Lexington Weather

Lexington, Massachusetts, USA

41°F
12/15/2019 3:10am 
  • Lexington Conditions: Clear
  • Temperature: 41.2°F / 5.1°CColder 2.1°F than last hour.
  • Dew Point: 34.9°FDecreased 1.4°Fsince last hour.
  • Relative Humidity: 78%Increased 2.0% since last hour.
  • Wind: Wind from W W 3 mph, 10-min avg: 4 mph, gust: 8 mph
  • Barometer: 29.25 inRising 0.04  inHg/hr Rising Slowly
  • Visibility: 10 miles
  • Rain Today: 0.00 in
  •   
Southeast Middlesex County - Wind Advisory
 - SEE ALL NEARBY ALERTS -

Area Forecast Discussion
for Boston, MA

        
000
FXUS61 KBOX 150613
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
113 AM EST Sun Dec 15 2019

.SYNOPSIS...

Mainly dry but windy weather is on tap for the region today. A 
frontal system passing just south of Southern New England may 
spread a period of snow to wintry mix Monday night into Tuesday.
A strong Arctic front then ushers in a period of scattered snow
showers and much below normal temperatures by mid to late week.
Gradual moderation by the weekend, through temperatures should 
remain at below normal levels.

&&

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

110 AM Update...

Lingering shortwave energy behind the cold front was resulting
in a few sprinkles across the region. Otherwise, mainly dry
weather overnight other than a few snow showers along the east 
slopes of the Berks.

Winds will continue to increase overnight as the intense low
pressure that has lifted into Quebec combines with steepening
lapse rates. WSW wind gusts of 25 to 40 mph anticipated toward
daybreak in most locations and near wind advisory criteria
across portions of the high terrain and Islands.

While it will turn cooler, it will remain above freezing in 
most locations overnight. Low temps will only drop into the 
middle 30s the lower 40s by daybreak.

&&

.SHORT TERM /6 AM THIS MORNING THROUGH 6 PM MONDAY/...
Sunday...

Strong west winds will be the main story as deep low pres lifts
north across eastern Canada. Steep low level lapse rates will 
promote a deep and well mixed boundary layer with soundings 
supporting gusts 40-50 mph. Strongest gusts expected over the 
higher terrain and Islands. We expanded the wind advisory 
across much of MA along and north of the Pike as well as east 
coastal MA and Cape Cod. Otherwise, a mainly dry day with 
sunshine becoming mixed with clouds as some low level moisture 
is present. However, can't rule out a few lake effect snow 
showers spilling across the Berkshires. Most cloud cover will be
in the interior. Highs will range from upper 30s to mid 40s. 

Sunday night...

Still gusty into the evening with potential for 35-45 mph gusts
but winds will gradually diminish through the overnight as
gradient relaxes and high pres approaches from the west. 
Mainly clear skies with lows dropping into the upper teens to 
mid 20s.

&&

.LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...
Highlights... 

* Increasing confidence for plowable snow, and/or snow and icy 
  roads, Monday overnight into Tuesday across interior MA/CT into 
  RI. Impact to the Tuesday AM commute from snow and/or mixed 
  precip. Accumulations to depend on thermal structure and p-type 
  changes, details on both are still uncertain. 
  
* Scattered snow showers and possible snow squalls Wednesday 
  afternoon into early evening ahead of Arctic cold front.

* Well below normal temperatures, with single digit to below-zero 
  wind chills, for Wednesday night into Thursday. 

* Gradual moderation, though just less below-normal temps by 
  Friday into the weekend. 
  
Details...

After a brief shot of colder air Sunday night, temperatures start to 
moderate somewhat with warm advection into the first part of Monday. 
GFS and ECMWF ensemble means then show a lead shortwave trough 
moving out from the Plains states into the Northeast states late 
Monday into Tuesday. This lead feature, with some interaction from a 
stronger polar trough digging SE from the northern Canadian Prairies 
into the Great Lakes, will spread a surface low/frontal system from 
the OH Valley to Southern New England and look to spread a period of 
wintry wx for Monday night into Tuesday. The polar trough mentioned 
then becomes the primary feature, with a shot of modified-Arctic air 
overspreading a good part of New England to end the workweek. Though 
there's greater spread in the guidance beyond Friday, the shot of 
well below normal temperatures looks generally short, with 
moderating temperatures for the weekend.

Monday: 

Chilly start, though gradual warming through much of the day amid 
weak high pressure, modest warm advection and daytime sun becoming 
increasingly filtered moving through the day. BUFKIT soundings still 
show considerable dry air below 800 mb across much of the region so 
looking for a dry forecast through sunset; but the dry air from low-
levels to sfc is an indication that temperatures may cool to wet-
bulb values once steadier precip gets going later Monday night. 

