Ice Storm in Washington, February 8 to 9, 2017

Area Forecast Discussion
National Weather Service Seattle WA
1130 AM PST Wed Feb 8 2017

.SYNOPSIS...A strong warm front will move through Western
Washington this afternoon and evening. Snow levels will rise and the
air mass will warm rapidly. A cold front and low pressure system will
move through the area Thursday and Thursday night, and the snow
level will fall Thursday night and Friday. An upper ridge of high
pressure will build into the Pacific Northwest for the weekend with
dry weather developing and persisting into next week.

&&

.SHORT TERM...Update...the webcam at the Sequim valley airport showed
the runway turn from black to white with the snow falling there
today, and the webcam at Fairchild airport near Port Angeles is also
wintry, so added a winter weather advisory for the areas from Sequim
to Sekiu for 1-3" of new snow til 3pm...when the snow should turn to
rain.

A strong warm front will move north through Western Washington
through this evening. Areas of wet snow will turn to rain
--there may be some accumulation in the spots that normally do that
in easterly flow, the hills around Hood Canal for instance, and on
the route from Port Angeles out to the coast. I re-wrote most of the
zone forecasts to mention some wet snow, and I also removed the snow
level forecasts which are too precise and nonsensical. The air mass
warms rapidly through this evening, for instance the 925mb temp
warms from around 0c this morning to 2 or 3c in the afternoon, with
much warmer air than that poised just off the south coast. By 9pm
tonight that 925mb temp should range from +6c at Bellingham to +10c
on the south coast and by late tonight all of Western Washington
will be around 9c. Surface temps will lag a little, especially up
where the Fraser outflow keeps Whatcom county cold for awhile, but
it is hard to imagine that by daybreak even that area wont be
melting fast.

Thursday through Friday will be wet and windy at times as the rest
of the frontal system and a vigorous low or trough move through the
area.

.LONG TERM...A period of dry weather is likely with an upper level
ridge building over the region over the weekend...and persisting
into next week.

&&

.AVIATION...A warm front will lift north through the area today
and tonight with west to southwest flow aloft. The air mass is moist
and stable.

Ceilings will gradually lower today as a warm front brings rain to
the area. MFVR and local IFR have already developed over the south
interior and these conditions will gradually spread north today.

Precipitation could initially start as a rain/snow mix in places but
should change to all rain pretty quickly. The exception is western
Whatcom county, especially away from the water and towards the
Canadian border where trapped cold air could lead to several hours
of snow and then freezing rain before things warm up tonight.

KSEA...VFR low clouds will prevail today. MVFR low clouds are
possible as rain increases. The 18Z TAF will probably include at
least a tempo for this. A rain/snow mix is likely for the next few
hours but accumulations are not expected. Temperatures will warm in
the afternoon which should end any snow threat. Northerly wind 4-8
knots will become south 5-10 knots tonight. Schneider

&&

.MARINE...A warm front will lift through the waters and bring
gales to the Coastal Waters later today and tonight. A trailing cold
front will likely bring another round of gales to the Coastal waters
later Thursday and perhaps into Thursday night. The current forecast
might be a little light with the winds for Thursday night.

Elsewhere, small craft advisory winds are likely all other waters
for the next couple of days. Strong onshore flow will follow the
cold front on Friday. High pressure will bring a period of lighter
winds Saturday and Sunday.

The combination of high astronomical tides, low pressure, and strong
winds could result in minor tidal overflow for the inland waters
tonight. A coastal flood advisory for minor tidal overflow will
probably be issued later today. Schneider

&&

.HYDROLOGY...A warm, wet Pacific frontal system will spread rain
across the area late tonight through Friday. At this point, looks
like around 3 inches of rain may fall in the 30 hours from Wednesday
morning through Thursday afternoon, with possibly of higher amounts
over the Olympics. Strong S-SW winds at 850 MB will over the
Olympics will result in a decently strong orographic enhancement to
the rainfall there. Rivers will rise and there may be some potential
for one or more of the more flood prone rivers to reach flood stage
Thursday or Friday. Melting snow in the lower elevations leading up
to the storm might also contribute to putting moisture into the
ground and thereby slightly increasing the runoff efficiency. Any
leftover low elevation snow will likely melt during the storm
potentially contributing a minor amount to the runoff.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Storm Warning from 2 PM this afternoon to 6 AM PST
Thursday for Cascades of Pierce and Lewis Counties-Cascades      of
Snohomish and King Counties-Cascades of Whatcom and      Skagit
Counties.

     Flood Watch from Thursday morning through Friday morning for
  Hood Canal Area-Lower Chehalis Valley Area-Olympics.

     Winter Storm Warning until 4 AM PST Thursday for Western
Whatcom      County.

PZ...Gale Warning until 4 PM PST Thursday 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 until 4 AM PST Thursday for Central U.S.
   Waters Strait Of Juan De Fuca-Northern Inland Waters
Including The San Juan Islands.

     Gale Warning until 4 AM PST Thursday for West Entrance U.S.
 Waters Strait Of Juan De Fuca.

&&

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.weather.gov/seattle/gafd/latest_webafd.html