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FXUS66 KPQR 192135
AFDPQR

Area Forecast Discussion
National Service Portland OR
234 PM PDT Mon Aug 19 2019

.SYNOPSIS...Morning low clouds are giving way to afternoon sunshine
today across much of the forecast area, with temperatures near
mid-August normals. High clouds will increase and thicken Tuesday
ahead of an approaching frontal system, with rain possible along the
coast by late Tuesday night. Wednesday will be a damp day for much of
the forecast area as a cold front moves through. Expect a return to
more seasonable conditions Thursday and Friday, though a weak system
passing by to the north may clip our north coastal zones with some
light rain or drizzle by the weekend.

&&

.SHORT TERM...This afternoon through Thursday...Latest visible
satellite imagery shows low clouds clearing from inland areas,
retreating back to the Lower Columbia and Willapa Hills. Overall this
is turning out to be a fairly typical mid-August day, and high
temperatures should generally top out near seasonal normals. Skies
will remain mostly clear this evening, then the 12z HREF suggests low
clouds will redevelop along the coast with a limited push inland
Tuesday morning. Most areas will drop into the 50s overnight tonight,
though some cooler valleys could dip into the mid to upper 40s. These
cooler, wind-sheltered valleys could also see patchy fog Tuesday
morning due to optimal radiational cooling.

Attention then turns to the "main event" of this week, in the form of
a very impressive moisture plume crossing the Pacific. Total
Precipitable Water (TPW) values in excess of 2 inches are now
reaching east of 150W, between 35N-40N. This moisture-rich air mass
traces back to Tropical Storm Krosa, which made landfall in southern
Japan back on August 14th. The moisture is being pulled into a
well-developed low pressure system centered near 42N/145W, spawning a
very early-season atmospheric river that will eventually make
landfall on the Pac NW coast late Tue night or Wednesday. While TPW
values will likely moderate some by that time, the incoming frontal
system will still be loaded with moisture as GEFS and EC ensemble
means are suggesting TPW values near 1.50 inches for Salem Wednesday.
This is in the neighborhood of 4 standard deviations above the
climatological mean, with a return interval of 10-30 years for the
3-week period centered on Aug 21.

While it is apparent that the incoming system will have plenty of
moisture to work with, it is less clear how much forcing will be in
play to wring out that moisture. Much will depend on the interplay
between the above-mentioned low now near 42N/145W and the northern
stream upper trough emerging from the Gulf of Alaska. Most model
guidance shows some phasing together of the two systems, with some of
the stronger runs showing an unusually powerful-for-August and
potentially sub-985 mb low making landfall on the British Columbia
coast early Wednesday. The earlier the phasing occurs, the more jet
energy that will become available for forcing to the south of the
system, across western WA/OR. At this point, most models do not phase
the systems fast enough to allow much more than a 50-60 kt secondary
jet max as far south as our CWA; instead, they keep the bulk of the
jet energy well to our north and aimed toward the British Columbia
coast. This seems to indicate only weak forcing across our forecast
area as the plume of deeper moisture moves through Wednesday, and
less likelihood for waves of low pressure to develop along the front
to provide the necessary focus for heavier rain.

All this discussion is to make the point that while most of the
forecast area will see increasing clouds and humidity Tuesday and
some rain Wednesday, amounts should generally be light for most of
the district...especially considering the depth of moisture in play.
Currently expecting less than 0.25 inch for the Willamette
Valley...especially west of I-5. Portions of SW Washington,
particularly the higher terrain, will see a bit more QPF with up to 1
inch possible. However, given the moisture available, subtle changes
(like earlier phasing and/or stronger lows developing along the front
as it moves onshore) could result in much more QPF, especially along
the coast and in the higher terrain. Winds associated with the front,
as shown by current guidance, appear to remain modest though gusts
25-35 mph will be possible for the capes along the coast.

