Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 300341
AFDPQR

Area Forecast Discussion
National Weather Service Portland Oregon
828 PM PDT Thu Sep 29 2016

.SYNOPSIS...The upper ridge that brought mild dry weather to
southwest Washington and northwest Oregon that past couple of days
will slowly move east as a low pressure system moves south well off
the coast tonight, then approaches the coast Friday and swings
onshore Friday night and Saturday. Look for increasing clouds,
showers, and cooler temperatures with some snow possible over the
higher Cascades, especially Friday night and Saturday. A second low
is expected to follow quickly behind the first and swing in Sunday
and Sunday night, with the bulk of the energy heading into southern
Oregon and northern California but still affecting our forecast area.
Another splitting system is expected late Monday into Tuesday.
&&

.SHORT TERM...(Tonight through Sunday)...Satellite imagery show a
band of clouds over the Cascades this evening moving slowly east.
Radar indicates that a few light showers aloft are occurring over
Hood River County but nothing reaching the ground this evening. We do
have a slight chance of showers or thunderstorms forecast for the
Cascades, but the emphasis will be on slight as there really is no
strong mid level forcing for elevated convection. Schneider

A low pressure system continues to dig south out between 130W and
140W this evening, and will set up off the Washington and Oregon
coasts tonight and Friday closer to 130W. The low will slowly
approach the coast Friday, with a band of vorticity likely associated
with a possible frontal boundary moving onto the coast mid day Friday
and into the valleys late in the day for a few showers. This feature
weakens and lifts north Friday evening. This will be followed by the
main upper low area and its associated vorticity center that will
swing onshore later Friday night and inland and through Oregon on
Saturday. This will be the best period of showers over our forecast
area. Snow levels will drop to the 5000 to 5500 foot range by
Saturday with some light snow accumulations possible in the higher
Cascades.

A second low follows closely on its heels and swings into southern
Oregon and northern California Sunday. Nevertheless, some of the
moisture and showers will spread through our area, with the greatest
coverage in the south part of our forecast area, like near Eugene.
Snow will remain possible in the higher Cascades. Tolleson

.LONG TERM...(Sunday night through Thursday)...Cooler, Fall pattern
continues into next week. As the weekend system drops off to the
south and east, a quick-moving shortwave will bring a quick round of
rain Tuesday morning, mostly drying out Tuesday evening. Beyond
Tuesday the forecast gets more difficult as the models diverge on a
solution for later next week. While the GFS builds weak ridging over
the area drying us out until later next week, the ECMWF keeps us
under a more zonal flow pattern which would mean a chance for showers
every day next week. As the GFS is the outlier with a closed-off low
next weekend, thinking that it likely is building the ridge too far
north ahead of this system, so leaned more toward climo and the ECMWF
with a slight chance of showers through the week. Still think there`s
a better chance than not Wednesday and Thursday will be dry before
precipitation returns next weekend. -McCoy
&&

.AVIATION...Expect marine stratus to continuing moving into the
coastal areas late this evening into tonight for MVFR conditions.
Meanwhile, VFR conditions continues this evening across the
interior as a thick band of high cirrus continues to cross the
region. A lower stratus deck with MVFR cigs around 1500 ft
expected to push along the Columbia River, reaching Kelso between
09z-12z. Lower confidence in the extent of the development into
the Willamette Valley, but models continue to suggest the best
chance for development with be both from north into the Portland
metro terminals and south into KEUG generally after 13z through
17z, with the lowest chance in the mid-Valley, including near
KSLE. Local low clouds linger inland early Friday morning, but
increasing mid/high clouds ahead of disturbance. Front will spread
rain into the coastal terminals by late afternoon with generally
MVFR or lower conditions.

KPDX AND APPROACHES...VFR conditions early tonight. Expect that
some stratus will develop along the Columbia River, with about a
40 percent chance of MVFR developing at the terminal after 13z
through 18z. Expect any lingering lower cigs to break around 18z,
but increasing mid/high clouds through day ahead of approaching
disturbance. Cullen

&&

.MARINE...Seas will remain 4 to 6 feet through Friday. However,
expect rather choppy conditions with the dominant period remaining
around 9 seconds. Light N-NW winds will continue tonight, but turn
S on Friday as a low pressure system moves south just beyond the
outer coastal waters. Winds will increase late Friday as the low
moves closer to the waters, with gusts to 20 kt possible. A
stronger low moves through the waters on Saturday night, and a
Small Craft Advisory for winds may be required for at least a
portion of the waters. Seas will gradually increase through the
first part of next week and build to near 10 ft by the middle of
next week.  Cullen

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...None.
&&

$$

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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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