Gale Warning 8, NWS Portland, OR

Area Forecast Discussion
National Weather Service Portland OR
850 PM PST Tue Feb 7 2017

.SYNOPSIS...A warm warm front moves into the Southern Oregon coastal
waters late tonight then slowly moves north Wednesday. Offshore low-
level flow strengthens late tonight and Wednesday, which will pull
colder air into the Columbia Gorge. A very moist frontal zone
reaches the coast Wednesday afternoon and slowly pushes inland
Thursday. Residual cold air in and near the Gorge will bring the
potential for mixed precipitation as far west as the eastern
Portland/Vancouver metro area Wednesday morning. There is high
potential for significant icing in the Western Columbia Gorge
Wednesday through Thursday morning. Snow will be the predominant
precipitation type Wednesday and Wednesday night for the Central
Gorge, Hood River Valley and Wind River Valley. The freezing rain
threat will persist in the Central Columbia Gorge, Hood River Valley
and valleys near Mt. Adams Thursday. After a few lingering showers
Friday, high pressure is expected to develop late Saturday and hold
through Sunday.

&&

.SHORT TERM...Tonight through Friday...No significant changes to the
upcoming multi-faceted event slated to impact just about every area
in the CWA over the next couple days. Steady rain and snow has
largely dissipated although light shower activity is starting to
increase under the leading edge of the incoming system. Decided to
drop the current set of Winter Weather Advisories as accumulation
rates are currently not and will not be sufficient to justify
keeping the statement.

There is talk about potential freezing rain for the Metro area late
tonight and tomorrow morning. Have trimmed back any threat to the
far east Metro areas closest to the mouth of the Gorge. TTD-DLS
pressure gradients will be a little slower to increase based on
latest hourly high-res model runs and the 00z NAM. Also feel that
when the winds do kick in, it will help keep the near surface air
temperatures mixed and remaining just slightly above freezing. Still
could not rule out the threat for Troutdale/Camas and north Gresham
(plus the Gorge) but did limit the threat to a slight chance.

Did issue a flood watch early in the shift covering a large part of
the forecast area for the slug of moisture detailed below. Several
rivers are currently forecast to rise above flood stage over the
next couple days. However, as with any atmospheric river event, we
will need to see specifically where the narrow ribbon of very heavy
rain sets up before having high enough confidence to pull the
trigger on any flood warnings. /JBonk

Remainder of the previous discussion: The frontal boundary weakens
through tonight, but really never goes away. The primary weather
event will be Wed morning through Thu.
Another deepening low pressure area in the Central Pacific this
evening throws out a warm front over the coastal waters late tonight.
This new warm front becomes attached to the remnant frontal boundary
left over from today. In any event, strong warm advection develops
late tonight and Wed. This also strengthens the offshore low-level
flow through the Gorge. This system has some decent precipitable
water associated with it...around 1.50 inches with a moisture tap
that goes SW across Hawaii. It also has impressive integrated water
vapor transport, on the order of 1000 to 1100 kg per m per second
directed at the south and central Oregon coast 12Z Thu. However, the
directional transport is from the SSW, not as favorable as SW to W
direction. In any event, this system will produce another round of
significant precipitation Wed afternoon through Thursday. By 06Z Thu
the GFS and NAM indicate an impressive 60-80 kt SW 850 mb wind into
the Oregon Coast Range, which will result in premium orographic
enhancement to the precipitation. At this point, it appears this
system may produce around 2 inches of QPF for the inland valleys, 2-4
inches for the coast, and 3-6 inches for the higher terrain by the
time the parent upper trough swings through Thursday night. Given the
warmer temperatures and existing snowpack in the lower elevations of
the Coast Range and Cascades, there are definitely some hydro
concerns with this system.

The Columbia Gorge, Hood River Valley and areas around Mt. Adams in
the South Washington Cascades will be under the gun for significant
winter weather precipitation. A NAM sounding near near KCZK suggests
light snow Wed morning, but transitioning to moderate -FZRA by early
afternoon. Columbia Basin temperatures this afternoon are generally
in 20s, so there is a decent source of cold air available to get
pulled into the Gorge Wed by the strengthening offshore flow. Enough
low-level cold air may linger in the PDX metro for some brief
freezing rain Wed morning, but for the most part it appears the ice
storm threat will be in the Gorge. Given the copious QPF associated
with this system, there may be some damaging ice accumulations in the
Gorge. Will be converting the Winter Storm Watch for the Western
Columbia Gorge to an Ice Storm Warning valid 15Z Wed through 18Z Thu.
The most likely zone for significant icing Wed through Thu seems to
be from near Multnomah Falls to around KCZK. Areas to the east and
also the Wind River Valley should see more snow, or a later
transition to -FZRA.  The big winners for snowfall will likely be
eastern Skamania County, where up to 2 feet of snow may occur near
Mount Adams...including Trout Lake (just across the border in
Klickitat County). Cold air is notoriously stubborn in these areas in
easterly flow, and much lower snow levels will likely linger in
eastern Skamania County than for western portions of the county. Will
go with a Winter Storm Warning for these areas Wed morning through
Thu afternoon.