Aside from a couple lingering early morning showers in the Cascades,
Thursday should be mainly dry across the forecast area. Based on 12z
NAM time-heights and soundings, morning clouds will likely give way
to afternoon sunshine and temperatures climbing back up to the upper
70s or lower 80s. Northerly flow will likely dry out the humidity
left behind by Wednesday's front, setting the stage for a cool early
autumn-like night with many spots likely to dip into the 40s. Weagle

.LONG TERM...Friday through Wednesday Morning...In the extended
period, a shortwave passes over northwest Oregon/southwest
Washington on Friday night. This shortwave, coupled with decent
upper air support in the jet stream, will bring another chance of
rain to the area Friday night into Saturday morning. Much of the
rain would fall along the north coast and Willapa Hills, with very
little expected to make it inland over the foothills.

After the shortwave passes and rain dissipates, a ridge begins to
build over the west coast on Sunday at 500 mb. This is the point
where models begin to diverge. The EC creates a more amplified ridge
with high pressure building over the Pacific Ocean while the GFS,
while still bringing in high pressure, is much less dramatic and
takes on a flatter, more zonal synoptic pattern. This is creating
some uncertainty, especially regarding temperatures as the GFS is
the cooler of the two models. If the EC model output pans out,
temperatures easily could be in the mid 80s, while the GFS would
keep temperatures cooler in the upper 70s. With either solution,
conditions are expected to be mostly clear and dry. /Muessle

&&


.AVIATION...VFR is expected to prevail over the area through
tonight. Expect the marine stratus to reform along the coast
starting around 05Z Tuesday bringing MVFR cigs to areas north of
KONP around 09Z Tuesday. Areas around and south of KONP expect
MVFR/IFR conditions following a similar time frame. KONP could
see some LIFR vis and cigs starting around 12Z Tuesday.

Valley locations will be a challenge tonight. The marine layer
isn't expected to be as deep as the past few nights. Combining
that with the S/SW wind shift expected over the next 24 hours,
expect some pockets of MVFR cigs within the valley but not a
full-fledged intrusion. Often times, this kind of scenario will keep
northern locations VFR, but satellite is showing a stratus deck
around 3000 to 3500 feet lingering along the western part of the
Columbia River. This maintains the possibility that marine clouds
might push back further inland though the morning hours and could
provide an MVFR deck for KPDX and other locations along the
Columbia River.

KPDX AND APPROACHES...Predominately VFR conditions expected
through Tuesday morning. However, a stratus deck around 3000 to
3500 feet lingering along the western part of the Columbia River
could bring back MVFR cigs to the terminal later tonight or early
Tuesday morning. /42

&&

.MARINE...Weak high pressure offshore is producing relatively
quiet conditions over the waters. Seas remain in the 4 to 6 ft
range, mainly from a NW swell. North to northwest winds are
expected to stay at 15 kt or less through early Tuesday.

Significant changes begin Tuesday, with winds becoming S-SE
ahead of an approaching frontal system. A small craft advisory
has been issued starting on Tuesday afternoon for our outer
waters. Expect these small craft winds to slowly progress
towards the inner waters by late Wednesday afternoon. The
strongest winds are expected from Tuesday night through Wednesday
with gusts of 30 kt in our outer waters, and some isolated gusts
higher than 30 kt could be experienced.

Seas will generally start to build towards 6 to 8 feet with the
potential for some steep square seas too. The wind wave and swell
associated with this system could push seas towards 10 feet
Wednesday and Thursday. The area that would have the best chance
of having 10 foot seas is our northern waters due to larger
swells and potentially stronger wind. /42

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory from 2 PM Tuesday to 11 AM PDT Wednesday
for Waters from Cape Shoalwater WA to Florence OR from 10
to 60 NM.

Small Craft Advisory from 8 PM Tuesday to 5 PM PDT Wednesday
for Coastal Waters from Cape Shoalwater WA to Florence OR
out 10 NM.

&&

$$


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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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