Flood Watch 16, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
945 AM PST Thu Dec 28 2017

.SYNOPSIS...A frontal system will bring rain and a mix of snow,
rain, and freezing rain to the mountains through tonight. The next
system on Friday will bring significant precipitation to the
Cascades, falling as heavy freezing rain in some passes and a mix
elsewhere in the Cascades. Drier weather will return to the region
this weekend and possibly continue through the middle of next week.

&&

.SHORT TERM...Forecast updates are needed to keep up with
changing wintry conditions in the Cascades and Wrn Whatcom county.
Expect adjustments to the current warnings/advisories (Cascades and
Wrn Whatcom). Revised statements and updated forecast information
should be completed by Noon. The watch/warnings/advisories at the
bottom will likely change by that time. Check back for updates.

The main short term focus is on the freezing rain occuring across
Wrn Whatcom county. Some areas have warmed above freezing close to
the water and just south of Bellingham. Temperatures are below
freezing over the majority of the county just inland from the water.
Bellingham airport is still reporting freezing rain with an ice
accumulation now likely over a tenth of an inch. From around
Ferndale and further inland, there are reports of sleet and freezing
rain accumulating on trees, roadways, and other surfaces. Travel
will remain dangerous through the day as northeasterly flow at the
surface keeps cold air trapped. Warmer air aloft is infiltrating Wrn
Washington and a change over to rain is still anticipated partially
inland at places like Ferndale and Blaine before noon. Further
inland, freezing rain/sleet may persist longer into the afternoon.
Will likely extend the winter storm warming through the rest of the
afternoon and indicate most of the ice accumulation today will be
confined further inland toward Lynden and Sumas. Total ice
accumulation could reach a quarter inch in central parts of the
county and slightly higher near the Canadian border. Less than two
tenths will fall just inland from the water including Bellingham
before the switch to rain occurs before noon. Forecasts should be
monitored as the duration and areas affected by ice could change.

The next issue is the combination of snow and rain in the Cascades
through Friday night. Warmer air aloft is beginning to infiltrate
parts of the Cascades this morning with freezing rain at Snoqualmie
pass. Precipitation is still fairly light, but is expected to pick
up and become heavier this afternoon and evening. The winter weather
advisory looks OK for the Cascades of King county north to Whatcom.
Over 6 inches should fall at Mount Baker, with a mix of freezing
rain and sleet in Snoqualmie and Stevens pass. Snow amounts may not
be very significant in those locations but some snow combined with
perhaps a quarter inch of ice (in the form of freezing rain or
sleet) warrants a continuation of the advisory. Stevens will
probably have the most snowfall with  up to 6 inches but also mixing
later today and tonight. Snoqualmie could change over to just rain
tonight as a weak low moves inland, but timing of this is uncertain.

Another surface low and warm front, even stronger than today's
system, will approach the area this evening. Easterly flow will
increase through the Passes and possibly some northeasterly flow
could develop again over Western Whatcom. This raises a freezing
rain concern for the passes, especially Snoqualmie. Mostly snow or a
mix at Stevens, and mostly snow at Baker with up to nearly 1 foot
Friday and Friday evening. It now appears that temperatures could
remain too warm in Cascades of Lewis and Pierce and will need to
take down the warning with lack of precipitation today and mostly
rain by tonight at Paradise. Will need to evaluate Wrn Whatcom for a
freezing rain threat over interior parts of the county, but the
current forecast will remain as rain for now.

The surface low will bring a trailing cold front across Western
Washington on Friday evening, with precip then tapering off to
showers on Saturday, before ending on Saturday night.

