Lexington Weather

Area Forecast Discussion
for Lexington, MA

        
000
FXUS61 KBOX 082054
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
354 PM EST Sun Dec 8 2019

.SYNOPSIS...
Strong high pressure centered over Georges Bank this evening shifts 
farther offshore tonight, which will start a return flow of 
increasingly milder and more moist air. Scattered showers late 
tonight will turn into a soaking rain Monday into Tuesday. Strong 
southwest winds are expected in southeast Massachusetts and all 
coastal waters late Monday afternoon and Monday night. A cold front 
moves through Tuesday night, ushering in much colder air that will 
likely change rain to snow and linger into Wednesday with 
accumulations possible. Cold and dry into Friday before 
temperatures begin to moderate Friday night. Next system brings 
a period of rains by the weekend, with temperatures trending 
above normal.

&&

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

345 PM update ...

Tonight ...

With ridge axis now offshore WAA pattern underway with positive 
thermal advection at 850 mb and 925 mb. Surface temps will fall with 
sunset given light winds, dry airmass and just high clouds across 
the region. However as WAA strengthens and clouds increase by late 
evening and especially after midnight temperatures will rise thru 
the 30s and into the 40s along the coast. Big difference from the 
past two nights. 

Eventually column saturates in the WAA pattern with risk of 
scattered showers 09z-12z. SSW winds gradually increase overnight as 
low level jet increases to 35-45 kt at 925 mb. 

&&

.SHORT TERM /6 AM MONDAY MORNING THROUGH MONDAY NIGHT/...

*** Periods of moderate to heavy rain ***

345 PM update...

Monday ...

Strong thermal and moisture advection ahead of approaching short 
wave with 925 mb winds increasing to 60-70 kt. This results in PWATs 
surging to about 1.25 inches by end of the day. This synoptic scale 
lift combined with anomalous moisture advection will yield periods 
of moderate to heavy rain. This strong low level WAA results in 
surface dew pts rising into the low 50s across eastern CT/RI and 
eastern MA. These high dew pts combined with plenty of SSW wind 
should yield appreciable snow melt and combined with moderate to 
heavy rain at times, expecting lots of standing water for the late 
day/evening commute. Thus hydroplaning could become a concern. These 
increasing dew pts streaming over the snowpack will also increase 
the probability of patchy dense fog. 

Strongest wind gusts up to 30 mph will be over southeast MA where 
dew pts climb into the low 50s and easy low level inversion. Inland 
with dew pts only the 40s should be enough of an inversion to keep 
winds 20 mph or less. 

As for thunder, not expecting much if any given lack of elevated 
instability/poor mid level lapse rates.

Monday night ...

*** Periods of Moderate to Heavy Rain Likely *** 

Periods of moderate to heavy rain continue thru the evening as 
robust mid level short wave moves across the area and accompanied by 
60-70 kts of SSW flow at 925 mb and advects PWATs up to 1.4 inches 
across RI and eastern MA. This strong low level WAA results in 
surface dew pts continuing to surge into the low and mid 50s. Thus 
more snowmelt combined with periods of moderate to heavy rain during 
the event will continue to yields lots of standing water on area 
roadways. 

SW winds could gusts up to 40 mph over southeast MA including Cape 
Cod and the Islands. This strong low level WAA pattern will result 
in temps remaining in the 50s overnight! These increasing dew pts 
streaming over the snowpack will also keep the risk of patchy dense 
fog. 

Short wave moves east of the region second half of the night with 
weak short ridging building across the area. Thus a downward trend 
in precip is likely after midnight. 

&&

.LONG TERM /TUESDAY THROUGH SUNDAY/...

345 PM update ...

Highlights... 

* Cloudy but mild Tues with another round of showers mainly late day

* Rain early Tues nite changes to a period of accumulating snow 
  (potentially plowable in interior and urban corridor) overnight 
  into Wed. Increasing confidence for adverse impact to Wed AM 
  commute. 
  
* Cold and dry Wed nite through Fri. Moderating temperature and 
  moisture levels Friday night.

* Next system for the weekend, with rainy conditions and above-
  normal temps. 
  
Details...

Tuesday: 

In the wake of expected rains Monday night, weather pattern remains 
unsettled for Southern New England. Most of the region will be the 
warm sector and a cloudy one at that. Will keep chances for showers 
going through the first part of the day, but tried to focus highest 
PoPs into the Likely range more tied with the cold front, which 
approaches our western counties late Tuesday. Cold frontal precip 
should prove much less than the rains for Monday/Monday night, with 
additional QPF of a quarter inch or less. Highs mid/upper 50s to 
near 60. 

Tuesday night into Wednesday: 

Colder air rushes southeastward Tuesday night, and as at least 
sheared-out mid-level vort max ripples northeastward from the 
central Appalachians, will be looking at rain changing to wet snow 
on the cold side of the cold front.

Did note more ECMWF Ensemble members indicating snow for Tuesday 
night into Wednesday than 24 hours ago, with ensemble probabilities 
for 3" or more of snow in the 24-hour period 18z Tue - 18z Wed now 
up to 50% across western MA/western Hartford County, the hills in 
central MA and Tolland County eastward into the northern Providence 
suburbs. Recent trends in the GFS have also trended snowier in this 
period. Global models show anomalously strong mid and upper level 
jets (~ 100 kts at 500 mb, 150 kts at 250 mb, with Southern New 
England positioned in the favorable equatorward entrance region of 
each jet), and there are hints in the guidance at some bands (axis 
of 850 mb -EPV, some though unfocused FGen @ 850) during the 
overnight hrs. 

