Area Forecast Discussion
Issued by NWS Portland, OR

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109
FXUS66 KPQR 120406 AAA
AFDPQR

Area Forecast Discussion UPDATED
National Weather Service Portland OR
805 PM PST Mon Nov 11 2024

...EVENING UPDATE, for aviation weather...

.SYNOPSIS...Post frontal showers today ahead of a warm front
that will move inland Tuesday night. Snow over the Cascades with
periods of heavy rain for the next 24 hours. Heavy stratiform
rain and strong winds Tuesday night into Wednesday. Gusts as
high as 55+ mph possible along the coast. Tidal overflow
concerns with high surf mid-week. Rain continues through the
week.

&&

.SHORT TERM...Now through Tuesday Night...Satellite and radar
show post frontal showers trailing a cold front that moved
through the area today. Onshore westerly flow is present with
cold air aloft. Snow has been falling over the Cascades with
snow levels above 5500 feet. The passes are clear at this time.
There remains a 10-15% chance for thunderstorms today with a 20%
chance along the coast.

On Tuesday a closed low centered in the Gulf of Alaska will
begin to shift southward. Within the flow around this low is a
shortwave which is associated with a warm front that will make
landfall on Tuesday. This warm front has ample support aloft as
the jet streak will move nearly directly over the forecast
area. This jet streak is coupled at both 250 mb, 500 mb, with a
weak coastal jet at the surface. As the winds become more
southerly Tuesday night, that coastal jet will compress against
the coast. Wind gusts are expected to exceed 50 mph along the
coast - especially the beaches and headlands. There is around a
50% chance for gusts to meet High Wind Warning criteria (58 mph
gusts) along the beaches and headlands of the north Oregon and
south Washington coasts until early Wednesday morning. Have
issued a High Wind Watch at this time for those areas due to
these probabilities and the pattern. Will note that the hi-
resolution models seem to be running a bit high for the wind
speeds with the 90th percentile gusts reaching nearly 70 mph.
Again, this is the outlier in all of the ensembles both hi-
resolution and the NBM. If winds shift more to the west or even
southeast, winds will not be nearly as strong. It is still a
good idea to anchor loose outdoor objects, outdoor decorations,
and prepare for the case there are any power outages. Inland
sites will be gusty from the south with gusts as high as 35 mph.

Wind is just one of the components of the forecast for this
system as rain too will be a feature to watch. Rainfall will
start off stratiform with topographically driven amounts. The
Coast Range and Cascades will see the highest precipitation
totals. Let`s look at thee NBM 24 hour rainfall 25th-75th
percentile rainfall totals for example. Around the Long Beach
Peninsula, QPF ranges from 2.0-2.8 inches, and the central
Oregon Coast Range ranges from 2.0-3.2 inches. Salem will see
anywhere from 0.8-1.5 inches, and the south Washington Cascades
will see precipitation amounts of liquid water around 2.5-3.7
inches. Snow levels will be 5000 ft during the period of the
most rain. Thus, snow related hazards are not expected. As the
days have progressed, river flooding concerns have subsided. For
the flashiest rivers there is still elevated attention being
paid in case rainfall amounts are higher than expected.

There are concerns for tidal overflow and high surf conditions.
For more details on these hazards, see the "Beach Hazards"
section below.-Muessle

.LONG TERM...Thursday through Monday...The long term forecast
is highlighted by a cool and showery weather pattern typical of
mid November. WPC`s cluster analysis for 500 mb heights and
anomalies suggests excellent agreement between the GEFS/EPS/CMC,
showing an upper level trough over the coastal waters late
Thursday shifting inland over the western CONUS Friday. At 500
mb, there is a weak ridge attempting to build in but it is
unsupported at the surface. Even with this being the case,
northerly flow will dominate and cannot rule out clearing skies
though which will support much cooler overnight lows. The
probability for low temperatures near freezing overnight Friday
into Saturday morning have increased since previous forecasts.
For rural areas of the inland valleys and near Eugene, there is
around a 30% chance of below freezing temperatures. Even higher
probabilities (near 60%) for the valleys of the Cascade
Foothills around Sweet Home, Creswell and Molalla. The greater
Portland-Vancouver metro area will see less than a 5% chance of
below freezing temperatures.

This cooler and drier pattern will quickly come to an end as the
flow becomes zonal on Sunday with increasing onshore flow. This
onshore flow will send a Pacific frontal system inland bringing
more significant rain amounts late in the weekend. That being
said, expecting minimal impacts with this system as winds do not
look significant and ensemble guidance for QPF suggest rain
amounts will not be high enough for hydro concerns.

