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Area Forecast Discussion (AFD)

Issued by Boston / Norton, MA (BOX)

FXUS61 KBOX 180548

Area Forecast Discussion
National Weather Service Boston/Norton MA
1248 AM EST Fri Jan 18 2019

Low pressure brings light wintry precipitation to southern New 
England late tonight into early Friday. High pressure brings a 
brief break in the precipitation Friday afternoon into 
Saturday. A major winter storm will impact the region Saturday 
night into Sunday, but exact precipitation types and impacts are
uncertain. Arctic air follows this system into early next week.
Another weather system may bring rain and snow next Wednesday.


10 pm update...

A deck of high clouds has moved into southern New England this
evening ahead of Friday morning's light snow. Regional radar 
shows the main swath of snow moving over northern New Jersey at 
this hour. Beginning to see some light returns over our area but
anything falling from these won't make it to the ground as 
model soundings indicate exceedingly dry lower levels that will 
take several hours to saturate. Dewpoint depressions across the
regions range from 10 to 20 degrees. Given the lack of 
significant lift and antecedent dry atmosphere, still expecting 
light snow amounts of a dusting to a half inch.

7 pm update...

Dry weather persists this evening...but weak warm advection will
result in an increase in cloudiness overnight. Still expect some
scattered light snow in some locales toward daybreak through
about 12 or 13z Friday...but a lot of the guidance is rather 
meager with QPF. Quite dry in the low levels too, so generally a
dusting to one half inch in most locations.

Lows tonight should be mainly in the teens, except some lows 
20-25 along the immediate shoreline and Cape/Islands. 

Pressure gradient is not very tight over our area during this event. 
Surface winds light from the SE tonight becoming S/SW 5-10 mph 
Friday. Should have some decrease in cloud cover as Friday afternoon
wears on. High temps on Friday mainly in the 30s across the 
interior, upper 30s to low 40s along eastern MA and most of RI.


During Friday night surface high pressure ridging builds into our 
area. Expecting clearing skies, though we may have some cirrus
later in the night far ahead of the low pressure to affect our 
area. Winds from the W then NW bring some CAA and allow 
dewpoints to lower overnight. Lows in the teens and 20s 
forecast, coldest in the far interior. Some lows in the single 
digits are not out of the question if skies remain clear thru 
the night.


* Major Winter Storm Sat night/Sun but uncertainty in regards to 
  precipitation types and exact impacts

High pressure over eastern Canada will keep the weather dry across 
most of the region through mid to late Saturday afternoon...but it 
will be cold.  A major winter storm will impact the region Saturday 
evening through Sunday afternoon, but specific precipitation 
types/impacts remain uncertain at this time.  We will break things 
down a bit more below.

1) Snow Amounts and Uncertainty:

Snow will overspread just about the entire region by early Saturday 
evening.  A strong east to southeasterly low level jet of 50+ knots 
will result in strong a period of 1 to 2 inch per hour 
snowfall rates are possible.  However, model guidance indicates that 
this low level jet will advect a warm nose roughly in the 800 to 900 
mb layer.  In fact, the latest EC/NAM/GFS all have it near route 2 
if not further north by 12z Sunday. 

It is important to keep in mind that we are still 60 to 84 hours out 
in the model world from this event, so changes will still likely 
occur.  However, based on the warm nose we expect a change to 
sleet/freezing rain up to route 2 and perhaps further.  The greatest 
risk for freezing rain/ice accretion will be south of the MA 
Turnpike and perhaps portions of our CT zones into RI/SE MA.  The 
amount of shallow cold air to our north and the track of the surface 
low will keep temperatures sub-freezing anywhere northwest of the 
Boston to Providence corridor.

As for snow amounts, given our time range they remain uncertain. Our 
current estimates are 3 to 6 inches south of the Pike, 4 to 8 inches 
near the Pike, and 8 to 16 inches across northern MA with the 
highest of those amounts most likely near the NH border. 
Again...these amounts certainly may shift north of south depending 
on the track but this is just a starting point.

