735
FXUS66 KPQR 092243
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
243 PM PST Sat Nov 9 2024

.SYNOPSIS...Strong onshore flow through the week with a series
of fronts passing through the Pacific Northwest. The first front
is impacting the area today but is very slow and thus having
minimal impact at this time. The second front arrives late
Sunday/early Monday morning but will be showery. There is around
a 15-20% chance of thunderstorms through Monday. A much more
robust system arrives on Wednesday with widespread heavy rain.

&&

.SHORT TERM...Now through Monday Night...Radar shows a nearly
stationary front sitting over the north Oregon and south
Washington coast this afternoon. This front has produced
variable rainfall amounts along the coast with Astoria seeing
0.01-0.10 inch and the Long Beach Peninsula receiving 0.30 inch
in the last 6 hours. Some showers have formed inland but
accumulation has been less than 0.05 inch at this point.
Onshore flow will intensify through the evening as the shortwave
trough (associated with the front) nears the coast. The
precipitation will fan out with the most concentrated band still
along the northern most portions of the forecast area. It will
be a quick shot of rain though as it begins to dissipate early
Sunday morning.

At 500 mb, there is a weak shortwave ridge that forms over the
area, but it is transient and will have little overall impact.
Will cause showers to become weaker and bring some cloud breaks
but overall, non-impactful. Sunday night into Monday though is
another story as yet another system drops to the south. This
incoming system is associated with a broad area of low pressure
that is anchored over the Gulf of Alaska. The trough that
extends from this low reaches as far south as northern
California. There is quite a bit of variability on the
deterministic precipitation accumulation and timing. The ECMWF
is attempting to bring in a frontal system faster than the
higher resolution systems. Based on HREF guidance, rainfall is
expected to start after 11PM Sunday. Now use this timing with a
grain of salt because it likely will be the "precursor" showers
ahead of the bulk of the fast moving front. 24 our rainfall
totals ending Monday night vary from 1.30-2.20 inches
(25th-75th percentile) around Lincoln City, 1.0-1.60 inches
around Corvallis, and 0.8-1.26 inch near Molalla.


Cooler air aloft will filter in which will cause an increase in
potential convective action. These thunderstorms could locally
increase precipitation amounts. With the cold air in place, snow
levels will lower too. While it spills into Tuesday, moderate
snow is expected over the Cascades. Between Monday morning and
Tuesday morning, the crest (above 5500 ft) could see up to 2 ft
of snow, with the passes seeing up to 8 inches in that 24 hour
period. Have decided against any winter products at this time
due to the evolving nature of this system and the level of
uncertainty.

A feature to take note of is the increasing
pressure gradient and strengthening southerly flow. Will see
winds increase significantly with a 40% chance of seeing gusts
greater than 30 mph along the beaches and headlands, and in the
southern Willamette Valley. Gusty conditions also likely in
southern portions of Skamania County as well. -Muessle


.LONG TERM...Tuesday through Saturday...As we transition into
mid- week, the weather takes a turn for the wetter. The low
pressure in the Gulf of Alaska advects southward and the
pressure gradient increases significantly. The orientation of
the low will cause winds to become more southerly and quite
gusty. The pressure gradient between Portland and Eugene (which
is a proxy used for the strength of southerly winds) reaches
around -5 mb. This pressure gradient coincides with gusts
around 35 mph. However, there is around a 40% chance of gusts
exceeding 50 mph along the beaches and headlands and less than a
5% chance of similar wind speeds south of Corvallis. Rain will
also be heavy at this time but there is a lot of variability in
the flow, track, and intensity. Based on the GEFS and ECMWF
ensembles, there is a split between a southerly track (GEFS) and
a northerly track (ECMWF). The GEFS would lead to a split
solution with wetter conditions along the Washington coast and
northern California, and drier conditions here. The ECMWF though
is significantly more wet. These cases are specifically looking
at inland locations; the coast will see similar rainfall totals
no matter the pattern direction. Will say, the NBM appears to
be trending moreso to the ECMWF and Canadian than the GFS with a
more robust rainfall accumulation.