Monday Night Through Tuesday Night: 

** Confidence on Wintry Weather Increasing for Monday Night thru 
Tuesday ** 

Warm front will begin to lift across coastal RI into southern CT 
early Monday night, with precip spreading northward across most of 
Southern New England late Monday night and into Tuesday. This again 
looks to be a mixed precip event with multiple p-types/changes for 
parts of the area, with greatest period of impact appearing to be 
Monday overnight into the first part of Tuesday. 

Biggest sources of uncertainty on forecast accumulations and impact 
are from thermal structure, potential for bands and event total QPF. 
Wet-bulb temps look cold enough off most models to fall as frozen 
precip at least initially. Eastern/SE MA into central/southern RI 
should eventually see temps warming to near/above freezing levels. 
Models also continue to vary on how quickly to advance a warm nose 
northward into the colder air, which would affect precipitation 
types and changeovers even in the interior. The past few runs of the 
GFS have extended its warm nose further north the soonest; so the 
GFS would offer a somewhat shorter period of snow and a quicker 
transition to mixed precip and/or rain by Tuesday afternoon. ECMWF 
remains the coldest, and would keep snow hanging on longest at least 
into western/central and northern MA, and more of a snow to mixed-
precip scenario across northern CT into central/northern RI. 
Canadian GEM is something in between when it comes to thermal 
structure. GFS does indicate a ribbon of 850-700 mb frontogenetic 
forcing, on the nose of 850 mb southerly speed convergence, lifting 
from S to N overnight into early Tuesday morning that may force 
steadier/heavier periods of precip in that period of time. While 
models have trended somewhat wetter compared to morning runs, I do 
note that synoptic features in mid-levels aren't closed off, giving 
an impression of a generally progressive system with (at least at 
the moment) a relatively short accumulation period. 

Ensemble probabilities of "plowable" snow (> 3 inches) are around 60-
70 percent across interior MA into northern CT/northern RI on the 
ECMWF ensemble; the more warmer GFS shifts this axis more into 
interior New England. Think this does convey the forecast 
uncertainty especially hinging on thermal structure and p-type 
transition zones.

In terms of accumulations, a first shot would be something in the 3-
5" range along the Route 2 corridor into NE MA away from the coast, 
amounts then may look like a 2-4" with a bit of freezing rain from 
the Mass Pike/I-95 southward through Bristol County MA into the 
Hartford/Providence corridor (with a transition from less snow and 
more PL/FZRA accretions further south one goes). Slushy C-2" and 
mainly cold rain along the E/SE MA coast into coastal S RI. Above 
initial scenario more closely follows the international guidance 
than the milder GFS. Uncertainties stated earlier may increase or 
decrease those totals, perhaps by a few inches. It is possible that 
some areas could see marginal warning-criteria accumulations towards 
northern MA though see this as being at least an Advisory event for 
the interior; given questions on p-types throwing a monkey wrench 
into accumulations, confidence wasn't high enough to hoist Watches.

Confidence is increasing on the overnight into the Tuesday AM 
commute being adversely impacted with snow and/or iced up roads, 
especially areas near/north of Route 6 in CT-RI and near/NW of I-95. 
Snow and/or sleet should be mixing in Tuesday across northern into 
central/west-central MA, with sleet/freezing rain transition zone 
trudging northward. Coastal MA/RI into part of SE MA may only see a 
shorter period of wintry weather before changing to a cold rain, 
with mild water temps resulting in warmer 2-m temps that may switch 
p-type from wintry to rain sooner. All areas change back to a brief 
bit of snow as surface low pulls away from the coast by Tuesday 
evening.

Wednesday: 

Cold advection plunges in on NW winds on early Wednesday, as we 
await the strong northern stream trough and sharp Arctic cold front 
for the second part of the day into early evening. BUFKIT soundings 
show steep low-level lapse rates and shallow wintertime convective 
instability during the mid-aftn to early evening hrs with some 
shallow moisture. That could open the door for scattered snow 
showers or even some snow squalls with the frontal zone. Timing this 
front through will be critical and there exists the potential for 
low-visibility snow bursts/snow squalls during the Wednesday PM 
commute. 

