Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 051122

Area Forecast Discussion
National Weather Service Portland Oregon
322 AM PST Mon Dec 5 2016

.SYNOPSIS...A cold front moving down from the nnw today will
move through sw WA and nw OR bringing low elevation snow showers to
much of the region. Another shortwave trough aloft will move se
across the region late tonight and early Tue for a continued chance
of rain and snow showers. Dry and cool weather with strong east
winds Tuesday night and Wednesday will set the stage for more winter
weather Wednesday night and Thursday.

.SHORT TERM...Today through Wednesday...A weak vort max and
shortwave were seen in water vapor pictures early this morning
moving se along the s Vancouver Island coast. A cold front
accompanies this wave, and is expected to move through the n part of
the forecast area this morning, then through the south this
afternoon. Showers seen on radar have been increasing ahead of the
front, and with the moist onshore flow coupled with the strong
frontal dynamic will carry cateogorical pops today. Based on surface
temps and precipitation reports overnight, appears snow levels on
the order of 500 to 1000 feet still representative for most of the
forecast area today. Heavier showers, esp those associated with the
front this morning in the north, will continue to cause snow levels
to temporarily dip to valley floors. Model qpf fields have remained
reasonably consistent with this system, and so several inches of
snow continue to look likely above 1000 feet today, while surface
air and ground temps will make it difficult to see much if any
accumulations below 500 feet. Overall expect to make little change
to existing winter weather advisories today, except will add an
advisory to the central oregon coast range where several inches of
accumulation appear likely today above 1500 feet, and will extend
the advisory for the Oregon Cascade foothills until midnight tonight
as baroclinic zone will be slow to clear out there.

main baroclinic zone appears to sag south of lane county this
evening. A shortwave though seen on water vapor pictures near 51n
136w early this morning will move se over the region tonight into
tue morning, with a moist marginally unstable air mass remaining in
the onshore flow below 500 mb. expect to see showers continue under
the upper trough Mon night into early Tue. With models showing
little change to the thermal profile, does not appear likely that
elevations below 1000 feet should be prone to any significant
accumulations in the showers, but given some opportunity for cooling
in the broken cloud cover, it is not possible to rule out some light
low level accumulations mon night into early tue. That will need to
be monitored to the possible need of another winter weather
advisory at low elevations.

Tue through Wed sees a transition to an increasingly strong offshore
flow. This results in colder continental air pushing through the
gorge and spreading throughout sw WA and nw OR. This air mass will
be dry, but will prime the region with cold air for the system
approaching off the Pacific Wed night.

.LONG TERM...No Changes. Previous discussion follows...Wednesday
night through Sunday...Concern is increasing that the Wednesday
Night/Thursday storm system could be very high impact for all of the
forecast area except for coastal locations. The cold air which will
move into the area on Tuesday/Wednesday will be locked into place
and sustained by offshore flow. The last few model runs have trended
stronger with the high pressure in the Columbia Basin which further
increases confidence for high impact winter weather.

As the front initially approaches, the entire column will be cold
enough for precipitation to start as snow. During this time, a few
inches of accumulation is possible even down to the valley floor.
The timing of this storm system is especially poor for the Thursday
morning commute with snow Wednesday night changing to freezing rain
some time during the morning hours Thursday. How long will the east
winds persist in the Portland metro? That is the million dollar
question. Current model solutions seem to indicate winds shifting
southerly at some point Thursday afternoon. However, the GEM has a
nightmare scenario of east winds continuing through Friday morning
with close to 2 inches of QPF in the period. Will choose to ignore
the GEM at this time and hope for the 00Z to switch winds southerly
much earlier. Even though GFS and ECMWF switch winds southerly
Thursday afternoon, would not be surprised to see the east winds
persist into the late afternoon/early evening as east winds are
notorious for holding on longer than models forecast for the
Portland metro in these type of events. While the majority of the
focus and greatest impact will likely be in the eastern Portland
metro area, it is important to note that most of the Willamette
Valley will likely have some ice Thursday morning before w inds
shift southerly.

In addition to the snow and ice concerns, strong winds will
complicate things for the eastern Portland metro and western
Columbia River Gorge. Wind gusts at PDX will approach 40 knots with
mid 40 knot gusts at TTD. Some type of wind headline will likely be
needed as the event draws closer. These winds will exacerbate
impacts, especially if there is any appreciable ice build up on
trees and power lines.

