Lexington Weather

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 101801
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
101 PM EST Tue Dec 10 2019

.SYNOPSIS...
Unseasonably mild temperatures will continue today. Another 
round of showers spreading in this afternoon. A strong cold 
front crosses the region this evening, followed by much colder 
temperatures. Showers will change to a period of accumulating 
snow later tonight into Wednesday morning, especially south of 
the Massachusetts Turnpike where a significant impact may occur 
during the morning commute. Snow will end by Wednesday 
afternoon. Large high pressure will bring cold and dry 
conditions Thursday and Friday. Developing low pressure across 
the southeast U.S. will shift up the coast with the potential 
for periods of heavy rain late Friday into Saturday, possibly 
beginning as a period of light snow across interior 
Massachusetts. Light rain may linger into early Sunday, then 
drier and cool weather returns late Sunday into Monday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
No major changes to the forecast this morning. Updated the
timing and sped up precipitation chances based on obs and near
term guidance. Precipitation likely this afternoon across much 
of the eastern half of the CWA. Increased surface winds and
associated gust a bit slower than the previous forecast to more
closely match obs. 

Previous Discussion...

Cloudy skies and mainly dry conditions in place early this
morning. Noting a few bands of showers lingering off the S
coast, which should pass across Nantucket and Martha's Vineyard
through mid to late morning. Another area of light precip 
across central NY may push across western Mass by mid morning or
so as it shifts E-NE, while one more well organized area across
central and SW PA is also moving toward the region. This area 
should reach western MA into N central CT by around midday and 
will linger into the afternoon as it moves into central and 
northeastern areas.

The day is starting off quite mild with temperatures in the
upper 40s across portions of the CT valley ranging to the mid to
upper 50s from near KBVY-KHFD southward to the Cape and islands.
With the mainly dry conditions through at least mid morning,
temps should rise a bit further before the next area of rain
arrives.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM WEDNESDAY/...
* Accumulating snow expected late tonight into Wed AM for much 
  of the region, but a sharp cutoff is possible in northern MA

* Significant impact to Wed AM Rush hour possible, especially 
  across parts of northern CT, Rhode Island and Southeast MA

Tonight and Wednesday...

Quite the challenging forecast tonight into Wednesday morning. 
This revolves around snow amounts and a potentially significant 
impact to the Wed morning commute. Greatest concern is across 
parts of CT, RI/SE MA with a possible sharp cutoff in 
appreciable snow in northern MA.

A cold front will cross the region this evening with 
temperatures quickly falling through the 40s and then into the 
30s. However, an approaching shortwave will keep our region in 
southwest flow aloft into Wed morning. Very strong jet dynamics 
are in place as southern New England will be in the right rear 
quad of a 180 knot jet streak. This leads to a narrow, but very 
strong area of mid level frontogenesis in the 600 to 700 mb 
layer. Snowgrowth also looks favorable especially across 
portions of CT/RI and SE MA, where NAM indicates 20+ units of 
omega in the snowgrowth region for a short time.

The problem is that the northern extent of the mid level 
frontogenesis and sufficiently deep moisture remains uncertain. 
It will also be short-lived and offset by low level drying with 
WNW winds just off the deck. This is what makes these anafrontal
waves so difficult to forecast even inside 12 to 24 hours. 
There often can be a narrow area of moderate to heavy snow and a
sharp cutoff to the north. 

Based on model consensus and overall pattern recognition, 
thinking a general 1 to 4 inches of snow across the region. 
Greatest risk for the higher amounts will be south of the Pike 
across portions of CT/RI and SE MA. If the dry air ends up being
a little stronger than expected, snow accumulations will be on 
the order of a coating to 2 inches with little if any 
accumulations north of the MA Turnpike. On the flip side of the 
coin, if forcing/deeper moisture ends up a bit further north may
see a swath of 4 to 6 inches of snow. 
  
