River Flood Warning 34, NWS Portland, OR

Area Forecast Discussion
National Weather Service Portland OR
245 AM PST Mon Nov 20 2017

.SYNOPSIS...Wet and unsettled weather this week. A cold front will
move southeast across the area today. The front returns tonight and
Tue, lifting north as a warm front. A cold front will push into the
region off the Pacific Tuesday night and Wednesday. Another cold
front will move through on Thanksgiving, with showers lingering into
Friday. Another front is expected for the weekend.
&&

.SHORT TERM...Tonight through Wednesday...A fine line on KLGX
radar, and buoys showed the cold front finally approaching the coast
early this morning, with coastal surface pres gradients beginning to
weaken. Expect the winds to diminish once the front moves onshore
during the early morning hours. Heaviest precipitation was out in
front of the front, where sw 850 mb winds from 50 to 60 kt were
coupled with precipitable water values of 1.1 to 1.2 inches. The
focus of the heaviest rains will shift south ahead of the front
today, with the front ultimately expected to drop down into sw
Oregon. Surface observations in the south WA Cascades were warming,
reflecting rising snow levels. Expect to be able to drop the winter
storm warning a little early there with the new forecast, as
precipitation turns over to rain over all but the highest
elevations.

Models continue to show the upper level ridge quickly rebuilding
over the Pacific NW tonight and Tue, once the shortwave moving
across WA heads off to the east. This brings the old frontal
boundary over sw OR back north tonight and Tue, lifting the front
back up into WA. Good moist isentrpic lift is noted ini the lower
levels, seen along the 290K isentrope tonight, so will see
categorical pops spread back to the north overnight into early Tue.
Deep moisture is noted with the air mass south the of the front and
feeding up over the front, with precipitable water values around 1.3
inches. the front moves up pretty quickly however, shifting the
focus for the heaviest rains back north into WA by Tue. A mild and
very moist air mass however remains over the region into Wed.
Dynamics however are rather weak, as a rather amorphous-looking old
baroclinic zone drifts in from the west. With the deep moisture,
will need keep a decent chance for pops going through Wed, but not
enough confidence to keep categorical pops everywhere, so wlll back
off on pops a bit for Tue night and Wed. South of the warm front Tue
through Wed, will see temps above normal with snow levels well above
pass levels.

.LONG TERM...Wednesday night through Sunday. No changes.
Previous discussion follows. Expect steady rain across the region
during any pre-holiday travel. For Thanksgiving, a front crossing
the area will bring rain to southwest Washington and northwest
Oregon, but expect temperatures to be several degrees above seasonal
normals, with afternoon highs in the mid to upper 50s. Snow levels
look to remain well above the Cascade passes. There might be a
slight break in precipitation at the start of next weekend, but it's
looking more unlikely as a new system enters into the area during
the weekend. Bishop/Tolleson &&

.AVIATION...Cold front slowly working south along the WA coast.
Front is moving ashore on the south WA coast as of 2 AM and will
continue to slowly drop south and east this morning. Ahead of
the front mainly MVFR cigs with occasional IFR cigs and vis at
the coast and north of KSLE inland. Lower conditions will fill in
south of KSLE as the front nears. Model soundings suggest better
mixing after 18Z Mon for improvement to an MVFR to VFR mix.
Cascades and higher terrain become frequently obscured. A warm
front lifts from south to north tonight into Tue morning. Expect
MVFR or lower conditions with this system.

KPDX AND APPROACHES...Moderate rain in the area but generally
MVFR, but may see IFR cigs through 18Z as the front moves
through. Low-level instability after 18Z will help to improve
flight conditions to an MVFR to VFR mix. /mh Weishaar

&&

.MARINE...Front is slow to drop south along the coast. It appears
to have made landfall near Willapa Bay as of 2 AM. It appears to
be weakening as well although slowly. Will continue Gale Warning
through early morning. There should be a short period of Small
Craft Advisory winds and seas later in the morning then ease
quickly as weak high pressure build over the waters. This quiet
period will be benign as the front returns as a warm front
tonight into Tue. This system will probably bring gale force
winds, especially to the outer zones later tonight into Tue.
A series of systems move south to north offshore Wed and may
bring S winds 20 to 30 kt at times through Thu.

With the slower movement of the front, combined seas remain near
20 ft and remains 3 to 4 ft above spectral bulletin forecast. Seas
forecast to drop to near 10 ft or less later this afternoon. Seas
climb back well into the teens Tue and look to remain at that
level through Thanksgiving. /mh Weishaar

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Gale Warning until 5 AM PST early this morning for Coastal
     Waters from Cape Shoalwater WA to Cascade Head OR out 60
     NM.

     Gale Warning until 7 AM PST this morning for Coastal Waters
     from Cascade Head OR to Florence OR out 60 NM.

     Small Craft Advisory for Rough Columbia River Bar until 9 AM
     PST this morning.
&&

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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.