Several waves will move through the area in the Thursday through
Sunday timeframe. With snow levels hovering between 2000 and 3000
feet, snow will be measured in feet in the Cascades. Will need to
monitor the potential for flooding below the snow level as
probabilities of IVT> 250 are nearly 100% with the Thursday/Friday
system and with topographic influences, rates could be intense
enough for some flooding concerns, especially in the Coast Range.

.AVIATION...A trough of low pres will spread precipitation through
the region from NW to SE this morning. There is a fairly cold air
mass in place, and surface temps have dropped into the mid 30s for
most locations overnight. Expect that we will see a period of
mixed rain/snow for inland areas around KSLE and northward this
morning through around 18Z. Surface temps should be warm enough to
preclude much if any snow from accumulating on runways, however.
Think that the coastal TAF sites and KEUG will see mostly rain,
although they could see a few snow flakes mixed in as well.
Conditions likely come down to MVFR, with localized IFR during
heavier precip this morning, before returning to VFR for this
afternoon and evening.

KPDX AND APPROACHES...Expect a rain/snow mix and MVFR conditions
at the terminal to develop between 12Z and 18Z. Brief IFR possible
during heavier showers. Surface temps should be warm enough to
prevent accumulation on the runways, but could see it briefly
stick during any heavier showers before quickly melting. Showers
taper off this afternoon, and VFR returns for this afternoon and
evening. Pyle


.MARINE...A weak surface trough will move through the waters
today, keeping NW winds gusting 20 to 25 kt. There will be showers
as well, and any heavier showers could produce brief gusts to 30
kt. A small craft advisory for winds is in effect through this
afternoon. There will then be a relative lull in the winds for a
couple of days. A low will slide by just outside the waters and
into S Oregon tonight into Tue without having much impact for our
waters. The next significant frontal system will be approaching
Wed night and moving through Thu. The fcst models are coming into
pretty good agreement moving a strengthening surface toward
Vancouver Island or just north, with a strong frontal system
bringing a period of very gusty offshore winds late Wed into Thu.
The offshore winds could be strong enough to produce low-end gales
in some portions of the coastal waters. Winds shift to southerly
during the day Thu, with a continued threat of gales. Gusty post-
frontal winds likely continue into early Fri.

A westerly swell train will continue to impact the waters, keeping
seas around 12 to 14 ft through Tue. Seas drop below 10 ft Wed.
The late week frontal system should push seas at least back into
the mid teens, with the potential for 20 ft seas or higher
depending on the track of the low and how strong the winds end up
being. Pyle


OR...Winter Weather Advisory until 2 PM PST this afternoon for
     Central Columbia River Gorge-Upper Hood River Valley-
     Western Columbia River Gorge.

     Winter Weather Advisory until 4 PM PST this afternoon for
     Cascade Foothills in Lane County-Northern Oregon Cascade

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

     Winter Weather Advisory until 1 PM PST this afternoon for
     Central Willamette Valley-Greater Portland Metro Area.

     Winter Weather Advisory until noon PST today for Coast Range of
     Northwest Oregon-Lower Columbia.

     Winter Weather Advisory until 6 PM PST this evening for Central
     Coast Range of Western Oregon.

WA...Winter Weather Advisory until 2 PM PST this afternoon for
     Central Columbia River Gorge-South Washington Cascades-
     Western Columbia River Gorge.

     Winter Weather Advisory until 1 PM PST this afternoon for
     Greater Vancouver Area.

     Winter Weather Advisory until noon PST today for I-5 Corridor
     in Cowlitz County-South Washington Cascade Foothills-
     Willapa Hills.

PZ...Small Craft Advisory for winds until 4 PM PST this afternoon
     for Coastal Waters from Cape Shoalwater WA to Florence OR
     out 60 nm.

     Small Craft Advisory for hazardous seas until 4 PM PST Tuesday
     for Coastal Waters from Cape Shoalwater WA to Florence OR
     out 60 nm.

     Small Craft Advisory for Rough Columbia River Bar until 4 AM
     PST Tuesday.


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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.