Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 202219
AFDPQR

Area Forecast Discussion
National Weather Service Portland Oregon
219 PM PST Fri Jan 20 2017

.SYNOPSIS...A 975 mb low was centered just west of buoy 5 this
afternoon, around 300 miles off Astoria. An initial frontal band was
lifting north through southwest Washington this morning and early
afternoon and will then be followed by showers. Another short wave or
band will swing through the forecast area on Saturday for more rain
and mountain and Gorge snow. Another low will deepen to near 960 mb
off the coast near buoy 5 Saturday night and Sunday for another round
of rain and mountain and Gorge snow changing to showers. The low will
spread more showers into the area into Monday before dropping south
into California. Expect drier weather for a good part of next week.
&&

.SHORT TERM...Tonight through Monday...A 975 mb low is centered just
west of buoy 5, a little more than 300 miles west of Astoria. A
frontal/deformation band was lifting north through southwest
Washington this afternoon. There are a lot of showers offshore south
of the low headed toward the west coast of the U.S., but much of that
area of showers will head south into California. Some will spread
onshore to the north into northwest Oregon and southwest Washington
overnight tonight, but coverage and QPF will be less in our forecast
area than to our south. Snow is having trouble accumulating in the
Gorge and will drop the Winter Weather Advisory for tonight but not
for later time periods.

A second decent short wave which is actually the wraparound band
currently on the west side of the offshore low will rotate around the
low and swing through our forecast area on Saturday for another surge
in showers or possibly a period of steadier precipitation. Look for
more rain and Gorge and Cascade snow with this band.

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 near the position of buoy 5 Saturday night
and Sunday. This low brings another period of rain and Gorge and
Cascade snow later Saturday night into early Sunday before turning to
decreasing showers.

Look for periods of brisk east winds through the Gorge that will keep
the Gorge cold enough for snow from these bands. The surface pressure
gradient even along the coast will stay easterly, which should
prevent high wind from developing near the coast from these 2 strong
lows off the coast.

When all is said and done, seems reasonable to see total snowfall for
today through Sunday of 2 to 6 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 south into California Sunday
night and Monday. 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. Tolleson

.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...A band of steadier rain has set up over the northern
Willamette Valley and I-5 corridor of SW Washington this
afternoon. This is bringing occasional MVFR conditions. Otherwise,
VFR in control across the region. Showers should taper off this
evening and overnight. Expect VFR conditions to persist through
Sat morning. A surface trough will bring increasing rain and
possibility of MVFR conditions between 18Z Sat and 00Z Sun.

PDX AND APPROACHES...A band of steady light rain will persist
through the late afternoon. Expect mainly VFR during this time,
but occasional MVFR is possible. Rain tapers off this evening and
overnight, with VFR expected tonight and Sat morning. Increasing
rain and possible MVFR conditions move in between 18Z Sat and 00Z
Sun. Easterly winds gusting 25 to 30 kt near the western end of
the gorge may impact easterly approaches. Pyle
&&

.MARINE...A complicated weather pattern to impact the coastal
waters over the next several days. A surface low around 975 mb
near 46N/132W has brought gusty SE winds mainly to the outer
waters today. Allowed the Gale Warning to expire earlier today and
have replaced with a Small Craft Advisory. Expect south to
southeast wind gusts to 25 kt to continue tonight. Another surface
trough will move through tomorrow, turning winds to the southwest
and increasing speeds. Think we will remain below gales, but
frequent gusts of 25 to 30 kt are likely. Another low currently
near 40N/155W will approach from the southwest and is modeled to
strengthen
rapidly as it nears the coast late Sat and Sun. The low occludes
off the Oregon coast on Sun and remains nearly stationary into
early next week. The models indicate that it will reach a strength
of around 965 mb by Sun morning. The timing of system is now in
pretty good agreement in the latest model runs. However, there is
still spread in the positioning of the low. The last couple of
runs of the GFS have been further offshore from the NAM and ECMWF
solutions, which would result in significantly less wind for our
coastal waters, likely only moderate gales. The closer ECMWF and
NAM solutions would likely support storm force winds over at least
a portion of our waters. For now, went with a blend of these two
scenarios and issued a gale watch for all of the waters from late
Sat evening through Sun evening. The low is modeled to finally
move south down the CA coast late Mon into Tue.

Seas will remain in the mid teens through this evening. Then a
significant long-period westerly swell will arrive late tonight
and Sat. Expect seas to build into the low to mid 20s by early Sat
morning. With the long periods and the primary energy out of the
west, we should easily meet high surf advisory criteria for a good
chunk of tomorrow. The strong low over the weekend will produce
significant swell as well, but it looks like the bulk of this
will head south onto the southern Oregon and northern California
coast. However, think we will see seas remain above 20 ft for much
if not all of the weekend. Seas finally subside early next week,
likely dropping below 10 ft by Tue as high pres settles over the
waters. Pyle
&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...High Surf Advisory from 4 AM to 10 PM PST Saturday for Central
     Oregon Coast-North Oregon Coast.

     Winter Weather Advisory from 10 AM Saturday to 6 PM PST Sunday
     for Central Columbia River Gorge-Upper Hood River Valley-
     Western Columbia River Gorge.

WA...High Surf Advisory from 4 AM to 10 PM PST Saturday for South
     Washington Coast.

     Winter Weather Advisory from 10 AM Saturday to 6 PM PST Sunday
     for Central Columbia River Gorge-Western Columbia River
     Gorge.

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

     Gale Watch from Saturday 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 Saturday for
     Coastal Waters from Cape Shoalwater WA to Florence OR out
     60 nm.

     Small Craft Advisory for hazardous seas until 10 PM PST
     Saturday for Coastal Waters from Cape Shoalwater WA to
     Florence OR out 10 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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