape and
Islands), that Small Craft Advisory thresholds of 5 foot waves 
will be exceeded for the waters off the south coast, to just 
east of Cape Cod. Already have the Small Craft Advisory in 
effect, and based on latest guidance, it appears those rougher 
wave conditions will continue through Saturday night...perhaps 
even into Sunday for the waters east of the Cape. 

Outlook /Sunday through Wednesday/...

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

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

Monday: Winds less than 25 kt. Seas locally approaching 5 ft. 

Monday Night: Winds less than 25 kt. 

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. Slight chance of rain showers.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Dense Fog Advisory until 8 AM EDT Saturday for CTZ004.
MA...Dense Fog Advisory until 8 AM EDT Saturday for MAZ004>007-
     012>022-026.
     High Rip Current Risk from 6 AM EDT Saturday through Saturday 
     afternoon for MAZ022-024.
RI...Dense Fog Advisory until 8 AM EDT Saturday for RIZ001>005.
MARINE...Small Craft Advisory until 8 AM EDT Sunday for ANZ235-237-
     254>256.

&&

$$
SYNOPSIS...Chai/Nash/Field
NEAR TERM...Field 
SHORT TERM...Nash 
LONG TERM...Chai 
AVIATION...Field/Chai/Nash 
MARINE...Field/Chai/Nash
      

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