High Wind in Washington, November 12 to 13, 2020

Area Forecast Discussion
National Weather Service Portland OR
940 PM PST Thu Nov 12 2020

.SYNOPSIS...A series of storm systems will bring mountain snow,
periods of substantial rain and windy conditions to the area over the
course of the next week.

&&

.SHORT TERM...Tonight through Saturday night...Water vapor satellite
imagery shows a shortwave trough sliding southeastward out of the
Gulf of Alaska towards southern British Columbia and the Pacific
Northwest. A doppler radar mosaic shows widespread steady rain
spreading across southwest Washington and northwest Oregon this
evening as a warm front pushes into the region. Hourly rainfall rates
have been a tenth of an inch or less per hour, but should begin to
pick up over the higher terrain over the next few hours as stronger
south-southwest flow aloft impinges on the topography. A trailing
cold front with decently strong low level frontogenesis underneath
strong PVA aloft should provide an extra boost to rainfall rates as
the trailing cold front pushes southeastward
across the area late tonight into Friday morning. There are some
hints in HREF guidance that it will be slightly more organized as it
pushes through the central and southern Willamette Valley Friday
morning, but confidence in this level of detail is still pretty low
given the lack of observations upstream. 24 hour rainfall amounts of
0.50-1.50" appear likely across the lower elevations with 2-5" of
rain likely across the higher elevations of the Coast Range and
Cascades. This should not present too many problems hydrologically
given current river levels, but will certainly need to monitor
rainfall rates over the burn areas later tonight into Friday morning.

Snow levels are currently hovering in the 2000-3000 feet range across
the south Washington Cascades and in the 3000-4000 ft range across
the Oregon Cascades. Expect snow levels to rise into the 4000 to 5000
ft range temporarily late tonight into Friday morning as the
aforementioned warm front pushes into the region before falling back
down below 3000 feet late on Friday. Given the QPF totals, plenty of
snow will fall across the higher elevations above 4500-5000 feet, but
will be more limited at the passes. The current Winter Storm Warning
seems to capture this well enough that no changes are needed.

Otherwise, the biggest story with this storm system will likely be
the wind. While the majority of model guidance backed off on the idea
of a strong surface low pressure barreling towards the south
Washington coast later tonight, not all model guidance has abandoned
this idea. The GEM and a number of members within the EPS still
suggest a closed  985-990mb low making landfall near the mouth of the
Columbia River. Given satellite imagery and the lack of a
cyclogenesis signature near the location a low pressure should be
forming according to these models, think this scenario does not look
promising. With that said, models have trended stronger with the
front itself and its accompanying pressure gradients over the past
12-24 hours. HREF and EPS wind gust guidance have followed suit and
trended notably stronger with their wind gust fields along the Oregon
coast and even in the Willamette Valley. Opted to extend the High
Wind Warning to the north Oregon coast and increase wind speed
wording per the aforementioned guidance and modeled 925mb winds.
Given the strongest 925mb winds tend to remain out of the
south to south-southwest, held off on extending the warning into the
Coast Range. With that said, 850mb winds appear more favorable so we
may still need to expand the High Wind Warning to include the Coast
Range as well. Farther inland, operational model soundings support
wind gusts in the 30 to 45 mph range. However, higher resolution
models like the NAMnest and HREF guidance suggest potential for wind
gusts upwards of 50 mph or greater in the Willamette Valley,
particularly in the central and south valley. Did not go quite this
strong given the pattern, but find it hard to ignore the higher
resolution guidance given their recent performances so went ahead
with a Wind Advisory for the Willamette Valley for late tonight and
early Friday morning.

Expect precipitation to become more showery behind the front on
Friday. Colder air aloft transitioning across the region will bring
the potential for thunderstorms across our waters by late morning and
inland during the afternoon. Otherwise, expect showers to
decrease Friday night as shortwave ridging quickly transitions across
the area. The next storm system will be quick on its heels, and
likely spread widespread steady rain back across the area on Saturday
as a developing low pressure pushes towards the region. Still not
sure QPF will be there for too many hydro concerns given the system
appears rather transient, but it will certainly be priming the pump
if some of the wetter solutions in the GEFS and EPS systems pan out
for early next week. /Neuman

.LONG TERM...Sunday through Wednesday...An active weather pattern
will likely continue through early next week. There is general model
agreement of zonal flow aloft Sunday as onshore flow will likely
prolong shower activity through the day. The pattern will become
more amplified early next week as an upper level trough sets in over
the north Pacific and ridging builds across the Pacific Northwest.
There remain differences between exact placement of these features
among the models and ensemble members, leading to varying scenarios
that could play out next week. In general, models are suggesting a
large low pressure system develops in the eastern pacific associated
with the upstream trough. Models are trending towards more warm air
advection across the area Monday as a warm front lifts northward,
followed by a cold front sometime Tuesday. Should see periods of
rain along with periods of drier weather early next week, but
details will likely not be worked out for a few more days. The NBM
is trending temperatures early next week warmer than normal, with
highs into the upper 50s and lower 60s not out of the realm of
possibilities.	 /DDH

&&

.AVIATION...Warm frontal rains spread into the area this evening.
Cigs have lowered to the MVFR category over southwest WA. This
trend will continue to spread southeast overnight so expect
MVFR cigs and vis as rain increases. May have periods of IFR cigs
and vis later tonight and Friday morning as the cold front slides
north to south, but forecast confidence is not high at this
time.

