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Area forecast discussion 
National Weather Service Portland or
706 PM PST sun Jan 25 2015

Synopsis...high pressure aloft gradually shift east tonight and
Monday. Strong valley inversions will continue tonight...but start to
weaken Monday afternoon. A very weak frontal system nears the coast
Tuesday for a threat of some light rain or drizzle. However...high
pressure and dry weather are expected to return around middle-week and
last into next weekend.

Update...dense fog is filling in quickly for the interior valleys
this evening and have issued a dense fog advisory. The easterly
surface pressure gradient from Troutdale to The Dalles has increased
to 4.5 mb...which is enough for weak easterly winds to keep the coast
mostly fog free tonight. The only change with the evening update was
to add visibility wording with the fog for the inland valleys
tonight and Monday morning. Tj

Previous short term...tonight through Wednesday...very strong
subsidence inversions are resulting in persistent fog throughout the
Willamette Valley and Clark County in SW Washington. Had to extend the dense
fog advisory through noon...but as of 21z areas of dense fog
continue. The magnitude of the inversion is quite impressive.
Elevations above 2000 feet in the Oregon Coast Range were in the middle
60s to middle 70s at 21z. Marys 4137 feet in the central Coast
Range...was 67 degree at 2025z. Equally as impressive was the 21z 63 degree
reading at the 6000-foot timberline sensor with a humidity of just 11
percent. Several Cascade RAWS sites between 2500 and 4500 feet were
in the 70s as of 22z.

Primary features on water vapor imagery early this afternoon are the
high pressure cell centered over NE California and a closed low just
south of Baja California. 12z model forecasts show 850 mb temperatures peaking in the
16c to 18c range tonight...which would be close to the warmest 850 mb
temperature recorded in January in northwest Oregon since records became available
in the 1950s. The upper ridge will start to move east tonight and
continue to do so Monday...resulting in moderating temperatures for the higher
terrain. GFS has 850 mb temperatures lowering to 10 to 13c by 00z Tuesday.
Higher parts of the Coast Range and Cascades could see maximum temperatures Monday
fairly early. Valleys are likely to see persistent low clouds and fog
as the inversion weakens during the day Monday.

Models are consistent in depicting a weak front pushing onto the
coast Tuesday as the upper flow flattens out. Model cross sections
indicate moisture with this decaying front will be rather
shallow...generally below 800 mb late Monday night and Tuesday. Not looking
at much precipitation with this...with drizzle more likely than rain. The
European model (ecmwf) has 500 mb ridging starting to build Wednesday...but time-height and
model cross-sections show a fairly deep marine layer in place.
Thus...have left low probability of precipitation in the forecast for Wednesday. Weishaar

Long term...Wednesday night through Sunday...a passing shortwave
late Wednesday night may provide enough lift for a few showers...mostly
along the north coast and Coast Range and into SW Washington. Models
remain in good agreement that a ridge of high pressure aloft rebuilds on
Thursday and persists through the end of the week. This will maintain dry
conditions but bring daily fog and low clouds to Interior Valley
locations. The ridge will likely break down and shift inland some
time next weekend...but still some uncertainty as to if this will be
Sat or sun. The 12z GFS is a bit more aggressive in bringing a moist
frontal system to the region by late Sat night...while the European model (ecmwf)
would keep things dry until Sun night. Increased probability of precipitation a bit for next
weekend...particularly for the northwest portion of the forecast
area...though restricted to lower end chance for Sat night through
sun given the support for the later timing from the European model (ecmwf) and
numerous gefs members. Cullen

Aviation...Astoria has cleared out...meaning all coastal sites
are now VFR. They should remain VFR overnight with a light
easterly/southeasterly downslope wind. Fog is still lowering ceilings
and visible at South Valley sites to LIFR/vlifr. Fog over the northern
Willamette Valley is burning off as expected...improving all but
khio to VFR. Khio should improve to MVFR by 23z...and possibly to
VFR after 00z before fog redevelops around 02z. Keug may still
make it to MVFR this afternoon around 23z as fog slowly Burns off
in the South Valley. There may be a brief period of VFR after 00z
before the sun GOES down and fog starts to rapidly redevelop. Ceilings
and visible over the south and Central Valley will drop to LIFR/vlifr
this evening around 02-04z...while the north Willamette Valley
will drop to LIFR/vlifr a bit later around 03-06z. LIFR/vlifr ceilings
and visible expected to persist again overnight tonight as dense fog
redevelops over the inland terminals. Timing of fog burn-off
should be similar to today...with North Valley sites improving to
VFR by 20-22z and South Valley sites struggling to improve to
MVFR/VFR by 00z.

Kpdx and approaches...VFR conditions will start to drop as fog
begins to redevelop around 04z. Ceilings and visible drop to LIFR/vlifr
around 05z and will remain with 1/4sm visible fog overnight. Timing of
improvement Monday should be similar to today...with fog rapidly
clearing to VFR around 21z. -McCoy


Marine...seas currently sitting around 9 to 10 feet with light
winds over the waters. Southerly winds will surge northward along
the outer waters late this evening...with a brief period of gusts
around 20 to 25 kts along the leading edge before stabilizing
around 15 to 20 kts. Gusts above 20 kts will not persist long
enough to warrant a Small Craft Advisory over the outer waters.
Seas should drop to around 8 to 9 feet later this evening...but
could come back up near 10 feet over the outer waters with gusty
southerly winds Monday morning along the leading edge of the
southerly surge. Monday evening seas will start to subside...
falling to around 5 feet by Tuesday night or Wednesday. -McCoy


Pqr watches/warnings/advisories...
Pz...Small Craft Advisory for hazardous seas until midnight PST
tonight for coastal waters from Cape Shoalwater Washington to Florence
or out 60 nm.

Small Craft Advisory for rough Columbia River bar from 6 PM to
11 PM PST this evening.



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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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