Lexington Weather

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 081809
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
109 PM EST Thu Dec 8 2022

.SYNOPSIS...
High pressure building across eastern Canada will bring dry 
conditions with a cooling trend through Friday. Onshore winds on 
Saturday may lead to a few rain showers or nighttime flurries near 
the coast. Low pressure from the Ohio Valley redevelops south of the 
coast and may bring a period of accumulating snow to western New 
England late Sunday into early Monday. Mainly dry and cool weather 
follows through the middle of next week.

&&

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

Though still on the mild side with temps in the 40s to lower 50s
to this point, it's a much drier day with dewpoints falling into
the 20s to mid 30s through the day. 

It is fairly breezy outside especially across eastern MA and RI
where northwest gusts have been in the 25 to 30 mph range. Good
mixing today given cold thermal advection steepening lapse rates
but mixed layer winds are progged to decrease by early to mid 
aftn; that decrease should bring gusts into the 20 to 25 mph 
range until sundown.

Aside from the NW breezes though, it's otherwise a really nice
day with only modest rises in temps from current values as cold
thermal advection offsets diabatic heating from full sun. 

Previous discussion:

Strong high pres builds across eastern Canada today. Rather dry NW 
flow with PWATs decreasing below 0.25" will result in plenty of 
sunshine although some increase in cirrus is expected this afternoon 
assocd with the upper level jet. Modest cold advection today but low 
level temps are starting out fairly mild so temps will be above 
normal with highs ranging through the 40s, except lower 50s near the 
south coast. It will be breezy at times as shallow but well mixed 
boundary layer supports 20-30 mph gusts.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/...
6:30 AM Update... 

Some patchy low ceilings that were lingering across SE MA and
Rhode Island have scattered out, giving way to abundant
sunshine! Low clouds continue to drape over the high terrain of
the Berkshires, but anticipating that these clouds will 
dissipate too as dry air works its way in today. Breezy NW flow 
expected today but with seasonably mild conditions, expecting a 
very pleasant afternoon. 

Tonight into Friday...

Strong high pres across eastern Canada remains the dominant feature 
through Friday with a persistent north wind. Very dry airmass as 
PWATs drop to near 0.10", about as dry as it gets. This will ensure 
clear skies tonight, although some ocean effect clouds are expected 
to develop over the ocean and may brush the Cape/Islands tonight. 
Full sunshine Friday with barely a cloud in the sky. Temps tonight 
are tricky as radiational cooling may be limited somewhat by a 
steady northerly wind. Expect lows mostly in the 20s, except lower 
30s near the immediate coast where winds will be strongest and 
remain gusty at times. On Fri, highs will recover to near seasonable 
normals, in the 40s.

&&

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

* Trend towards a mostly dry weekend continues outside of some ocean 
  effect showers and flurries Saturday and Sunday. Minor 
  accumulations of snow across eastern MA possible Saturday night

* Another shot for accumulating snow Sunday night, primarily across 
  western MA and CT

* Unsettled pattern continues into next workweek

Saturday and Sunday...

With strong high pressure to our north, to the tune of 1050mb, 
approaching low pressure from the Ohio River Valley will be 
suppressed to our south. With high pressure to the north and low 
pressure to our south, we will be stuck in a prolonged easterly 
flow pattern. While winds will prevail anywhere form northeast to 
southeast through the weekend, the easterly component advecting 
warmer marine air over portions of eastern MA and the Cape will be 
enough to generate some ocean effect showers Saturday afternoon. 
Given the dry airmass in place, with PWATs only around 0.3" west of 
I-95, any advected moisture will be restricted east of Worcester. 
Not anticipating much if any precipitation across western MA and CT 
during the period. Showers have the potential to change to flurries 
and light snow showers overnight Saturday as temperatures fall. The 
maritime airmass will keep temperatures above freezing along the 
immediate coast and Cape, but temperatures in the 20s across the 
interior may allow very, emphasis on very, minor accumulations of 
snow, less than 0.5" on grassy surfaces, to fall along the I-95 
corridor south of Boston. Confidence on accumulating snow is low, 
with the greatest chance in Norfolk and Bristol counties in MA. 


Sunday Night and Monday...

Guidance has honed in on a better chance for accumulating snow 
during the late Sunday night and early Monday morning timeframe as 
low pressure moves tracks from upstate NY to south of Long Island. 
With high pressure still to our north, anticipating that the brunt 
of precipitation will be suppressed to our south and west, but an 
amplifying shortwave behind the low looks to provide enough forcing 
to generate precipitation. Uncertainty remains in how much snow can 
be expected, but both deterministic and ensemble ECWMF and GEFS 
members increase our confidence in the possibility that at least 
minor accumulations of snowfall will be associated with this system. 
There are a few factors that look to limit snow amounts and 
geographic range of accumulating snow. The first limitation will be 
the lack of moisture in place over the region. While the 
shortwave/low will bump PWATs up to about 0.5", values will only be 
about 120% of normal. Secondly, the easterly wind component 
advecting warmer marine air over our region will still be present, 
especially across eastern portions of our CWA, which will 
significantly impact not only surface temps, but the entire thermal 
profile. Subprime 925mb temps around -2 to -4C will not be enough to 
overcome above freezing surface temps across eastern MA. However, a 
colder thermal profile and subfreezing surface temperatures across 
western MA and CT look to be in place by Sunday evening. Given a 
supportive thermal profile and slightly higher PWATs, the thought is 
that the probability of accumulating snow will be greatest across CT 
and portions of MA west of Worcester. Given the low will track to 
our SW, anticipating that Hartford County and the downslopes of the 
Berkshires could see the highest accumulations in our CWA. "Highest" 
should be used lightly in this circumstance, with GEFS and ECWMF 
ensemble mean snowfall maxing out around 2" in our CWA, with higher 
totals restricted to central/upstate NY. The 00Z deterministic GFS 
again hinted at the potential for greater accumulations, but at this 
time remains an outlier and given this system is still in the Day4/5 
timeframe, we continue to rely heavily on ensemble guidance. GEFS 
ensemble probs suggest a less than 30% chance for 3"+ of 
accumulation in our area, with ECWMF ensemble probs falling much 
lower. 

