Lexington Weather

Lexington, Massachusetts, USA

53°F
10/17/2021 9:11pm 
  • Lexington Conditions: Partly cloudy
  • Temperature: 52.7°F / 11.5°CColder 1.1°F than last hour.
  • Dew Point: 47.3°FDecreased 0.5°Fsince last hour.
  • Relative Humidity: 82%Increased 2.0% since last hour.
  • Wind: Wind from SE SE Calm, 10-min avg: 1 mph, gust: 5 mph
  • Barometer: 29.67 in Steady
  • Visibility: 10 miles
  • Rain Today: 0.35 in
  •   

Area Forecast Discussion
for Boston, MA

        
000
FXUS61 KBOX 121758
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
158 PM EDT Tue Oct 12 2021

.SYNOPSIS...
High pressure remains in control through Friday with dry 
weather and temperatures well above normal. A strong frontal 
system brings breezy conditions and periods of showers or 
possible thunder Saturday night into early Sunday. Turning more 
seasonable and blustery for Sunday into Monday, though 
temperatures to trend less above normal into early next week.

&&

.NEAR TERM /THROUGH TONIGHT/...

2 PM update: 

Quite the weather for mid October, temps at 1:30 pm in the upper
60s and lower 70s, abundant sunshine and light winds. Normal
high this time of year is 60-65, so approaching 10 degs warmer
than normal. Earlier discussion below.

==============================================================

Low clouds and fog continue to slowly burn off at mid morning
and will eventually give way to mostly sunny & warm conditions,
courtesy of drier air aloft mixing to the surface. This is seen
on the SPC mesoanalysis with PWATs less than 1 inch along with
drier than normal 850 mb dew pts. Warm temps aloft with +13C at
925 mb supporting highs in the low to mid 70s this afternoon. 
These temps are about 10 degs warmer than normal (60-65). Cooler
along the immediate coast due to seabreezes, but not too cool 
given ocean temps are in the low to mid 60s. 

&&

.SHORT TERM /WEDNESDAY/...

Tonight...

High pres overhead will result in light to calm winds. Evening
will start out clear with good radiational cooling, then expect
areas of stratus and fog to develop. However, areal 
coverage/extent expected to be less than early this morning.
Some mid/high clouds may move in from the NW overnight ahead of
weak shortwave. Lows will range from the upper 40s in the 
normally colder locations to 50s elsewhere. 

Wednesday...

High pres remains in control and slips south of New Eng with
light SW flow developing. Some mid/high clouds expected as 
shortwave passes to the north but any shower activity should 
remain to the north and west, with partly to mostly sunny skies
in SNE. Another warm day for mid October with highs into the 
mid 70s and possibly some upper 70s in the CT valley and NE MA. 
A bit cooler along the immediate south coast.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/...

Highlights...

* Dry weather with above normal temps through Fri.

* Strong frontal passage around Sat nite or early Sun brings breezy 
  conditions and a period of gusty showers (possible thunder?). 
  Timing still not set in stone.
  
* Still unsettled and blustery Sunday into Mon. Cooler, 
  though temperatures still trend less above normal.

Details:

Through Friday Night: 

Amplified pattern across CONUS with building ridging across the east 
coast, with a deep trough carved out across the center of the 
country. Weak sfc high remains in place across New England as well. 
Pattern transitions toward geopotential height rises later Wed nite 
with mid-level ridge axis over New England and the mid-Atlantic 
states Thurs into Fri. 

Should be a generally dry period on the whole, though with some 
diurnal cloudiness around. Above-average temperatures still remain 
highly probable in this period, with highs in the low to mid 70s 
(possibly some upper 70s moving into Fri) and lows in the 50s. Nice 
stretch of weather for outdoor autumn activities!  

Saturday through Sunday: 

Main source of more active weather is centered in this period, as a 
potent shortwave disturbance rounds the mean central US trough and 
induces a robust frontal system to cross New England. 

Confidence in the timing is still closer to moderate, as the 00z 
guidance has adjusted a few hrs slower and more westward. There's 
still some variation in when the frontal passage occurs across 
models, with the GFS clearing the cold front through by early Sunday 
morning, while the ECMWF/Canadian GEM offer a late-morning to early 
afternoon clear. 

