Area Forecast Discussion
National Weather Service Boston/Norton MA
707 AM EST Mon Dec 16 2019

.SYNOPSIS...
A weak ridge of high pressure brings dry weather today into 
this evening. Low pressure passing south of our region will 
bring snow and ice to the region overnight into Tuesday evening.
A major Arctic cold blast will move in Wednesday night, 
accompanied by strong wind gusts, scattered snow squalls and 
very low wind chill values. Dry and cold conditions continue 
Thursday and Friday. A weak weather system may push across 
Saturday afternoon into Sunday, which may bring a low chance  
for rain or snow showers.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...

650 AM Update...

High pressure ridge from NY state to central NJ will cross the
region today. However, already noting a thin veil of high clouds
across southern New England as seen on latest GOES-East Night 
Fog and Nighttime Microphysics RGB satellite imagery.

Noting generally light W-NW winds early this morning, the 11Z
observation from KORH reported W winds gusting to 23 kt and KORE
had a NW wind gust to 20 kt, though these gusts have been
diminishing over the last few hours.

With generally westerly winds in place aloft thanks to a nearly
zonal flow aloft, will see lower clouds move east through this
morning but should remain dry as high pressure ridge moves out 
of NY state through midday. 

Noting some spotty light radar echoes across central NY into N 
central PA, but the latest observations remain dry except for 
some light snow at KUNV at 12Z. Steadiest precip noting across 
most of MD into DE and S NJ with temps in the low to mid 30s. 

Have brought near term conditions current and incorporated into
this morning's forecast.

Previous Discussion...

A brief ridge of high pressure builds over the region today. 
The result will be quiet weather with much less wind than 
yesterday. We will see an increase in mid/high level cloudiness 
especially south of the MA Turnpike in advance of low pressure. 
Highs will be in the 30s, but with much less wind compared to 
yesterday certainly not a bad day.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH TUESDAY NIGHT/...
Highlights...

* Accumulating snow/ice significantly impacts the Tue AM 
  commute near and especially south of the MA Turnpike

* 2-5" of snow seem reasonable with the higher most likely near 
  the MA Turnpike. Light icing possible south of Pike.

* Ptype likely changes to rain southeast of the BOS-PVD 
  corridor Tue afternoon, but should change back to light 
  ice/snow Tue night
 
Details...

Tonight Through Tuesday night...

Dry weather should generally prevail through the evening, but 
clouds will be lowering and thickening ahead of low pressure 
approaching from the southwest. We have high confidence that 
snow/ice will impact the Tuesday AM commute near and especially 
south of the MA Turnpike. Across northern MA, it is more 
uncertain, but they may escape with just light snow 
overspreading the region between 5 and 8 am. 

Low pressure approaching from the southwest will increase the 
forcing for ascent. Snow should overspread most of the region 
from south to north between 1 and 7 am Tue. Across northern MA, 
they might be able to escape with just some light snow/flurries 
during most of the Tue am commute before intensity increases. 
Areas near and south of the MA Turnpike will likely see a 
significant impact to the AM commute. A modest southerly LLJ 
develops and results in a band of strong frontogenesis with good
snowgrowth. Moderate to perhaps briefly heavy snow may be 
falling at 1"+ per hour toward daybreak. However, warming mid 
levels should result in the duration of that short-lived across 
much of CT/RI and SE MA. Nonetheless, may see a quick burst of 2
to 4 inches of snow and then a change to sleet/freezing rain as
warmer air aloft moves into the region. Mainly light icing is 
possible generally south of the MA Turnpike on top of the snow. 
We may have to watch for a narrow swath of one tenth to two 
tenths of inch in a narrow corridor of CT into northwest RI, if 
the warmer guidance aloft verifies but probably does not result 
in any significant issues. Ptype should change to rain for a 
time during the afternoon southeast of a Boston to Providence 
corridor.

Still uncertainty regarding axis of heaviest QPF along with 
specific snow/ice amounts. Overall, we are thinking that this is
a 2 to 5" snow event with the higher amounts more likely near 
the MA Pike. This is where we are more confident in the heavier 
QPF along with thermal profiles remaining cold enough to support
snow for an extended period of time. Areas near the NH border, 
there is more uncertainty in whether the heavier QPF makes it 
that far north. 

