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FXUS66 KPQR 161146
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
346 AM PST Sun Dec 16 2018

.SYNOPSIS...Increasingly wet and windy weather will affect southwest
Washington and northwest Oregon through Monday night, possibly
lingering into Tuesday. Heavy rainfall and strong coastal winds
appear likely Monday night into Tuesday as a cold front and
associated atmospheric river move slowly southward across the
forecast area. Flooding is possible, despite mainstem rivers starting
off low due to a relatively dry autumn. Rainfall rates may be
sufficient to result in urban and small stream flooding as well.
Active weather may persist through the end of the week, while periods
of heavy surf continue to batter the coast.

&&

.SHORT TERM...Today through Tuesday...The most active weather so far
this season appears likely over the coming days for southwest
Washington and northwest Oregon.

Satellite imagery continues to show a very broad and complex low
pressure system over the Gulf of Alaska and NE Pacific. A very strong
Pacific jet stream lies to the south of this system, with some of
this jet stream energy going toward developing a wave of low pressure
off the northern California coast. With the strong jet stream still
aimed toward our south, today will be wet but not excessively so, and
the strongest winds today appear destined for the southern
OR/northern CA coast. Cool high pressure remains jammed against the
east slopes of the Cascades, with a 6.5 mb easterly MSLP gradient
from KDLS-KTTD. This will provide some gusty east winds through the
Columbia Gorge and portions of the Portland/Vancouver metro area
until the front moves through this afternoon. Snow levels have
generally risen above the Cascade passes, and precipitation today
should be in the form of rain for all but the higher ski resort
elevations.

It should be noted that the aforementioned surface low pressure in
the Gulf of Alaska created a large westerly swell that is currently
moving towards the region. The most powerful waves will likely arrive
along our coastline this afternoon and evening so the High Surf
headlines look on track.

As the upper level trough currently found in northeast Pacific
finally pushes ashore Sunday evening, 500mb temperatures will drop to
~-29C, which may be enough to support a thunderstorm or two across
our waters. Modestly strong south-southwest low level flow should
allow some showers to push inland behind the front, but orographic
effects will be somewhat limited so snow amounts should remain well
below advisory criteria across the Cascades Sunday night despite snow
levels lowering to 4000-5000 feet.

Weak shortwave ridging should allow most of the area to dry out
briefly early Monday, but the next storm system will be quick on its
heels and likely begin to spread rain into the area Monday afternoon
and evening. A 160 kt WSW-ENE oriented jet and attendant atmospheric
river will then take aim at Oregon and likely result in one of the
wetter 24 hour periods of the winter season for the region.
Integrated water vapor transport values will likely exceed 750 kg/ms
per the 00z GEFS, which would classify this as a strong atmospheric
river event. The strong southwesterly flow in this event will not be
an ideal direction for orographic enhancement across at least some of
the drainages of the Coast Range and Cascades, but given the IVT
values forecasted by the models, would not be surprised to see a
couple of our typically wetter sites in the Coast Range and Cascades
register around 2" of rain in a 6 hour period late Monday night or
Tuesday morning.

In total 1-2" of rain seems like a fair bet for many Willamette
Valley locations late Monday through Tuesday, with 3-6" of rain and
locally more likely for the higher terrain of the Coast Range and
Cascades. In addition, strong SW flow aloft, on the order of 65-75 kt
at 850 mb per the 00z GFS/NAM, should be strong enough to carry some
of the heavier rain east of the Coast Range and into normally rain
shadowed areas, which should bring notable rises to those rivers as
well. It is still too early to say where the heaviest rain will fall
across our CWA given the inevitable wobbles in the flow that the
models simply cannot accurately resolve at this point in time, will
likely determine where the atmospheric river will sit the longest.

Finally, models have been consistent in depicting strong southerly
winds developing offshore in advance of the front. As opposed to many
of our frontal systems so far this season, models continue to suggest
that enough of a southerly MSLP gradient component will exist to
spread strong winds inland from the beaches and headlands. Given the
65-plus knots down to 900-925 mb Monday night, we decided to expand
the High Wind Watch into the coastal communities and elevations above
1000 feet in the Coast Range and Willapa Hills. Based on the latest
guidance, western Oregon looks a bit more likely than SW Washington
to see strong winds, but this still may change in future model runs.
At this point, gusts to 70 mph appear likely for the beaches and
headlands and higher elevations of the Coast Range, and gusts to 60
mph appear likely for the coastal communities. Wind-sheltered valley
communities such as Vernonia, Grande Ronde, and Alsea will likely
feel considerably lighter winds than the higher terrain. Strong winds
may also develop in the Willamette Valley to the south of the front
Monday afternoon and evening, but there is a chance southerly
gradients will not develop in time to tap into the strongest winds
aloft. At this point, our forecast does show low-end advisory-level
winds for the Willamette Valley, but confidence is not yet high
enough to issue an advisory. Weagle/Neuman

