Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 132207

Area Forecast Discussion
National Weather Service Portland OR
206 PM PST Fri Dec 13 2019

.SYNOPSIS...Northwest flow aloft over the forecast area through the
weekend will result in a continued chance of light precipitation.
High pressure builds offshore Sunday night and gradually shifts
toward the coast Monday. The ridge shifts east of the Cascades by
Tuesday afternoon. A more progressive and wetter pattern is expected
for the latter half of next week.


.SHORT TERM...Tonight through Monday...Early afternoon water vapor
imagery revealed an extensive moisture band extending from the
Sierras to near Hawaii in associated with a 150-170 kt jet stream. A
short-wave was also noted near 48N 138W and a second, deeper upper
level trough could be seen near 42N 142W.

The forecast area continues to be split by two weather environments
associated with the strong jet stream. North of the jet a cool and
unstable air mass exists, which covers the majority of the forecast
area. South of the jet is a milder and more stratiform air mass.
Rainfall rates across the Lane County Cascades and foothills have
been generally under a tenth of an inch per hour. The 12Z GFS and NAM
show weak isentropic lift along the 290K surface over the southeast
quarter of the forecast area late this afternoon through around 03Z
Sat. Downglide, or negative isentropic advection develops after 03Z
as surface high pressure noses into SW Oregon. Meanwhile, a weak
upper level short-wave is forecast to move into western Washington
and northwest Oregon overnight. This will help shunt the baroclinic
boundary further south, into extreme southern Oregon and northern
California. Have included the possibility of drizzle for the southern
half of the forecast area overnight into Sat morning due to a nearly
saturated layer to around 800 mb and weak lift. The second upper
level short-wave slides southeast toward SW Oregon late Sat. The
combination of cyclonic 500 mb flow and weak short-wave dynamics
leads to a continued chance of some light precipitation. The south
part of the forecast area will be more favored and have the higher

By Sat night/Sun morning a 500 mb ridge amplifies along 130W.
However, models still indicate some degree of cyclonic northwest flow
over the area Sat night. NAM model time-height section for KPDX shows
a more shallow saturated lower level air mass Sat night into Sun
morning as drier north flow descends to 700 to 750 mb. The GFS hints
at some light QPF generally along the coast and over the higher
coastal terrain, but the NAM is a little less aggressive. The upper
ridge axis reaches the coast Mon. However, the 12Z ECMWF shows some
over-running light QPF along the coastal areas Mon. the 18Z NAM shows
a well-defined surface feature just off the central coast Sun night
and then moves it inland near KONP early Mon morning. Think this
solution is more of an outlier. Would also expect areas of valley
fog/stratus Sun night into Mon morning.  Weishaar

.LONG TERM...Monday night through Friday...Above-average forecast
confidence in the first half of the extended period. The 12Z
operational runs and respective ensembles are in good agreement with
the upper ridge slowly moving across the Pac NW early next week. The
12Z ECMWF appears to be a little quicker pushing the ridge east of
the Cascades. This is likely due to the ECWMF maintaining an open,
but elongated upper level trough along 130W, while the CMC and GFS
show a distinct closed low near 40N 130W. About half of the GFS
ensemble members show this closed low. The WPC forecast valid 00Z Wed
is more in line with the 12Z operational GFS.

Forecast confidence lowers Wed through Fri. However, there is
reasonable confidence that a return to a more progressive and wetter
pattern is in store for the latter half of next week. Models show a
more consolidated jet stream directed at Oregon and northern
California beginning Wed night. Again, it appears southwest Oregon
and northern California will be the more favored QPF areas. Model
1000-500 mb thickness values valid Fri are expected to be near 537 dm
across SW Washington to the mid 540s for Lane County. Thus, snow
levels will hover near to just below the passes across the north and
near the passes in the Central Cascades. Weishaar


.AVIATION...18Z tafs: Low stratus remains entrenched over the
majority of NW Oregon and far SW Washington. Weak low pressure is
starting to cross the region and should help to further
destabilize the lower atmosphere but it may take until closer to
13/22Z for conditions to improve everywhere. Until then, would
suggest planning for extra fuel requirements as confidence on any
specific terminal clearing out earlier is fairly low. Feel
decently confident there will be a several hour break in the low
clouds before clearing skies aloft allow stratus to reform
14/03-06Z tonight. Many locations will drop further into fog
overnight and/or will see drizzle produce IFR vsbys. Most areas
will not see the fog lift until closer to 14/18Z with the stratus
possible lifting/dissipating much later.

KPDX AND APPROACHES...Low stratus should begin to lift
shortly, but may hang on until 20-21Z. Meanwhile, weak
instability above that layer could bring the occassional shower
across the field to break up the layer earlier. Do expect several
hours of minimal clouds/vsby impacts thereafter. Around 14/05Z, a
stratus cig could develop between 008 and 015 AGL. Later near
14/11Z, that layer should be in place with light drizzle further
deteriorating flight conditions likely to solid IFR, but possibly
to LIFR. Wherever conditions settle in at, do not expect
significant improvement until at least 14/18Z. /JBonk


.MARINE...Main impact continues to be the slowly easing westerly
swell generated by the parent low pressure area well northwest
of the waters. Will continue to see this swell ease tonight and
Saturday. It still appears the swell will keep seas above 10 ft
through at least Saturday afternoon while still possibly linger
for a few hours Saturday evening.

Following that period, no significant impacts from winds or seas
until late Monday when a large low pressure area sets up well
offshore. It will swing multiple smaller scale lows just west of
the waters Tuesday through Friday of next week. Weather models
are showing broad agreement in this pattern with periods where
winds will range 20-30 kt and swell possibly reaching back into
the mid to upper teens. However, have not seen much day to day or
run to run consistency with details yet from any given model. So
generally recommend a plan for rogh conditions affecting smaller
vessels and waiting until next week to see how the details shake
out. /JBonk


PZ...Small Craft Advisory until 4 PM PST Saturday for coastal waters
     from Cape Shoalwater WA to Florence OR out 10 NM-Columbia
     River Bar-Coastal waters from Cape Shoalwater WA to
     Florence OR from 10 to 60 NM.


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