Area Forecast Discussion
Issued by NWS Portland, OR

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303
FXUS66 KPQR 120502
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
859 PM PST Mon Dec 11 2017

.SYNOPSIS...High pressure remains over the region much of the week,
though a weak disturbance will approach the northern Oregon coast
tonight through early Tuesday. Otherwise, little change to the
pattern until late in the week when a front will bring a return of
onshore flow and a return to a wet weather pattern by the weekend.

&&

.SHORT TERM...Tonight through Thursday...The strong gradients driving
the east winds are beginning to relax and have come down to around
8mb from Troutdale to The Dalles. The inversion capping the gap flow
through the Gorge lowered slightly this afternoon as seen in
Troutdale
profiler gap flow tool and that resulted in stronger east winds even
while the gradient was coming down. Gusts of 79 mph were reported at
Corbett at 4 pm. Winds have been slowly decreasing in all areas
especially farther down the river near Portland. As the gradients
continue to lower and winds decrease we should expect to see longer
and better radiational cooling overnight tonight with slightly cooler
conditions Tuesday morning than Monday. Most locations in the valley
will be below freezing except for possibly a small area in the
western Gorge where the winds will keep things more mixed. Some
outlying locations away from cities will be in the lower 20s and
frost
is likely. Schneider

Previous discussion follows...Little change to the overall pattern
with a high amplitude ridge aloft firmly centered over the northern
Great Basin and extending into the Inland Northwest. Meanwhile, a low
pressure system in the northeastern Pacific and continues to move
northeast with the associated front moving into the coastal waters
later tonight. Forecast models continue to suggest some light rain
approaching the coast, but with the front weakening as it approaches,
think there remains a rather minimal chance of measurable rain
onshore. Furthermore, model cross-sections indicate very little
moisture making it to the coast and weak vertical forcing, so have
maintained a dry forecast for the interior. Most notable impact may
be some additional cloud cover across portions of the region. A
leading line of clouds will move through the forecast area late this
afternoon, but again no rain expected.

Thus, this weak boundary will do little do change the prevailing
pattern through midweek. As a result, have maintained the air
stagnation advisory for the interior valley locations (other than the
Portland area) through 2 PM Thursday given little expected change
through the week. While the surface pressure gradient from The Dalles
to Troutdale peaked near 12 mb yesterday, it will gradually weaken
through midweek. However, until that time, expect that conditions
will still remain breezy through the western Columbia River Gorge and
the far eastern portions of the Portland metro area. Will need to
continue to monitor conditions, but expect that the window for
stagnant conditions around Portland will be sufficiently short to
preclude the need to include the zone in the advisory. The upper
ridge quickly rebuilds on Wednesday, but becomes centered offshore.
This will further ease the cross-Cascade surface pressure gradient
later in the week. Flow aloft will turn more onshore by late in the
Thursday ahead of the next frontal system.   Cullen

.LONG TERM...Thursday night through Monday...Rain moves in late
Thursday night or early Friday.  Models are still not converging on
timing of this system. The timing could be critical, with a chance
for a short period of freezing rain at onset in the central Columbia
River Gorge and around Hood River. Amounts should be light if any,
but could be impactful to travel in this area for a few hours. For
the rest of Southwest Washington and Northwest Oregon, temperatures
should be above freezing when rain starts and rain amounts should be
relatively light. Snow levels will be falling through this event,
bringing snow down to around 3000-4000 ft by Saturday morning.

Precipitation should come to an end early Saturday, with a break in
the weather under a shortwave ridge of high pressure. Forecast gets
more uncertain later in the weekend going into early next week as
there has been some run-to-run differences in how far north the
ridge of high pressure will push the jet stream/storm track back up
on Sunday into Monday. Previous runs showed us staying wet through
the weekend into early next week, but today`s model data suggests
the storm track shifting farther north. This keeps the southern half
of our area dry the rest of Saturday through early next week, with
precip only clipping far northern Oregon and southern Washington.
Right now have left a chance for rain across our southern counties,
but if this trend continues, will probably take pops out of these
areas. In any case, do not see any significantly impactful weather
through early next week. -McCoy

&&

.AVIATION...Offshore under high pressure aloft will continue to
bring predominantly VFR conditions areawide. The main exception
to this will be across the southern half of the Willamette Valley
where some fog will likely develop tonight and bring LIFR
conditions to KEUG between ~06-20z Tuesday.

PDX AND APPROACHES...Decreasing easterly winds will continue to
bring VFR conditions through 06z Wednesday. /Neuman

&&

.MARINE...High pressure aloft will bring relatively little change
in the weather through the end of the work week. A couple swells
moving in from the southwest and west will generally result in
seas hovering between 5-10 ft for the rest of the work week. A
relatively weak front will likely bring an increase in southerly
winds over the weekend. /Neuman

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Air Stagnation Advisory until 2 PM PST Thursday for Central
     Willamette Valley-Lower Columbia-South Willamette Valley.

WA...Air Stagnation Advisory until 2 PM PST Thursday for Greater
     Vancouver Area-I-5 Corridor in Cowlitz County.

PZ...None.


&&

$$

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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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