Area Forecast Discussion
Issued by NWS Boston, MA

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527
FXUS61 KBOX 200320
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
1020 PM EST Sat Jan 19 2019

.SYNOPSIS...
A significant winter storm will impact the region tonight into
Sunday afternoon with heavy, wet snow, ice, and rain. Arctic
air follows this system into early next week. Another weather
system may bring rain and snow next Wednesday.

&&

.NEAR TERM /THROUGH SUNDAY/...

10 PM Update...

Light snow continues across the region with some locations
initially mixed with some freezing drizzle. As the stronger
forcing arrives any spots mixing with freezing drizzle should
switch over. Expecting snow to ramp up in intensity as the
stronger forcing moves north over the next few hours. A band of
strong 850 mb frontogenesis currently over Long Island is
producing very heavy snowfall at this hour. This together with
the associated WAA and speed convergence at the 925 mb level
moving north will bring a period of heavy snowfall on the front
end. Have adjusted surface temperatures toward the latest HRRR
which continues to capture the cold air at the surface.
Contining to be concerned with the threat of freezing rain in
the CT river valley into portions of interior MA Sunday morning
and this will need to be reevaluated with the latest data later
tonight.

Previous Discussion...

Not too many changes at this point. Not wanting to wipe away
immediately the 4 pm update. Evaluating the latest trends, the
2m temperatures via the HREF are too cold whereas the latest
HRRR and NBM are close to spot on. Thermal fields aloft continue
to differ as does the low-level jet. 18z guidance has shifted
ever so subtly but not in any one particular direction. At this
point it`ll be short-term / near-term updates going forward.

*/ Highlights...

 - Significant Winter Storm tonight into Sun afternoon

 - Heavy rain and possible urban/poor drainage flooding SE New
   England Sunday

 - Flash Freeze Eastern MA/RI Sunday mid/late afternoon: Very
   Hazardous Travel

*/ Overview...

12z guidance suite has continued the northward trend and warmer
scenario as we are dealing with a more amplified S stream short-
wave. GFS remains on the western envelope of solutions but even
the NAM/ECMWF has low tracking across SE New Eng on Sunday. We
are leaning toward NAM/ECMWF solution.

850/700 mb lows tracking mainly north and west of SNE which is
not a favorable track to keep deep cold air in place. Also,
persistent mesoscale banding will be to the north of the mid
level low track. Result will be a bit less snow due to rapid mid
level warming, although still looking at a decent front end
thump tonight. Highest risk for appreciable ice accum also
shifting north across interior MA.

This southern stream system will have abundant moisture with
PWATs up to 3SD above normal. Southerly low level wind
anomalies also around 3SD above normal which will bring a heavy
QPF event to SNE. The main challenges are snowfall amounts
tonight in the intense warm advection before ptype transition,
location/extent of icing in the interior, timing of flash freeze
eastern New Eng Sun afternoon, and flood potential for SE New
Eng.

1) Snow/Ice Accumulations and Uncertainty...

Steady snow develops this evening and should quickly ramp up
with a period of heavy snow as a band of strong 850 mb
frontogenesis lifts north across SNE. Snowfall rates of 1-2
inches/per hour are expected, especially in the interior.
However, mid level warm layer expected to advance north across
SNE after midnight likely reaching NH border by 12z Sun. This
will result in snow flipping to rain from the south coast
northward along the I95 corridor after 06z, with a changeover to
sleet and freezing rain in the interior where low level cold air
will persist. Much of the snow in northern MA will likely be
over by 12z Sun. Given that the warm layer aloft is more
pronounced and low level cold layer is more shallow, we are
expecting more freezing rain than sleet in the interior. The
main risk of freezing rain on Sunday will be across interior MA
west of I495 and possibly into the CT Valley in northern CT.

Our snowfall forecast has 1-3 inches along the south coast,
increasing to 3-6 inches across N CT and along the I95 corridor
from N RI to Boston and coastal Essex county. The max snowfall
will be north of the Mass Pike where generally 6-10 inches are
expected, highest amounts near the NH border, with a risk for a
foot of snow across NW MA. The exact timing of changeover will
be critical to accumulation forecast as heavy snow will be
falling during the transition time. Amounts could be off by 2 or
3 inches if the timing changes by a few hours.

