Flood and High Wind in Washington, December 19 to 21, 2020

Area Forecast Discussion
National Weather Service Portland OR
309 PM PST Sat Dec 19 2020

.SYNOPSIS... A windy and wet system will move across the forecast
area tonight through Sunday night as an atmospheric river event
unfolds, with active weather continuing into early next week. High
pressure will bring a period of drier weather by the middle of
next week as offshore flow develops.

&&

.SHORT TERM...Tonight through Tuesday...A very wet system is
still on track to impact the Pacific Northwest through this weekend.
Expect precipitation to start along the coast late this afternoon and
move inland by this evening and, persisting thorough Sunday night.
During this time a very strong mid/upper level jet will help enable
the transport of subtropical moisture into upper latitudes, creating
a moderate to strong atmospheric river (AR) with integrated vapor
transport (IVT) values greater than 500 kg/(ms). In addition to the
mid/upper level jet, a broad area of weak upper level ridging along
with a weak surface high pressure located off the coast of California
will help in focusing this subtropical moisture towards the Pacific
Northwest. This AR will be the dominant synoptic feature for the area
through the weekend, as a result a hydrological section has been
added to the AFD that will provide further details on precipitation
amounts, flood potential and timing of this event.

As this system pushes through, perturbations within the flow will
produce some marginal high wind gusts up to 60 mph this afternoon
through the early evening hours for the beaches and headlands. High
winds should extend into coastal communities but will be less intense
with gusts up to 50 mph. Therefore, a High Wind Warning has been
issued for the south WA/north OR coast through 10 PM this evening and
for central OR through midnight. Interior valleys could see gusts up
to 40 mph.

Monday, the aforementioned broad upper level ridge will slowly start
to meander eastward, which looks to bring a surface low and
associated fronts across the area. This will continue the
precipitation but, will bring 850 mb temperatures around -3C to -5C
behind the cold front on Monday night. These cooler temperatures will
bring snow levels down to around 2000 to 2500 feet for the start of
the week. Expect several inches of new snow for the Cascades Monday
into Tuesday. However, Tuesday a significant change in the overall
pattern will start as models are in agreement that a strong upper
level ridge will start building across the area. Cold temperatures
also return starting Tuesday. /42

&&

.LONG TERM...Tuesday night through Friday night...A period of cooler
and dry weather can be expected by the middle of next week. An upper
level ridge slides eastward over the Pacific Northwest Tue night and
Wednesday. High pressure builds across the region Tue night. There
will be a good chance for fog development late Tue into Wed morning
from the lingering moisture along with light winds and possible
clearing. By Wed afternoon think more of an offshore flow develops
allowing for more clearing. Otherwise high pressure should maintain
dry conditions through Christmas Eve (Thursday night). Mostly clear
skies will likely support widespread sub-freezing temperatures Wed
night and Thu night, although if the easterly gap flow out of the
Columbia Gorge is strong enough, it could keep temperatures above
freezing through the Portland/Vancouver metro areas. Afternoon
temperatures will generally be in the 40s. Strong subsidence
inversions may also result in poor circulation in the interior
valleys through Thursday, but the ridge is not likely to hang around
long enough for stagnant conditions to develop.

A broad upper trough moves toward the Pacific NW replacing the ridge
Friday. Models show an associated front approaching the coast early
Friday, with timing and strength uncertain. Will keep a chance of
precipitation on Christmas Day (Friday). Snow levels should creep
upwards, with the possible exception near Hood River Valley. Will it
snow on Christmas? Not in most areas, but cold air may remain
trapped in the Upper Hood River Valley and the central Columbia Gorge
Friday as the precipitation arrives for a chance of snow or a wintry
mix Christmas afternoon or evening. The Cascades above 3000 feet may
also receive some Christmas snow. Otherwise chance of rain continues
through Fri night. DH

&&

.AVIATION...Mostly overcast VFR through the next hour or so ahead
of a warm front that is quickly approaching. Expect flight
conditions to deteriorate quickly as it makes landfall this
evening. This front will cause southerly winds to ramp up along
the coast and will bring a significant amount of rain to the
area. Southerly winds, while strongest along the coast, will be
breezy inland and along the Cascades. Expecting gusts up to 50+
kt along the coast and Cascades, and up to 30 kt through the
Willamette Valley, potentially slightly stronger near KSLE and
KUAO. Expect LLWS along the leading edge persisting through
around 06Z Sunday with winds around 50 kt at 2000 ft. Winds will
ease after 12Z Sunday as the front moves inland. Rainfall wise,
heaviest precipitation will be through 12Z Sunday. Strong
rainfall rates combined with increased winds may reduce
visibility at times. In addition, with the heaviest rain,
expecting CIGs to drop significantly to low MVFR/IFR levels.
Improvement in conditions is likely as the front shifts eastward
and rains lighten.