2) Sleet and Freezing Rain Threat:

As we mentioned in the prior paragraph, the amount of warm air aloft 
overspreading the region should allow for snow to change to sleet. 
In fact, significant amounts of sleet are possible given the 
depth/strength of the low level cold air near and north of the MA 
Turnpike.  While we can not rule out some freezing rain north of the 
the Pike too, feel the greater risk is across portions of our CT 
zones and particularly into RI/SE MA.  This is where the warm nose 
is lower and the depth of the sub-freezing air is more shallow.  It 
is a very difficult forecast and tough to pinpoint this far out.  If 
the coastal front allows temps to climb above freezing, would expect 
some light ice accretion as temps will certainly drop quickly on 
Sunday.  However, if those areas stay below freezing ice accretion 
of one quarter to one half inch would be possible across portions of 
this region.  Whether or not this occurs remains to be seen, but 
will need to be watched for the risk of scattered power outages if 
higher ice accretion amounts are realized.

3) Transition Back To Snow/Flash Freeze Potential:

As the low pressure system tracks near or south of the 
Islands...mixed precipitation will transition back to snow later 
Sunday morning/afternoon across most of the region.  While some 
minor additional snow accumulations are possible, the bigger story 
will be the rapid falling temperatures on the coastal plain. High 
resolution models indicate that much of the region may fall into the 
teens by Sunday afternoon.  Areas near and especially southeast of 
the Boston to Providence corridor, may experience a Flash Freeze 
where temperatures briefly reach or exceed 32 and then rapidly 
plunge.  This will have to be monitored closely.

4) Main Impacts/Concerns:

A major winter storm is on tap for the region Saturday evening into 
Sunday.  Significant snow and sleet accumulations are expected for 
much of the region.  While the heavier snow/sleet amounts are 
expected north of the MA turnpike, the greater impact potential 
might be south of of the Pike. This would be if higher amounts of 
freezing rain occur across parts of CT and especially RI/SE MA, 
resulting in the risk of scattered power outages.  In 
addition...areas near and southeast of the Boston to Providence 
corridor may experience a flash freeze with rapidly falling 

Sunday night-Monday...

Weekend storm departs through the Maritimes Sunday night, but 
continues to draw colder air south into Southern New England. High 
pressure over the Great Lakes and the storm in the Maritimes will 
mean a strong pressure gradient over New England, maintaining the 
cold advection and resulting mixing over our area.

Temperatures upstream in the Nrn Plains early this morning were in 
the single numbers with dew points below zero. Expect Sunday evening 
temps to continue falling through the night, reaching those levels 
by morning. Mixing from 950 mb suggests Monday max sfc temps near 
10F. We used 5-15F inland and around 20F on the Cape and Islands.

Tuesday through Thursday...

High pressure moves over New England Tuesday. Radiational cooling 
Tuesday morning will allow min temps in the single numbers and below 
zero. The high pressure will continue dry weather through Tuesday.

Low pressure that moves on shore on the Pacific coast this weekend 
will move across the country next week, bringing rain or snow to 
Southern New England Wednesday and possibly Thursday.


Forecaster Confidence Levels...

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

Short Term /through Saturday/...High confidence.

Today...VFR conditions at all sites will begin to see localized
IFR/MVFR conditions as scattered light snow showers currently 
entering western MA slide east....mainly impacting TAF sites 
between 8z and 12z. Lingering over the south coast so ACK and 
perhaps FMH and HYA will see these lingering into late morning. 
Some runways may receive a coating of snow. SN may briefly
change to -RA before ending in RI/E MA. Mainly MVFR CIGS with
IFR more likely over RI and southeast MA. 

Tonight...VFR. Could become breezy along east coastal 
MA/Cape/Islands w/NW gusts 15-20 kts. 

Saturday: Mainly VFR, with local MVFR possible.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF. 

