Ice Storm Warning 4, NWS Portland, OR

Area Forecast Discussion
National Weather Service Portland OR
1023 AM PST Tue Feb 14 2017

.SYNOPSIS...One more dry day today, but we will see increasing
clouds. Then back to damp weather as a front pushes into southwest
Washington and northwest Oregon tonight and Wednesday, continuing
over the area Wednesday night before moving onshore and through the
area early Thursday. There will be an initial surge of wind near the
Oregon coastal headlands later tonight and early Wednesday, followed
by stronger more widespread winds along the coast Wednesday afternoon
into early Thursday. With brisk east winds in the Gorge reinforcing
cold air in the Columbia River Gorge, there is a threat of
significant ice east of about Multnomah Falls and in the Hood River
Valley. We will see showery weather for the remainder of Thursday
through next weekend, with snow levels just below the passes in the
Cascades.
&&

.SHORT TERM...Today through Thursday...One more dry day today with
brisk winds continuing through the Gorge and the patchy fog that did
form this morning having mostly cleared.

Attention now turns to the organizing baroclinic or frontal zone off
the coast. The models show it slowly moving onshore tonight. An
initial wave along the coast could bring some headland high winds
along the Oregon coast later tonight and Wednesday morning and have
issued a High Wind Watch for that. A second stronger surface low
moves north along the coast later Wednesday and Wednesday night with
50 kt boundary layer winds and over 90 kts of wind at 850 mb. Have
issued a second High Wind Watch for the entire coastal area for that.

The precipitation type in the Gorge is the next problem. The brisk
east winds that will continue into Wednesday night and model
soundings near Hood River suggest freezing rain will be the main
hazard from about Multnomah Falls eastward. It is 26 degrees at The
Dalles this morning, so it is plenty cold. Precipitation amounts
support a lot of ice, from 0.5 inch to an inch or more. Will
maintain the Winter Storm Watch for that.

This system will produce substantial rains as well. Look for rain
most areas Wednesday into Thursday morning, heaviest along the south
Washington and north Oregon coast and the adjacent coastal mountains,
as well as the south Washington Cascades. Currently, no rivers
approach flood stage but expect significant rises.

The front moves through Thursday morning with showers spreading in.
Snow levels will lower to near the passes in the Cascades later
Thursday. Tolleson

.LONG TERM...No Changes. Previous discussion follows...
Thursday Night through Monday...Models continue to show the moisture
plume shifting south late Thursday. However, this is also when they
start to diverge from previous runs that showed the upper level
trough moving across the region. Instead, they now show the trough
digging into the northern California offshore waters on Friday before
moving inland late Friday. This could nudge a weak upper level ridge
over the forecast area on Friday that could bring a brief period of
dry weather, but confidence in this scenario is low. The most likely
scenario is that cool and showery conditions persist through
Saturday. Snow level generally remain above 2000 ft so expect light
snow showers in the Cascades. The active weather pattern continues as
models then show another upper level trough approaching the Pac NW on
Sunday. This will likely bring another round of rain and Cascade snow
Sunday and Monday. /64
&&

.AVIATION...VFR conditions prevail across most of the region this
morning with only a high overcast deck. Only exception remains local
IFR fog in the southern Willamette Valley, mostly from KEUG south.
This should improve through 19z this morning. Increasing high/mid
clouds through day ahead of next frontal system. Light rain reaches
coast this evening 03-06z and pushes inland overnight by 10z.
Heavier, steadier rain and MVFR cigs/vsbys push onshore by 10z and
into interior closer to 12-15z. MVFR conditions and RA continue into
Wed. Gusty south winds develop at coastal terminals late overnight,
while strong east wind through gorge will continue at KTTD.

KPDX AND APPROACHES...VFR through Tue afternoon. Gusty east winds
will persist out of gorge through Tuesday, but less of an impact at
KPDX terminal. Lowering cloud deck late overnight through early Wed
with light rain, but MVFR cigs and steady rain hold off until after
12z.  Cullen
&&

.MARINE...A complex weather pattern will result in several lows
affecting the marine weather through the weekend. The most immediate
concern is the winds which will increase this afternoon. Gale force
gusts will materialize this afternoon beyond 60 nm. East component to
the winds near the coast will hold off gales for waters within 10 nm
until tonight, but think waters within 10 nm will also see gale force
gusts tonight despite the models keeping the higher winds well
offshore. Have therefore modified the Gale warnings to account for
these changes. There will be a second burst of strong winds Wednesday
afternoon and Wednesday night with the potential for storm force
gusts, and have made minor modifications to the Storm Watch.  Seas
will build this afternoon and exceed 10 feet late tonight. The seas
will be confused and a combination of two separate swell trains one
from the NW and another from the SW with strong differences in wave
periods. The SW swell will rapidly build to above 20 feet Wednesday
morning as a smaller (6 ft) longer period(18 seconds) swell slowly
builds. Have leaned towards the NWPS for Significant wave heights
which is a tad higher than the ENP, but tends to perform better under
these conditions as it is initialized from the NWS forecast winds.
THe NWPS is forecasting seas up to 26 ft late Wed night. ~TJ

Seas may actually rise above 10 feet in the outer waters shortly
before the gale warning begins, but for the sake of simplicity, will
not issue a small craft advisory for seas for a short, 6 hour period.
Otherwise, seas still appear to rise to between 20 and 25 feet
Wednesday night. /Bentley
&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Storm Watch from late tonight through late Wednesday
     night for Central Columbia River Gorge-Upper Hood River
     Valley-Western Columbia River Gorge.

     High Wind Watch from 9 PM PST this evening through Wednesday
     morning for Central Oregon Coast-North Oregon Coast.

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

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

WA...High Wind Watch from Wednesday evening through Thursday morning
     for South Washington Coast.

     Winter Storm Watch from late tonight through late Wednesday
     night for Central Columbia River Gorge-Western Columbia
     River Gorge.

PZ...Small Craft Advisory for Rough Columbia River Bar until 4 PM
     PST Wednesday.

     Small Craft Advisory for winds until 1 PM PST this afternoon
     for Waters from Cape Shoalwater WA to Florence OR from 10
     to 60 nm.

     Storm Watch from Wednesday morning through late Wednesday night
     for Waters from Cape Shoalwater WA to Florence OR from 10
     to 60 nm.

     Gale Warning from 1 PM this afternoon to 10 AM PST Wednesday
     for Waters from Cape Shoalwater WA to Florence OR from 10
     to 60 nm.

     Storm Watch from Wednesday evening through late Wednesday night
     for Coastal Waters from Cape Shoalwater WA to Florence OR
     out 10 nm.

     Gale Warning from 6 PM this evening to 6 PM PST Wednesday for
     Coastal Waters from Cape Shoalwater WA to Florence OR out
     10 nm.

&&

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

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.