For detailed regional Pac NW weather information, go online to:
http://weather.gov/zse

KPDX AND APPROACHES...MVFR with light drizzle ahead of a warm
front will turn into heavier rain and gusty southerly winds
through the evening. Expecting LLWS along the leading edge of the
front through at least 06Z Sunday, with amplified winds
persisting through 12Z Sunday. With heavy rain expected there is
a chance for pooling water and IFR conditions. Improvement is
likely after 15Z Sunday as the front pushes further east and
heavy rain becomes lighter. -Muessle

&&

.MARINE...Made little changes to the overall timing and strength
of the winds with the afternoon forecast package. South winds are
increasing over the waters ahead of a strong front, and expect
Gales through the evening to the north and into early Sunday
morning for the south (near Newport). A coastal jet may develop
within 10 NM of the coast for possibly a brief period (1 to 3
hours) with gusts of 45 to 55 kt near the immediate coast. The
best time and location for a coastal jet is along the S
Washington coast to just south of the Columbia Bar entrance in
the early evening (4 to 6 pm). This front will result in heavy
rain this evening through tonight which may reduce visibilities
over the waters at times. This front stalls and weakens over the
southern waters early Sunday morning then lifts northward Sunday
afternoon and evening.

A cold front is expected to bring another round of gale force
winds and elevated season on Monday. A coastal jet is possible
with this system as well which could provide a brief period of
stronger winds closer to shore. High pressure should then shift
across the waters Tuesday through Thursday for a period of
offshore winds. ~TJ

&&

.HYDROLOGY...An atmospheric river funneling subtropical moisture off
the Pacific was pointed at western Washington Saturday afternoon.
The very moist air, coupled with strong low and mid level winds
including west-southwest 850 mb winds reaching 50 to 60 kt, will
transport copious moisture onto the coast and inland tonight into
Sunday. As the expected band of heaviest rains sags south tonight
into northwest Oregon, a threat of flooding returns to much of the
forecast area tonight into Sunday. Will continue with the flood
watch as previously issued. As if often the case, a few rivers
originating on the west slopes of the Willapa Hills and north and
central Oregon Coast Range are likeliest to threaten flood stage
later tonight. As the inland transport of moisture will be good with
this event, the threat of flooding extends inland as well, with
another area of heaviest rains expected on the west slopes of the
Cascades. This is expected to bring sharp rises on some of the
Willamette tributaries by Sunday. The anticipated heavy rains will
be a concern too for the burn scars tonight into Sunday, which if
the intensity of the rain is sufficient, could make debris flows and
flash floods possible. Finally, flooding threats are not necessarily
limited to rivers and burn scars. Expect some minor ponding and
flooding in low-lying areas and urban areas as well.

When flooding is imminent or already occurring, Flood Warnings
and/or Areal Flood Advisories will be issued as necessary.

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Flood Watch through Sunday evening for Cascade Foothills in
     Lane County-Cascades in Lane County-Central Coast Range of
     Western Oregon-Central Columbia River Gorge-Central Oregon
     Coast-Central Willamette Valley-Coast Range of Northwest
     Oregon-Greater Portland Metro Area-Lower Columbia-North
     Oregon Coast-Northern Oregon Cascade Foothills-Northern
     Oregon Cascades-South Willamette Valley-Western Columbia
     River Gorge.

     High Wind Warning until 10 PM PST this evening for North Oregon
     Coast.

     High Wind Warning until midnight PST tonight for Central Oregon
     Coast.

WA...Flood Watch through Sunday evening for Central Columbia River
     Gorge-Greater Vancouver Area-I-5 Corridor in Cowlitz
     County-South Washington Cascade Foothills-South Washington
     Cascades-South Washington Coast-Western Columbia River
     Gorge-Willapa Hills.

     High Wind Warning until 10 PM PST this evening for South
     Washington Coast.

PZ...Gale Warning until 10 PM PST this evening for coastal waters
     from Cape Shoalwater WA to Cascade Head OR out 60 NM.

     Gale Warning until 4 AM PST Sunday for coastal waters from
     Cascade Head OR to Florence OR out 60 NM.

     Small Craft Advisory until 4 PM PST Sunday for Columbia River
     Bar.

&&

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