.LONG TERM...Tuesday night through Saturday...Previous discussion
follow...Weather pattern remains active later this week going into
the weekend. Tuesday night into Wednesday we will still be dealing
with the remnants of the atmospheric river that moved through Monday
night into Tuesday. While it tries to pass to our south Tuesday night
into early Wednesday, it stalls across the southern half of our area
as the front is weakening. The front also gets cut-off from its
tropical moisture feed by the next system out over the north-central
Pacific, which makes the front relatively anemic by Wednesday
morning. This system over the central Pacific actually ends up
picking up this dilapidated frontal boundary and merges it with its
warm front. The warm front then lifts north across our area during
the day on Wednesday for another round of steady, stratiform rain.

Models diverge once we get to late Wednesday into Thursday. The GFS
lifts this warm front completely north of our area for a dry day on
Thursday. However, the ECMWF doesn't quite clear the northern half
of our area, then moves the cold front through quicker which spreads
the rain across our area again by Thursday evening. Have went with a
blend of the two which is somewhat reflected in the NBM, but also
did some manual tweaking. As of now, the bulk of the moisture with
this late Thursday/Friday system makes landfall north of our area,
then moves quickly southeastward across our area. This is not a very
heavy rain/flooding scenario for our area as the front never stalls.
Still would be a good idea to monitor this time period in case the
track of the low changes.

Unfortunately, with how warm these systems are, neither the rain on
Wednesday or the next round Thursday night into Friday will drop
much snow in the Cascades. As we get later into Friday and into
Saturday, we should be between systems, which may mean a break in
the rain. -McCoy

&&

.AVIATION...Front well offshore will continue pushing towards the
region, with rain and gradually deteriorating conditions. At 2
am, generally VFR. But, will see increasing MVFR CIGS/VIS along
the coast around 16Z, and by 19Z inland as the front pushes
onshore. Higher terrain will be frequently obscured in clouds
and precip after 18Z. As rain decreases this evening, will see
return of mostly low VFR, but think will see have areas of MVFR
CIGS through the evening.

PDX AND APPROACHES...VFR this am, but will see cigs lower to MVFR
between 18Z and 20Z as front starts pushing inland. Front will
pass across PDX between 22Z and 01Z, with cigs gradually
improving to low VFR after 02Z. Rockey.

&&

.MARINE...First front now sits about 200 miles offshore, and will
push across the coastal waters this am. Currently have southerly
gales on the waters, with seas running 18 to 20 ft. But, as front
pushes to the coast, will see surface low over south Oregon
coastal waters push north along the coast. This will help to
weaken the southerly pressure gradients, especially north of
Cascade Head. So, think the winds will weaken this afternoon, and
have opted to end Gale warning bit at 1 pm, rather than 4 pm.

Seas currently 17 to 23 ft, with higher seas well offshore. Seas
will increase a bit more today, with 20 to 25 ft swell spread
across the coastal waters later this afternoon, and continuing
through Mon. Now, periods with this swell running 18 to 20 sec,
which means waves will pack rather powerful punch to the surf
zone. Have High Surf Advisory until 3 pm today, then High Surf
Warning as the more power-punched swell arrive. This is NOT the
time to be beach-combing or getting up on rocks near the surf
zones. Consider using some sense, and stay alive for presents.

Another strong front will arrive Mon night into Tue, with another
round of strong southerly gales, possibly even storm force gusts.
Seas continue to run at 22 to 28 ft, and may even touch 30 ft in
few spots. Rockey.

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...High Surf Advisory until 3 PM today for the North and
Central Coast of Oregon.

High Surf Warning from 3 PM today through tonight for
North and Central Coast of Oregon.

High Wind Watch from Monday afternoon through late Monday night
for Coast Range of Northwest Oregon-North Oregon Coast.

High Wind Watch from Monday afternoon through Tuesday morning
for Central Coast Range of Western Oregon-Central Oregon
Coast.

WA...High Surf Advisory until 3 PM PST today for South
Washington Coast.

High Surf Warning from 3 PM today through tonight for the
South Washington Coast.

High Wind Watch from Monday afternoon through late Monday night
for South Washington Coast-Willapa Hills.

PZ...Gale Warning until 1 pm today on all coastal waters.

Small Craft Advisory for Rough Columbia River Bar today
through Mon am.

&&

$$

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This discussion is for Northwest Oregon and Southwest Washington
from the Cascade crest to 60 nautical miles offshore. This area is
commonly referred to as the forecast area.
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