Ice Storm Warning 1, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
1004 AM PST Fri Dec 29 2017

.SYNOPSIS...A surface low will cross Northwest Washington early
this evening, preceded today by heavy rain, as well as localized
freezing rain near Bellingham and possibly in the mountain passes.
The low will be followed this evening by windy conditions and
lowering snow levels. Lingering showers should end on Saturday as
higher pressure builds over the region. Dry weather is expected
from Sunday through New Years Day.

&&

.SHORT TERM...A 998-1000 mb surface low will track E-NE across the
northwest Olympic Peninsula to near Bellingham late this afternoon
and early this evening. The low is being preceded by lots of
large-scale lift, owing in part to strong warm advection lift and
jet dynamics. This is adding up to fairly high snow levels and
lots of rain. The 12z UIL sounding had a freezing level of 6900
feet. Meanwhile, the very cold air mass over the interior of B.C.
is currently supporting a Bellingham-Williams Lake gradient
of -15.1 mb. This gradient should strengthen a little more as the
surface low approaches Northwest Washington. This will certainly
keep the cold, shallow northeast wind going over Western Whatcom
County. Air temperatures north and northeast of Bellingham are
currently in the 30F-32F range, and this should not budge until
the northeast wind ends this evening. So have issued a new Winter
Weather Advisory for icing, especially north and northeast of
Bellingham.

Freezing rain is also possible in the passes as long as the
shallow, cold east wind continues in the passes. Current
temperature is 32F and 31F at Snoqualmie and Stevens Passes
respectively.

With heavy rain and high snow levels, Flood Watch is in effect to
cover the possibility of minor flooding on a few rivers. See Hydro
section below.

Once the surface low passes east of our longitude early this
evening, the weather will quickly change. First, modestly strong
southwest and west winds will develop along the coast, through the
Strait and up the Puget Sound lowlands. Wind Advisories are in
place to cover the expectation of 45 mph gusts early this evening.
Second, the wind shift will blow out the shallow cold air over
Western Whatcom County and in the mountain passes, ending the
threat of freezing rain. A cold front (relative to temperatures
aloft) will closely trail the low center, lowering snow levels to
2500 feet or a bit less. Post-frontal showers will continue
through tonight, especially where there is orographic lift or lift
near a Puget Sound Convergence Zone.

Upper ridging will quickly pop up off the coast on Saturday,
helping to stabilize the air mass and taper off the post-frontal
showers.

Low-level offshore flow will develop on Saturday evening, and the
air mass will continue to dry. A dry, quiet Sunday is expected,
but there should be plenty of stubborn fog patches on Sunday,
especially over Southwest Washington valleys. Haner

.LONG TERM...The upper level ridge will shift onshore Monday with
high pressure will gradually weaken Tuesday through Thursday.
This will allow a few weak systems to brush the area area
resulting in what looks to be a few periods of light
precipitation.

&&

.AVIATION...A moist weather system will bring rain to the area
today. A cold front will move through the region tonight. Flow aloft
will remain westerly. At the surface, a deep low will move northeast
across the north part of the area this evening. Light wind will
become strong southerly. Strong onshore flow will follow the front
tonight.

Ceilings are mostly IFR around 1000 ft in rain. In western Whatcom
County, winds are NE and a cold air mass is at the surface. Freezing
rain is possible there today. Winds will turn SW tonight and warm
the lower air mass, ending any freezing rain around KBLI. Some
improvement in ceilings, to MVFR, is likely late tonight as onshore
flow develops.

KSEA...Ceilings likely to stay around 1K ft today into this evening,
with some improvement after 03Z or so. Visibility will also be
restricted in rain today. Light north winds this morning will become
southerly this afternoon, then southwest 15g25kt this evening. CHB

&&

.MARINE...A deep low is forecast to move northeast across the north
Olympic peninsula and into the north Cascades this evening. Gales
are likely south of the low and the warnings are up. Strong westerly
flow is likely behind the low later tonight, and gale warnings are
in effect in the strait for that.

Winds will ease considerably Saturday and remain mostly below 20 kt
into next week. Northerly and offshore flow could give small craft
advisory strength winds at the west entrance and over the northern
inland waters at times. CHB

&&

.HYDROLOGY...Heavy rain will likely bring a few rivers to flood
stage later today and tonight. Models continue to focus the heaviest
rain over the Olympics and central and south Cascades. The rivers
most likely to reach flood stage are still the Newaukum, Snoqualmie,
White, and Skokomish. There is already a flood warning in effect for
the Newaukum River in Lewis County; it is likely to reach flood
stage around noon and is forecast to crest right at the moderate
flood stage late this afternoon. A number of other rivers in the
southern part of the state may get close to flood stage but no other
flooding is forecast at this time. River basins in Skagit and
Whatcom Counties will receive the least rain and rivers in those
counties are not likely to flood.

Rain will end tonight and a colder air mass will settle over the
region. No more heavy rain events are forecast for the next week or
so. After the rivers crest tonight, flooding is unlikely. Burke

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Watch from 4 PM PST this afternoon through Saturday
     afternoon for Admiralty Inlet Area-Bellevue and Vicinity-
     Cascades of Pierce and Lewis Counties-Cascades of Snohomish
     and King Counties-Central Coast-East Puget Sound Lowlands-
     Eastern Strait of Juan de Fuca-Everett and Vicinity-Hood
     Canal Area-Lower Chehalis Valley Area-North Coast-Olympics-
     Seattle and Vicinity-Southwest Interior-Tacoma Area-Western
     Strait of Juan De Fuca.

     Wind Advisory from 6 PM this evening to 4 AM PST Saturday for
     Admiralty Inlet Area-Bellevue and Vicinity-Bremerton and
     Vicinity-East Puget Sound Lowlands-Eastern Strait of Juan
     de Fuca-Everett and Vicinity-Hood Canal Area-San Juan
     County-Seattle and Vicinity-Southwest Interior-Tacoma Area.

     Winter Storm Warning until 10 PM PST this evening for Cascades
     of Snohomish and King Counties-Cascades of Whatcom and
     Skagit Counties.

     Winter Weather Advisory until 9 PM PST this evening for Western
     Whatcom County.

     Wind Advisory until 10 PM PST this evening for Central Coast-
     North Coast.

     Wind Advisory from 3 PM this afternoon to 4 AM PST Saturday for
     Western Strait of Juan De Fuca.

PZ...Small Craft Advisory for rough bar from 3 PM this afternoon to 6
     AM PST Saturday for Grays Harbor Bar.

     Gale Warning until 10 PM PST this evening for 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 10 AM PST Saturday for Coastal Waters
     From Cape Flattery To James Island 10 To 60 Nm-Coastal
     Waters From Cape Flattery To James Island Out 10 Nm-West
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Gale Warning from 7 PM this evening to 10 AM PST Saturday for
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 1 PM this afternoon to 10 AM PST
     Saturday for Admiralty Inlet-Northern Inland Waters
     Including The San Juan Islands.

     Small Craft Advisory until 7 PM PST this evening for Central
     U.S. Waters Strait Of Juan De Fuca.

     Gale Warning from 4 PM this afternoon to 4 AM PST Saturday for
     Puget Sound and Hood Canal.

&&

www.weather.gov/seattle