Winds will increase as well peaking between 08Z-12Z north of
Lincoln City and 08-15Z south of Lincoln city. South wind 45 to
55 kt possible right along the coast during this time. Inland
winds will become gusty 06Z Fri becoming. The strongest winds
30-40 kt is expected 10-15Z Fri. Breezy conditions will continue
through the day Fri.

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

KPDX AND APPROACHES...Expect MVFR cig and vis to develop over
the next few hours. There is less then 30 percent chance of IFR
conditions late tonight and early Friday. South winds should
break through 07-09Z Fri with strongest winds gusting 25 to 30
kt in a 10-15Z time frame. Winds become southwesterly and
decrease later Friday morning. Low level windshear (LLWS) is a
possibility through 07-09Z, particularly at KTTD & KPDX where
east wind prevail with stronger southerly winds aloft.

&&

.MARINE...Update for the evening is to strengthen winds a bit as
new models have indicated. Satellite loop not showing much low
pres development yet so will have to monitor closely. Have
upgraded the Gale Warning in the waters Cascade Head to Florence
to a Storm Warning through mid morning Friday as a cold front
works it way south along the coast. Small impulses riding along
the front may slow the progress of the front, delaying the easing
of the winds.

Previous discussion follows...
A complicated weather pattern will play out over the next 24 to
48 hours. A shortwave will sling shot around the base of the
parent low in the Gulf of Alaska and will reach the coastal
waters Friday evening. This shortwave will strengthen into a
defined low and wind speeds around midnight tonight will likely
reach 40 possibly 45 kt across all the waters. Things get a
little complicated overnight, the cold front from the shortwave
mentioned is expected to move across the outer waters around 3 AM
and winds in the inner waters could reach up to 50 kt before 6
AM. However, the timing of these winds may change because at the
same time this front comes another shortwave will move south
along the Canadian coastline and possibly merge with the
shortwave from the southwest. According to the 18Z NAM and GFS
the Canadian shortwave will likely weaken by the time it crosses
the waters but if it strengthens then there could be another
round of high end Gales by tomorrow afternoon. Persistent
northwesterly winds will continue in the cold sector of the
front, therefore intermittent periods of gale force gusts will
continue Friday afternoon through Friday night.

Although the fronts have been particularly difficult to time
winds have been more reliable than seas. The persistent
northwesterly winds will set up a decent fetch producing
northwesterly swell around 15 ft according the 12Z ECMWF wave
model. According to the 12Z GEFS probabilistic wave heights
there was a 60-70% chance significant wave heights would build
greater than 20 ft, with the current 18Z run there is only a
10-20% chance. Most likely scenario for Friday evening into
Friday night is significant wave heights will trend between 15-17
ft, between 12 to 15 sec with 20 ft seas likely but not
widespread as intermittent gale gusts will likely build
significant wind waves in some places.

The next storm system is expected to swing a cold front across
the waters Saturday afternoon which will likely bring another
round of gales. Seas will remain between 10 to 15 ft through the
beginning of next week and wind gusts will likely stay above 21
kt through next week. -BPhillips

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Storm Warning until 8 PM PST Friday for Cascades in Lane
     County-Northern Oregon Cascades.

     High Wind Warning from 1 AM to 10 AM PST Friday for Central
     Oregon Coast.

     Wind Advisory from 2 AM to 10 AM PST Friday for Central
     Willamette Valley-South Willamette Valley.

     High Wind Warning until 6 AM PST Friday for North Oregon Coast.

     Wind Advisory from 2 AM to 8 AM PST Friday for Greater Portland
     Metro Area.

WA...Winter Storm Warning until 6 PM PST Friday for South Washington
     Cascades.

PZ...Gale Warning until 7 PM PST Friday for coastal waters from Cape
     Shoalwater WA to Cascade Head OR out 60 NM.

     Storm Warning from midnight tonight to 9 AM PST Friday for
     coastal waters from Cascade Head OR to Florence OR out 60
     NM.

     Small Craft Advisory from midnight tonight to 5 AM PST Friday
     for Columbia River Bar.

     Small Craft Advisory from noon Friday to 4 PM PST Saturday for
     Columbia River Bar.

&&

Interact with us via social media:
www.facebook.com/NWSPortland
www.twitter.com/NWSPortland