Tuesday and Beyond...

Unsettled pattern looks to stick with us through the majority of the 
week. While Tuesday and Wednesday look primarily dry, there is a 
signal for heavy precipitation event sometime late next week, as 
PWATs grow to 300% of normal. While details will need to be 
ironed out, should that system look to impact our area, it will 
be primarily rain. Trends will be our friend while analyzing 
systems late in the period.

&&

.AVIATION /18Z THURSDAY THROUGH TUESDAY/...
Forecaster Confidence Levels:

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

18z TAF Update: 

Rest of the Afternoon (thru 00z Friday): High confidence. 

VFR, but more SCT-OVC bases around 035-050 in the interior/high
terrain. Breezy with NW winds 10-15 kt gusts 20-28 kt, 
strongest eastern and southeast MA. Gust speeds ease by mid to
late aftn with cessation of gusts around/after sundown. 

Tonight: High confidence, though moderate on ceilings across
Cape Cod and the Islands. 

VFR for most, but ocean effect stratocu develops with VFR/MVFR
bases (020-035) for HYA, FMH and ACK. Uncertainty on timing of
onset and how pervasive it may be (e.g. it may oscillate between
VFR and MVFR bases). NW/NNW winds 8-12 kt for most, but stronger
Cape and Islands (12-18 kt with gusts 25-30 kt). 

Friday: High confidence.  

Stratocu near Cape and Islands shifts offshore early Fri to VFR
all airports. N to NNE winds 8-12 kt for most. Breezy for the
Cape and Islands, around 18-22 kt with gusts up to 30 kt. 

Friday Night: Moderate confidence. 

VFR for much of the period, but returning stratocu toward early
Sat AM could lower ceilings to MVFR levels. NE winds for most
around 6-10 kt; strong NE winds 15-20 kt with gusts around 30 
kt Cape and Islands.

KBOS Terminal...High confidence in TAF. VFR. Decreasing gust
speeds thru sundown.

KBDL Terminal...High confidence in TAF. VFR.

Outlook /Friday Night through Monday/... 

Friday: VFR. Windy with areas of gusts up to 30 kt. 

Friday Night: Mainly VFR, with local MVFR possible. Windy with
areas of gusts up to 30 kt. 

Saturday: Mainly VFR, with areas MVFR possible. Breezy. Slight
chance RA.

Saturday Night: Mainly VFR, with areas MVFR possible. Breezy. 

Sunday: MVFR/IFR conditions possible. Breezy. Chance RA, chance
SN.

Sunday Night: MVFR/IFR conditions possible. Breezy. Chance SN.

Monday: Mainly VFR, with local IFR possible. Windy with local
gusts up to 30 kt. Slight chance RA, slight chance SN.

&&

.MARINE...
Forecaster Confidence Levels:

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

350 PM Update:

Overall high confidence.

Relatively strong pressure gradient through Fri will result in gusty 
N/NW winds today veering to N tonight and Fri. Gusts to 25 kt 
expected through tonight, especially eastern waters, increasing to 
30 kt Fri. There is some risk for a brief period of marginal gale 
force gusts late Fri/Fri evening but confidence not high enough for 
a gale watch at this time. SCA all waters today, and extended into 
Fri for eastern waters.


Outlook /Friday Night through Monday/...

Friday: Moderate risk for Small Craft Advisory winds with gusts
up to 30 kt. Rough seas up to 11 ft. 

Friday Night: Strong winds with gusts up to 30 kt. Rough seas
up to 12 ft. Slight chance of rain. 

Saturday: Moderate risk for Small Craft Advisory winds with
gusts up to 25 kt. Rough seas up to 12 ft. Chance of rain. 

Saturday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Rough seas up to 12 ft. Slight chance of
rain. 

Sunday through Sunday Night: Winds less than 25 kt. Rough seas
up to 11 ft. Chance of rain. 

Monday: Moderate risk for Small Craft Advisory winds with gusts
up to 25 kt. Rough seas up to 11 ft. Chance of rain. Local
visibility 1 to 3 nm.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 1 PM EST this afternoon for ANZ230-
     233>237.
     Small Craft Advisory until 7 PM EST Friday for ANZ231-232-250-
     251-254>256.

&&

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