What remains uncertain is the timing of the changeover, forecast 
liquid-equivalent precip and the northwestern extent of accumulating 
snows. Each of the above factors renders confidence in snow 
accumulations at low to moderate levels. Did note that most models 
have again increased QPF in this period, though there may be a sharp 
cutoff in precip N/W of the Mass Pike and especially north of 
northern MA/Route 2 corridor. WPC's Day-3 accumulation seems fairly 
reasonable as far as a first-cut at accumulations goes.

In spite of stated uncertainties, do think confidence is increasing 
in potential for an adversely impacted Wednesday morning commute for 
at least a part of the region, especially across the central part of 
the interior (Hartford/Springfield, Worcester/Providence into the I-
95 corridor). This is roughly where axis of greatest liquid-
equivalent QPF seems to lie, though some variability in models 
exists to be re-evaluated with forthcoming guidance. Pavement's also 
apt to be wet at onset and sharply colder temperatures filtering 
that could freeze this before or during the period of accumulating 
snow, the potential for slippery travel into the AM commute remains 
a possibility. 

Wednesday Night through Friday Night: 

1040+ mb high becomes anchored across much of New England. Quite 
chilly and dry through Thursday night with 850 mb temps in the -10 
to -15C range. Will show highs in the 20s to near freezing, with 
lows in the teens. A few single digit lows are possible across 
northern MA.

Southerly warm advection pattern commences Friday into Friday night 
as high pressure shifts into the Canadian Maritimes. Temperatures 
and moisture levels should begin to recover. Will see increased 
clouds into the nighttime hrs Friday night, and while the GFS brings 
in warm frontal precip late, likely too dry to support PoPs higher 
than low Chance. 

Saturday into Sunday: 

Weekend remains unsettled with the next chance of significant precip 
by the weekend, though models continue to deviate on timing. As 
mentioned, the GFS brings rain in as soon as Friday night, while the 
ECMWF is more into late Saturday into Saturday night, and the 
Canadian GEM is still slower than the ECMWF. Could be a decent rain 
maker for most of southern New England, with another round of gusty 
southerly winds across the waters. 

A return to above-normal temps looks likely for the weekend with 850 
mb temps rising into the mid/upper single digits Celsius. Will show 
highs in the upper 40s to mid 50s with clouds and rain likely 
accompanying the warmest temps, though potential for even warmer 
with cloud breaks.

&&

.AVIATION /21Z SUNDAY THROUGH FRIDAY/...
Forecaster Confidence Levels...

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

Short Term /through Monday Night/...High confidence.

1815z ...

Thru Midnight ... VFR, dry weather and increasing SSW winds.

After midnight ... VFR lowering to MVFR in scattered showers
toward morning. 

Monday ... MVFR with embedded IFR especially inland. Periods of
rain, moderate to heavy at times. LLWS 020/19045kt. Surface 
south at 20-25 kt, lighter winds elsewhere. Areas of fog will 
restrict vsbys in the MVFR/IFR category. 

Monday night ... A mix of MVFR/IFR in periods of rain, moderate
to heavy at times. Areas of fog too. Rain possibly tapering off
after 06z. LLWS 020/19050kt

KBOS Terminal...High confidence in TAF trends, lower confidence
on exact timing details.

KBDL Terminal...High confidence in TAF trends, lower confidence
on exact timing details.

Outlook /Tuesday through Friday/...

Tuesday: Mainly MVFR, with local VFR possible. Windy with local
gusts up to 30 kt. SHRA likely.

Tuesday Night: MVFR/IFR conditions possible. Breezy. SHRA
likely, SHSN likely, PL likely, FZRA likely.

Wednesday: Mainly VFR, with areas IFR possible. Breezy. Chance
SHSN.

Wednesday Night through Thursday: VFR. Windy with local gusts
up to 30 kt. 

Thursday Night through Friday: VFR. Breezy. 

&&

.MARINE...
Short Term /through Monday Night/...High confidence.

345 AM update:

** Gale Warning Continues late Monday/Monday Night **

Tonight ... 

1030+ mb high pressure over Georges Bank continues moving seaward 
overnight. Gradual increase in SSW winds. Dry weather and good vsby 
thru midnight but then risk of showers toward daybreak.

Monday ...

SSW winds increase to gale force late in the day along with building 
seas. Areas of dense fog develop possible in the afternoon along 
with milder conditions. Periods of moderate to heavy rain limit 
vsby. 

Monday night ...

SSW gales continue with periods of moderate to heavy rain during the 
evening along with areas of dense fog possible. Vsby may improve 
after midnight as rain moves offshore and SSW winds become more SW.

Outlook /Tuesday through Friday/...

Tuesday: Moderate risk for Small Craft Advisory winds with
gusts up to 30 kt. Rough seas up to 14 ft. Chance of rain
showers. 

Tuesday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Rough seas up to 10 ft. Rain showers likely.

Wednesday: Winds less than 25 kt. Local rough seas. Rain
showers likely, snow showers likely. Local visibility 1 to
3 nm.

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

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

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

Friday: Winds less than 25 kt. Local rough seas. 

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Gale Warning from 6 PM Monday to 6 AM EST Tuesday for 
     ANZ231>235-237-250-251-254>256.
     Small Craft Advisory from 4 PM Monday to 6 AM EST Tuesday for 
     ANZ230-236.

&&

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