Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 201021
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
320 AM PDT Fri Oct 20 2017

.SYNOPSIS...Cool and unsettled weather will prevail today, as an
upper level trough moves eastward across the Pac NW. Showers and
perhaps a couple thunderstorms will tend to decrease late this
afternoon and evening as the upper trough moves into the Rockies.
Steadier rain will increase again late tonight and Saturday as a very
wet and breezy frontal system begins to impact the forecast area.
Snow levels will start near the Cascade passes today, then lift well
above pass level as a warm front moves through early Saturday. Rain
will be heavy at times this weekend as an atmospheric river of
moisture stalls over SW Washington and NW Oregon. Drier weather is
expected early next week as high pressure builds over the western
United States.

&&

.SHORT TERM...Today through Sunday...Plenty of showers persist across
SW Washington and NW Oregon this morning, as a cool upper level
trough begins to push onshore. The cool air aloft will keep our
weather unsettled throughout the day, with a couple thunderstorms
possible. Low to mid level shear exists, but looks rather weak; with
0-3km shear values are generally around 20 kt today on the 06z NAM
soundings for the Willamette Valley. Plus it appears the core of
coolest air aloft will move across the forecast area this morning,
with temps warming aloft and gradually capping convection as this
afternoon progresses. While there may be some heavier showers that
produce brief downpours and spotty small hail, it appears doubtful
any thunderstorms will approach severe limits today. Snow levels will
be low enough for a few more inches of snow on the Cascade passes,
with more substantial snow for the higher ski resort elevations. The
Winter Weather Advisory looks marginal for the Cascades, but we will
allow it to continue as plenty of showers look to be on their way.

Showers will decrease this evening as the air mass stabilizes with
the loss of solar heating and warm advection aloft. However, this
warm advection aloft will be the beginning of a period of isentropic
lift which will cause steadier stratiform rain to increase as a warm
front approaches tonight. Strong low-level winds will enhance
precipitation both due to more efficient isentropic lift and
orographic lift in the higher terrain; both the latest NAM and GFS
suggest 850 mb winds will be W-SW around 50 knots by 12z Sat morning.
This will result in impressive rainfall rates Saturday morning,
especially in the higher terrain.

Assuming the warm front moves east across the Cascades early Saturday
afternoon as the 06z NAM/GFS suggest, much if not all of the forecast
area will be in the warm sector Saturday afternoon, with snow levels
rising above 8000 feet. Southwest winds approach 50 kt all the way
down to 925 mb Saturday afternoon. This will often result windy but
relatively dry weather for the Willamette Valley while the Coast
Range, Cascades and Willapa Hills get hammered with persistent
orographic rain. Models suggest the frontal zone will linger near SW
Washington Saturday afternoon and evening before finally sagging back
southward Saturday night and Sunday morning. Rain will gradually ease
later Sunday as the frontal zone drifts south and weakens.

The progression of this front and the magnitude of 850 mb flow are
somewhat reminiscent of the Halloween 2015 heavy rain event across
the PDX metro, with one major difference; that event occurred close
to evening rush hour and this one will occur during the late
night/early AM period in the middle of a weekend. Nonetheless, there
will probably be urban and small stream flooding issues if this
forecast pans out. Some smaller rivers may even make a run at minor
flood stage; the Grays in Rosburg and a couple of coastal drainages
in the Coast Range come to mind. Additionally, rainfall rates appear
likely to reach values of concern for our burn scar areas; especially
the Eagle Creek burn scar which is near where we expect the heaviest
rain to fall. Considered issuing a Flood Watch on this shift, but
will allow our hydrologist and the day shift to take a look at 12z
runs before making the final call.

Overall rainfall totals this weekend look to range from 5 to 10
inches in the Cascades, Coast Range, and Willapa Hills north of
Highway 20. Coastal areas will likely be in the 3 to 6 inch range,
while 2 to 4 inches appear likely for the inland valleys. Of our
forecast area, Lane County appears likely to spend the least time
under the aim of this atmospheric river. Therefore QPF totals for
Lane County will probably be about half of the values listed above.

Will also need to consider the wind associated with this system,
given the 45-60 kt W-SW flow just above the surface. As mentioned
above, most of the forecast area will be in the warm sector by Sat
afternoon if all goes according to plan. This will allow good mixing
of the boundary layer, and pressure gradients turn more
south-to-north during the day Saturday. Cannot rule out south wind
gusts 35-45 mph for the Willamette Valley Saturday afternoon and
evening; the higher end of which would warrant a Wind Advisory,
especially considering most trees are still bearing plenty of leaves.
Again, with this situation being borderline, will allow the day shift
to take a look at 12z runs before making any decisions on an advisory
inland or regarding the High Wind Watch for the Coast and Coast
Range. This could end up being one of those situations where winds
along the coast are only slightly stronger than the winds inland.

