Lexington Weather

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 211118
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
618 AM EST Thu Jan 21 2021

.SYNOPSIS...
An upper level disturbance moves off to the east this evening, 
followed by colder air tonight and Thursday. Mainly dry Thursday
night into Friday as a clipper low passes to our north. A strong
cold frontal passage Friday night ushers in much colder air and
blustery conditions for the weekend. Low pressure in the Ohio 
Valley will likely redevelop off the mid- Atlantic coast early 
next week. It remains uncertain if this system may affect our
area, and will depend on a favorable storm track.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
615 AM Update:

Overall no significant chagnes needed to the going forecast; did
opt to slow arriving cloud cover back by a couple hrs but
expectation of increasing clouds by late morning looks on track. 
Current temps are colder than we've seen recently but still are
seasonable in the teens to the mid 20s. At least some diurnal
warming and winds becoming SW should boost temperatures up to
highs in the mid to upper 30s, near 40. 

Previous discussion: 

Our region will be on the southern periphery of a low pressure
moving across southeast Canada today. The warm front of this 
low pressure should move to our north later this morning, will 
the cold front of this system approaching from the west late 
today. A mid level cold pool is expected to remain to our north 
as well, indicating meager lift across southern New England. The
limiting factor for precipitation is moisture, which will be 
limited. Thinking most of our region will remain dry but cloudy,
with the exception being the higher terrain of western MA. Even
there any snow accumulations should be light, generally an inch
or less.

Expecting near to slightly above normal temperatures with
southwest winds developing behind a warm front.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/...
The weak cold front associated with a low pressure across 
southeast Canada should move through this evening. This will 
mean clearing skies with colder air tonight into Friday. The 
presence of the low pressure and mid level cold pool to our 
north may keep some lingering clouds north of the Mass Pike. 
Winds turn westerly tonight, and become gusty Friday.

Mainly dry conditions with temperatures trending above normal
once more.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
Highlights... 

* Blustery and much colder Sat and Sunday. Sub-zero wind chills 
  possible Sat nite in the interior, with below-normal temps. Low 
  prob of Wind Advisory-level northerly gusts Sat nite/Sunday 
  eastern MA/Cape.

* Montoring a low pressure near the mid-Atlantic region later Mon 
  that may affect our area late Mon night/Tues. A suppressed 
  solution is becoming more favored but can't rule out a wintry 
  impact as yet.  

* Near to below normal temps through the period, coldest 
  temperatures this weekend.

Details: 

Friday Night:

A weak Clipper low passes into the northern Gulf of Maine early 
Friday night. The main story for our area, however, is the passage 
of a strong but dry cold front. This feature ushers in a 
strong shot of colder air for especially the second half of the 
Fri night timeframe (continuing into Sat). By daybreak, 925 mb 
temps should fall to around -9 to -14C. North to northwest winds 
will also be increasing to around 15 mph with gusts around 20-25 
mph. Will start to see wind chills in the single digits to low teens 
across interior MA and the higher terrain (Berkshires/Worcester 
Hills) and into the mid-teens for the rest of Southern New England. 
Low temperatures mid teens to low 20s, but as mentioned the wind 
will make it feel colder than that.

The Weekend: 

A blustery/windy and rather cold weekend remains in the offing for 
Southern New England for Sat into Sunday. High pressure to our west 
combined with low pressure over the Maritimes will support a fairly 
robust northerly pressure gradient, and steep lower-level lapse 
rates owing to the cold advection aloft will facilitate good 
momentum transport for breezy to at times gusty conditions.

Coldest period looks to again be on Sat, with 925 mb temps 
continuing to fall to values around -12 to -17C following the ECMWF. 
Full sunshine should be ineffective at producing much diurnal 
warming and have shown highs in the upper teens to mid 20s in 
western/central MA and the terrain, mid to upper 20s for CT into 
northeast MA, and 30-32F along and southeast of I-95. This will 
likely be the first day this January that Boston and Providence 
record their first sub-freezing high temperature in the month. 
Nearly 3 weeks into the month, it speaks to just how mild 
January has proven to be. Daytime wind chills around 5 to 10 
above in the terrain, and mid teens to around 20F for the 
remainder of the area. Even colder wind chills are anticipated 
for Sat night, with sub-zero wind chills looking to be the case 
for most of western and central MA up into the Merrimack Valley.
We do have to watch for wind gusts Sat night that may get close
to Wind Advisory levels across eastern MA and Cape Cod. May be 
more limited-impact were Advisory gusts to materialize, and its 
primary effect being to drive wind chills down. Low temps mainly
in the low-mid teens, a little higher than the colder guidance 
based on the winds staying up most of the night. 