Another concern with this system is wind along the coast. The latest
NAM not as impressive as its previous runs with the 850-900 mb wind
field. The 12Z run shows 55-70 kt 900 mb wind along the coast Thu
morning, with the most favorable north-to-south surface gradient
along the central and north Oregon coast. The ECMWF is not quite as
strong with the wind field. The 12km WRF-NAM suggests coastal jet
development late Wed night through Thu morning with 55-60 kt wind
gusts. Will go with a High Wind Watch valid 08Z Thu through 00Z Fri.
The primary baroclinic feature finally pushes through the region late
Thu or Thu evening. However, the upper level trough does not rotate
through until Fri. Models hint at a cold front or well-organized
surface trough reaching the coast 12Z Fri and pushing across the area
during the day. Snow levels fall to 3000 to 4000 ft Fri. Weishaar

.LONG TERM...Friday Night Through Tuesday...The upper level trough
moves well east of the area Friday night, with high pressure
developing in the Eastern Pacific. Shower activity diminishes Friday
night, but will linger into Saturday morning. The ECMWF actually
holds on to some showers across the far north into Saturday
afternoon. Models in general agreement showing an upper level ridge
building across the Pac NW late Saturday through Sunday. This should
bring a reprieve from the wet weather this weekend, but with abundant
surface moisture and relatively light winds expect patchy fog to
develop across the area at night and potentially linger well into the
morning hours through early next week. A weak wave dents the ridge
Mon, but not expecting any precipitation from this feature. High
pressure and associated offshore low-level flow returns Mon night and
Tue. Weishaar

&&

.AVIATION...A mixture of IFR, MVFR, and VFR cigs this evening
will likely be mainly IFR and MVFR as a weak warm front lifts
north through the area tonight and a stronger warm front moves
north through the area Wednesday and early Wednesday evening.
There is a chance of some of freezing rain Wed morning at KTTD,
with significant freezing rain and snow in the Columbia River
Gorge itself.

KPDX AND APPROACHES...Expect a mixture of MVFR and IFR conditions
as a pair of warm fronts lift north past the airport later
tonight and again on Wednesday afternoon and early evening. Will
have occasional light rain tonight and early Wednesday, that
could become heavy at times by Wednesday afternoon. The main
chance of any freezing rain in the next 24 hours will be on
eastern approaches. pt
&&

.MARINE...Winds will remain rather light tonight as a very weak
warm front lifts north. Winds will increase to Small Craft
Advisory levels Wednesday morning ahead of the next stronger warm
front, reaching gale force from the E and SE in the afternoon
and evening as the strong warm front moves north. Behind the warm
front the strongest gales will occur as the winds turn
southerly, possibly getting close to storm force, especially in
the central Oregon coastal waters. The winds will ease a bit but
remain on the high side of Small Craft Advisory once the main
cold or occluded front moves onshore mid day Thursday. A
secondary low and front could bring another round of possible
gales on Friday. Winds over the weekend will be much weaker as
higher pressure builds in, with a thermal trough possible on the
south Oregon coast on Sunday.

Seas are around 4 or 5 ft this evening and will continue in that
range overnight tonight. Seas start to build above 10 ft in the
central Oregon waters Wednesday morning and in the northern
waters in the afternoon. Wave models have the seas building
Wednesday night and Thursday, peaking just above 20 ft thursday
morning. Seas will probably stay in the teens through Friday,
with a possible peak of 15 to 20 ft midday Friday from a
secondary system. Seas fall below 10 ft this weekend.
&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Storm Warning from 7 AM Wednesday to 4 PM PST Thursday
     for Central Columbia River Gorge-Upper Hood River Valley.

     Ice Storm Warning from 7 AM Wednesday to 10 AM PST Thursday for
     Western Columbia River Gorge.

     Flood Watch from Wednesday afternoon through late Friday night
     for Cascade Foothills in Lane County-Central Coast Range
     of Western Oregon-Central Oregon Coast-Central Willamette
     Valley-Coast Range of Northwest Oregon-Greater Portland
     Metro Area-Lower Columbia-North Oregon Coast-Northern
     Oregon Cascade Foothills-South Willamette Valley.

     High Wind Watch from late Wednesday night through Thursday
     afternoon for Central Oregon Coast-North Oregon Coast.

WA...Winter Storm Warning from 7 AM Wednesday to 4 PM PST Thursday
     for Central Columbia River Gorge-South Washington Cascades.

     Ice Storm Warning from 7 AM Wednesday to 10 AM PST Thursday for
     Western Columbia River Gorge.

     Flood Watch from Wednesday afternoon through late Friday night
     for Greater Vancouver Area-I-5 Corridor in Cowlitz County-
     South Washington Cascade Foothills-South Washington Coast-
     Willapa Hills.

PZ...Gale Watch from Wednesday afternoon through Thursday afternoon
     for Coastal Waters from Cape Shoalwater WA to Cascade Head
     OR out 60 nm.

     Small Craft Advisory for winds from 6 AM to 1 PM PST Wednesday
     for Coastal Waters from Cape Shoalwater WA to Cascade Head
     OR out 60 nm.

     Gale Watch from Wednesday morning through Thursday afternoon
     for Coastal Waters from Cascade Head OR to Florence OR out
     60 nm.

     Small Craft Advisory for winds from 6 AM to 11 AM PST Wednesday
     for Coastal Waters from Cascade Head OR to Florence OR out
     60 nm.

     Small Craft Advisory for Rough Columbia River Bar from noon to
     6 PM PST Wednesday.

&&

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This discussion is for Northwest Oregon and Southwest Washington
from the Cascade crest to 60 nautical miles offshore. The area is
commonly referred to as the forecast area.