Heavy Snow in Washington, February 10 to 13, 2019

Area Forecast Discussion
National Weather Service Seattle WA
956 AM PST Sun Feb 10 2019

.UPDATE /time period/...
Active weather is expected today through Tuesday as several storm
systems affect the area. A few adjustments have been made this
morning to mainly the POP, QPF, and Snow Amount grids.

A glance at water vapor imagery this morning depicts a quick
shortwave ridge traversing the area with a trough of low pressure
extending to our south along the Pacific coast into the Great
Basin vicinity. To our north we can spot our next winter weather
maker-a closed low migrating southward and currently located
across the southern BC coastline.

For today...this upper low is expected to track SE along the
Washington/BC border with associated sfc low traversing Vancouver
Is before cutting across the center of the CWA. This track will
allow the system to pick up moisture from the Pacific where it
will then spread it across western Washington from west to east
today, mainly in the form of snow. This will be a quick moving
system, affecting the coast and Olympic Peninsula this afternoon
before spreading east into the Sound vicinity this evening. A few
additional inches seem reasonable for most lowland locations,
however, potential for a heavier band of snow is possible-perhaps
across Snohomish County. Keep in mind snow bands are never easy to
predict so it is never easy to determine exactly who will see
enhanced snowfall. Overall most locations should see 1-3 inches
of new snow by Monday morning, with some mesoscale banding
allowing for up to 4 inches or so. Winter Weather Advisory for
this system remains in effect.

The next, stronger, system will move in quickly Monday late
morning bringing yet another round of winter weather. This system
is much trickier and details will continue to be refined with
upcoming forecast updates. The low responsible for this next event
is still over the Aleutian Islands as of this AFD update. 24 hours
from now it will set up shop west of Vancouver Is. This system
will do a better job than the previous several in advecting in
some modified Pacific air. This looks to potentially make for a
headache in determining precip type. After parsing thru some of
the 12z guidance, any precip that spreads north into the CWA
Monday morning looks to be snow. Then as warmer air moves in, a
weak warm nose looks to develop just above the sfc as sfc temps
also try to increase. This could make for a several hour period of
a snow, sleet, freezing rain mix from Olympia to Seattle south
Monday afternoon and evening. The questions that remain to be
answered include: how deep the warm nose is, how high do sfc temps
try to rise, and will there even be a noticeable warm nose vs
just an isothermal layer around 0C? After this potential period
of mixed precip, areas from Seattle south look to transition to a
rain/snow mix or a cold rain overnight Monday. Could cool down
enough early Tuesday to transition back to snow (no mix expected).
For areas north of the mix potential, snow is likely with high
accumulations possible, especially if a convergence zone develops.
Preliminary estimates range from 6-8 inches at this time with
higher amounts likely in isolated areas. A Winter Storm Watch
still covers this winter storm and will likely be upgraded this
afternoon. Wind looks to be less of a concern for both of these
winter events, but will likely be breezy.

Kovacik

&&

.PREV DISCUSSION... /issued 343 AM PST Sun Feb 10 2019/

.SYNOPSIS...Snow will develop along the coast this afternoon and
move through the interior overnight as a developing low pressure
system off Vancouver Island moves southeast across the region. A
potentially stronger system will move into the area Monday
afternoon through Tuesday bringing the potential of heavy snow to
the northern half of Western Washington and a wintry mix over
southern portions of the area. Abnormally cool conditions will
continue Wednesday through next weekend and there will be a chance
of rain or snow at times as weather systems move across the area.

&&

.SHORT TERM /TODAY THROUGH TUESDAY/...Satellite imagery shows
clear skies across much of the area this morning and temperatures
have plummeted in locations where the winds have calmed. Several
locations have fallen into the teens and Olympia briefly dropped
to 9 degrees before some shallow fog developed there.

Models show low pressure developing to the lee of Vancouver Island
as another upper level trough drops southwestward from the
interior of British Columbia. Clouds and precipitation is expected
to rapidly develop midday over the coastal areas and on the
Olympic Peninsula then spread into the interior late this
afternoon or this evening. Models have come into agreement that
between 1 to 4 inches of snow can be expected throughout the
lowland zones while up 4 to 8 inches can be expected over the
Cascades from Snohomish County southward. So, winter weather
advisories are being issued for these areas.

There will be a brief few-hour break between systems during the
morning hours on Monday. Models then drop an upper level trough
now over southern Alaska ESE to near 50N 135W midday Monday.
Moisture wrapping around the low will spread northeast into the
area bringing another round of snowfall to the region Monday
afternoon and night. The precipitation becomes heavy during the
overnight hours as the trough entrains mild air and moisture from
east of the deep low that is NE of Hawaii. This creates
precipitation type problems for much of the area as temperatures
aloft warm to near freezing from about Seattle and Forks southward
and low level temperatures remain rather cold with cold air in
place and a rather deep snow cover.

With the exception to the Experimental FV3 solution that keeps
conditions cold and gives insane amounts of snow to much of the
southern 2/3 of the area, the models show about 5-8 inches of snow
in the interior north of Seattle and Shelton and change
precipitation south of there to a rain, or a freezing rain-snow-
sleet mix. Will opt for the mixture given the cold air in place
and the tendency for models to mix it out too quickly. While
snowfall amounts would be cut back to the south of Seattle in this
most likely scenario late Monday night into Tuesday, freezing
precipitation can pose huge impacts to roadways and the power
grid. The mountains are expected to see large amounts of snow with
this system. The winter storm watch will be changed to midday
Monday through Tuesday and will be segmented as appropriate to
differentiate the frozen precipitation from the mixed frozen and
freezing precipitation threats.

