Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 221107
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
407 AM PDT Sun Oct 22 2017

.SYNOPSIS...Moderate to heavy rain continues across much of northwest
Oregon this morning as a frontal zone slowly sags southward across
the area. A potent atmospheric river is feeding this front with deep
moisture, resulting in impressive rainfall totals and some localized
flooding. Strong southwest winds ahead of the front will gradually
ease early this morning, and rain will taper to scattered showers as
the front moves south and east of the forecast area. Showers will
mostly end by this evening, as strong high pressure builds into the
Pac NW. Valley inversions will likely develop under this high
pressure, resulting in locally dense valley fog the next few nights.
Otherwise mostly sunny and mild weather is expected much of this
week, save a chance for showers Wednesday as a weak system clips by
to our north.

&&

.SHORT TERM...Today through Tuesday...Rainfall totals have been
impressive over the past 24 hours, as a strong atmospheric river
event impacts southwest Washington and northwest Oregon. Moisture
flux on the order of 4 standard deviations above normal is feeding a
frontal zone moving slowly southward across the forecast area. Lees
Camp appears to be the big winner thus far, with 24 hour rainfall
totals approaching 10 inches. A swath of 4 to 8 inch rain totals
extends across the higher terrain roughly north of a Marys Peak to
Santiam Pass line all the way up through southwest Washington. Lower
elevations have generally seen about half that, with 2.65 inches thus
far at the office in NE Portland (as of 345 AM). Totals have yet to
reach an inch in Eugene, but much more is still on the way. So even
the south Willamette Valley should end up in the 1-2 inch range by
the time all is said and done with this event.

Luckily this heavy rain event has not produced widespread significant
flooding, thanks to rivers starting out low going into this event.
More about this and our flood issues in the Hydrology section below.

This system has also produced fairly strong winds, especially in the
higher terrain. Had to add the S WA/N OR Cascades to the High Wind
Warning, as Three Corner Rock RAWS has been gusting to 80 mph over
the past few hours. Similar gusts can be expected on exposed ridges
and ski areas in the north Oregon Cascades, though gusts at pass
level are more likely to be in the 40 to 55 mph range. With the
combination of strong wind and heavy rain, as well as the threat of
landslides, travel in the higher terrain is treacherous early this
morning even though snow is not a factor. Rockhouse RAWS in the
Oregon Coast Range has gusted as high as 78 mph. Winds were a bit
lighter along the coast itself, but it appears the warning marginally
verified there with a few places gusting to around 60 mph. The
orientation of the front, combined with strong high pressure south of
the area, opened up the coastal communities to similar gusts as the
beaches and headlands, so this was quite the windy event for them
with gusts 45-60 mph. The strongest wind gusts in the Willamette
Valley were around 40 mph. The winds have since eased in the lower
elevations, so all wind warnings/advisories have been cancelled aside
from the Oregon Coast Range and above-mentioned Cascades zones, which
will likely be allowed to expire at 5 AM.

Rain will taper to showers today as the front pushes south of the
forecast area. Showers will taper off quickly this evening as high
pressure builds into the Pac NW, and skies will probably clear
sufficiently for some locally dense valley fog tonight. We then
settle into a pattern of mainly clear skies Monday through Tuesday,
aside from locally dense night and morning valley fog. Temperatures
will remain mild, with highs generally in the 60s Monday and Tuesday.
 Weagle

.Hydrology...Moderate to heavy rainfall has been steady across the
north Oregon Coast Range and Willapa Hills in SW Washington. Rainfall
rates have now been in excess of 0.3" per hour for over 12 hours with
some locations such as Lees Camp above 0.5" per hour through the
period. Maximum precipitation rates reached as high as 0.8" per hour
in some of these areas. This has lead to flooding along the Nehalem,
Grays, Trask, and Wilson rivers. The Grays river crested less than
half a foot below major flood stage and will start to retreat in the
next few hours. The Wilson River has now risen above moderate flood
stage and is expected to crest near or slightly above 17 feet in the
next hour. Would not be surprised if the Wilson reaches slightly
higher than this given the extreme rainfall upstream approaching 10
inches at Lees Camp in the last 24 hours. Also monitoring the Siletz
River as it appears to be making a run towards flood stage. /Bentley

