Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 270955

Area Forecast Discussion
National Weather Service Portland OR
255 AM PDT Mon Mar 27 2017

.SYNOPSIS...An upper level trough will maintain showers today,
decreasing during the afternoon and evening as high pressure builds
into the region. This will bring a brief break in the rain for most
of the forecast area tonight and Tuesday. The next frontal system
will cause rain to increase along the coast Tuesday night, then
spread inland for a generally wet day Wednesday. The parent upper
trough will keep showers into Thursday, then perhaps some drier and
milder weather Friday as high pressure builds across the region.
Another system may bring some rain back for next weekend, but details
on this remain uncertain.


.SHORT TERM...(Today through Wednesday)...The progressive pattern
continues across the North Pacific and into the Pac NW, with a fast,
mostly zonal jet stream extending from China to the U.S. West Coast
and British Columbia. Only brief and modest changes are expected in
this overall pattern for the next 5-7 days at least; mostly in the
Wed/Thu time frame.  This pattern will likely continue to push
frontal systems into the Pac NW every 2-3 days, with the next one
likely to affect our region Tue night/Wed.

Focusing on what`s going on now, showers persist in onshore flow
coupled with an upper level trough pushing into the Pac NW. GOES-16
low-level water vapor channel seems to show a decent dry slot
approaching the Oregon Coast, which will probably be the back edge of
any organized shower activity. The relatively drier air should result
in increasing sunbreaks today, but 00z NAM/GFS soundings suggest
low-level lapse rates will probably be steep enough to allow shallow
showers to persist for much of the day...especially over the higher
terrain. ODOT webcams suggest snow levels have lowered below
Government Camp (4000`), and the ESRL profiler at Troutdale seems to
confirm that snow levels are around 3500` at the moment. Expect snow
levels to remain in the 3000 to 3500 ft range today, but relatively
strong late March solar energy should allow for wet or slushy roads
over most passes by midday today. Still, anyone traveling across the
Cascades should be prepared for snow today, especially this morning.
We will maintain the Winter Weather Advisory for snow in the Cascades
through this afternoon. SNOTEL reports seem to suggest 5-10 inches
have already fallen, with another 2-5 inches likely in snow showers

Warm advection aloft and subsidence associated with higher pressure
building into the region will likely bring an end to the showers this
evening. A flat upper level ridge will temporarily push the Pacific
jet stream north toward Vancouver Island tonight and Tue, allowing
for a mostly dry and mild day Tuesday for areas south of a Newport to
Portland line. Eugene may push 60 degrees Tuesday depending on how
many breaks there are in the high clouds. Given the flatness and
positive tilt to the upper ridge, it appears unlikely that complete
clearing will occur anywhere in our CWA Tue. SW Washington and the
North Coast of Oregon may get clipped with occasional light rain Tue
as a shortwave trough races across BC/WA.

Models are finally showing decent agreement that an upper trough from
the Gulf of Alaska will generate a surface low off the Pac NW coast
Tue night, pushing another occluding frontal system onshore late Tue
night/Wed. This will cause rain to increase along the coast Tue
night, spreading inland late Tue night and Wed. 00z ECMWF/06z NAM
suggest this system will have a bit better of an atmospheric river
associated with it than the past couple of systems, so rainfall
amounts may be heavier...with up to 2.5 inches in the higher terrain
and 1-2 inches along the coast. Significant downsloping will probably
keep rainfall totals around 0.50 inch for most inland valleys. With
much of our coast already in the warm sector, southerly gradients may
be sufficient to produce some decent wind with this system Tue
night/Wed, especially along the coast. Gusts up to 50 mph will be
possible for beaches and headlands, with gusts up to 35 mph possible
for the Willamette Valley Wed. Due to the prior ridging, snow levels
will likely be well above the passes until the upper trough pushes in
behind the front late Wed/Wed night.  Weagle

.LONG TERM...(Wednesday night through Sunday)...Main challenge in the
extended this morning was to put together a forecast disregarding the
00z GFS, which seems to cut off an upper low way too soon off the Pac
NW coast during the weekend. Strongly prefer the 00z ECMWF for days
4-7, which remains more progressive through the weekend. The ECMWF
shown far more run-to-run consistency than the GFS has demonstrated
the last few days; both with our midweek system and the evolution of
the next system over the weekend. Model agreement is better through
Friday, showing our Wednesday rain tapering to showers Wed night/Thu
as a cool upper trough pushes in behind our Wednesday frontal system.
The upper trough will cause snow levels to drop briefly below the
passes Wed night/Thu, with a few inches of snow likely in the
Cascades from the post-frontal showers. As mentioned earlier, leaned
heavily on the 00z ECMWF for the forecast Fri-Sun, which still shows
decent warming/drying Fri and temps possibly reaching the 60s for the
inland valleys. The ECMWF then increases high clouds Fri night, with
a weak frontal system bringing a chance for rain Sat and cooler temps
with a chance of post-frontal showers Sunday.  Weagle


.AVIATION...A weak surface trough approaching the coast will
bring enhanced shower activity through mid-morning. Think that
there will be a mix of VFR and MVFR conditions during this time.
Showers will decrease somewhat through the rest of the day as
higher pres builds in aloft and begins to stabilize the
atmosphere. Conditions should be predominantly VFR from late
morning through tonight, but occasional brief MVFR possible with
any heavier showers.

KPDX AND APPROACHES...Showers continue through tonight. Heavier
showers possible this morning, with a mix of VFR and MVFR. Then
expect predominantly VFR the rest of today and tonight. Pyle


.MARINE...A weak surface trough is moving through the coastal
waters this morning. There will be occasional gusts of 20 to 25
kt, but they shouldn`t be frequent enough to justify a small
craft advisory. Then higher pres will build over the waters later
today and tonight. A frontal boundary will sag south into the
waters on Tue, bringing southerly winds gusting 25 to 30 kt. A
low forming along the front will approach from the southwest Tue
night into Wed morning. This will strengthen the wind field and
may bring some gale force gusts. The low is modeled to move
onshore over Vancouver Island Wed afternoon. Then high pres will
begin to build over the NE Pac, bringing a period of benign
conditions with northerly winds Thu and Fri. There is some
uncertainty going into next weekend, but it looks likely that the
NE Pac high will persist.

Seas are running around 7 to 8 ft this morning. A larger westerly
swell will be arriving today. Seas will rise back above 10 ft
later this morning, and are expected to reach 12 to 14 ft
tonight. Seas will then remain above 10 ft for the next few
days. Then should finally drop below 10 ft on Thu and remain
below 10 ft into next weekend. Pyle


OR...Winter Weather Advisory until 4 PM PDT this afternoon for
     Cascades in Lane County-Northern Oregon Cascades.

WA...Winter Weather Advisory until 4 PM PDT this afternoon for South
     Washington Cascades.

PZ...Small Craft Advisory for hazardous seas until 5 PM PDT Tuesday
     for Coastal Waters from Cape Shoalwater WA to Florence OR
     out 60 nm.

     Small Craft Advisory for Rough Columbia River Bar until 7 AM
     PDT this morning.

     Small Craft Advisory for Rough Columbia River Bar from 3 PM
     this afternoon to 8 PM PDT this evening.



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This discussion is for Northwest Oregon and Southwest Washington
from the Cascade crest to 60 nautical miles offshore. The area is
commonly referred to as the forecast area. is the U.S. government's official web portal to all federal, state and local government web resources and services.