Area Forecast Discussion
National Weather Service Portland OR
923 PM PST Thu Dec 14 2017

.SYNOPSIS...A front approaches the region tonight and spreads light
rain across the area Friday. A series of fronts will move into the
region after Friday with lowland rain and mountain snow at times
through at least the first half of next week.


.SHORT TERM...Tonight through Sunday...Satellite imagery this evening
shows most of Western Oregon in a break from high coverage of cirrus,
which has allowed temperatures to drop below freezing at sites in the
southern Willamette Valley and outlying inland areas. Adjusted
overnight minimum temperatures to account for this cooling. Also
moved precipitation chances inland to 15Z Friday as most models are
showing a little bit of a later arrival. This should give enough time
for a few degrees of morning warming such that freezing precipitation
shouldn`t be an issue. However, if some very light rain or drizzle
arrives in the valley before 15Z, could see some patches of it
freezing. Woudldn`t expect any significant or widespread impacts even
in that case, with warming following shortly as winds switch onshore
with the front. Bowen

Rain will transition to showers later Friday evening, with
diminishing shower coverage overnight. Snow levels will fall behind
the front with accumulating snow down to around 2,500 feet on
Saturday. However, snow amounts should be rather modest with less
than an inch in the upper foothills and lower Cascades, and 1-3
inches at pass elevation. Snow levels will then rise above the
Cascade passes late Saturday night as a warm front moves through the
region. This will bring another round of rain on Sunday and have
maintained chance PoPs across this area with highest chances over
southwestern Washington. However, would not be surprised if areas
south of a Newport to Salem line remain dry over the weekend.

.LONG TERM...No Changes. Previous discussion follows...Sunday night
through Thursday...Models are in decent agreement the area will be
under zonal flow with the jet stream remaining across southern
British Columbia and far northern Washington Sunday night and Monday.
This will leave our more northern zones open to a period or two of
light rain as a weak front drags southward towards the region, but
rain chances appear considerably less towards Lane County.

Models seem to also be coming into agreement a shortwave trough and
attendant front will drop southeastward out of the Gulf of Alaska
and usher in a good shot of valley rain and mountain snow Tuesday
and Tuesday night. As upper level ridging builds northward into
British Columbia behind this feature late Wednesday and Thursday,
our flow should become increasingly northeasterly. This will in turn
likely lower snow levels to near the valley bottom, but
precipitation will almost certainly have ended by that point. If
precipitation were to linger across the area Wednesday night and
Thursday, our flow will likely not be quite as northeasterly, and as
a result, snow levels will likely be higher.  /Neuman


.AVIATION...VFR with light to moderate offshore flow this
evening, except for some low clouds causing MVFR and mountain
obscurations around the Hood River Valley. Offshore flow is
lighter in general tonight, and may allow a little more fog to
develop in the south Willamette Valley overnight. Otherwise,
expect VFR and high clouds to persist until cloud bases start to
lower with the approach of a cold front Fri morning. Will add
roughly 4-6 hours of -RA and MVFR to all TAFs Friday; starting
in KAST around 14z then spreading south and east with the front
as the day progresses. This front will finally bring an end to
the strong E winds through the Columbia Gorge Fri morning, as our
general flow pattern turns more onshore.

PDX AND APPROACHES...VFR with decreasing E-SE winds through 12z
Fri. Cold front will bring lowering cigs Fri morning, with -RA
and MVFR likely by 18z Fri. Rain will taper to showers behind the
front Fri afternoon.  Weagle


.MARINE...Light SE winds will shift to a N-NW wind later
tonight/early Fri as a weak cold front moves across the waters.
The SCA for winds in the outer waters Friday looks marginal, but
most models do show 25 kt winds in the boundary layer, which
indicates some gusts may reach that high. Decided to shorten the
tail end of the advisory a bit, as high pressure should cause
winds to ease Fri evening. Seas will likely remain just below 10
feet for the most part with this system, though shorter periods
may make it a bit rough late Fri night/Sat. Latest ENP guidance
shows lighter seas in between fronts later Sat through early
Sun, then another weak frontal system is expected later Sunday.
Swell energy from the Gulf of Alaska will again push seas close
to 10 ft behind this front Sun/Mon. A stronger cold front may
bring low-end gales Tue, with seas potentially pushing into the


OR...Air Stagnation Advisory until 10 AM PST Friday for Central
     Willamette Valley-South Willamette Valley.


PZ...Small Craft Advisory for winds from 7 AM to 8 PM PST Friday for
     Waters from Cape Shoalwater WA to Florence OR from 10 to
     60 NM.



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.