High Wind in Oregon, November 24, 2016

Area Forecast Discussion
National Weather Service Portland Oregon
932 PM PST Wed Nov 23 2016

.SYNOPSIS...An active weather pattern will bring periods of valley
rain, mountain snow and windy conditions at the coast for the next
several days.

&&

.SHORT TERM...Tonight through Friday...Water vapor satellite imagery
reveals an impressive shortwave trough encompassing much of the Gulf
of Alaska this evening. Infrared satellite imagery shows plenty of
cooling cloud tops off the Pacific Northwest coast due to large scale
lift ahead of the shortwave trough and an advancing low level front.
A strong jet on the backside of the trough is beginning to result in
the shortwave trough digging. This will result in the approaching
front to become more oriented from southwest to northeast as it drops
southeastward across the area late tonight and Thursday.

A conditionally unstable airmass has led to numerous showers this
evening across the region. 3 hourly QPF has been relatively light
with just a couple hundredths of an inch in the Willamette Valley and
upwards of a quarter inch in the wettest spots near the coast and in
the Cascades. Snow levels continue to hover around 3500-4000' per
surface observations in the Cascades. It should be noted that
temperatures have fallen across the upper Hood River valley and we
could see precipitation begin to mix with snow above 2000-2500 ft
later tonight and Thursday morning. The NAM & WRF-ARW wet bulb zero
heights support this idea as cold air banks up against the Cascades.
Do not foresee major accumulations at these elevations, but still
something worth monitoring.

Relatively mild ocean temperatures have resulted in convection
continuing off the coast this evening. Given the high shear
environment, a few of these cells have shown some modest rotation a
couple thousand feet AGL so will continue to monitor radar the next
few hours. Mid level warm advection should diminish this threat
fairly quickly later this evening.

Gale force winds have spread across the waters in advance of an
approaching cold front. This cold front is beginning to make an
appearance on the KLGX radar in the form of a disorganized band of
higher echoes ~50 miles off the WA coast. Expect winds to strengthen
across the waters in advance of this front and then spread onto our
beaches and headlands towards midnight, particularly along the Oregon
coast. Pressure gradients remain oriented pretty offshore during this
event so it will be difficult for the strongest winds to reach the
coastline, but we should still see a few gusts around 60 mph in our
more exposed locations along the central & north Oregon coastline.
The current high wind warning covers this well.

Convergence along the front will continue to be quite strong. This
will likely produce a narrow band of locally heavy rain that will
slowly drop southeastward across the region on Thursday morning.
Interestingly, this atmospheric river event is not all that unusual
for this time of year in terms of return intervals for PW and IVT
values so it will likely take the front sitting over a basin or area
to produce higher rainfall rates. With that said, the NCAR Ensemble
puts greater than a 50% chance of seeing 1-hourly rainfall rates in
excess of 0.50" along portions of the coast Thursday afternoon. Even
the raw 1 hourly GFS output suggests a rainfall rates will likely
peak out between 0.30"-0.50" along the coast. While our river basins
can handle an hour or two of these rainfall rates, if these rates
persist over any basin for more than a couple hours, the odds of a
river or two reaching minor flood stage increase dramatically. And
this is where the uncertainty lies...

Models have varied considerably on how fast this front will drop
southeastward across the area, which has led to considerable
uncertainty in the QPF and river forecasts. The GEM, EC & WRF-ARW
models hold the heaviest QPF over the Nehalem, Wilson, Nestucca &
Siletz River Basins on Thursday while the latest GFS & HRRR runs
suggest the front will likely drop southward through Thursday morning
and end up over the Alsea and Siuslaw River drainages by midday. The
GFS & HRRR runs would be less likely to produce river flooding as the
heaviest rainfall would likely not sit over any given drainage too
long. However, it's hard to ignore the EC & GEM solutions so will
keep the current flood watch as is for now.

An embedded shortwave currently out near 150W will race towards the
region Thursday afternoon and evening. The EC and the GEM to some
degree insist this will remain an open wave while the GFS and its
followers such as the NAM, suggest a surface low pressure will deepen
to around 995mb and move northeastward towards the Washington coast
Thursday night. This latter scenario would likely bring a secondary
bout of high winds to our coastal zones Thursday evening while the
former scenario will increase winds along our coast during this time,
but not likely not to high wind levels. Given the position of this
shortwave relative to the jet, a weaker and more open wave seem more
plausible, but confidence is certainly not high this will be the
case. As a result, will keep the high wind watch intact for our
coastal zones for Thursday evening.

Meanwhile, all this rain will fall in the form of snow across the
higher elevations Thursday and Thursday night. However, snow levels
will vary quite a bit across the Cascades during this time. Cold air
will likely be stubborn to scour out across the south Washington
Cascades especially near Mt Adams where 10-20" of snow appear likely
to fall above 3500' and somewhere between 15-30" above 4500'. Farther
south, warm air streaming northeastward ahead of the front should
result in snow levels rising to ~4500' Thursday afternoon and evening
across the Oregon Cascades. This process will be slowest to occur
near Mt Hood so the current Winter Wx Advisory looks on track. The
good news is that this should bring a period of improved travel
conditions for those traveling across the Cascade passes on Thursday.

Cool, but substantially quieter weather will likely follow on Friday
before more showers produce valley rain and mountain snow showers
above 3500-4000 feet Saturday and Saturday night. /Neuman

.LONG TERM...(Saturday night through Wednesday)...No Changes...
Previous Discussion from Wednesday afternoon follows...The active
weather pattern looks to continue through much of the long term
forecast as a series of storm systems move across the Pac NW. Models
continue to show an upper level trough moving across the forecast
area Saturday night and Sunday which will maintain showers over the
region. Snow levels look to lower to around 2500 ft on Sunday, with
an additional 3 to 6 inches possible at the Cascade passes. If you
are planning on traveling over the Cascades, prepare for hazardous
road conditions.