All in all, it appears our autumn storm season is beginning in
earnest this weekend, if it hasn`t already. We strongly urge people
to check their storm drains for clogs due to fallen leaves today, as
this is a common cause of significant urban flooding.  Weagle

.LONG TERM...No Changes. Previous discussion follows...
Sunday night through Thursday....Models are in agreement, that
shortwave ridging will develop over the Pacific NW early next week.
This will bring a period of dry weather that will likely be followed,
by a weak trough towards the middle of the week. This will provide
the chance for more rain Tuesday night into Wednesday. Models are
suggesting that another ridge of high pressure could bring a dry
period late next week. Bishop

&&

.AVIATION...VFR prevailing in showery weather as cool upper
trough tracks east across the area today. MVFR possible as well,
particularly between 12Z and 18Z as a somewhat organized shower
band moves through. There is a slight chance of thunderstorms
at the coast, spreading inland in the afternoon before ending
tonight. Next system approaching the coast tonight will bring
increasing winds that may produce wind shear at coastal locations
late tonight.

KPDX AND APPROACHES...VFR conditions are expected to dominate
remains showery today. But it is likely that there will be
occasional temporary MVFR conditions into Fri associated with
showers. Surface wind continues out of the south to southwest
today. Gusty conditions this morning will subside in the
afternoon. /mh

&&

.MARINE...Today will be a transition from showery weather pattern
to a warm frontal system moving into the waters late tonight and
Sat. Gales are expected with the warm front tonight and Sat
morning. Gusts 45 to 50 kt possible as the warm front nears the
coast early Sat morning.  There may be a several hours of sub-
gale winds around midday Sat but gales are likely to return in
the afternoon and night as the cold front arrives. Expectations
are for this system to track a little further south than
originally anticipated, meaning the strongest and more persistent
gales are likely across the southern waters Sat night. But as a
whole expect the entire coastal waters to have gales this
weekend. High pressure begins to build over the area early next
week for less winds and seas.

A long period westerly swell currently around 20-22 ft is still
on track to subside below 20 ft by late morning. The swell is
expected to slowly subside, eventually nearing 15 ft by late this
evening. Another swell train arrives from the west Sat night, so
expect seas to remain in the 14 to 19 ft range through at least
then. Gales Sat and Sat night may build enough short period
waves to bring seas above 20 ft. Seas will lower later Sun but
is expected to remain above 10 ft through Mon night.

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory until 5 PM PDT this afternoon for
     Cascades in Lane County-Northern Oregon Cascades.

     High Surf Advisory until 10 AM PDT this morning for Central
     Oregon Coast-North Oregon Coast.

     High Wind Watch from Saturday morning through late Saturday
     night for Central Coast Range of Western Oregon-Central
     Oregon Coast-Coast Range of Northwest Oregon-North Oregon
     Coast.

WA...Winter Weather Advisory until 5 PM PDT this afternoon for South
     Washington Cascades.

     High Surf Advisory until 10 AM PDT this morning for South
     Washington Coast.

     High Wind Watch from Saturday morning through late Saturday
     night for South Washington Coast-Willapa Hills.

PZ...Small Craft Advisory for winds until 2 PM PDT this afternoon
     for Coastal waters from Cape Shoalwater WA to Cascade Head
     OR out 10 NM-Coastal waters from Cascade Head to Florence
     OR out 10 NM-Waters from Cape Shoalwater WA to Cascade
     Head OR from 10 to 60 NM-Waters from Cascade Head to
     Florence OR from 10 to 60 NM.

     Gale Warning from 2 AM Saturday to 5 AM PDT Sunday for Coastal
     waters from Cape Shoalwater WA to Cascade Head OR out 10
     NM-Coastal waters from Cascade Head to Florence OR out 10
     NM-Waters from Cape Shoalwater WA to Cascade Head OR from
     10 to 60 NM-Waters from Cascade Head to Florence OR from
     10 to 60 NM.

     Small Craft Advisory for hazardous seas until 2 AM PDT Saturday
     for Coastal waters from Cape Shoalwater WA to Cascade Head
     OR out 10 NM-Coastal waters from Cascade Head to Florence
     OR out 10 NM-Waters from Cape Shoalwater WA to Cascade
     Head OR from 10 to 60 NM-Waters from Cascade Head to
     Florence OR from 10 to 60 NM.

     Small Craft Advisory for Rough Columbia River Bar until
     midnight PDT tonight.

&&

$$

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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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