River Flood Warning 8, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
844 PM PST Mon Jan 6 2020

.SYNOPSIS...An active weather pattern will persist over the next
week as storm tracks are focused on the Pacific Northwest. Steady
warm frontal rain, heavy at times, will cause rivers to rise
tonight and Tuesday. Several additional systems will bring lowland
rain and mountain snow through the week. Temperatures will trend
colder by the end of the week and into next week.

&&

.SHORT TERM /TONIGHT THROUGH THURSDAY/...Active weather through
the short term. Widespread precipitation continues this evening
as a frontal system remains draped across the northern interior of
Western Washington. Radar indicates that rain has gradually started
to move northward this evening. This front will continue to pose
as the main concern tonight and into tomorrow. Snow levels will
continue to rise this evening, with recent observations indicating
that snow has transitioned to rain at Stevens Pass. Precipitation
may become lighter overnight before a cold front moves through on
Tuesday.

From previous discussion...Headlines for the weather concerns
that exist into mid week:

Lowland Rain: Widespread stratiform rain across Western Washington
this afternoon. QPF amounts over the last 12 hours are generally
0.25-0.75 inches for the Lowlands with the rain shadow effect for
northern King/Snohomish counties where amounts of up to 0.10" of
an inch has fallen. Amounts up to 2-3.5 inches for the Olympics
with upslope southwesterly flow aloft and 1-3 inches for the
Cascades. Stratiform rain will lift northward into this evening as
the warm front slides through. Steadier rain for late Tuesday
morning into Tuesday afternoon as the cold front moves through.
Refer to the hydrologic outlook for information regarding rising
rivers and flooding potential.

Mountain Snow: Snow levels are rising this evening with warmer
southwesterly flow aloft. Snoqualmie Pass transitioned to rain
early this afternoon. Snow levels will rise to 4500-5000 feet this
evening into tonight ahead of the cold front. Stevens Pass is
expected to transition to rain/snow this evening with light rain
at times overnight. Light easterly flow developing Tuesday morning
may transition locally lower snow levels over Stevens Pass.
Winter Storm Warning has been extended through Tuesday for the
North Cascades and South Cascades of Washington, including Mt.
Baker Ski Area, as snow levels fall with the cold front with
additional snowfall.

Winds: Southerly winds will increase ahead of the front Tuesday
morning for breezy across Western Washington. Elevated gradients
into Tuesday afternoon with wind gusts 35-40 MPH for most, with
the highest gusts most likely along the coast.

Tuesday afternoon may also feature a few isolated thunderstorms
along the coast with a rather unstable post-frontal environment,
though instability is generally limited. Temperatures will be
rather warm this afternoon through Tuesday afternoon prior the
cold front moving through with highs in the low 50s.

Hi-res guidance does suggest the development of a Puget Sound
Convergence Zone Tuesday evening through Tuesday night with a
strong westerly push through the Strait and onshore flow. The most
likely region of development will be across Snohomish/King
Counties. Snow levels will also lower significantly by late
Tuesday night and be around 1500-2000 feet by Wednesday morning.
Locally lower snow levels are possible within the convergence zone
but is expected to be above 1000 feet. Unsettled post-frontal
scattered showers into Wednesday afternoon.

A shortwave will drop along the coast of the Pacific Northwest
Wednesday night into Thursday. Although the bulk of the energy
will be south of the area, guidance does suggest enough moisture
for the chance of precipitation late Wednesday night into Thursday
morning. Snow levels will be rather low around 1000 feet for most
of the Puget Sound with snow levels around 500 feet across the
North Interior (Whatcom/Skagit). Although spotty rain showers will
be the primary precip type, a few wet snowflakes will be possible,
especially for the North Interior and Foothills, though no
accumulation is expected with marginal temps. Forecasts will need
to be monitored. JD/SB

.LONG TERM /FRIDAY THROUGH MONDAY/...From previous discussion...A
rather strong weather system will move through the Pacific
Northwest on Friday as a compact shortwave trough moves through.
Guidance does suggest enough Pacific influence to raise snow
levels through the day to 1500-2500 feet. This aspect of the
forecast will need to be monitored however with temperatures in
the mid-upper 30s in place early Friday morning at the onset of
the precipitation. Otherwise Friday will feature Lowland rain and
possibly heavy mountain snow as the system moves through into
Saturday morning.