We do expect steadier snow to overspread northern MA Tuesday 
morning. However, they may not get the burst of strong forcing 
that area further south can expect. The frontogenesis does 
weaken a bit as it moves north. This is a result of strong 
northern stream energy that cuts off the better forcing/lift as 
the surface low is forced east. We still think that light to 
moderate snow will impact this region through Tuesday afternoon 
as a result of modest lift/saturation. In fact, light snow may 
linger well into Tuesday evening, as a weak inverted trough may 
keep things going. In fact, while areas southeast of the Boston 
to Providence corridor probably change to rain for a time colder
air will work back in behind the low Tue night. This should 
allow ptype change back to ice and light snow, so something will
have to keep an eye on with later forecasts.

So in a nutshell, our best estimate is for 2 to 5 inches of 
snow with the higher amounts in the vicinity of the MA Turnpike.
The biggest concern is the Tuesday AM commute, near and 
especially south of the MA Turnpike where snow may be falling 
moderate to heavy before changing to ice and rain along the 
immediate coast. Further north, the snow arrives later and 
probably with less intensity. However, it will continue through 
the afternoon and probably into the evening. With that said, 
Winter Weather Advisories have been posted for the entire 
region, except for the Cape/Islands.

&&

.LONG TERM /WEDNESDAY THROUGH SUNDAY/...
Highlights... 

* An upper level disturbance brings scattered snow showers 
  across, with a few snow squalls possible from late Wednesday 
  morning into Wednesday evening

* A major blast of Arctic air arrives Wednesday night...Expect 
  strong, gusty winds and Wind Chill Indices from -10 to -20 
  degrees across central and western areas early Thursday 
  morning

* Large high pressure slowly crosses the northeast Thursday and
  Friday...Gusty winds linger through midday Thursday

* A weak frontal system may approach around midday Saturday, 
  bringing a low chance for snow showers into Sunday
  
Details...

Wednesday...

May see some leftover rain or snow showers across the outer 
Cape and Nantucket early Wednesday morning, otherwise will see a
mix of clouds and sunshine by mid morning across the region. 
West to northwest winds at 15-20 mph with some higher gusts 
along the coast and across the higher inland terrain will move 
in late in the day. 

Expect a cool day with highs ranging through the 30s, coldest 
across the higher terrain. 

Clouds will increase across most areas this afternoon, with 
scattered snow showers and possibly some snow squalls pushing 
into western Mass and N central CT and also developing across 
Cape Cod and the islands by late in the day. 

Wednesday night... 

A cutoff H5 low pressure will swing SE from northern NY state 
into northern New England Wednesday night. Its surface 
reflection and arctic cold front will swing across the region, 
with a chance for a band of snow squalls moving through around 
03Z or so, though could linger across the Cape and islands 
through most of the night.

The big change will be increasing W-NW winds moving in from 
late evening through the overnight hours. Gusts will increase 
to 30-40 mph with the higher winds across the higher terrain 
and along the immediate coast, and possibly reaching 45 to 
possibly 50 mph across outer Cape Cod and Nantucket. 

Temps will sharply drop as the arctic air moves in, with 
readings bottoming out in the single digits to around 10 
degrees across the interior, ranging to 15 to around 20 along 
the immediate coast. With the very strong winds in place, wind 
chill indices will plummet to zero to as low as around -15 
across portions central and western Massachusetts where Wind 
Chill Advisories will likely be needed. 

Thursday through Friday...

Large high pressure will build slowly across the Appalachians 
during Thu into Thu night, which will keep dry conditions in 
place. The stronger NW winds will linger through at least 
midday Thursday, then should start to slowly diminish from W-E 
late Thu into Thu evening. Will still see gusts up to 25-30 kt 
across Cape Cod and the islands through the day. However, air 
temps will only reach the mid teens to mid 20s, coldest across 
the higher inland terrain. These readings will run 15 to 20 
degrees below seasonal levels! Bundle up!

The pressure gradient will start to relax as the high moves 
slowly E Thu night, the mainly clear skies will allow for 
radiational cooling. So, low temps early Fri morning will be 
close to the day before (from the single digits inland to the 
teens across the coastal plain), but would not be surprised to 
see some spots across N central/NW Mass fall a bit lower. As 
the wind continues to stir, will see wind chills from -5 to -10
well inland, ranging through the single digits around 5 above 
along the immediate coast.

On Friday, as the large high pressure area slowly moves across 
New England, winds will become light NW through the day. With 
mostly clear skies in place, temps will recover to the mid 20s 
to lower 30s, which will run around 5 degrees below seasonal 
levels. 

Saturday and Sunday... 

