Area Forecast Discussion
Issued by NWS Portland, OR

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623
FXUS66 KPQR 191130
AFDPQR

Area Forecast Discussion
National Weather Service Portland Oregon
323 AM PST Thu Jan 19 2017

.SYNOPSIS...An upper level trough traverses the forecast area today,
with showers gradually decreasing.  Cold air remains trapped in the
Columbia Gorge and Upper Hood River Valley, which will maintain a
threat of freezing precipitation through this evening...but any new
icing would be light. Offshore low level flow will strengthen tonight
and Friday as another low pres area approaches the region. Wet
weather continues this weekend, with snow at times over the Cascades
and in the Hood Valley/Columbia River Gorge. Drier but seasonable
weather returns next week.
&&

.SHORT TERM...Today through Saturday....Upper trough that is offshore
this am will push across the region today. Showers will gradually
decrease this afternoon as the trough axis shifts east of the region.
Will maintain a slight chance of thunderstorms from the Coast
Range/Willapa Hills westward. This supported considering the rather
cold tops on the convection offshore. Otherwise, will remain mild
with seasonable temperatures across the region.

Now, do still have cold air stuck in the Columbia Gorge, Hood River
Valley, and in valleys of the south Washington Cascades. Most of the
Columbia Gorge has temperatures in the lower to middle 30s, but at or
just below 32 deg from Stevenson/Cascade Locks eastward through Hood
River and also in the Hood River Valley. Not a lot of change from
that pattern today, but temps could warm up a deg or two. But
overall, will see showers and main threat of freezing rain/sleet
decrease today. But any shower this am will bring spotty light
accumulations of sleet or freezing rain to those areas. Will maintain
Freezing Rain Advisory for those areas through 6 pm today, for new
accumulations of ice generally around a tenth of an inch or less. As
the overall air mass cools tonight, would favor sleet over freezing
rain. But again, moisture decreasing so not much of any new impacts.

Models continue to indicate a break in the action this evening,
though will keep some threat of showers around. But another low well
offshore will approach region late tonight, with precipitation
spreading across the region late tonight into early Fri. Snow levels
will sit around 2500 to 3500 feet, with several inches expected over
the Cascades. At this time, likely to get 2 to 5 inches for the
Cascades, not enough snow to warrant advisories.

But different story for the Central Columbia Gorge and Hood River
Valley. As the offshore pres gradients increase tonight, will see
colder air seep westward through the Columbia Gorge tonight and Fri.
Will see cold air deep enough that any snow falling will survive to
the valley floor, with 1 to 2 inches likely for those areas late
tonight into Fri, with less than an inch as far west as Bonneville.
Will let day shift reevaluate the snow potential, but may need a
Winter Weather Advisory for that area late tonight and Fri.

Another break in the action Fri night and Sat, with showery but cool
air mass over the region. Again, snow levels sit at 2500 to 3000
feet, but lower in the Gorge/Hood Valley.  Fortunately, not much warm
air aloft, so mainly snow at times for the Gorge/Hood Valley, but
accumulations remain generally on the light side.Rockey.


.LONG TERM...Saturday night through Tuesday night...Models are in
good agreement that a low will develop off of the Southern Oregon
coast Saturday night and move north just offshore the Oregon coast
Sunday before moving inland either across Oregon or Southern
Washington on Monday. There are variations in the models on how near
to the coast the low will track, and where the low will move inland.
Despite the model differences there is potential for strong south
winds along the coast Sunday night, as well as rain and Cascade snow
Saturday night and Sunday with showers on Monday. Snow levels are
expected between 2500 and 3500 Saturday night and Sunday then
lowering down to around 1500 feet as the showers decrease on Monday.
There will be periods of east winds through the Gorge at times with
this low, and there is some uncertainty of what the precipitation
type will be for the Gorge and Upper Hood River Valley.

High pressure builds over the area behind the low for a dry weather
and clearing Tuesday. Depending on the cloud cover Tuesday night,
there is potential for radiative cooling to result in a cold
night Tuesday with widespread temperatures below freezing. ~TJ
&&

.AVIATION...Mostly VFR conditions through the period except for
occasional MVFR ceilings during the heavier showers. Mostly
southerly winds through the area except near the mouth of the
Columbia River Gorge where east winds have resumed.

KPDX AND APPROACHES...Mainly VFR in showers today, with
occasional MVFR possible in heavier showers. Winds have returned
to easterly with a slight offshore gradient through the Gorge,
however this gradient is light enough that occasional wind shifts
to southerly cannot be ruled out. /Bentley

&&

.MARINE...Winds have strengthened this morning with gusts to 25 to
30 knots as a surface wave approaches the area. An occasional gust
may approach gale force, but considering the 06Z ASCAT pass did
not have any gale force winds and the NAM is the only model
showing boundary layer winds over 30 knots, a gale warning is not
warranted at this time. It appears when winds weaken this
afternoon, gusts will stay right around 21 knots, so elected to
extend the small craft for winds through 12Z Friday. Near this
time, the potential for gale force winds will be increasing.
Contemplated a gale watch on this shift, but given the complicated
surface low development, elected not to issue any headlines for
that approaching low at this time. A stronger surface low will
move up from the south and replace this low over the weekend.
Depending on the strength and location of this surface low, gale
to possibly storm force winds will be possible. Currently the
stronger solutions, GFS and ECMWF both have a surface low near
960mb. This storm will need to be watched closely as it could have
significant marine impacts over the weekend.

Seas will remain in the 14 to 17 foot range through the remainder
of the work week but continued gale force winds offshore will lead
to a building fresh swell with waves into the low to mid 20 foot
range by Saturday morning. Seas will be complicated over the
weekend with the combination of the long period swell from the
approaching low, the shorter period fresh swell from the nearby
strong surface low, and the building wind wave from the gale force
winds. Bottom line, between the winds and seas over the weekend,
there will be several periods of treacherous marine conditions.
/Bentley

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Freezing Rain Advisory until 6 pm today for...
Central Columbia River Gorge
Upper Hood River Valley.

WA...Freezing Rain Advisory until 6 pm today for...
Central Columbia River Gorge
South Washington Cascades.

PZ...Small Craft Advisory for winds and hazardous seas
until 4 AM Fri on all Coastal waters.

     Small Craft Advisory for Rough Columbia River Bar
conditions today.

&&



$$

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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.



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