Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 140422
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
822 PM PST Wed Dec 13 2017

.SYNOPSIS...Little change to the overall pattern through Thursday as
a ridge of high pressure remains over the region with low-level
offshore flow. The next frontal system moves in off the Pacific
Friday, bringing a return of onshore flow and chances for lowland
rain and mountain snow. This more seasonally typical weather pattern
then persists through much of next week.

&&

.SHORT TERM...Tonight through Saturday...Not much change needed to
the forecast this evening. Moved freezing fog back to late tonight
for the southern Willamette Valley and Lower Columbia areas as
temperatures and dewpoint depressions are much higher at this point
tonight than they were last night. Relatedly adjusted sky cover for
this evening. The pressure gradient through the Gorge hasn`t
increased much since this afternoon, but still expect some tightening
overnight for even a bit more wind. Bowen

Meanwhile, the surface pressure gradient through the gorge continues
to slowly increase, as expected - 8.6 mb from The Dalles to Troutdale
as of 7 PM. A generally persistence-based forecast should hold
through Thursday. Forecast models suggest the gradient could
strengthen to around 10 mb by early Thursday morning. While this is
not quite as strong as last weekend and early this week, this will
still bring a return of windy conditions to these locations.

The long-awaited breakdown of the ridge still appears on track for
early Friday. Latest model data continues to suggest a slowing of the
disturbance as it approaches, so have continued to trend the forecast
this direction. As this system is the initial one to
move into the longwave ridge, and moisture remains quite modest, will
continue to maintain PoPs into the chance category or lower and with
relatively little QPF. Offshore flow through the Gorge is likely to
continue right up until the time the front moves through Friday
morning. As such, the temperatures around Hood River and in the Hood
River Valley are likely to remain right around freezing right until
the front moves through. Will continue to indicate a slight chance of
freezing rain in the Hood River area at onset of precipitation late
Friday morning, but confidence is beginning to increase that any
period of freezing rain will be very short and with very limited
amounts of moisture. It should be noted that if the front does speed
up with rain arriving in the pre-dawn time period Friday and before
onshore flow breaks the surface inversion, localized patches of
freezing rain may be possible to the west (i.e., cooler, more
sheltered locations in the northern Willamette Valley north through
the Lower Columbia).

A few showers may continue through the day Friday behind the front.
Shortwave ridging begins to build aloft on Saturday, for an
increasingly dry trend across the southern portions of the forecast
area. However, maintained some mention of PoPs in far northwestern
Oregon and SW Washington as some moisture may attempt to spread over
the top of the ridge. Nonetheless, Saturday may end up being a dry
day for most.Cullen

.LONG TERM...No Changes. Previous discussion follows...Saturday night
through Wednesday. Transient shortwave ridging is quickly replaced by
the next disturbance, with precipitation chances back for at least
the northern half of the area. However, the models quickly begin to
differ in the evolution of these disturbances over the weekend. For
the first half of next week, these model differences quickly
accumulate to provide for even lower than typical confidence in the
details of the pattern that will emerge. Nonetheless, utilizing
ensemble data, there is broad agreement in a generally more
progressive pattern through long term period (i.e. the first half of
next week), and have maintained the forecast that largely follows a
climatologically typical pattern.

&&

.AVIATION...Offshore flow pattern in place with strong high
pressure east of the Cascades and weak thermal trough along the
Oregon Coast. East winds have ramped up again through the
Columbia Gorge, with gusts nearing 40 kt at times for KTTD.
Surface winds are lighter PDX westward, resulting in areas of
LLWS especially west and north of downtown Portland due to the
stronger winds above 500-1000 ft MSL. Light winds south of KSLE
may allow for some patchy fog later tonight/early Thu; especially
around KEUG. Otherwise VFR will likely prevail through Thu.

PDX AND APPROACHES...VFR conditions through 00z Friday. Strong
easterly Gorge winds of up to 40 kt between 500 to 2500 ft MSL
may result in some LLWS at times on western approaches tonight
and early Thu. The stronger winds extend all the way down to the
surface KPDX eastward into the Columbia Gorge.Weagle

&&

.MARINE...Offshore flow is increasing again across the coastal
waters this evening, as high pressure strengthens east of the
Cascades. Expect E-NE winds locally gusting 25-30 kt below gaps
in the coastal terrain, but winds in general should be a bit
lighter than that. A weak cold front will begin to break down
the high pressure system Thu evening, then move across the
coastal waters early Fri morning. NW winds behind the front may
push winds up to SCA criteria later Fri/Fri night, but it will be
very marginal and most likely to impact the northern waters.
Swell energy from the WNW and SW will will combine with an
increasing wind wave component to push seas up to 7-9 ft Thu
night/Fri. After a break in the swell Saturday and early Sunday,
swell energy from the Gulf of Alaska will push seas back up
toward 10 feet or slightly higher early next week.  Weagle

&&

.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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