Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 232242
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
242 PM PST Thu Nov 23 2017

.SYNOPSIS...A series of fronts will continue periods of wet weather
into next week. Snow levels will remain above the Cascade passes
today, then lower to near the Cascade passes tonight, but with light
precipitation. There is a better chance for snow to impact the passes
Sunday and Monday.

&&

.SHORT TERM...Tonight through Sunday...Rain continues across SW
Washington and NW Oregon this afternoon as a cold front moves across
the region. Satellite imagery shows the back end of the front just
about to reach the SW Washington coast. Open-cell cumulus over the NE
Pacific indicate colder and more unstable air will follow the  front.
Snow levels will lower later this afternoon through tonight behind
the front, and likely reach the Cascade passes.

Showers will accompany the colder air, but satellite derived
precipitable water values drop behind the front, and suggests that
the showers will not generate significant precipitation. Rain totals
behind the front will generally be less than 0.20 inch tonight
through Friday morning which may materialize to spotty 1 to 2 inches
of snow near or above 4500 feet. Showers will decrease Friday and be
limited to SW Washington and extreme NW Oregon coast late Friday
afternoon.

The Grays River at Rosburg is receding, but at a slightly slower rate
than the current forecast. The river will likely remain above flood
level a few hours beyond the current forecast time of 5 PM. We will
further analyze and modify the forecast for this river later this
afternoon.

Have reduced the probability of precipitation for Saturday morning as
the models have slowed the progress of the next system which will
approach from the south west. A warm front will produce rain late
Saturday morning or early Saturday afternoon. Snow levels will rise
above the Cascade passes Saturday afternoon and evening. Models are
still not in agreement on the strength and track of the low
associated with this frontal system, which continues to challenge the
wind forecast. However, there is a chance that there could be a burst
of gusty winds along the coast and coastal mountains as the low moves
north along the coast Saturday evening.

The cold front will move across the area overnight Sunday morning,
with the snow levels lowering back down to the Cascade passes behind
it in the afternoon. ~TJ

.LONG TERM...Sunday night through Thursday... A colder upper level
trough will support snow levels staying near the Cascade passes with
showers over the region Sunday night and Monday. The models
vary on how quickly the upper trough will move east, but generally
agree that an upper ridge will replace the upper trough Monday night.
As Monday`s upper trough advances into the Great Basin and Plains,
models remain in reasonable agreement and suggest rising heights
across the Pacific Northwest into Tuesday. This may bring a bit of a
break in the precipitation, though there is some hint that the
northern extent of the forecast area (primarily north of a Tillamook
to Mt. Hood line) may be clipped by a front associated with a low
moving into central B.C. As a result, have lowered PoPs across the
southern and southeastern portions of the forecast area. Models then
diverge more significantly as we move into the second half of next
week, so have held the forecast closer to climatological normals.
Cullen

&&

.AVIATION...Back-side of the surface front is moving onto the
coast right now, with improving conditions along the coast behind
the front. Inland has gone predominantly VFR as well this
afternoon. Predominantly VFR conditions should continue through
early this evening as clouds start breaking up behind the front.
With clearing skies comes the threat for fog or low stratus
tonight across our area. Will likely see a mix of low
MVFR/IFR/LIFR conditions develop around 06-08Z across the area.
This mix of conditions will likely persist through midday Friday,
only starting to break up in the early afternoon.

KPDX AND APPROACHES...VFR conditions this afternoon and early
evening. Will see clouds start to break up a bit this evening as
the back-side of the front moves through. With clouds breaking
up, will see a good chance for fog or low stratus to develop
tonight after 06Z. Low MVFR/IFR conditions to persist through
most of Friday morning, not lifting until around 21Z. -McCoy

&&

.MARINE...Relatively benign conditions to continue through
Friday. A broad area of low pressure remains offshore, keeping a
strong enough gradient for gusts to 25 kt and will likely keep
seas up around 10 ft out near buoy 89 and in our northern outer
waters (beyond 10 nm from the coast north of Cascade Head). Seas
have generally subsided to 7 to 9 ft today and will remain around
7 to 9 ft except for aforementioned seas getting back up to
around 10 ft out near buoy 89.

The next system we focused the most on today is the system coming
in on Saturday night. Have issued a gale watch for as models are
growing more confident in a strong system Saturday night into
early Sunday. There is some chance for storm force winds with
this system as well, but right now much more confident in at
least gale force southerly winds with this system. With this,
seas will build fairly quickly on Saturday, getting up near 20
ft in the evening and persisting in the upper teens or near 20 ft
through much of Sunday morning.

The next time period to watch will be Sunday night as another
system moves through, but there is large differences in the
models on the track of this Sunday system. For now, went with a
blend of the GFS and the ECMWF and kept with the trend of
much weaker winds with the system on Sunday. Seas should be
gradually coming down as well.

Another weaker system looks to move through on Monday night, but
Tuesday and beyond looks to be much more tranquil weather as
high pres builds over the region. If trends continue, could see
northerly winds 10 to 15 kt for good part of next week. But seas
will vary, due to various swell fetches arriving next week.
-McCoy


&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Gale Watch from Saturday afternoon through late Saturday night
     for Waters from Cape Shoalwater WA to Florence OR from 10
     to 60 NM.

     Small Craft Advisory until 10 AM PST Friday for Waters from
     Cape Shoalwater WA to Cascade Head OR from 10 to 60 NM.

     Small Craft Advisory for Rough Columbia River Bar from 5 PM
     this afternoon to 10 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. This area is
commonly referred to as the CWA, or forecast area.



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