Regarding ice accumulation, we shifted the axis of max
accumulation to the north, from NW Hartford county to interior
MA along and a bit north of the Pike. This is where locally
0.25" to less than 0.50" ice accretion is possible. This would
likely be in a narrow location but would increase power outage
threat which would be significant problem given bitter cold air
to follow Sun night.

Winter headlines are largely unchanged but upgraded Hartford
county to a warning for combination of 3-6" snow and up to
0.25" ice.

2) Flash Freeze across Eastern MA/RI: Very Hazardous Travel

We have high confidence that a flash freeze will occur but
lower confidence on timing. Given the further north trend in
the guidance, much milder air is expected across the coastal
plain with temps into the 40s with a chance of some 50s as well.
However, a strong coastal front in the interior with temps in
the 20s to the west will crash to the coast sometime in the
afternoon with temps falling sharply into and through the 20s.
Current timing is 3-6 pm from BOS-PVD to the south coast and a
bit later for Cape Cod but timing could be off by a few hours.
This is a big concern as any wet surfaces will rapidly freeze
resulting in very hazardous travel. In addition, rain may end as
a period of sleet or freezing rain here as deep cold air is
lagging the low level cold air by a few hours.

3) Heavy rainfall and Flood Potential

Given anomalous PWATs and low level jet with strong forcing
expect rainfall of 2-3 inches across RI and SE MA, with locally
1-2 inches possible in a 6 hour period 12-18z Sun. This will
result in areas of urban and poor drainage flooding so a flood
watch will be issued. Some river flooding is also possible in RI
if these rainfall amounts are realized.

4) Strong winds

Strong low level jet will be moving across the Cape/Islands on
Sunday. Thermal profiles are inverted which will limit gusts,
but could see a few gusts to 45 mph so wind advisories will be
issued for the outer Cape and ACK.

&&

.SHORT TERM /SUNDAY NIGHT/...
Sunday night-Monday...

Weekend storm departs through the Maritimes Sunday night. High
pressure over the Great Lakes and the storm in the Maritimes will
generate a strong pressure gradient over New England, maintaining
the cold advection and resulting mixing over our area. Strong north
winds will turn from the northwest overnight and continue to draw
arctic air into Southern New England.

Temperatures upstream in the Nrn Plains started the past couple of
days in the single numbers with dew points below zero. Expect Sunday
evening temps in the teens, but falling into the single numbers by
morning. A few spots in Western and Central Mass and Northern CT.
Mixing from 950 mb suggests Monday max sfc temps near 10F. We used 5-
15F inland and around 20F on the Cape and Islands.

&&

.LONG TERM /MONDAY THROUGH SATURDAY/...
Tuesday through Thursday...

High pressure moves over New England Tuesday. Winds diminish with
the high, which will generate radiational cooling Tuesday morning.
The clear skies and light wind will allow min temps in the single
numbers and below zero.

Low pressure that moves onshore on the Pacific coast this weekend
splits into two parts. One moves across Canada and the northern USA
will move across the country next week, bringing rain or snow to
Southern New England Wednesday.  The second part of the system dives
southeast to the Gulf of Mexico by Wednesday, then moving up the
East Coast Thursday or Friday maintaining a chance of rain or snow.

&&

.AVIATION /03Z SUNDAY THROUGH THURSDAY/...
Forecaster Confidence Levels...

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

Short Term /through Sunday Night/...High confidence on trends. Lower
confidence on exact timing of precip type transitions.

Tonight...
Widespread IFR / LIFR with -SN / SN developing, transitioning to
PL / FZRA towards the N and W with time, changing to RA along
the BOS-HFD line by 12z Sunday. With transition, may see improved
VSBYs, however CIGs will remain IFR / LIFR. E winds increasing,
gusts of around 35 kts towards morning with LLWS over SE New
England 2 kft agl 40 to 60 kts.

Sunday...
Continued IFR / LIFR conditions. PL / FZRA over N/W MA through
roughly 18z while RA / +RA for HFD-BVY and points S/E. VSBY
issues with either. LLWS for SE New England through 21-0z at the
latest, continued 2 kft agl 40 to 60 kts. E winds initially
through roughly 18z with gusts up to 40 kts across E coastal MA
terminals. Winds shifting W late, will see precip dissipate,
conditions quickly improve, CIGs lifting towards VFR. Concern
for SE New England with residual, standing water freezing on
runways.

