High Wind in Oregon, December 20, 2018

Area Forecast Discussion
National Weather Service Portland OR
858 PM PST Wed Dec 19 2018

.Updated aviation and marine sections.

.SYNOPSIS...A warm front will lift north through western Oregon and
into Washington this afternoon and tonight. The trailing cold front
will move through on Thursday, as the low pressure center lifts NE
into Vancouver Island. Active weather pattern continues into early
next week, with lower snow levels starting this weekend.

&&

.SHORT TERM...Today through Saturday...We have some returns on radar
this afternoon, but with ceilings generally up around 10,000 feet, no
rain is being observed on the ground today. Surf along the coast
today has been lower than models were forecasting, so we have
cancelled the high surf advisory.

Our next system is currently sitting offshore around 42N, 138W. The
associated warm front is draped to our south, situated right around
the California-Oregon border, but offshore. We are going to see this
warm front lift north this afternoon and evening, which should
generate some rain along the north Oregon coast and the south
Washington Cascades this evening. Some rain may spill over into the
northern Willamette Valley, but confidence isn't high as this front
isn't doing much inland right now to our south.

Once the warm front lifts to our north, expect mostly dry conditions
overnight as we move into the warm sector of the cyclone. This will
be the time period when winds start to ramp up along the coast as the
cold front starts to move closer. We have issued a high wind warning
for southerly winds peaking during the morning hours with gusts up to
65 mph on beaches and headlands, and up to 60 mph in northern coastal
communities.There is some uncertainty in the winds along the central
Oregon coast. The ECMWF remains the outlier with a stronger low
moving into Vancouver Island. Right now steering the forecast away
from that solution as it is the outlier, but if it proves to be true,
central Oregon coastal communities could see wind gusts up to 60 mph
as well. Peak winds will occur Thursday morning, helping to drive up
seas, but most of the energy will be going due north, so there
shouldn't be any impacts along our beaches.

Have issued a minor tidal overflow advisory out for the south
Washington coast. Total tide is expected to reach just above 12 ft
around the 10:36 AM high tide.

The cold front will move through midday Thursday, bringing moderate
rainfall. As this system will move through relatively quickly, and
the moisture transport is oriented north as opposed to due east, we
aren't expecting a large amount of rain with this system. Really only
0.75 inch to 1 inch of rain along the coast, coast range, and
Cascades. Inland valleys will probably only see around 0.5 inch of
rain. Behind this front, showers will linger overnight Thursday night
into early Friday morning. Snow levels will be falling behind the
front, dropping to around 1500 to 2000 feet just in time for the
showers to come to an end. The Cascade passes should see an inch or
two of snow as showers come to an end Friday morning. Cascade
foothills may see some flurries or a patchy dusting of snow early
Friday morning, but it shouldn't accumulate to much.

High pressure builds over the area on Friday, for relatively quiet
conditions. Saturday, the models diverge on whether or not we will
see a weak shortwave move through, bringing precipitation Saturday
morning to south Washington and far northern Oregon. I don't think
this is going to happen, but have left a 25% chance of PoPs due to
the ECMWF model solution. If the ECMWF proves true, snow levels will
be around 1000-1500 feet, and amounts should be light. -McCoy

.LONG TERM...Saturday through Wednesday... The GFS 12Z and ECMWF 12Z
agree that precipitation will arrive along the coast Saturday night.
However, the models seem to be disagreeing on when the precipitation
will affect the forecast region based on where they are forecasting
the center of the low. The GFS shows the low pressure center farther
north than the ECMWF resulting in a lag in when this initial system
passes fully over the forecast area. Therefore the GFS has
precipitation from this initial system lingering for a longer period
of time. Whereas, the ECMWF has a small break before a second low
moves in early Monday morning at the same time the GFS first system
is dissipating.

Models come back into agreement by Tuesday showing a split system
with a high pressure affecting the forecast region and reducing
precipitation Tuesday night into Wednesday. Temperatures will become
progressively cooler  and snow levels progressively lower throughout
the extended which brings a chance of mountain snow through
Wednesday. -BPhillips
&&

.AVIATION...VFR currently and is expected to continue through
about 20/14-16z. Spotty light rain will continue as a warm front
passes over the area. Cigs may lower a tad toward that time,
which could begin to impact visual approaches at KPDX and KEUG.
Then, will see the cold front approach which should increase
rain rates and start to bring cigs down into a 015-025 MVFR
range. Cold front will slowly cross the region from mainly west
to east reaching the coast around 20/19z and the I-5 corridor
near 20/21z. Gusty prefrontal winds appear to briefly reach 40-45
kt at KONP and KAST with inland gusts mainly around 30 kt,
although a brief isolated gusts 35-40 kt is a slim possibility.
Post frontal cigs will minimally improve, if at all, under
showery onshore flow and likely stay MVFR at 025-030 across the
board through 21/06z.

PDX AND APPROACHES...VFR with cigs mainly staying above 040
through the boxer arrivals. Cigs will likely fall under 040
nearer to 18z before falling to 025 with the cold frontal
passage around 21z. East wind 10 to 15 kt will continue until
southeast to south winds increase with gusts 25-30 kt beginning
near 13z. Currently looks like peak southerly winds of 20g30kt
will occur from roughly 16z until the frontal passage at 21z.
/JBonk

&&

.MARINE...Overall conditions are running a few feet and several
kts below forecast so far this evening. Have adjusted the
forecast downward to match current and expected reality for the
next few hours. Have then begun to taper back toward the previous
package keeping the 4 AM conditions forward. This means our Gale
Warning is somewhat overdone for now but the adjusted forecast
does bring gales in overnight. Thus decided to leave the timing
alone. Storm force gusts still looking very favorable across the
northern waters and perhaps not so much for the central coast.
Didn't have enough confidence to say they will not occur over a
couple hour period and so also kept that warning in place. The
low bringing about these winds will deepen to about 982 mb as it
moves northeast to Vancouver Island tonight and Thursday.

As the deepening low lifts northeast tonight, it will push a
very strong cold front eastward. Winds will increase to
widespread 30 to 40 kt out of the south with gusts as high as 55
kt Thursday morning. Have extended the Storm Warning to the
waters off the central Oregon coast, but winds to actual storm
level will likely be more brief over that area than further
north.

Seas will build with the approaching low and associated winds,
likely topping at 23 to 26 ft Thu morning, then quickly
dropping back down to 20 ft or even lower Thursday afternoon.

Lull in the weather for Fri, but active pattern continues with
another front arrive on Sat, and yet another on Sun. JBonk/Bowen

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...High Wind Warning from 1 AM to 1 PM PST Thursday for North
     Oregon Coast.

     High Wind Warning from 4 AM to 1 PM PST Thursday for Central
     Oregon Coast.

WA...High Wind Warning from 1 AM to 1 PM PST Thursday for South
     Washington Coast.

     Coastal Flood Advisory from 6 AM to 1 PM PST Thursday for South
     Washington Coast.

PZ...Storm Warning from 1 AM to 4 PM PST Thursday for Coastal Waters
     from Cape Shoalwater WA to Cascade Head OR out 60 NM.

     Gale Warning until 1 AM PST Thursday for Coastal Waters from
     Cape Shoalwater WA to Cascade Head OR out 60 NM.

     Storm Warning from 3 AM to 1 PM PST Thursday for Coastal Waters
     from Cascade Head OR to Florence OR out 60 NM.

     Gale Warning until 3 AM PST Thursday for Coastal Waters from
     Cascade Head OR to Florence OR out 60 NM.

     Small Craft Advisory for Rough Columbia River Bar until 6 PM
     PST Thursday.

&&

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