River Flood Warning 3, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
855 AM PST Mon Jan 6 2020

.SYNOPSIS...An active weather pattern over the next week as storm
tracks are focused on the Pacific Northwester. Steady warm frontal
rain, heavy at times, will cause rivers to rise today and Tuesday,
with heavy mountain snow this morning into this afternoon before
snow levels rise. 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 the weekend.

&&

.SHORT TERM /TODAY THROUGH WEDNESDAY/...Active weather for the
short term for the area. Widespread precipitation this morning as
a warm front slides northward across Western Washington. The warm
front will pose the main concern through today as it slowly slides
through and stalls across the northern tier into BC later today
into tonight. Numerous weather concerns exist through the next few
days with the frontal system:

Recent MRMS radar loops show widespread stratiform rain pushing
across the local area this morning, with the heaviest in the
vicinity of current warm frontal position near the Coast/SW
interior. SPC mesoanalysis reveals strong warm air advection in
the 925-700mb layer and strong 925 frontogenesis coinciding
unsurprisingly well with the area of heaviest rainfall. As the
warm front continues to lift thru the day, expect these conditions
to persist and the rain to move northward. Aiding in these
factors will be a LLJ of ~45-50kts between 925/850mb which should
act to increase precip intensity and overall strength of the warm
front.

Taking a closer look at sensible weather impacts this morning and
for today, snow levels are currently relatively low ahead of the
warm front, roughly between 2-3k feet. Snow levels are currently
rising this morning with the warm front. Rain/snow mix reported at
Snoqualmie Pass as of 8AM. VWP suggests transition to
southwesterly flow aloft which will rise snow levels through this
afternoon. Exact timing of snow levels rise is difficult, refer
to WSWSEW for information on winter hazards.

In addition to mountain snow, heavy rainfall is expected at lower
elevations through today, and eventually for the mountains as the
warmer, Pacific air moves in. Given the the strong LLJ/WAA,
locally heavier pockets of rain are expected thru the day, which
will cause rivers to rise. Several rivers across the Olympic
Peninsula and east of the Sound are expected to rise into minor
flood over the next 24 hours. For this reason, a Flood Watch has
been issued for the area beginning this morning. Refer to
hydrology section below for more details.

As the aforementioned Pacific trough near 155W moves east,
perturbations within the cyclonic flow should be strong enough to
help organize a lower level frontal system and carry it towards
the PacNW. Steady stratiform precip is expected to continue in the
warm sector into Tuesday with the sfc cold front moving rather
quickly east across the Pacific. This will bring yet another round
of rain to the area. This will not help the hydrologic concerns
as rivers will already have been on the rise. In addition, SPC has
made note of steepening mid level lapse rates in the vicinity of
the front strong enough to initiate a few lightning strikes
offshore and near the coast Tuesday afternoon/evening. Agree with
this thinking so added slight chance thunder.

Scattered nuisance showers persist into Wed in an unstable post-
frontal environment. Yet another shortwave will drop south across
the eastern Pacific, this time with storm track into Oregon,
sparing western Washington of the deeper later moisture.

JD/Kovacik

.LONG TERM /THURSDAY THROUGH SUNDAY/...Although the bulk of this
forecast was focused on the impactful near term, there are several
things to note in the long term. The long term period begins on
Thursday, where the local area will be under the influence of NW
flow aloft in the wake of the Oregon shortwave. A weak speed max
is forecast to dive south from W British Columbia and quickly
graze the area. It appears strong enough to enhance shower
activity across the area before it continues south. Snow levels
are rather low, especially across the northern CWA enough to
mention chance of some light rain/snow. Elsewhere in the lowlands,
looks like some chilly rain showers. It should be noted that
overall moisture is not impressive with this quick clipper type
system given its continental influence.

Next organized cold front is on tap for Friday. Given the current
track of this system, looks to be enough Pacific influence to
raise snow levels through the day to 1500-3000 feet. Should make
for another round of rain and mountain snow.

The passage of this system will help build better troughing
across the entire region, with snow levels dropping steadily
through Saturday. Very cold arctic air is noted across BC, with
storm track having the potential to bring some of this cold air
southward. Model guidance is pretty set on colder temperatures
late in the long term (highs 30s and lows 20s for lowlands). At
this time it is unclear if an arctic front will drop south across
western Washington, remain bottled up in BC, or be shoved E of
the Cascades. In addition, it is unclear how much moisture there
would be to work with if an arctic front were to drop south, as
continental polar/arctic airmasses cannot hold much moisture. At
this point the forecast has trended towards colder temps with
decreasing moisture as snow levels drop to the sfc. Will be
important to monitor this forecast in the coming days should
moisture and cold temps appear more significant together.
&&

.AVIATION...An upper ridge will be over the area through Tuesday
with westerly flow aloft. A warm front draped over the area will
lift slightly north today and tonight. A trailing cold front will
move onshore 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 morning with a warm front over the
area. Southerly wind 10-20 knots with gusts to 30 knots possible.

&&

.MARINE...A warm front will gradually lift north through the area
today and tonight with small craft advisory strength southerly
winds most waters. Gales are likely for the Coastal Waters and
possible for the northern interior waters later tonight and
Tuesday morning as a cold front approaches. There will be a
period of onshore flow later Tuesday behind the cold front with
easing winds in Wednesday.

Winds will be relatively light Wednesday night and Thursday as a
surface low will moves inland to the south. Another strong frontal
system will move into the area on Friday.

&&

.HYDROLOGY...A Flood Watch has been issued for all counties in the
CWA with the exception of Kitsap, Island, and San Juan. A warm
front will continue 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
frontogenesis and WAA in the 925-820mb layer, supported by a LLJ
of 40-50kts in the wake of the front will allow for increasing
intensity of the rainfall in the warm frontal vicinity. Wind
direction will be oriented orthogonal to the Olympic Mountains,
likely contributing to precip enhancement. Several rivers are
expected to rise to minor flood stage as early as later this
afternoon and especially over the 24 hours with 3-6 inches of
rainfall expected through Tuesday across the mountainous terrain
(locally higher amounts likely). The lowlands can generally expect
1-2 inches of rain thru Tuesday with several rivers east of Puget
Sound expected to reach minor flood stage. Rain will taper behind
the system late Tuesday into Wednesday.

&&

.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 10 AM PST this morning for Cascades
     of Pierce and Lewis Counties.

     Winter Storm Warning until 4 PM PST this afternoon for Cascades
     of Snohomish and King Counties.

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

PZ...Small Craft Advisory until 4 AM 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 from 10 PM this evening to 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 late tonight through Tuesday afternoon for
     Admiralty Inlet-East Entrance U.S. Waters Strait Of Juan De
     Fuca-Northern Inland Waters Including The San Juan Islands.

     Small Craft Advisory until 10 AM PST this morning for Admiralty
     Inlet-East Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 4 PM PST this afternoon for Northern
     Inland Waters Including The San Juan Islands.

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

&&

www.weather.gov/seattle

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