Troughing will build into the Pacific Northwest with the passage
of the system. Snow levels will steadily drop through Saturday.
This will begin a colder weather pattern for much of the Western
US as ensembles suggest fairly anomalous mean troughing through
into next week. However, there is rather significant uncertainty
as to the magnitude of cold air across Western Washington heading
into next week. Guidance differs on the magnitude of the ridging
across the North Pacific next week, which in turn impacts the how
much cold air drops south into Western Washington. In addition,
there is uncertainty as to how much moisture exists for systems to
work if an arctic front drops south, especially headed into early
next week. The forecasts will need to be monitored through this
week to address these aspects of the forecast. JD

&&

.AVIATION...An upper ridge over the area will shift inland on
Tuesday with an upper trough moving to the coast. Westerly flow
aloft. A warm front will be draped over the area tonight. A
trailing cold front will move through the area on Tuesday. The air
mass is moist and stable. It will become slightly unstable near
the coast on Tuesday. Low clouds will be widespread through
Tuesday

KSEA...Low clouds and occasional reduced visibilities in rain
will persist through Tuesday with a warm front over the area
tonight followed by a cold front on Tuesday. Southerly wind 10-20
knots with gusts to 30 knots possible.

&&

.MARINE...A frontal system will move through the area tonight and
Tuesday. Gales are likely for the Coastal Waters later
tonight/Tuesday. Small craft advisory strength southerly winds
are expected most other waters. A westerly gale is also possible
in the Strait behind the frontal system Tuesday night.

Onshore flow will gradually ease on Wednesday with winds turning
more northerly on Thursday. Another strong frontal system will
move through the area on Friday. Northerly offshore flow will
develop behind this weather system on Saturday.

&&

.HYDROLOGY...The Flood Watch for much of western Washington. The
warm front is still expected to lift north across the area today,
allowing for significantly rising snow levels and the transition
from heavy mountain snow to rain from south to north. Strong winds
in the low levels will continue to produce orographic enhancement
over the Olympic and Cascade Mountains. 1 to 4 inches of rain have
fallen with another 2-4 inches of rainfall expected now through
Tuesday across the mountainous terrain (locally higher amounts
likely). The lowlands can generally expect 1-2 inches of rain thru
Tuesday. Rain will taper off behind the system late Tuesday into
Wednesday.

Flooding has begun and several more rivers are expected to
rise to minor or moderate flood stage overnight and tomorrow. While
many of the rivers in the flood watch area are at risk of flooding,
some of the rivers most at risk are the Tolt, Snoqualmie,
Stillaguamish River, Nooksack and White Rivers. Check the latest
river forecasts on the AHPS website for details.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Watch through late Tuesday night for Admiralty Inlet Area-
     Bellevue and Vicinity-Cascades of Pierce and Lewis Counties-
     Cascades of Snohomish and King Counties-Cascades of Whatcom
     and Skagit 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
     Skagit County-Western Strait of Juan De Fuca-Western
     Whatcom County.

     Winter Storm Warning until midnight PST Tuesday night for
     Cascades of Pierce and Lewis Counties-Cascades of Whatcom
     and Skagit Counties.

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

PZ...Small Craft Advisory until 4 PM PST Tuesday for Grays Harbor Bar.

     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.

     Gale Warning until 4 PM PST Tuesday 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.

     Gale Watch from Tuesday evening through late Tuesday night for
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 6 PM PST Tuesday for East Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 4 AM PST Wednesday for Admiralty
     Inlet-Northern Inland Waters Including The San Juan Islands.

     Small Craft Advisory until 4 AM PST Wednesday for Puget Sound
     and Hood Canal-West Entrance U.S. Waters Strait Of Juan De
     Fuca.

&&

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