High Wind in Oregon, November 4, 2023

Area Forecast Discussion
National Weather Service Portland OR
345 AM PDT Sat Nov 4 2023

.SYNOPSIS...Wet weather continues through the weekend as a
pair of atmospheric rivers impact the Pac NW, with the heaviest rain
expected Saturday morning. Active weather will then continue early
next week as a fairly strong low pressure system brings more heavy
rain, gusty winds, and a 15-30% chance of thunderstorms. Trending
cool and showery on Tuesday before high pressure brings a break from
precipitation Wednesday into Thursday morning.

&&

.SHORT TERM...SATURDAY MORNING THROUGH MONDAY NIGHT...Active weather
continues Saturday morning. From 1:00-3:00 AM Saturday, radar
observations showed multiple fast-moving bands of heavy rain moving
eastward across southwest WA and far northwest OR, heaviest over the
Portland/Vancouver metro and north OR Coast Range. In fact, most
locations across the Portland metro had received anywhere between
0.25-0.5 inches of rain over the past three hours. At the NWS
Portland Weather Forecast Office, 0.25 inches of rain was measured in
a 45 minute period (1:00-1:45 AM) with the first band of heavy rain.
This has resulted in ponding of water in some areas across the metro,
especially in low-lying areas with poor drainage. May need to issue
an urban Flood Advisory later this morning with the last band of
heavy rain that is expected to move through sometime between 7-11 AM
with the passage of a strong cold front, unless the band of rain is
able to move through the metro very quickly. Regardless of the speed,
this last band of rain looks to be the heaviest as the latest
iteration of the HRRR suggests rain rates will peak between 0.25-0.3
in/hr. KLGX radar observations already show this band of heavy rain
approaching the Long Beach Peninsula.

Motorists should drive slowly to avoid hydroplaning. As always, be
sure to turn around if you encounter a flooded roadway (if possible).
Despite the heavy rain, still not expecting any mainstem river
flooding, however sharp rises will occur. In addition to the rain,
the cold frontal passage will also bring gusty south winds up to
25-35 mph, except up to 50-60 mph along beaches and headlands at the
coast. A marginal High Wind Warning remains in effect for the beaches
through 9 AM Saturday, however this warning may need to be cancelled
a bit early if the cold front pushes inland faster than expected.
Minor tree damage and isolated power outages are possible Saturday
morning as the cold front sweeps towards the Cascades. Also cannot
rule out a rumble or two of thunder Saturday morning as at least one
lightning flash has already been observed to the southwest of Salem
within a heavy rain band earlier this morning.

The cold front should reach the high Cascades Saturday afternoon
while a few post-frontal showers develop to the west. The latest
suite of CAM guidance suggests isolated to scattered showers will
then continue Saturday night before a brief break arrives Sunday
morning ahead of the next weather system. This break will be
short-lived as the next system is set to move inland Sunday
afternoon, bringing more rain with it (albeit not as heavy as
Saturday's rain). However, rain is expected to become heavier again
Sunday night into Monday as a closed surface low over the coastal
waters approaches the Long Beach Peninsula and sends a frontal system
inland. Depending on the track of this surface low, gusty south winds
will also be a possibility from the coast to the Cascades. As of
right now, it appears winds will peak Monday afternoon with gusts up
to 40 mph across all of southwest WA and northwest OR, except up to
55 mph in higher terrain and at the immediate coast. There is still
some uncertainty regarding the exact track of the surface low,
however the NBM is already showing a 30-50% chance of max wind gusts
up to 40 mph across the Willamette Valley and Portland/Vancouver
metro. The NBM is also showing a 15-30% chance of thunderstorms on
Monday, which seems reasonable as most models show SBCAPE values
increasing to at least 200-400 J/kg. Gusty and erratic winds can be
expected with any thunderstorms that do develop on Monday. In fact,
cannot rule out funnel clouds or a weak tornado with some storms on
Monday as NAM soundings show a very favorable wind profile with
southerly surface winds veering to the west with height. Speed shear
also looks good, and helicity values increase to 300-450 J/kg from
Kelso to Portland to Salem to Eugene.