Outlook /Saturday Night through Tuesday/... 

Saturday Night: Mainly IFR,. Strong winds with gusts to 40 kt. 

Sunday: Mainly IFR, with local MVFR possible. Strong winds with
areas gusts to 40 kt. FZRA, PL, chance SN.

Sunday Night: Mainly VFR, with local IFR possible. Strong winds
with local gusts to 40 kt. Chance FZRA, slight chance SN.

Martin Luther King Jr Day: Mainly VFR, with local MVFR
possible. Windy with gusts to 35 kt. 

Monday Night: VFR. Windy with gusts to 35 kt. 

Tuesday: VFR. Breezy.


Forecaster Confidence Levels...

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

Short Term /through Saturday/... 

Tonight and Friday...An approaching weather system will spread 
light snow and rain over the waters after midnight tonight, with
reduced vsbys late tonight and Friday morning. SE winds around 
10 kts tonight becoming SW 10-15 kts during Friday. Seas below 5

Friday night...Winds become NW and increase, with gusts to 25
kts possible. Seas building to 4 to 5 feet on the eastern outer
coastal waters.  

Outlook /Saturday Night through Tuesday/...

Saturday: Winds less than 25 kt. Areas of seas approaching
5 ft. Slight chance of snow. 

Saturday Night: Moderate risk for gale force winds with gusts
up to 40 kt. Rough seas up to 12 ft. Rain and snow. Areas of 
visibility 1 nm or less.

Sunday: Moderate risk for gale force winds with gusts up to
40 kt. Rough seas up to 16 ft. Rain with a chance of sleet. 
Visibility 1 to 3 nm.

Sunday Night: Moderate risk for gale force winds with gusts up
to 40 kt. Rough seas up to 14 ft. Chance of snow, chance of
freezing spray. Visibility 1 to 3 nm.

Martin Luther King Jr Day: Moderate risk for gale force winds
with gusts up to 40 kt. Rough seas up to 12 ft. Chance of
freezing spray, chance of snow. Visibility 1 to 3 nm.

Monday Night: Low risk for gale force winds with gusts up to
35 kt. Rough seas up to 11 ft. Chance of freezing spray, slight
chance of snow. Local visibility 1 to 3 nm.

Tuesday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Local rough seas.


Approaching deep low pres has track has been trending on a somewhat 
south coastal solution with the last few model runs.  The 
consequences are twofold: 1. Extended period of long fetch onshore 
flow for the E coast and 2. Low lvl jet energy is oriented with a 
stronger E component, compounding the surge.  

By the time of high tide Sun for the east coast (roughly 10AM 
local), surge should be approaching 2.0 ft, which combined with 
offshore wave action around 15 ft should lead to localized minor to 
moderate E coastal flooding given a high astronomical base tide in 
Boston around 11.6 ft. There are still slight low pres track errors, 
so these will be better resolved, but confidence is high enough for 
a Coastal Flood Watch at this time, with a particularly focus from 
Scituate to the New Hampshire border.  

For the south coast, surge will not be as high given the wind 
trajectory, likely between 1.0-1.5 ft. While this may lead to 
some pockets of minor coastal flooding around the 7AM Sunday 
tide, confidence is not as high. So will hold off on headlines 
for the time being.

Will also need to monitor Mon AM high tide (11.9 ft in Boston 
Harbor).  As continued wave action and lingering swells may lead to 
pockets of minor flooding along the E coast as well, even as winds 
shift to a stronger westerly component.


CT...Winter Storm Watch from Saturday afternoon through Sunday 
     evening for CTZ002>004.
MA...Coastal Flood Watch Sunday morning for MAZ007-015-016-019-022-
     Winter Storm Watch from Saturday afternoon through Sunday 
     evening for MAZ002>021-026.
RI...Winter Storm Watch from Saturday afternoon through Sunday 
     evening for RIZ001>007.
MARINE...Gale Watch from Saturday evening through Monday morning for