Weak ridging attempts to build in once again early next week but
will be difficult to overcome the pervasive troughing from this
last week.-Muessle/TK

&&

.AVIATION...Unstable air mass, with plenty of showers through
tonight. Expect mix of lower VFR and occasional MVFR with passing
showers.  Does appear will have a period when showers decrease late
tonight into tomorrow am, when will have more MVFR CIGS than VFR.
Higher terrain frequently obscured in clouds.

MVFR clouds will gradually break by midday Tue, with variable mid
and high clouds remaining. This as next system approaches. Will see
VFR through the afternoon into early evening, with rain spreading
inland towards Tue evening.

PDX APPROACHES...Mix of VFR and occasional MVFR with passing showers
tonight into Tue am, though think will have more MVFR CIGS of 2500
to 3000 ft after 10Z until 18Z Tue. Then, VFR with variable mid/high
clouds Tue afternoon.

&&

.MARINE...An active and progressive weather pattern will continue
to facilitate bouts of strong winds and fairly high seas over the
next several days. Currently, an area of low pressure continues
to move across the coastal waters to our north with wind gusts up
around 25-30 knots persisting into the evening hours, gradually
decreasing closer to 20 knots overnight. This same low pressure
system has also facilitated isolated thunderstorms over the inner
and outer waters today; a 20-30% chance of thunderstorms lasts
until Tuesday morning any of which could produce erratic wind
gusts and lightning. Seas have subsided to around 9 to 10 ft but a
mid-period westerly swell begins to push into the waters as the
evening progresses, building seas into the upper teens tonight
through Tuesday. Expect significant wave heights to peak around 19
to 22 ft after midnight. Seas are very likely (greater than 90%
chance) to remain near or above 15 ft through Tuesday, the current
Hazardous Seas Warning will remain in place through Tuesday
afternoon.

Then, all eyes are on the next frontal system rapidly approach
the coastal waters later Tuesday. Southerly winds associated with
this strong disturbance are expected to peak Tuesday evening/night
with the NBM and HREF still suggesting gusts likely exceed 48-50
knots (60-90% chance). Given the model consistency resolving the
general magnitude and timing of these winds the Storm Force Watch
has been upgraded to a Storm Force Warning between 4 PM Tuesday
and 7 AM Wednesday for all coastal waters. The latest models
continue to suggest the development of a coastal jet with this
system around 500-1000ft off the surface. Should these winds get
mixed to the surface as the front passes it`s within the realm of
possibility(20-30%) for isolated gusts closer to 60 knots. Seas
will likely remain elevated and chaotic, building back into the
upper teens to lower 20s as a fresh southerly swell also develops.
Winds will gradually ease behind the front Wednesday through
Thursday, while seas likely remain in the mid to upper teens.
-Schuldt/DH

&&

.BEACH HAZARDS...It still appears a westerly swell leads to seas
building to around 19-21 ft with a period of 15 seconds later
tonight into Tuesday morning. This will produce large waves and
hazardous surf conditions with breakers up to 22-25 ft. As a
result a High Surf Advisory is still in effect for Tuesday. A
high surf will affect beaches, producing rip currents, sneaker
waves and beach erosion. Remain alert for exceptionally high waves
and stay a safe distance from the water`s edge.

While unrelated to the current series of weather disturbance, the
perigean spring tides (sometime referred to as the king tides) will
begin Friday November 15, increasing high tides along with a strong
weather system on Wed into Wed night will bring possibility of tidal
overflow and minor coastal flooding.        -Schuldt/DH

&&


.PQR WATCHES/WARNINGS/ADVISORIES...

OR...High Surf Advisory from midnight tonight to 4 PM PST Tuesday
            for ORZ101>103.

     High Wind Watch from Tuesday evening through late Tuesday night
            for ORZ101>103.

     Winter Weather Advisory until 10 AM PST Tuesday for ORZ126>128.

WA...High Surf Advisory from midnight tonight to 4 PM PST Tuesday
            for WAZ201.

     High Wind Watch from Tuesday evening through late Tuesday night
            for WAZ201.

PZ...Small Craft Advisory until 10 PM PST this evening for PZZ210.

     Hazardous Seas Warning until 4 PM PST Tuesday for PZZ210.

     Hazardous Seas Warning until 4 PM PST Tuesday for PZZ251>253-
             271>273.

     Storm Warning from 4 PM Tuesday to 7 AM PST Wednesday for
             PZZ251>253-271>273.

$$

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