Debris Flow and Flooding in Oregon, January 10 to 16, 2021

Area Forecast Discussion
National Weather Service Portland OR
256 AM PST Sun Jan 10 2021

.SYNOPSIS...Some showers will linger this morning in the wake of a
cold front, but it will otherwise be a mostly dry day.	Rain returns
Monday, with periods of heavy rain likely Monday night and Tuesday as
an atmospheric river event is expected to impact the Pacific NW. There
is potential for heavy rain to continue into Wednesday. Snow levels
to be well above the Cascades passes the first half of next week.
Large surf and high tides will result in periods of hazardous
conditions for the beaches and low-lying coastal areas today through
at least Tuesday.

&&

.SHORT TERM...Today through Tuesday night...Satellite and radar
imagery shows a weakening cold front moving across SW Washington and
NW Oregon early this morning. The front will be east of the Cascades
by 12z with post-frontal showers mostly ending well before noon. The
best chance for showers late this morning and early afternoon are
across the higher terrain. Hazardous conditions are expected on the
beaches today as a 22-24 foot west swell will produce high surf and
sneaker wave conditions through the early evening. Wave run-up will
be especially high around the strong high tide late this morning, as
the tide will correspond with the time the highest surf is expected.

Dry weather is expected this evening and tonight except for extreme
SW Washington as the southern edge of a warm front will generate a
chance of rain. The break from rain will be short-lived as a cold
front will likely produce rain for the coast and coast range early
Monday morning, and moving into the Willamette Valley late Monday
morning or the early afternoon. Snow levels are expected to be 4500
to 5000 feet through Monday morning then rise to 5500 to 6000 feet
Monday afternoon as a warm front arrives. Maximum rain totals with
the cold front are expected for the coast, coast range, and the SW
Washington Cascades, where 0.5 to 0.75 inch of rain is expected from
early Monday morning through the afternoon. A threat for minor tidal
overflow is expected with the high tide late Monday morning which may
result in minor flooding along the beaches and low-lying areas close
to the shore and near coastal rivers.

Higher rainfall amounts are expected with the next frontal system as
models are suggesting the possibility of an atmospheric river event.
The 0z GEFS and 0z ECMWF ensembles are honing in on IVT Plumes
between 750 and 1000 kg/ms which correspond to a strong atmospheric
river. Model variations of the track and characteristics of the low
associated with the frontal system results in uncertainty on where
the heaviest rain will set up, but can generally expect a period of
heavy rain across W Washington and NW Oregon Monday night through
Tuesday afternoon, and possibly into Tuesday night if the following
frontal system catches up with this one.

The current forecast is for 3 to 6 inches of rain for the higher
terrain Monday night through Tuesday night with 1 to 2 inches for the
interior valleys. With the uncertainty on the location of heaviest
rain, expect adjustments to the rain amount forecast over the next
few forecast cycles. Snow levels are generally expected to be above
6000 feet through this period and may add to hydrologic concerns.
With the expected heavy rain, rural,urban, and river flooding are a
possibility as well as an increased risk for land slides. Would like
to see better agreement in the models on the location of the heaviest
rain before issuing any hydrologic headlines though. Besides the
rain, there will be breezy conditions Monday night and Tuesday but do
not anticipate the need for any wind highlight.  Also, there will be
another threat for high surf and minor tidal overflow flooding on
Tuesday. ~TJ

.LONG TERM...Wednesday through Saturday...The aforementioned
potential AR event likely runs through Wed. The operational GFS and
ECMWF show the baroclinic boundary slowly sagging southeast Wed, with
the main focus generally extending from the Cascades to the south
Oregon coast. Snow levels will rise above 7000 feet during this time,
adding another layer to potential hydro concerns.

The deterministic runs are in decent agreement with the 500 mb
cluster analyses showing a fairly sharp 500 mb ridge moving over the
Pac NW Thu. There are a handful of ECMWF ensemble members that
maintain some QPF over much of the area Thu morning and push it north
in the afternoon. Another weak front pushes across the area Fri, but
snow levels appear to remain above the Cascades passes. All in all,
high confidence exists that mild conditions will persist through the
extended period with daytime temperatures 5-10 degrees above average.
Do not see any indication of low-elevation snow or unseasonably cold
air through the next 10 days. Weishaar

&&

.AVIATION...As of 2 am, front pushing across the region. Has
been enough mixing and lift that have seen most of areas with VFR
conditions. But, will see pockets of IFR to MVFR CIGS along the
coast as well as against the Cascades Foothills through 15Z. As
the upper trough axis, which is near the coast as of 2 am, shifts
on across the region through 18Z. With this, will see scattered
showers, with mix of VFR and occasional MVFR CIGS. Afterwards,
will see improving conditions with rapidly decreasing showers.
Likely just VFR for the afternoon, with variable mid and high
clouds. Not much change Sun night, as next system offshore brings
thicker high clouds.

For detailed regional Pac NW weather information, go online to:
http://weather.gov/zse

KPDX AND APPROACHES...Front shifting across region this am, with
mix of VFR and occasional MVFR with showers. East winds will
gradually weaken through 18Z, but think gradients such that will
maintain lighter east to southeast winds through afternoon. VFR
this afternoon into tonight, with increasing high clouds.

&&

.MARINE...Not a lot of change. Front has shifted onshore this am,
with winds mostly 10 to 20 kt today. Seas will be more
problematic today. As of 2 am, seas were running 12 to 16 ft.
But, that will change this am. Buoys 46006 and 46005, both well
offshore, peaked at 30 to 32 ft Saturday evening, then dropped
back into the middle to upper 20s. This swell continues to push
towards our area. Models fairly good with projections at 46006
and 46005, so will go with their projections of 22 to 25 ft seas
on our waters this am into early afternoon. Current forecasts are
on track, so will make only minor tweaks. High Surf Warning still
on track for today. Seas will gradually ease back to the mid to
upper teens by late afternoon into this evening.

Seas will stay in the 16 to 18 ft range tonight, and 14 to 17 ft
for Mon. Another front will cross the waters Mon am, with
increasing winds. At this time, looks like will see marginal
gales, with gusts 35 to 40 kt for a brief time. Given this
sequence, have extended the SCA through late Mon.

A more significant surface low will mature and slide north
somewhere well offshore Monday night and Tuesday. Models have
been struggling with the details on this system with one model
now showing a single central low pressure of 954 mb while others
split the low and keep it much weaker. In either case, is appears
strong gale force gusts are likely with seas again potentially
reaching the low 20 ft range. Have issued a Gale Watch to cover
the winds, although, it too will send a series of westerly swell
trains across the waters with seas above 10 feet lasting well
into Wednesday. 			       Weagle/JBonk

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...High Surf Warning today for South and Central Oregon coast.

WA...High Surf Warning today for South Washington Coast.

PZ...Gale Watch for Mon night through Tue on all coastal waters.

     Small Craft Advisory today through Mon afternoon on all
	  coastal waters.

     Small Craft Advisory through Mon for Columbia River Bar.

&&

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