Elsewhere, there will be periods of moderate to occasionally heavy
rain this afternoon and again on Friday during periods of strong
warm advection lift. In the Cascades, heavy snow is expected. The
snow level will fluctuate from about 3000 to 5000 feet over the
next 2 days.  Mercer/Haner

.LONG TERM...Previous discussion...Upper level high pressure will
develop on Sunday and should persist into the middle of next week.
The ECMWF shows dry weather throughout this period with northeast
then easterly low-level flow. After coordination with surrounding
offices, lowered PoP into the slight chance category starting
Sunday, with the downward trend possibly continuing on later shifts.
However the 06z GFS shows an impulse passing through the ridge on
Tuesday with some precip. Most GFS ensemble members support the
drier ECMWF.
Haner

&&

.AVIATION...Strong westerly flow aloft. A warm front will lift
north across the area today and will be followed by a cold front
tonight. A fast-moving low pressure system will approach the area
from the west Friday. The air mass is moist and stable.

Widespread MVFR conditions with patchy IFR conditions early this
morning will deteriorate somewhat for a few hours midday through
early this afternoon as precipitation associated along the coast
pushes inland with the first frontal system today. Expect clouds
and precipitation to become more confined with the terrain late
this afternoon and early this evening as a cold front moves east
across the area and low level flow shifts from offshore to
onshore.

Late tonight and early Friday expect rain to increase again in
response to the next approaching low. CIGS and VIS conditions
will deteriorate to mainly IFR early in the morning and continue
through much of the day Friday.

Freezing rain is falling this morning in KBLI. Expect temperatures
to slowly move above freezing midday in Bellingham, but areas to
the northeast of the airport to the Canadian border will likely
stay freezing rain into the afternoon hours. Albrecht

KSEA...Expect CIGS around 020 to fall to 008 at times until 22Z with
light rainfall reducing VIS to 3-5 SM. Some improvement with CIGS
around 025 behind the cold front 22Z to 06Z. Cigs fall back to 015
with -RA after 06Z then 3SM -RA OVC007 after 10Z with rain from
next system. Winds south 8-12 kt. Albrecht

&&

.MARINE...A frontal system associated with a weak 1012 mb low will
move across the waters this afternoon through early this evening.
A 1003 mb low will move from well offshore into the coastal waters
Fri afternoon then well east of the area late Friday night.

Small craft advisory conditions are expected over most of the
waters as the first system moves through the area today. Small
craft advisory conditions may be needed for portions of the waters
for the second system on Friday. The strongest winds with this
second low will likely be to the south and west of the low track.

The flow will become westerly in the wake of the low Friday night,
then flow will weaken on Saturday.

Light offshore flow is expected Sunday into early next week as
high pressure develops east of the Cascades. Albrecht

&&

.HYDROLOGY...There will be heavy rain at times in the mountains over
the next 36 hours. One period of rain will be today, with the snow
level 3000-4000 feet. The area of heaviest rain will drift south
tonight, then move back north Friday. The snow level will be around
5000 feet south and 3000 feet north on Friday. Then a cold front
will move through the region Friday night, lowering the snow level
below 2000 feet and ending any heavy rain.

A few rivers could have minor flooding from this event. The most
likely flooding will be on flood prone rivers in the southern half
of the CWA, such as the Skokomish River in Mason County, the
Snoqualmie River near Carnation and the White River at R Street
bridge in Auburn, both in King County, and the Newaukum River in
Lewis County. Will issue a flood watch later, highlighting the
possibility of minor flooding on a few rivers. Burke

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory until 6 AM PST Friday for Cascades of
     Snohomish and King Counties-Cascades of Whatcom and Skagit
     Counties.

     Winter Storm Warning until 6 AM PST Friday for Cascades of
     Pierce and Lewis Counties.

     Winter Storm Watch from Friday morning through Friday evening
     for Cascades of Pierce and Lewis Counties-Cascades of
     Snohomish and King Counties-Cascades of Whatcom and Skagit
     Counties.

     Winter Storm Warning until 1 PM PST this afternoon for Western
     Whatcom County.

PZ...Small Craft Advisory until 10 PM PST this evening 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 from noon today to 10 PM PST this evening
     for East Entrance U.S. Waters Strait Of Juan De Fuca-
     Northern Inland Waters Including The San Juan Islands-Puget
     Sound and Hood Canal.

     Small Craft Advisory until 10 PM PST this evening for Admiralty
     Inlet.

&&

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