Temperatures in the forecast were warmed somewhat tonight due to
the precipitation and the cloud cover. Temperatures were warmed
for Monday night and Tuesday to account for the potential
intrusion of mild air aloft into the region. Albrecht

&&

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...While conditions will
generally be warmer than they have been for the past week or so,
temperatures are expected to remain well below normal for the next
couple of weeks as a strong Rex block remains along about 155W
longitude. Cold air will continue to filter in from the northeast,
but it is not expected to be as deep or severely cold as the upper
air pattern deamplifies somewhat and cold air to the northeast
gets depleted somewhat. Still, expect highs to generally be in the
30s to around 40 and lows to remain in the 20s to around 30.
Precipitation will generally be in the form of light snow or rain
at times with snow levels popping up to 1000-2000 feet at times
and falling to the surface at other times. Confidence in system
timing or strength is highly uncertain at this time, so only minor
changes were made to the Wednesday forecast to move it toward a
new model mean. Albrecht

&&

.AVIATION...Northwesterly flow aloft backing to west-northwesterly
later today as next cold upper trough over northern British Columbia
this morning shifts southward and deepens in the vicinity of YVR
this evening. VFR conditions will continue this morning will mid and
high level moisture increasing. MVFR ceilings with areas of light
snow with develop from OLM northward by 00Z and will lower to IFR at
times this evening with increasing snow. Some low level convergence
may lead to a period of moderate snow and visibilities below 1 SM
between KSEA and KPAE between 03Z and 09Z tonight. S-SE surface
winds will continue through early tonight then shift back to
northerly from about KSEA northward as a surface trough pushes
southeastward across the region between 08z-12z. Mainly dry from
09z-14z across the region before the next system pushes in.

KSEA...Increasing high and mid level clouds this morning. Snow
developing around 00Z with MVFR ceilings possibly deteriorating to
IFR for a period this evening. Expect 1 to 3 inches of new snow.
Southeast winds 5 to 10 knots to continue through today, increasing
some this evening from 10 to 15 knots as the system pushes through.
Winds will become northerly later tonight (between 09z-12z) as the
sfc trough pushes through. JD

&&

.MARINE...A pair of systems will affect the waters through
Tuesday. The first system will move through the waters tonight
with the second system arriving late Monday into Tuesday. Gusty
Fraser river will increase Monday with gales possible Monday night
into Tuesday. Gales are also possible over the coastal waters and
at the West Entrance to the Strait of Juan de Fuca Monday night.
Visibilities may be reduced in snow showers.

Offshore flow will continue behind the second system on Tuesday and
will continue until another system arrives late in the week. 33

&&

.HYDROLOGY...No river flooding is expected over the next 7 days.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory from 4 PM this afternoon to 4 AM PST
     Monday for Admiralty Inlet Area-Bellevue and Vicinity-
     Bremerton and Vicinity-Cascades of Pierce and Lewis
     Counties-Cascades of Snohomish and King Counties-East Puget
     Sound Lowlands-Everett and Vicinity-Hood Canal Area-San
     Juan County-Seattle and Vicinity-Southwest Interior-Tacoma
     Area-Western Skagit County-Western Whatcom County.

     Winter Storm Watch from Monday morning through Tuesday afternoon
     for Admiralty Inlet Area-Bellevue and Vicinity-Bremerton
     and Vicinity-Cascades of Pierce and Lewis Counties-Cascades
     of Snohomish and King Counties-Cascades of Whatcom and
     Skagit Counties-East Puget Sound Lowlands-Eastern Strait of
     Juan de Fuca-Everett and Vicinity-Hood Canal Area-Lower
     Chehalis Valley Area-North Coast-Olympics-San Juan County-
     Seattle and Vicinity-Southwest Interior-Tacoma Area-Western
     Skagit County-Western Strait of Juan De Fuca-Western
     Whatcom County.

     Winter Weather Advisory until midnight PST tonight for Central
     Coast-Eastern Strait of Juan de Fuca-Lower Chehalis Valley
     Area-North Coast-Western Strait of Juan De Fuca.

PZ...Gale Watch from Monday evening through late Monday night for
     Coastal Waters From Cape Flattery To James Island 10 To 60
     Nm-Coastal Waters From Cape Flattery To James Island Out 10
     Nm-Coastal Waters From James Island To Point Grenville 10
     To 60 Nm-Coastal Waters From James Island To Point
     Grenville Out 10 Nm-Coastal Waters From Point Grenville To
     Cape Shoalwater 10 To 60 Nm-Coastal Waters From Point
     Grenville To Cape Shoalwater Out 10 Nm-West Entrance U.S.
     Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 6 PM PST Monday for Coastal Waters
     From Cape Flattery To James Island 10 To 60 Nm-Coastal
     Waters From Cape Flattery To James Island Out 10 Nm-Coastal
     Waters From James Island To Point Grenville 10 To 60 Nm-
     Coastal Waters From James Island To Point Grenville Out 10
     Nm-Coastal Waters From Point Grenville To Cape Shoalwater
     10 To 60 Nm-Coastal Waters From Point Grenville To Cape
     Shoalwater Out 10 Nm.

     Small Craft Advisory from 6 PM this evening to 6 AM PST Monday
     for East Entrance U.S. Waters Strait Of Juan De Fuca.

     Gale Watch from Monday evening through Tuesday afternoon for
     Northern Inland Waters Including The San Juan Islands.

     Small Craft Advisory from midnight tonight to 6 PM PST Monday
     for Northern Inland Waters Including The San Juan Islands.

     Small Craft Advisory from 6 PM this evening to 6 PM PST Monday
     for Central U.S. Waters Strait Of Juan De Fuca-West
     Entrance U.S. Waters Strait Of Juan De Fuca.

&&

www.weather.gov/seattle