After collaboration with surrounding offices, opted to go with 
a Winter Weather Advisory late tonight into Wed AM for northern 
CT, RI, as well as south central and southeast MA. The biggest 
concern is the strong mid level frontogenesis coinciding with 
the Wed Am rush hour. While amounts may not reach advisory 
criteria across the entire area, moderate to briefly heavy snow 
may impact the morning commute and that can often be quite 
problematic. Therefore, felt the Winter Weather Advisory was 
warranted. Fortunately, this system is progressive with the snow
pretty much over by afternoon. High temperatures will remain in
the 30s, which will be a big change after today/s anomalously 
mild temps.

&&

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

* Dry conditions and colder than normal temperatures Wednesday 
  night and Thursday

* Temperatures moderate as next weather system brings the 
  potential for heavy rain & localized flooding late Friday into
  Saturday

* Leftover rain/snow showers linger into early Sunday as a cold
  front approaches
  
Details...

Wednesday night through Thursday night... 

Large 1040 hPa high pressure will shift east across the Ohio 
valley and across the northeast during this timeframe. 
Temperatures will run around 10 degrees below normal as H85 
temps run from -9C to -15C from S to N into Thursday. Will see 
dry conditions thanks to the strong subsidence as the high 
passes.

It will feel even colder on Wed night as W-NW winds at 10-15 kt
with gusts up to 25 to 30 kt across the higher terrain and 
along the immediate coast. Wind chill values will range from the
single digits to mid teens Wed night, then in the teens to 
lower 20s during Thursday though the winds will diminish during 
the afternoon.

The center of the high will allow winds to become light and 
variable or calm winds by Thu night, though will become light 
E-SE along the coast as the high shifts E after midnight. Lows 
will mainly be in the teens, ranging to the 20s along the 
immediate coast. Clouds will start to increase across the 
interior after midnight. 

Friday through Saturday night...

***Heavy Rain and Flood Potential***

A developing mid level wave across the southern tier states 
will start to take shape across the SE U.S. Thu/Thu night, will 
see SE winds beginning to bring low level moisture into the 
region. Noting H5 long wave trough digging out of the midwest to
the SE states, which will set up a strong SW mid level wind 
flow allowing the deep layer moisture to work into the northeast
late Friday through Saturday.

With the cold air in place at the onset of the precip, will 
start off as light snow across most areas except along the 
immediate S coast where temps will be around or above freezing. 
As winds shift to S-SW, the milder air may lift aloft, so could 
see a thin band of mixed precipitation. However, the colder air 
will continue to shift SE overnight, so should see any mixed 
precip change over to snow. 

GEFS model ensemble signaling a deep moisture plume working up 
the eastern seaboard as well. This plume will shift NE across 
the region late Fri or Fri night, with PWATs up to 2-3 SD above 
normal (about 1.2 to 1.5 inches). However, the H5 steering 
pattern, while amplified, remains progressive so should see the 
precip to shift NE.

The heaviest rain is expected from late Friday night through 
midday Saturday, with current forecast totals from 1.2 to 1.6 
inches regionwide. However, these amounts may change depending 
upon the timing and track of the area of heavier rain may fall. 

These rainfall amounts may combine with any snow to cause 
clogged catch basins which may result in localized urban and 
poor drainage flooding. This heavy rain, along with the rainfall
from today's event, may cause an increase in river flooding 
generally during Saturday. Will continue to monitor river levels
for any rises.

Expect temps on Saturday to be mild, ranging from 45-50 across 
the higher terrain to the mid 50s across the coastal plain. 

Sunday and Monday...

Low pressure will shift NE into Maine and the Maritimes by 
Sunday morning, though light rain may linger especially near and
N of the Mass Pike. Another cold front may wrap around the 
departing low late Sunday or Sunday night. May see the rain mix 
with or change to snow, but looks like the best chance may occur
across the E slopes of the Berkshires and possibly the northern
Worcester hills during the afternoon before tapering off.