With the addition of this rain to the previous system on
Sunday/Monday, river flooding is something we will be watching
carefully - especially in our flashier rivers. HEFS ensemble
River Level Probabilities is showing a wide range of
possibilities for the system on Tuesday night into Wednesday.
For example, the Wilson River near Tillamook as a 14% chance of
reaching major flood stage, and a 32% chance of reaching action
stage. This spread is going to be greatly dependent on a) how
the tributaries upstream react to the next few days of rain, and
b) the rain rates. If we see a rapid rainfall, the soils are
unable to take on the precipitation and rivers react much
faster. Will note though that the HEFS is calculated using the
GEFS ensembles which is the drier of the solutions. We will have
to continue to monitor in the coming days to see if there is
more consistency in the pattern and sensible weather.

Off and on showers persist through Friday. -Muessle


&&

.AVIATION...Southwest flow persists aloft with increasing mid to
high level clouds. A frontal system remains situated from the
Puget Sound area to offshore of the central Oregon coast as of 22Z
Saturday. Rain ahead of and along the front is stretching from
KTMK to KKLS as of 22Z and will very slowly move southeast,
filling into the Willamette Valley and northern Cascades by 00-03z
Sunday and pushing east of the Cascades by 08-10Z Sunday.
Lingering scattered showers expected behind the front. MVFR to
IFR ceilings expected to fall along the coast with the showers,
remaining into Sunday. Ceilings inland remain mainly VFR with the
showers, though there`s a 30-50% chance of MVFR ceilings behind
the front after 06Z Sunday. Winds will remain offshore generally
less than 10 kts.

PDX APPROACHES...Prevailing VFR with increasing mid level clouds.
Light rain expected to begin around 00-02z Sunday with a frontal
system. Chances for MVFR CIGs increase to around 50% after 06z
Sunday when rain turns to scattered showers. Southeast winds
around 5-6 kt expected through the day. -HEC

&&

.MARINE...Southerly winds will persist across much of the coastal
waters today ahead of a quasi-stationary front. Expect winds to
gust up to 20 kt. Buoys continue reporting wave heights around 8
to 11 ft with a period of 13 to 15 seconds. Expect seas to remain
elevated around 8 to 10 ft as the northwest swell persists
through this evening. A Small Craft Advisory remains in effect for
the outer coastal waters today, mainly for seas.

A stronger frontal system is expected to arrive later Sunday,
and move across the coastal waters Sunday night. Guidance
indicates there is an 80-90% chance of maximum wind gusts
exceeding 35 kt with this front, so have issued a Gale Warning for
all waters. Seas will also likely build in response, potentially
becoming steep and hazardous. Will maintain breezy westerly winds
through Monday as the parent low moves toward Haida Gwaii.

Still expecting a robust westerly swell to build seas at least
into the upper teens Monday night into Tuesday morning. Confidence
continues to grow for significant wave heights to exceed 20 ft,
with guidance suggesting there is greater than a 90% chance.

Active weather is expected to continue with the potential of another
strong, gale force frontal system Tuesday into Wednesday. Latest
guidance also suggests there is a 50-60% chance of maximum wind
gusts exceeding 48 kt with this system, so will need to monitor
the possibility of Storm Force winds for next week. -HEC/DH
More active weather returns to the region beginning today as
rain moves into the coast during the morning and then inland
later this afternoon. Active and wet weather continues through
much of the coming week, with a series of systems bringing
significant rainfall Sunday night-Monday and Tuesday night-
Wednesday, leading to potential hydro concerns by the middle of
the week. Lowering snow levels will also bring accumulating snow
back down to the Cascade passes Monday into Tuesday. -HEC/DH


&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Gale Warning from 1 PM Sunday to 4 AM PST Monday for
     PZZ251>253-271>273.
     Small Craft Advisory until 10 PM PST this evening for
     PZZ271>273.
&&

$$

www.weather.gov/portland

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