Sunday night...
Blustery NW winds, gusts upwards of 35 kts, conditions continuing
to improve VFR. Residual water along runways over SE New England
likely freezing with Arctic air building in.

KBOS Terminal...
SN ongoing likely to continuing into early morning by then will
see a brief flip to a wintry mix during the morning push before
quickly transitioning to rain. Have to watch LLWS closely for
roughly around the 9-18z Sunday period. Blustery NW winds Sunday
night, could see flash freeze conditions on area runways.

KBDL Terminal...
SN through roughly after midnight, then we`ll see the transition
to sleet then to freezing rain. Freezing rain lingering through
much of Sunday potentially, could see some significant ice
accretion on runways. Current forecast amounts of around 0.2 to
0.4 inches. Clearing out late Sunday with winds turning W / NW
and blustery.

Outlook /Monday through Thursday/...

Martin Luther King Jr Day through Tuesday: VFR. Windy with
gusts to 35 kt.

Tuesday Night: VFR. Breezy. Chance SN.

Wednesday: Mainly VFR, with areas MVFR possible. Windy with
gusts to 30 kt. RA likely, chance SN, chance PL, FZRA likely.

Wednesday Night: Mainly VFR, with areas MVFR possible. Windy
with local gusts to 30 kt. RA likely, FZRA likely.

Thursday: Mainly VFR, with local IFR possible. Strong winds
with areas gusts to 40 kt. RA likely.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Sunday Night/...

E winds increasing tonight with gale force gusts developing
after midnight into Sunday. Winds shifting to S over south
coastal waters Sunday, then near gale force N/NW winds develop
Sunday night behind arctic front. Freezing spray developing late
Sunday into Sunday night, becoming moderate to heavy further out
in time.

Outlook /Monday through Thursday/...

Martin Luther King Jr Day: Strong winds with gusts up to 35 kt.
Rough seas up to 12 ft. Freezing spray, chance of snow showers.

Monday Night: Moderate risk for gale force winds with gusts up
to 35 kt. Rough seas up to 12 ft. Freezing spray, slight chance
of snow showers.

Tuesday: Low risk for gale force winds with gusts up to 35 kt.
Areas of rough seas. Freezing spray likely.

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

Wednesday: Low risk for gale force winds with gusts up to
35 kt. Areas of rough seas. Rain likely, chance of snow.

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

Thursday: Low risk for gale force winds with local gusts up to
45 kt. Rough seas up to 11 ft. Rain likely.

&&

.TIDES/COASTAL FLOODING...
An approaching deep low pressure will induce a strong easterly
low level jet.

By the time of high tide Sunday along the east coast (roughly
10 AM 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 MLLW. Given the
high astro tide, continued a Coastal Flood Warning for eastern
MA and an advisory of the south coast, Cape, and Islands.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Winter Weather Advisory until 7 PM EST Sunday for CTZ003-004.
     Winter Storm Warning until 7 PM EST Sunday for CTZ002.
MA...Flood Watch from 6 AM EST Sunday through Sunday afternoon for
     MAZ017>022.
     Winter Weather Advisory until 7 PM EST Sunday for MAZ013-
     015>021.
     Coastal Flood Advisory from 8 AM to noon EST Sunday for MAZ007-
     015-016-019-022>024.
     Winter Storm Warning until 7 PM EST Sunday for MAZ002>012-014-
     026.
     Coastal Flood Advisory from 5 AM to 10 AM EST Sunday for
     MAZ020-021.
     Wind Advisory from 5 AM to 3 PM EST Sunday for MAZ022-024.
RI...Flood Watch from 6 AM EST Sunday through Sunday afternoon for
     RIZ002>007.
     Winter Weather Advisory until 7 PM EST Sunday for RIZ001>007.
     Coastal Flood Advisory from 5 AM to 10 AM EST Sunday for
     RIZ004>007.
MARINE...Gale Warning until 7 AM EST Monday for ANZ231>235-237-250-251-
     254>256.
     Small Craft Advisory until 7 AM EST Monday for ANZ230.
     Small Craft Advisory until 7 AM EST Monday for ANZ236.

&&

$$
SYNOPSIS...KJC/Sipprell
NEAR TERM...Sipprell/BW
SHORT TERM...Sipprell
LONG TERM...Sipprell
AVIATION...KJC/Sipprell
MARINE...KJC/Sipprell
TIDES/COASTAL FLOODING...



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