Models then show a weak upper level shortwave dropping southeastward
Sunday night and Monday. This system looks to bring another round of
rain to the area late Sunday/early Monday, with cool northwesterly
flow keeping showers going on Monday. High pressure looks to build
across the area Monday night and Tuesday bring with it a period of
quieter weather and raising snow levels to around 4500 ft. However,
this bring will be short as models show another upper level trough
dropping south out of the Gulf of Alaska Tuesday night and Wednesday
which could bring another round of heavy rain and Cascade snow, with
locally windy conditions. /64

&&

.AVIATION...Little change over the last several hours where
mainly VFR conditions prevail this evening with occasional dips to
MVFR both inland and at the coast. By later this evening most
terminals will drop to MVFR as the next slug of moisture moves
towards the area. Probably not much change to the MVFR Cigs for
much of Thursday. Cigs may rise back to VFR for several hours
Later Thursday afternoon and early evening.

Prefrontal winds have been underwhelming thus far but should
increase as the cold front approaches. Expect strongest southerly
frontal wind gusts along the coast near KONP around 40 knots and
around 30 knots at KAST. Should see 25 to maybe brief 30 knots for
the inland terminals peaking around 12Z with 20 to 25 knot gusts
the norm for the several hours around that. South winds appear to
increase again with gusts 30 to 40 knots Thursday afternoon and
evening but there is significant model uncertainty regarding the
track of a potential surface low and even it's possible existence
versus on open surface trough.

KPDX AND APPROACHES...Conditions will bounce between MVFR and VFR
this evening before settling to VFR for most of the overnight
period. Lower cigs are not expected until Thursday afternoon as
the front approaches. Winds will increase overnight and peak 20g30
knots around 12Z Thu plus another possible similar peak after
25/04z. JBonk/Bentley

&&

.MARINE...Gale gusts are underway at the inner buoys and am
eagerly waiting on an evening ASCAT pass to confirm the potential
for the outer waters. Offshore flow still dominates the near
shore waters where small craft advisory winds appear to be the
norm for now. Have seen sporadic gusts 35-40 kt under convective
showers embedded within the rain bands. This is still covered by
the ongoing gale warning. The cold front is sitting just outside
of the coastal waters at this hour and will move east- southeast
over the next 5-7 hours reaching the Columbia mouth somewhere
around 3-5 am PST. Do expect the strongest winds will occur for a
couple hour period with the frontal passage. Latest hi- resolution
models hint there may be occasional gusts 45-50 knots with the
frontal passage. Given the brief and marginal nature, cannot
justify issuing a storm warning for tonight's frontal passage.
Seas at buoy 46005 peaked at 23 feet several hours ago and was
likely driven by the local wind waves for a brief time before
settling down to around 20 feet for the following few hours.
Typical wave decay would bring that westerly swell in at 18-20
feet which remains in line with the current forecast for later
tonight. winds behind the front will drop of dramatically but
still likely sit in the SCA realm. The central waters, however,
have a chance at seeing low end gale gusts through the day
Thursday.

00z models have done little to change confidence fro the next
system to affect the waters later Thursday afternoon and Thursday
evening. The track and strength of this second system keeps
changing in small but impactful ways, leading to more uncertainty
with this second system. Have left the forecast from the previous
shift in place which would bring another round of gales, with a
chance for storm-force winds especially south of the Columbia
River Thursday evening and decreasing around 10pm Thursday night.
Have, unfortunately, still left the storm force watch in place
with the hopes the current front and the expected additional
satellite derived wind data will provide a clear picture. If a
choice had to be made right now, would be inclined to drop the
storm watch and go with a strongly worded gale warning...similar
to the current one with gusts primarily 40-45 kt and the
occasional gusts 50-55 kt possible. The current forecast would
bring seas back up to 20 to 25 ft Thursday evening with seas
starting to fall back down after the winds start to subside after
10pm Thursday night. Keep an eye on the forecast for this system
as it is subject to change.

Friday things start to calm down, with winds and seas coming back
down. Still think we will see small craft advisory winds through
most of the day on Friday, though winds should drop below 20 kt
by Friday evening. Seas should drop back down into the mid-teens
by Friday evening as well. Weaker fronts move across the waters
over the weekend, but generally seas expected to drop to around 9 ft
by Saturday night or Sunday, and winds stay fairly benign for this
time of year. JBonk/McCoy

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory from 4 AM Thursday to 4 AM PST Friday
     for Northern Oregon Cascades.

     High Wind Warning from 1 AM to 10 AM PST Thursday for Central
     Oregon Coast-North Oregon Coast.

     Flood Watch from Thursday morning through Friday evening for
     Central Coast Range of Western Oregon-Central Oregon Coast-
     Central Willamette Valley-Coast Range of Northwest Oregon-
     Greater Portland Metro Area-North Oregon Coast-South
     Willamette Valley.

     High Wind Watch from Thursday afternoon through Thursday
     evening for Central Oregon Coast-North Oregon Coast.

WA...Winter Storm Warning from 1 AM Thursday to 4 AM PST Friday for
     South Washington Cascades.

     Flood Watch from Thursday morning through Friday evening for
     South Washington Coast.

     High Wind Watch from Thursday afternoon through Thursday
     evening for South Washington Coast.

PZ...Storm Watch from Thursday morning through Thursday evening for
     Coastal Waters from Cape Shoalwater WA to Florence OR out
     60 nm.

     Gale Warning until 10 AM PST Thursday for Coastal Waters from
     Cape Shoalwater WA to Florence OR out 60 nm.

     Small Craft Advisory for Rough Columbia River Bar until 4 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. The area is
commonly referred to as the forecast area.