.LONG TERM...Tuesday night through Saturday...Strong ridge of high
pressure will buckle slightly Wednesday, allowing the tail end of a
cold front to clip mainly our northern zones with a chance for
showers. Models show decent agreement that we will otherwise be dry
through Saturday as a strong upper ridge builds over the NE Pacific
and Pacific Northwest. Given all the rain we have had this weekend,
there will be plenty of ground moisture for fog in the valleys. This
may cause some challenges regarding sky cover and temperatures later
in the week as valley inversions become established. Strengthening
inversions may also result in air quality and stagnation issues later
this week. Cooler air east of the Cascades may support high pressure
there and east winds through the Gorge, which would tend to limit the
fog and air stagnation issues in the Portland metro area, but it is
difficult to tell these details this far out in time.  Weagle

&&

.AVIATION...The heavier rain is now moving into the southern
Willamette Valley, with cigs improving to predominantly VFR
behind the front. The coast is holding onto MVFR cigs a bit
behind the front but should be improving to VFR as well behind
the front. Winds are shifting to southwesterly behind the front
and are starting to subside across the area. Expect improving
conditions this morning, with EUG being the last terminal to
improve to VFR. Post frontal showers will continue into this
afternoon and come to an end this evening.

KPDX AND APPROACHES...Cigs fluctuating between VFR and MVFR with
rain early this morning. Expect cigs to improve to VFR this
morning as PDX gets behind the front. Winds right now gusting to
20-25 kt will gradually decrease through the morning hours and
turn more west-southwesterly behind the front. -McCoy

&&

.MARINE...Gale force winds are coming to an end as the front
moves off to the south and east. Expect several hours of WSW-erly
small craft advisory winds behind the front, but gradually
decreasing through the morning hours. By this afternoon, winds
should be down below 20 kt. Seas, however, will still be up
around 12 to 14 ft through tonight. With dominant wave periods
still around 12-13 seconds, seas will continue to be very steep.

High pressure builds over the waters starting later today and
staying over the waters through the first part of this week. An
inverted surface trough along the coast strengthens Monday night
and Tuesday, which will produce North wind gusts around 20-25 kt.
A cold front drops south into our waters on Wednesday, bringing
moderate small craft northerlies behind this feature as well. The
inverted surface trough under high pressure returns late int he
week with small craft advisory wind speeds possible.

Seas are starting to gradually fall behind the front as well this
morning. Expect seas to remain above 10 ft through at least late
tonight, possibly through much of Monday as well. Once seas drop
down below 10 ft, they remain below 10 ft through Friday. -McCoy

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Flash Flood Watch until 2 PM PDT this afternoon for Central
     Columbia River Gorge-Western Columbia River Gorge.

     Flood Watch through late tonight for Central Willamette Valley-
     Coast Range of Northwest Oregon-Greater Portland Metro
     Area-Lower Columbia-North Oregon Coast-Northern Oregon
     Cascade Foothills-Northern Oregon Cascades.

     High Wind Warning until 5 AM PDT early this morning for Central
     Coast Range of Western Oregon-Coast Range of Northwest
     Oregon-Northern Oregon Cascades.

WA...Flood Watch through late tonight for Central Columbia River
     Gorge-Greater Vancouver Area-I-5 Corridor in Cowlitz
     County-South Washington Cascade Foothills-South Washington
     Cascades-South Washington Coast-Western Columbia River
     Gorge-Willapa Hills.

     High Wind Warning until 5 AM PDT early this morning for South
     Washington Cascades.

PZ...Small Craft Advisory for winds until 11 AM PDT this morning for
     Coastal waters from Cape Shoalwater WA to Cascade Head OR
     out 10 NM-Waters from Cape Shoalwater WA to Cascade Head
     OR from 10 to 60 NM.

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

     Gale Warning until 5 AM PDT early this morning for Coastal
     waters from Cascade Head to Florence OR out 10 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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