On Sunday, 925 mb temps are not as cold as they project to be on 
Sat, but will still be around -8 to -12C. Generally similar overall 
conditions to Sat with ineffective sunshine due to the colder air, 
though air temps should be a few degrees warmer than Sat. It 
will still be quite blustery, and should produce daytime wind 
chills around +5 to +10F in the terrain, with mid teens 
to low 20s wind chills elsewhere.  

Monday thru Wednesday: 

Pattern aloft changes to one of more zonally-oriented flow in this 
period, as opposed to persistent WNW flow from a dominant northern-
stream jet regime. 

We are still keeping close monitoring of trough energy in the 
southern stream that induces Southern Plains cyclogenesis on Monday. 
The track of this progged storm system thereafter is still unclear. 
Ensemble means and individual ensemble members generally support a 
system passing south of 40N/70W Mon night into Tues. There are a 
couple of members that still are close enough to SNE to bring some 
impacts, though a large majority keep the system suppressed to our 
south favoring a miss or a glancing blow for our southernmost 
areas. Deterministic guidance has suffered from run-to-run 
inconsistency, with the 00z GFS now weaker and more suppressed. 
The ECMWF remains the wettest (e.g. snowiest) solution and would
bring at least minor accumulations of snow to areas along/south
of the Pike, while the Canadian GEM looks more like the GFS in 
showing more of a mid-Atlantic region impact. Will maintain the 
NBM's slight to lower Chance PoP for Mon night into Tues. While 
we can't discount a greater-impact solution similar to the 00z 
ECMWF as yet, the general consistency across the GEPS/GEFS/EPS 
ensemble members in showing a more suppressed outcome may be the
preference. Will continue to monitor in subsequent model runs 
in the coming days. High pressure and modest northerly flow 
would then govern wx across Southern New England if a more 
suppressed solution verifies, with temps near to below avg.

&&

.AVIATION /11Z THURSDAY THROUGH MONDAY/...
Forecaster Confidence Levels.

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

Today...High confidence.

Mainly VFR with lowering ceilings. Could see some spotty MVFR- 
IFR visby -SHSN across the higher terrain but away from the 
TAFs. WNW to W winds become SW and increase to 7-10 kt with
occasional gusts to 18-20 kt. Low prob of LLWS as warm front
lifts northward late-AM to early-PM.

Tonight and Friday...High confidence.

VFR.

KBOS TAF...High confidence in TAF.

KBDL TAF...High confidence in TAF.

Outlook /Friday Night through Monday/...

Friday Night: VFR. Windy with local gusts up to 30 kt. 

Saturday through Sunday: VFR. Windy with gusts up to 35 kt. 

Sunday Night: VFR. Windy with local gusts up to 35 kt. 

Monday: Mainly VFR, with local IFR possible. Breezy. Slight
chance SN.

&&

.MARINE...
A weak ridge of high pressure will move offshore this morning.
Winds shifting to SW around daybreak as a warm front crosses the
southern waters this morning, and the eastern waters early this
afternoon. Winds expected to shift from the west tonight into 
Friday behind a weak cold front. Rough seas around 5 feet are 
most likely across the outer southern coastal waters into 
Friday.

Small Craft Advisories continue, especially across the southern
coastal waters. It is expected that there will be brief periods
where conditions improve to less than thresholds. The advisories
were simply continued to avoid having to make frequent changes.

Outlook /Friday Night through Monday/...

Friday Night: Moderate risk for Small Craft Advisory winds with
gusts up to 30 kt. Local rough seas. 

Saturday: Low risk for gale force winds with gusts up to 35 kt.
Areas of rough seas. 

Saturday Night through Sunday: Strong winds with gusts up to
35 kt. Rough seas up to 11 ft. 

Sunday Night: Moderate risk for Small Craft Advisory winds with
gusts up to 30 kt. Areas of rough seas. 

Monday: Winds less than 25 kt. Areas of seas approaching 5 ft.
Slight chance of snow. Local visibility 1 to 3 nm.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 7 PM EST this evening for 
     ANZ232>234.
     Small Craft Advisory until 11 PM EST this evening for ANZ235-
     237.
     Small Craft Advisory until 7 PM EST Friday for ANZ254>256.

&&

$$
SYNOPSIS...Belk/Loconto
NEAR TERM...Belk/Loconto
SHORT TERM...Belk
LONG TERM...Loconto
AVIATION...Belk/Loconto
MARINE...Belk/Loconto