On a final note, snow levels will remain well above the Cascade
passes through the weekend, with snow at pass level likely holding
off until Monday night/Tuesday.  -TK

.LONG TERM...TUESDAY THROUGH FRIDAY NIGHT...Models and their
ensembles are in good agreement regarding the synoptic scale pattern
Tuesday through Wednesday. This is highlighted well by WPC's cluster
analysis, as all four clusters show an upper level trough over the
Pacific Northwest on Tuesday giving way to a shortwave ridge on
Wednesday. This will result in cool and showery conditions on Tuesday
along with a slight chance of thunderstorms along and near the coast.
This trough will bring cooler temperatures aloft, likely lowering
snow levels to around 4000-4500 feet. As such, expect at least some
snowfall accumulation at pass level; anyone with travel plans over
the Cascade passes on Tuesday should be prepared for winter weather
driving conditions. Conditions will begin to improve Tuesday night
into Wednesday as the aforementioned shortwave ridge moves inland and
surface high pressure builds in. This will make for a decent day on
Wednesday with some breaks of sunshine and light winds expected. The
deterministic NBM suggests high temperatures will top out in the mid
50s across the lowlands.

NBM PoPs increase late Thursday into Friday as the next weather
system approaches the area. While the GEFS/EPS/CMC all agree this
system will eventually bring rain back to the area, there are notable
timing differences. For example, some of the EPS ensemble members
bring rain back into the area Thursday afternoon, while other members
hold off until Thursday night. NBM 6-hourly PoPs peak on Friday
between 65-85% before decreasing a bit Friday night.  -TK

&&

.AVIATION...12z TAFs: Current radar imagery and observations as of
10z Sat show a frontal system bringing widespread rain across the
region with a mix of IFR, MVFR, and low-end VFR cigs. From now
through 18z Sat, expect the potential for heavy rain which would
bring brief periods of vis and cig reduction (IFR or lower) at any
given terminal. After 18-21z Sat, the region transitions into a
post-frontal environment with lingering showers and potential
cig improvement to VFR in the Willamette Valley (20-30% chance of
MVFR cigs). The coast still maintains a 50-70% chance of MVFR cigs
mid-morning Saturday through the evening. It is also worth
mentioning that there is a 15-25% chance of thunderstorms across
the region through Saturday evening, however, confidence is not
high enough to add to TAFs.

Southerly winds may gust up to 35-40 kt along the coast and 20-25
kt in the Valley through 18z Sat. After 18z Sat, winds begin to
weaken across the region as the front moves further inland, gusting
up to 20-25 kt along the coast. Also expect some LLWS along the
coast (speed shear) and northern Willamette Valley (directional
shear) through 18z Sat.

**The new Aviation Weather Center website is live. The new website
can be found at aviationweather.gov **

KPDX AND APPROACHES...Expect a mix of MVFR and low-end VFR cigs
through 18z Sat with periods of heavy rainfall that could bring
vis reductions to IFR thresholds. After mid-morning/early
afternoon Saturday, cigs likely improve to VFR with lingering
showers. Easterly surface winds between 10-15 kt with SW flow
aloft may result in LLWS between 12-18z Sat.	   -Alviz

&&

.MARINE...Current observations as of 230am PDT at buoys 46029 and
46050 show southerly wind gusts approaching gale-force criteria
at 30-35 kt with seas 9 to 10 ft at 8 seconds. As the current
frontal system pushes across the waters, gales are forecast to
continue through mid-morning Saturday with wind gusts up to 40-45
kt. Meanwhile, seas early morning Saturday at 9 to 12 ft will
build up to 13 to 15 ft through the day and hold through Saturday
night. Once the front moves inland, southerly winds over the waters
Saturday afternoon will gradually weaken and gust up to 25-30 kt
(small craft criteria) through Sunday.

A low pressure system currently far offshore in the NE Pacific
is forecast to approach the waters Sunday night to Monday. Models
currently have this system coming in from the southwest and
turning winds more westerly as it tracks toward the northeast.
There is still some uncertainty with whether or not this would
bring gale-force gusts, however, current NBM guidance suggests a
60-80% chance of wind gusts exceeding 34 kt between Sunday night
to Monday night. Seas during this time frame also look to be
around 10 to 13 ft.	 -Alviz

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...High Wind Warning until 9 AM PDT this morning for Central Oregon

     Coast-North Oregon Coast.

WA...High Wind Warning until 9 AM PDT this morning for South
     Washington Coast.

PZ...Gale Warning until 3 PM PDT this afternoon for Coastal waters
     from Cape Shoalwater WA to Florence OR out 10 NM-Columbia
     River Bar-Coastal waters from Cape Shoalwater WA to
     Florence OR from 10 to 60 NM.

     Small Craft Advisory from 3 PM PDT this afternoon to 6 AM PST
     Sunday for Coastal waters from Cape Shoalwater WA to
     Florence OR out 10 NM-Columbia River Bar-Coastal waters
     from Cape Shoalwater WA to Florence OR from 10 to 60 NM.

&&

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