Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 130524 CCA
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
924 PM PST Tue Dec 12 2017

.SYNOPSIS...Little change to the overall pattern through Thursday as
a ridge of high pressure remains over the region. The next frontal
system approaches the coast by late Thursday night or early Friday,
bringing a return of onshore flow aloft and rain chances.

&&

.SHORT TERM...Tonight through Friday...Only change with this
evening`s update was to lower overnight temperatures some, although
temperatures this evening are a little bit warmer than they were at
the same time yesterday evening. There is still some lingering cloud
cover over the area, but as high pressure restrengthens, expect skies
to clear and temperatures to drop to well below freezing again in the
interior lowlands. With the temperature and dewpoint at Eugene both
28 degrees as of 9 PM, Freezing Fog Advisory still looks good. Bowen

Expect little change to the overall pattern through Wednesday, as the
upper level ridge remains in place and restrengthens over the Pacific
NW. The initial disturbance continues to move east today, leaving
behind some mid and high clouds contributing to more sky cover this
afternoon compared to past days. However, skies will slowly clear
through the evening, which could provide for additional fog coverage
late overnight through early Wednesday morning. Chances for fog (or
freezing fog) will be best in the southern Willamette Valley for the
Wednesday AM commute. This, coupled with the fact that fairly
widespread areas will see very reduced visibilities, has resulted in
a Freezing Fog Advisory being issued. Expect fog and freezing fog to
start forming shortly after midnight tonight and remaining through
most of the morning. More outlying and less frequently-travels roads,
as well as overpasses and bridge surfaces, will be the most likely
spots for ice to form. Motorists should plan ahead and travel
carefully during the Wednesday morning commute.

While the surface pressure gradient continues to relax
across the Cascades, expect that gradient from The Dalles to
Troutdale will remain in the 4 to 7 mb range for the next few days.
Therefore, gusts will likely remain noticeable from through the far
western Gorge. Forecast models suggest the offshore flow will again
strengthen late Wednesday through Thursday as the enhanced upper
ridge shifts to position more squarely over the forecast area.
Meanwhile, the far central Gorge (that is, around Hood River) will
continue to see no change with persistent stratus and temperatures
remaining around the freezing mark.

The pattern then begins to shift late Thursday and Friday as the next
frontal system moves into the region. Have trended the arrival of
this system a bit slower (in line with latest model consensus). At
this point, it looks like the moisture will spread onshore along the
coast late Thursday before pushing onshore Friday morning. Given the
persistent cold air in the central gorge, there remains the potential
for a brief round of freezing rain in the morning. However, the
timing of the wind reversal and arrival of the moisture are not yet
clear, so confidence in the details remains rather low. Additionally,
available moisture looks to be rather minimal with this onset of this
front. Therefore, any potential impacts should be relatively
short-lived. Of course, the known bias in the forecast models of
mixing out the surface inversion too quickly remains a factor in the
forecast. Thus, at this point, will continue to mention a chance of
both rain and freezing rain around Hood River and in the Hood River
Valley Friday morning. Cullen

.LONG TERM...No Changes. Previous discussion follows...Friday night
through Tuesday. The long term forecast period is marked by lower
than typical confidence in the specific details, given fairly large
disagreements among model guidance. However, there is some consensus
that heights will begin to fall by at least late Tuesday as an upper
trough (of varying strengths) drops across the Pacific Northwest.
Therefore, have trended the forecast closer to seasonal norms - both
in terms of temperature and precipitation (lowland rain/mountain
snow).

&&

.AVIATION...Offshore winds under high pressure aloft will
continue to bring predominantly VFR conditions areawide. The main
exception to this will be across the southern part of the
Willamette Valley, where fog will likely bring LIFR conditions to
KEUG tonight. This fog may also at least briefly develop around
KSLE, KUAO and KHIO towards daybreak on Wednesday, but confidence
is still relatively low at these sites.

PDX AND APPROACHES...Increasing offshore flow Wednesday will
bring predominantly VFR conditions through 06z Thursday. There is
a slight chance some MVFR conditions could briefly develop around
13-15z 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 much of the next 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.

     Freezing Fog Advisory from 2 AM to noon PST Wednesday for South
     Willamette Valley.

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

PZ...Small Craft Advisory for Rough Columbia River Bar until 3 AM
     PST Wednesday.

     Small Craft Advisory for Rough Columbia River Bar from noon to
     3 PM 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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