The large high will slowly move offshore Saturday. There is 
quite significant model solution spread with how a potential 
frontal system approaches sometime this weekend. Have carried 
low chance POPs for snow showers across most areas, though 
temps may linger around freezing along the immediate coast 
where rain showers may fall late Sat/Sat night, then should 
improve during Sunday. However, continued low confidence with 
this portion of the forecast. 

Temperatures will remain on the cold side on Saturday, but 
could recover to near normal levels on Sunday.

&&

.AVIATION /12Z MONDAY THROUGH FRIDAY/...
Forecaster Confidence Levels...

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

Short Term /through Tuesday Night/...

Today...High confidence.  VFR.  W winds of 5 to 15 knots.

Tonight...Moderate confidence. VFR this evening. Conditions
lower to IFR with perhaps brief LIFR conditions developing 
south of the Pike between 7z and 11z in a burst of moderate to 
briefly heavy snow. North of the Pike, light snow may arrive 
between 10z and 12z resulting in IFR conditions. 

Tuesday...Moderate confidence. Moderate to briefly heavy snow 
south of the Pike will be transitioning to sleet/freezing rain 
Tue morning and probably to rain for a time along the immediate 
coast. Steadier snow overspreads areas north of the Pike Tue am 
and continues into the afternoon. Total snow accumulations of 2 
to 5 inches likely with the higher amounts near the MA Turnpike.
IFR to brief LIFR conditions are expected Tue.

Tuesday night...Moderate confidence.  Mainly IFR conditions 
dominate during the evening probably in light snow, except 
mainly light rain towards the Cape Cod Canal point south and 
east. We may see improvement from west to east after midnight.

KBOS Terminal...Moderate confidence in TAF. The steady snow 
should be arriving towards 12z Tuesday.

KBDL Terminal...Moderate confidence in TAF. The Tuesday morning
push looks messy with snow and ice.

Outlook /Wednesday through Friday/...

Wednesday: Mainly VFR, with local MVFR possible. Windy with
local gusts up to 30 kt. Slight chance SHSN.

Wednesday Night through Thursday: VFR. Strong winds with gusts
up to 40 kt. 

Thursday Night: VFR. Windy with gusts up to 35 kt. 

Friday: VFR. Breezy. 

Friday Night: VFR.

&&

.MARINE...
Short Term /through Tuesday Night/...

Today...High confidence. Lingering SCA wind gust/seas this 
morning gradually diminish from west to east through the day as 
high pressure builds in from the west.

Tonight...High confidence. Winds/seas generally remain below 
SCA thresholds. However, approaching low pressure system may 
allow for conditions to approach SCA levels near daybreak in our
souther waters.

Tuesday and Tuesday night...Moderate to high confidence. Low 
pressure passing south of the waters will result in 20 to 30 
knot gust wind gusts Tue into Tue night. First form the NE 
during the day Tue shifting to the NW as the low pulls away. 
Seas build back to between 6 and 9 feet across our outer waters
Tue evening. Rain, sleet and snow will reduce vsbys for 
mariners Tue into Tue night.

Outlook /Wednesday through Friday/...

Wednesday: Low risk for Small Craft Advisory winds with gusts
up to 30 kt. Seas up to 5 ft. Slight chance of snow showers. 

Wednesday Night: Moderate risk for gale force winds with gusts
up to 40 kt. Rough seas up to 13 ft. Slight chance of snow
showers. Visibility 1 to 3 nm.

Thursday: Moderate risk for gale force winds with gusts up to
40 kt. Rough seas up to 13 ft. 

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

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

Friday Night: Winds less than 25 kt. Seas locally approaching
5 ft.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Winter Weather Advisory from 1 AM to 7 PM EST Tuesday for 
     CTZ002>004.
MA...Winter Weather Advisory from 1 AM to 1 PM EST Tuesday for 
     MAZ007-013>021.
     Winter Weather Advisory from 1 AM to 7 PM EST Tuesday for 
     MAZ002>006-008>012-026.
RI...Winter Weather Advisory from 1 AM to 1 PM EST Tuesday for 
     RIZ002>007.
     Winter Weather Advisory from 1 AM to 7 PM EST Tuesday for 
     RIZ001.
MARINE...Small Craft Advisory until 2 PM EST this afternoon for ANZ231-
     232-251.
     Small Craft Advisory until 11 AM EST this morning for 
     ANZ233>235-237.
     Small Craft Advisory until 7 PM EST this evening for ANZ250-
     254>256.

&&