What does appear fairly clear though is that the system should be 
accompanied by strong dynamics with the shortwave disturbance 
assuming a negative tilt and rather impressive southerly wind 
fields. GFS 950 mb SSW winds are around 40-45 kt Sat nite, with a 
surge in 850 mb temps in the mid teens Celsius and a rise in PWAT 
values (values around 2 standard deviations above 
normal/1.5-1.75"). Timing remains the biggest question mark to 
be resolved, but likelihood of a rather humid warm sector and 
the strong dynamics/wind fields could support something along 
the lines of a fine line of enhanced showers or even some 
thunder given K-indices progged around 30-35 units. As 
mentioned, comes down to the timing...which at this point 
appears to be around Saturday night into early Sunday but this 
will need some refinement. Despite the strong southerly wind 
fields, not clear at this point that southerly gusts will be 
strong enough for headlines as it's often tough to get gusty 
conditions in warm advection regimes where mixing tends more 
shallow. Something to monitor. Opted for Likely pops and left as
rain showers for now, but some potential exists for thunder in 
the pre-frontal environment. Highs in the 70s with lows in the 
50s to near 60F.

After frontal showers clear offshore early on Sunday, robust CAA 
then looks to usher in a more seasonable air mass that filters into 
Southern New England. 850 mb temps fall into the lower single digits 
Celsius range, and around +8 to +10C at 925 mb. WNW winds don't 
appear overly strong but cold advection likely should support good 
mixing. Expecting at least a cooler and blustery Sunday, though 
cooler is still relative - temperatures still look just less above 
normal with highs in the 60s. As winds subside Sunday night, should 
see one of the cooler nights experienced recently with lows commonly 
in the 40s!

Early Next Week: 

Mean longwave trough sets up across New England on Monday, which 
should keep our weather pattern unsettled but largely dry. ECMWF has 
a large area of high pressure that sets up Monday night. It's 
early/speculative but in such a pattern some of the typical radiator 
sites could drop into the upper 30s Monday night. Possible first 
frost? Maybe. 

Trough lifts out around Tuesday with quasi-zonal northern stream 
governing conditions. Should see a return to seasonably mild 
conditions by Tuesday.

&&

.AVIATION /18Z TUESDAY THROUGH SUNDAY/...
Low - less than 30 percent. 
Moderate - 30 to 60 percent. 
High - greater than 60 percent.

18z TAF Update:

This evening...High confidence.

VFR, dry weather and light winds, locally onshore. 

Overnight...moderate confidence.

VFR but trending toward MVFR/IFR with local LIFR possible in 
dense fog. Uncertainty centered on the areal extent of IFR/LIFR,
as mid clouds may preclude widespread IFR/LIFR from forming. 
However forecast confidence is low on how it will play out. 
Light SSW winds. 

Wednesday...

Any early IFR/LIFR in areas of fog, locally dense, improves to 
VFR by 14z/15z. Otherwise, another day of dry weather and light
SSW winds. 

Wednesday night...mainly VFR with areas of MVFR possible. Other
than a spot shower, dry weather prevails. Light SSW winds
prevail.  


KBOS Terminal...High confidence in TAF thru about 02z-03z then
some uncertainty on timing and duration of any IFR/LIFR late
tonight into Wed morning. 

KBDL Terminal...High confidence in TAF thru about 02z-03z then
some uncertainty on timing and duration of any IFR/LIFR late
tonight into Wed morning.

Outlook /Thursday through Saturday/...

Thursday through Friday Night: VFR. 

Saturday: 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.

Diminishing easterly winds 10-15 kt over the southern waters 
today, with variable winds generally less than 10 kts elsewhere.
However, seas will remain around 5 ft over southern waters where
SCA will continue today.

Light winds with seas dropping 5 ft tonight, then SW winds up 
to 10 kt Wed. 

Outlook /Wednesday Night through Saturday/...

Wednesday Night: Winds less than 25 kt. Local visibility 1 nm
or less.

Thursday through Friday Night: Winds less than 25 kt. 

Saturday: Winds less than 25 kt. Areas of seas approaching
5 ft. Chance of rain showers.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 8 PM EDT this evening for ANZ254-
     255.
     Small Craft Advisory until midnight EDT tonight for ANZ256.

&&

$$
SYNOPSIS...KJC/Loconto
NEAR TERM...Nocera
SHORT TERM...KJC
LONG TERM...Loconto
AVIATION...Nocera/Loconto
MARINE...KJC/Loconto
      

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