Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 220537
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
232 PM PST Tue Nov 21 2017

.SYNOPSIS...Wet and unsettled weather will continue this week. A
largely occluded frontal boundary is stretched over the region this
evening with breezy to windy conditions along the coast. The front
will drift south as a weak cold front tonight and Wednesday for more
rain, especially near the coast and in the north interior areas.
Snow levels in the Cascades will remain relatively high. A better
defined cold front sweeps across on Thanksgiving with snow levels in
the Cascades lowering close to, but likely remaining above the
passes Thursday night and Friday. Showers linger Friday before the
next system arrives on the weekend with rising Cascade snow levels.
Snow levels lower back near the Cascade passes again Sunday night
and Monday.
&&

.SHORT TERM...Tonight through Friday...COmplex structure to the air
mass over the CWA this evening. A frontal boundary lifted north as a
warm front this evening, but stalled a little south of where models
anticipated it would be. Have seen rain amounts over N Oregon/S
Washington coast and adjoining mountains exceed previous
expectations by about double with local areas near triple.
Unfortunately, one such area was over the headwaters of the Grays
River in Wahkiakum County. Runoff has brought the Grays River a
couple feet above flood stage as of the last 3 hour reporting block.
Issued a flood warning earlier this evening when the prior set of
data indicated the river had risen above flood stage after having
sat a few beet below. Rain rates across that area are running about
a third of an inch an hour. Satellite imagery and radar data implies
the rates should be getting weaker overall which would likely end
the rising waters, but will not know the actual resulting effect
until the next river stage data block is received from the
Washington Department of Ecology gage after 1130 PM. Other rivers
along the north Oregon/south Washington coasts are rising but not as
dramatically.

Other effects from the occluded front were a burst of warm downslope
wind for the South Willamette valley where Eugene spiked up several
degrees mid-afternoon resulting in a new record of 67 degrees for
the date. The warm air made it well north along the coast, but the
easterly winds through the Gorge managed to draw cooler air
westward. Hood River through about Bonneville are running about 10
degrees cooler than the rest of the forecast area with temperatures
in the lower to mid 40s.

Suspect the frontal boundary will not move all that much overnight
with temperatures not likely straying much from where they are now.
Did add additional QPF to the forecast across the northern coastal
zones, but still indicated overall diminishing to the rates until
the next frontal wave arrives tomorrow evening and overnight. /JBonk

Remaining previous discussion follows: The next cold front
approaches the coast Wednesday night and moves through the forecast
area midday on Thanksgiving. Snow levels will remain high initially
with this system. The front will move east of the Cascades later in
the day on Thanksgiving. There will be a fair amount of showers in
onshore flow behind the front Thursday night with snow levels
lowering to near Cascade pass elevations overnight, and the greatest
coverage and intensity over the Cascades and the coastal mountains.
The models indicate that showers will continue into Friday, but
there could be some drying in the south part of the area, from about
Salem southward, by afternoon.

Rainfall totals through Friday could reach 3-6 inches in the Cascades
and in the coastal mountains (heaviest amounts in the north), with
0.5-1.5 inches in the valleys. Tolleson

.LONG TERM...Friday night through Tuesday...The models say the next
system spreads in with more rain Saturday as snow levels rising well
above the Cascade passes. Snow levels drop back near the passes in
showers behind the front on Sunday. A cool system moves through
Sunday night and Monday with more rain and mountain snow, followed by
decreasing precipitation on Tuesday. Basically the unsettled weather
pattern will continue. Tolleson
&&

.AVIATION...Moist south to southwest flow will continue over the
region through Wed. Ill-defined frontal boundary along the coast
will shift slowly inland overnight and Wed. Generally a mix of
VFR and MVFR and this trend not expected to change much over the
next 24 hours. In general, expect increasing VFR coverage after
15Z Wed. Low-level offshore gradient continues to weaken this
evening with the 04Z KTTD-KDLS value down to -2.7 mb. Models show
this gradient becoming nearly neutral by Wed afternoon. All
inland TAF sites have enough wind to keep low-levels mixed and
prevent IFR or worse from occurring. However, sites such as KUAO
and possibly KHIO could see IFR cigs develop late tonight and Wed
morning should the wind go calm or light. Very moist low-levels
and reasonably high dew points will allow fog to form quite
rapidly in light or calm wind.

KPDX AND APPROACHES...VFR conditions to be the dominant flight
category through Wed afternoon. However, there will be periods
of MVFR vis in precipitation. There will be enough E-SE surface
wind to prevent IFR cigs from developing at the terminal and east
toward the Columbia Gorge. Weishaar

&&

.MARINE...Minimal changes to the forecast this evening. Will
extend the gale warning through 12Z Wed as 00Z guidance shows
boundary layer wind speeds generally 25-35 kt. Wind speeds ease a
little Wed. However, another surface low is forecast to develop
well offshore late Wed night and track northeast to the N
Washington coast Thu morning. This will tighten the pressure
gradient once again, resulting in another round of gale force
wind. Weak high pressure will bring short break in the active
weather pattern Friday. Longer range models show another strong
system impacting the waters Saturday.

Seas running 12-15 ft as of 04Z, which is at least 3 ft lower
than the 12 hr ENP guidance. Wave heights expected to hover in
the 12-15 ft range through Wed evening, then ease a little Wed
night and Thu. Seas forecast to fall near 10 ft Fri, but get
close to 20 ft Sat evening. Weishaar

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Gale Warning until 4 AM PST Wednesday for Coastal Waters from
     Cape Shoalwater WA to Florence OR out 60 NM.

     Small Craft Advisory for Rough Columbia River Bar from 4 AM to
     10 AM PST Wednesday.

&&

$$

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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 CWA, or forecast area.



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