Another cold front will sweep across the region by Monday 
morning, bringing gusty W-NW winds and colder air across the 
region. Current forecast suggests highs will run close to 
seasonal levels.

&&

.AVIATION /18Z TUESDAY THROUGH SUNDAY/...
Forecaster Confidence Levels...

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

Short Term /through Wednesday/...

Through 00z...
Conditions lowering to MVFR/IFR in stratus with showers
developing through the afternoon. Areas of LLWS and G25 kt over
the Cape/Islands. 

Tonight and Wednesday...Moderate confidence.
Winds shift to the NW tonight behind a strong cold front. Cigs 
may temporarily lift to VFR in the evening across portions of
the interior. Showers will change to a period of snow 00-06z
interior and 06-10z along the coast. MVFR/IFR conditions, with 
a period of LIFR and brief moderate/heavy snow possible across 
RI and SE MA late tonight and Wed morning. Conditions quickly 
improve to VFR with clearing skies from NW to SE late morning
into the afternoon. 

KBOS Terminal...Moderate confidence in TAF. Rain changing to
snow around 06z and ending 13-14z. 1-2 inches possible with 
an impact to the morning push.

KBDL Terminal...Moderate confidence in TAF. Rain changing to
snow 04-06z ending 12-13z. 1-2 inches possible with 
an impact to the morning push.

Outlook /Wednesday Night through Saturday/...

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

Thursday Night: VFR. 

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

Friday Night: Mainly IFR, with areas MVFR possible. Breezy. RA.

Saturday: Mainly MVFR, with areas IFR possible. Breezy. RA
likely.

&&

.MARINE...
Short Term /through Wednesday/...

Today...High confidence. SW winds with gusts of 20 to 30 knots
for the rest of today. Long southwest fetch will continue to 
result in 7 to 13 foot seas across the outer waters. Small Craft
Advisory in effect for the Coastal Waters. Showers become more 
numerous again late today.

Tonight and Wednesday...High confidence. Winds shift to the NW 
this evening behind a strong cold front. NW 20 to 25 knot gusts 
expected into Wed. Showers tonight will change to snow for a 
time Wed morning before things wind down by mid afternoon. 
Reduced vsbys expected. SCA headlines will likely be needed for 
seas and marginal 25 knot wind gusts across the open waters.

Outlook /Wednesday Night through Saturday/...

Wednesday Night: Low risk for gale force winds with gusts up to
35 kt. Local rough seas. 

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

Thursday Night: Winds less than 25 kt. 

Friday: Winds less than 25 kt. Seas locally approaching 5 ft.
Chance of rain. Local visibility 1 to 3 nm.

Friday Night: Moderate risk for Small Craft Advisory winds with
gusts up to 25 kt. Areas of rough seas. Rain. 

Saturday: Moderate risk for Small Craft Advisory winds with
gusts up to 25 kt. Rough seas up to 13 ft. Rain likely.

&&

.CLIMATE...
Record highs for Today (12/10). Best chance to tie or break 
record would likely be at Boston. 

BOS ... 64/1907
PVD ... 67/1946
BDL ... 67/1946
ORH ... 63/1946

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Winter Weather Advisory from 10 PM this evening to noon EST 
     Wednesday for CTZ002>004.
MA...Winter Weather Advisory from 10 PM this evening to noon EST 
     Wednesday for MAZ012-013-015>022.
RI...Winter Weather Advisory from 10 PM this evening to noon EST 
     Wednesday for RIZ001>007.
MARINE...Small Craft Advisory until 7 PM EST this evening for 
     ANZ231>234-251.
     Small Craft Advisory until 5 PM EST this afternoon for ANZ230-
     236.
     Small Craft Advisory until 1 PM EST Wednesday for ANZ235-237-
     250-254>256.

&&

$$
SYNOPSIS...Frank/EVT
NEAR TERM...Frank/EVT
SHORT TERM...KJC/BL
LONG TERM...EVT
AVIATION...KJC/EVT
MARINE...Frank/EVT
CLIMATE...