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FXUS66 KPQR 101824 AAA
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Area Forecast Discussion...UPDATED
National Weather Service Portland OR
1024 AM PST Sun Nov 10 2024

.SYNOPSIS...Active weather expected over the next few days as a
pair of strong frontal systems impact the area with heavy rain,
gusty winds, mountain snow, and possibly a few thunderstorms
tonight into Monday and again Tuesday night into Wednesday. This
will bring increased hydro concerns Tuesday into Wednesday,
especially for rivers draining the Coast Range and Willapa
Hills. Cool and showery weather lingers into the end of the
week.

&&

.SHORT TERM...Now through Tuesday Night...Will see a relative
lull in shower activity through later this morning as subtle
shortwave ridging crosses overhead, with showers starting to
increase in coverage again this afternoon as diffluent upper
level flow develops ahead of the next system approaching the
coastal waters. In all, expect minimal rainfall amounts of less
than a tenth of an inch across most of the area through early
this evening.

More substantial rain then arrives later this evening as a
strong front reaches the coast and sweeps across the area
tonight, driven by a potent upper level trough. Rainfall
amounts are more or less in line with previous thinking, with a
reasonable range of outcomes represented by NBM 25th-75th
percentile guidance bringing anywhere from 1.25-2.50" to the
coast and Coast Range and 0.50-1.50" inland through Monday
afternoon. The official forecast represents the middle ground
between these solutions, resulting in around an 1.50-2.0" on the
coast and 0.75-1.00" in the interior valleys for the 24 hr
period ending late Monday afternoon. The arrival of the front
will also bring gusty winds of 30-40 mph to beaches and
headlands this evening, with breezy conditions spreading inland
and bringing 20-30 mph gusts to the Willamette Valley tonight.
Cooler air aloft will filter into the region with the upper
level trough tonight into Monday, steepening mid level lapse
rates and bringing a 15-25 percent chance of thunderstorms that
will linger over the area through much of the coming week. Snow
levels will lower to around 4000-4500 feet by Monday morning,
bringing accumulating snow back to the Cascade passes through
Tuesday. Expect around 6-10 inches total through Tuesday
afternoon at Santiam and Willamette Passes, remaining slightly
below advisory criteria as accumulations will be spread out over
a period of 24-36 hours.

Expect another relative lull in activity with scattered showers
and more modest QPF totals Monday night through Tuesday as
another transient upper level ridge crosses the region but
maintains moist onshore flow ahead of the next system arriving
Tuesday night into Wednesday. -CB



.LONG TERM...Wednesday through Saturday...Guidance still points to this as
the stronger of the two systems, with the NBM interquartile
range indicating roughly another 2-3 inches of QPF for coastal
areas and 1.00-1.75" inland for the 24 hr period ending
Wednesday afternoon. Hydro concerns remain heightened Tuesday
into Wednesday given this second round of heavy rainfall in
relatively quick succession, with HEFS guidance still indicating
around a 10-15 percent chance for several rivers draining the
Coast Range to reach action or minor flood stage and even around
an 8 percent chance for the Wilson River near Tillamook to
reach major flood stage. It is also worth noting, as in the
previous discussion, that these HEFS probabilities are based
upon GFS guidance which is currently on the drier end of the
various ensemble suites, with the Euro ensemble system
indicating the highest amounts and the Canadian representing
the middle ground while staying a little closer to the GFS. So,
if the higher end scenarios represented by the Euro ensemble
were to verify, these river flood probabilities would increase.
That said, the most likely scenario at this time as indicated
by the available guidance is for area rivers to observe notable
rises early to mid week but to remain in their banks. Will
obviously continue to pay close attention to these trends in the
coming days. Aside from river flooding concerns, will also need
to contend with potential for ponding of water and clogged
storm drains given the abundance of leaves on the ground. Flash
flooding on recent burn scars does not appear to be a
significant concern at this time as hourly rainfall rates remain
well below thresholds for the duration of these events.

In addition to the hydro concerns, will also have to keep an
eye on winds Tuesday night into early Wednesday as the ECMWF
Extreme Forecast Index indicates potential for strong gusts
across much of the area. NBM guidance shows a high probability
of gusts exceeding 50 mph on beaches and headlands with the
front Tuesday evening, then around a 20 percent chance for
gusts as high as 40 mph in the Willamette Valley Tuesday night
into Wednesday morning. Also expect another round of
accumulating snow in the Cascade passes, but amounts are looking
rather marginal with respect to any advisory levels snow at
this time.

WPC ensemble clusters agree on maintaining upper level
troughing over the Pacific Northwest through the end of the
week, yielding cool and showery conditions with more minimal
QPF amounts across the area Thursday and Friday. -CB


&&

.AVIATION...Predominately VFR CIGs across the area this morning
with light winds, but scattered areas of mist with visibility
down to 4SM are expected to clear by 18-20z. Expect conditions to
deteriorate along the coast later this morning and through the
afternoon as the next frontal system approaches. Light rain
expected by 00z Monday, followed by heavier rain and reduced
visibility likely after 04-06z Monday. Conditions inland likely
remain predominately VFR through at least 00z Monday. Then,
chances for MVFR CIGs increase to around 40-70%. Offshore flow in
the lower levels will increase later today, with south to
southeast winds increasing inland to around 10-15 kt after 00z
Monday, while coastal terminals likely to see gusts up to 25-30 kt
this evening.

PDX APPROACHES...Predominately VFR with CIGs around FL050 through
this afternoon. Light rain expected by this evening, along with
increasing chances (50-70%) of MVFR conditions after 01z Monday.
Light winds this morning expected to become southeast 5-7 kt this
afternoon. -DH/HEC

&&

.MARINE...Southerly winds will gradually increase today ahead of
an approaching frontal system. Will maintain the Gale Warning for
all coastal waters beginning at 1 PM this afternoon, but strongest
winds with gusts up to 35-40 kt are expected between 4 PM and
Midnight. Expect the front to push onshore by Monday morning,
followed by breezy southwesterly winds with gusts up to 25 kt as
the parent low moves toward Haida Gwaii.

A robust westerly swell associated with this low is expected to
drive seas into the upper teens by Monday night, with significant
wave heights peaking around 20 to 22 ft, likely between midnight
Monday and 10 AM Tuesday. Confidence continues to remain high for
seas exceeding 20 ft with around 60-80% probabilities.

Active weather continues as the next weather system is expected to
move across the coastal waters later Tuesday into Wednesday. This
frontal system has the potential to produce Storm Force wind gusts
exceeding 48 kt (50-70% chance), especially with the strong
possibility of the development of a coastal jet. Seas will likely
remain elevated at least in the mid to upper teens as a fresh
southerly swell also develops. -DH

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory from 1 PM this afternoon to 4 AM PST
     Monday for PZZ210.
     Gale Warning from 1 PM this afternoon to 4 AM PST Monday for
     PZZ251>253-271>273.
&&

$$

www.weather.gov/portland

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