Area Forecast Discussion
Issued by NWS Portland, OR
FXUS66 KPQR 021028
Area Forecast Discussion
National Weather Service Portland Oregon
230 AM PST Fri Dec 2 2016
.SYNOPSIS...A warm front off the Pacific will approach the
coast today, and move inland at the end of the day. A cold front
from the nw will follow Fri night, moving se through the area.
Another front will move south into the Pacific NW Sat night and Sun,
ushering in a colder air mass for early next week that originated in
.SHORT TERM...Tonight through Sunday...Rain showing up off the WA
and far north OR coast early this morning on radar, well ahead of a
warm front that was estimated from satwellite out near 129W. Weak
mid level moist isentropic lift is seen in models on the 300k
isentrope early today, but more vigorous lift is not apparent until
low level isentropic lift along the 290k isentrope spreads south
late this morning and afternoon, so will see pops climb from nw to
se across the forecast area during the course of the day. Model time
height cross sections indicate mid levels drying considerably this
evening, so moisture will not be deep, limiting qpf potential with
this system tonight. Warmer air with rising snow levels will
temporarily move in this evening as the warm front moves in, then
will fall again as cooler air sinks south with a weak cold front
Moisture generally limited to low levels early Sat, with forecast
soundings suggesting shallow low level instability best across the
north part of the forecast area. Coupled with a weak onshore flow,
will keep a low pop in across most of the forecast area Sat.
Sat night sees a westerly flow aloft with sagging upper heights.
Models agree on a sharp but slow moving front sagging south through
sw WA into nw OR Sat night. Moisture cross sections show the slow
moving system picking up deeper moisture Sat night and Sun. Westerly
flow ahead of this system increases modestly to 20 kt at 850 mb,
which paired with the deeper moisture increases moisture transport.
With good moisture transport and additional dynamic lift associated
with the front will go with categorical pops. With cooling air mass
expect to see a good shot of snow down to passes in the northern
.LONG TERM...No Changes. Previous discussion follows...Sunday night
through Thursday...A series of shortwaves will bring cold air to the
Pac NW next week. Post frontal showers behind a departing shortwave
will continue to bring snow to the Cascades and Foothills, with snow
levels lowering to around 1000 ft on Monday. Models continue to show
snow levels lowering below 500 ft on Tuesday as another cold
shortwave moves across the region. This is the good news for those
looking for valley snow. The bad news is that limited moisture and
cold air filling in through the Gorge may act to decrease the snow
potential. Upper level ridging will briefly build across the area
late Tuesday/early Wednesday giving us a brief break from any precip.
Models then show another system approaching the region late Wednesday
which will bring another shot of valley rain and Cascade snow.
.AVIATION...Mix of conditions across region, with VFR over most
areas, then MVFR over inland areas from Portland metro northward
through Cowlitz Valley. Also, have patchy fog south of Portland.
Not much change this am, but will see MVFR increasing along the
coast as a front approaches. This MVFR and light rain will spread
across the interior after 18Z. Expect higher terrain to be
obscured in clouds and rain through this evening.
KPDX AND APPROACHES...As weak front pushes into the region, will
see little change today as MVFR will persist. Will see light rain
increasing after 17z. May see MVFR break up at times, but would
only brief briefly and even then will maintain lower VFR cigs. Not
much change through this evening. Rockey
.MARINE...Southerly winds will pick up a bit this am, with 15 to
20 kt and gusts in 20 to 30 kt range. Strongest winds will be to
north of Cascade Head. Once front moves onshore late this
afternoon into early this evening, winds will ease and turn more
west to southwest. Next boost of winds will be late Sat night into
Sun, as another strong cold front over the Gulf of Alaska drops
southeastward across the region.
Seas mostly around 8 to 9 ft this am, but will see seas running
at 9 to 11 ft today. The west to northwest swell train will
continue arriving tonight and Sat, with generally swell running
in the lower teens this evening, building to the mid and upper
teens for late tonight and Sat. In fact, with rather active
pattern over the Gulf of Alaska, will see persistent west to
northwest swell of 13 to 16 ft for Sun into Tue. Rockey.
PZ...Small Craft Advisory for winds today on all coastal waters.
Small Craft Advisory for hazardous seas today through Tue on
all coastal waters.
Small Craft Advisory for Rough Columbia River Bar conditions
today through Sat.
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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.