Wednesday Night into Thursday Night: 

Core of the Arctic air plunges into Southern New England behind the 
Arctic front Wednesday evening. GFS/ECMWF differ on how cold low-
level temperatures get, but there's multi-model signal for -15 to -
18C 850 mb air invading a large part of Southern New England 
Wednesday night. With lows in the single digits to low teens and 
winds 15-20 mph Wednesday night, likely looking at a period of sub-
zero wind chills by early Thursday morning from the I-95 corridor, 
and wind chills around 0-5 above towards the E/SE Mass coast into 
southern RI.   

Although Thursday looks dry as ridging builds into the region, it 
still looks quite chilly with highs only in the teens to low 20s and 
wind chills around 0-10 above. Winds slacken into Thursday night as 
high pressure centers itself over western New England, with good 
radiational cooling bringing single digit to mid teens lows.

Friday into the Weekend: 

Does look like the period of well below normal temperatures is 
pretty short-lived. While still below normal, slow/gradual 
moderation in temperatures then looks to prevail Friday into 
Saturday. Compared to the -15 to -18C 850 mb temps from Wed nite 
into Thurs, these are around -3 to -6C by 12z Sunday. This part of 
the forecast is overall on the dry side, with the next chance for 
precip coming from a Clipper system by the end of the weekend.

&&

.AVIATION /06Z SUNDAY THROUGH THURSDAY/...
Forecaster Confidence Levels...

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

Short Term /through Monday/...

Today...High confidence. Lingering MVFR CIGS will improve to VFR
toward daybreak. Otherwise...despite a scattered to broken deck
of mid level cloudiness mainly expect VFR conditions today. A
sprinkle/flurry or two is possible, but the main story will be
the winds. W winds will increase this morning with gusts of 35
to 40 knots in most locations. 40-45 knot gust possible over 
the high terrain, as well as portions of the Cape/Islands.

Tonight...High confidence in VFR conditions. W winds gusts
diminish a bit, but still expect many locations to gust between
20 and 30 knots for at least the first half of the night.

Monday...High confidence. VFR conditions despite some mid level
cloudiness, especially south of the MA Turnpike. W winds of 5 to
15 knots.

KBOS Terminal...High confidence in TAF. 

KBDL Terminal...High confidence in TAF.

Outlook /Monday Night through Thursday/...

Monday Night: Mainly IFR, with areas MVFR possible. RA, PL,
chance SN, FZRA.

Tuesday: Mainly IFR, with areas MVFR possible. Breezy. RA
likely, FZRA, PL likely, chance SN.

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

Wednesday: Mainly VFR, with areas IFR possible. Windy with
local gusts up to 35 kt. Slight chance SHSN.

Wednesday Night: VFR. Strong winds with areas of gusts up to
45 kt. 

Thursday: VFR. Strong winds with local gusts up to 40 kt.

&&

.MARINE...
Short Term /through Monday/...

Gale warnings in place for all waters tonight into Sunday
evening. Expect increasing SW winds tonight with gusts 35-40 kt
after midnight, and 40-45 kt west gusts over open waters on 
Sunday. Winds slowly diminish during Sunday night. Generally
improving visibilities through sunrise Sunday.

Outlook /Monday Night through Thursday/...

Monday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Areas of rough seas. Chance of rain, slight chance of
snow. 

Monday Night: Winds less than 25 kt. Areas of seas approaching
5 ft. Rain, snow likely, freezing rain. Local visibility 1 to
3 nm.

Tuesday: Winds less than 25 kt. Areas of rough seas. Rain. 

Tuesday Night: Winds less than 25 kt. Rough seas up to 10 ft.
Slight chance of rain, slight chance of snow. Local visibility
1 to 3 nm.

Wednesday: Low risk for Small Craft Advisory winds with gusts
up to 30 kt. Areas of rough seas. Slight chance of snow
showers. Visibility 1 to 3 nm.

Wednesday Night: Moderate risk for gale force winds with gusts
up to 35 kt. Rough seas up to 11 ft. Slight chance of snow
showers. Local visibility 1 to 3 nm.

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

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...Wind Advisory until 9 PM EST this evening for MAZ023-024.
     Wind Advisory from 7 AM this morning to 7 PM EST this evening 
     for MAZ002>016-018-019-021-022-026.
RI...Wind Advisory until 9 PM EST this evening for RIZ008.
MARINE...Gale Warning until 1 AM EST Monday for ANZ231>234-251-255-256.
     Gale Warning until 10 PM EST this evening for ANZ230.
     Gale Warning from 7 AM this morning to 7 PM EST this evening 
     for ANZ236.
     Gale Warning until 11 PM EST this evening for ANZ235-237.
     Gale Warning until 4 AM EST Monday for ANZ250-254.

&&

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

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