Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 211037
AFDPQR

Area Forecast Discussion
National Weather Service Portland Oregon
235 AM PST Sat Jan 21 2017

.SYNOPSIS...Broad low pressure continues to churn off the Pac NW
coast this evening, pushing bands of showers onshore into Oregon and
California. Next band of showers will swing north across the forecast
area today. Another low will deepen to near 960 mb well offshore
tonight into Sunday, sending yet another round of precipitation for
the region. Overall upper low will sink southward Monday and Mon
night, with showers decreasing from north to south. Dry weather with
seasonable temperatures still on tap for Tue through next weekend.
&&

.SHORT TERM...Today through Monday...Not a lot of change, as the
overall pattern stuck as broad upper low remains anchored over the
northeast Pac. This low is rotating several disturbances around it,
with these disturbances moving northward across western Oregon and
Washington.

Next disturbance sits off the south Oregon coast, and will move over
the region today, maintaining plenty of showers, with snow above 3000
ft in the Cascades. Best instability remains offshore and along the
coast. Will keep slight chance of thunderstorms for the west slopes
of the Coast Range and westward across the coastal areas for today.
Once this disturbance moves north of the region later today, showers
will decreasing from south to north.

Snow levels remain low across the Columbia Gorge and Upper Hood River
Valley, where precipitation will mostly fall as snow. Will maintain
current Winter Weather Advisory for today through Sun, as about 1 to
3 inches of snow seems reasonably for today, bit less tonight, but
bit more in way of snow for Sunday.

Our offshore low then weakens and moves north late Saturday as the
next system, currently out near 40N/155W, strongly develops out along
130W and deepens to 960 mb about 300 miles west of Astoria later
tonight into Sunday. This low wind send another front onshore Sunday
am, with rain and mountain snow. But will transition to showers in
the afternoon behind the front.  Will have to watch the coast for
stronger winds than current forecast, as the gradients could support
fair decent winds. But, with overall pressure pattern relatively low
already, the tight gradient round the low may result in keeping the
strong winds focused on the low, and not spread out as far as would
expect normally. Another mark against strong wind potential for the
coast is the orientation of the gradient. Feel that is way too much
southeasterly gradient over our area, so this will knock down speeds
quite a bit.  Still, bears watching.

When all is said and done, seems reasonable to see total snowfall for
today through Sunday of 1 to 5 inches in the Columbia Gorge (heaviest
east of Cascade Locks), and 4 to 8 inches in the Hood Valley, as well
as higher elevations of the Coast Range. For the Cascades, generally
12 to 15 inches of new snow on the passes, with up to 2 ft of new
snow on high peaks.

The low off the coast Sunday will drop southward towards northern
Calif Sunday night through Mon. This will bring decreasing east wind
and drying to the forecast area from the north. A few showers will
linger on Monday but decrease considerably, with the best chance in
our southern forecast zones.  Rockey.


.LONG TERM...Monday night through Friday...As the low pressure
system drops south of our area Monday night, a large upper-level
ridge builds over the area. As the ridge moves in the GFS brings one
last shortwave across the area. The GFS is the only model with this
feature, so right now have left precipitation out of the forecast,
though I did increase cloud cover on Tuesday. Behind this, the ridge
builds over the area drying us out through Wednesday. GFS and ECMWF
differ on how long the dry spell will last.  The ECMWF is quicker
with the next shortwave trough which it brings into our area on
Wednesday night. The GFS on the other hand doesn`t bring our next
system in until Thursday night-Friday. Either way, expect easterly
winds to develop ahead of this next system. Right now snow levels
look to stay up above 3000 feet. -McCoy

&&

.AVIATION...Mostly VFR conditions expected into Saturday morning.
Ceilings have dropped to MVFR at KONP and KHIO in the last hour.
Will hold out until 12Z TAF issuance before deciding whether to
include these MVFR cigs in a FM group. More widespread MVFR
cigs/visibility expected after 18Z when the next frontal wave
brings a round of rain through the area.

PDX AND APPROACHES...Mainly VFR conditions until late morning to
early afternoon when ceilings/visibility may temporarily drop to
MVFR as rain moves through the area. /Bentley

&&

.MARINE...Low end small craft winds continue this morning for all
waters and should remain mostly consistent through mid day when
winds will increase associated with the next frontal wave moving
through the waters. Some guidance is indicating a brief period of
gusts to gale force during the early to mid afternoon period. Will
forgo any gale warning for today given the borderline wind speeds
and the brief (~3 hour) window of concern.

Winds will subside to low end small craft Saturday night and then
increase again during the day Sunday as a strong (sub-970mb) low
moves north and eventually settles off the Oregon shore. Gales
appear likely for most of the day Sunday at least for the outer
waters. Given the southeasterly component of the BL winds, not
sure if the inner waters will reach be as consistent with the
stronger winds, but feel gale watch is warranted for the brief
periods where gales may occur, especially more than 6 miles
offshore. In addition, guidance was giving some storm force wind
gusts for outer waters, but elected to remove that mention from
the forecast at this time. The area of strongest winds will remain
quite close to the low pressure center, and models have been
trending westward with the surface low during maximum intensity.
Therefore, confidence is high that storm force winds will be
observed in some areas near the surface low, but am not sure those
stronger wind gusts will make it within 60 nm of shore in our area.

Watching seas closely this evening as they have been trending a
bit lower than expected. However, seas jumped TO 18 ft at Buoy 50
at 10Z, so the expected higher seas may finally be arriving.
Without upstream observations to confirm the swell, am relying
closely on model guidance, however the strength of the surface low
and presence of a dynamic fetch certainly support the >20ft seas
indicated by most model guidance. /Bentley

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...High Surf Advisory until 10 pm today for...
 North Coast of Oregon
 Central Coast of Oregon.

     Winter Weather Advisory today through Sunday for...
 Western Columbia River Gorge
 Central Columbia River Gorge
 Upper Hood River Valley.

WA...High Surf Advisory until 10 pm today for...
 South Coast of Washington.

     Winter Weather Advisory today through Sunday for...
 Western Columbia River Gorge
 Central Columbia River Gorge.

PZ...Small Craft Advisory for Rough Columbia River Bar until 10 PM
     PST this evening.

     Gale Watch from this evening through Sunday evening for Coastal
     Waters from Cape Shoalwater WA to Florence OR out 60 nm.

     Small Craft Advisory for winds until 10 PM PST this evening for
     Coastal Waters from Cape Shoalwater WA to Florence OR out
     60 nm.

     Small Craft Advisory for hazardous seas until 10 PM PST this
     evening for Coastal Waters from Cape Shoalwater WA to
     Florence OR out 60 nm.

&&



$$

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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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