Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 250440
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
940 PM PDT Wed May 24 2017

.SYNOPSIS...An upper level trough will reside over the Pacific
Northwest through Thursday. There remains a slim threat of late day
thunderstorms Thursday over the higher North Oregon and South
Washington Cascades. High pressure will return Friday for drier,
sunnier, and warmer weather. Warm and dry conditions continue through
the holiday weekend and early next week. There is a slim threat of
late-afternoon and evening thunderstorms over the higher Cascades
Sunday through Tuesday. Stronger southwest to west flow aloft
Wednesday pushes the thunderstorm threat east of the Cascades.
&&

.SHORT TERM...Tonight through Saturday...A broad upper level trough
moved through the Pac NW earlier today and is now over the Rockies.
However, a shortwave on the backside of the trough is diving south
through the Pac NW and will continue to influence our weather through
Thu.

Onshore flow continues tonight, although not nearly as strong as we
experienced yesterday. Satellite imagery shows clouds over most of
the northern half of the CWA, with skies mainly clear over Lane
County. The cloud fcst is a bit difficult overnight. Think that the
onshore flow into the Cascades will again lead to clouds backbuilding
off the foothills into the northern valley. The fcst models also
suggest that clouds again form along most of the south WA and north
OR coast. The southern portions of the fcst area may again stay
relatively cloud free overnight.

Although the main upper low will move east into the northern Great
Plains Thu, the elongated trough axis on the backside of the low will
pinch off and hang back over the Pac NW. The GFS shows channeled 500
mb vorticity through the interior valleys of Washington and Northern
Oregon Thu in north flow aloft. Expect there will be quite a bit of
clouds around Thu morning, particularly over the northern half of the
CWA, but the clouds should break out by late morning and provide
everyone with some sunshine. The GFS and NAM hint at some light QPF
Thu afternoon, primarily over the S Washington Cascades and N Oregon
Cascades, although the Lane County Cascades have a chance of a shower
as well. The GFS has surface based LI values of -2 to -4 C from
around Mt. Hood northward in the afternoon, so left the slight chance
of thunderstorms in the fcst near the Cascade crest. GFS indicates
850 mb temps warm about 5-8
deg C Thu, which should result in mid to upper 70s inland. NAM
suggests much less low cloud cover Fri morning as the 975 mb flow
veers more N to NE. Believe any morning stratus will be brief, with
850 mb temps warming another 4-5C, for inland highs getting into the
lower to mid 80s.

Models, particularly the NAM and ECMWF show An interesting
development Fri night. The NAM shows a south stratus surge up the
coast beginning Fri evening and continuing into Sat morning. It
appears as though the stratus surge will stop somewhere near KTMK Sat
morning. Adjusted the coastal sky grids to account for this trend.
Sat max temps look very similar to what is expected Fri. Models agree
that the 500 mb ridge axis will be directly over the forecast area
Sat afternoon. This provides a stable atmosphere with a mid-level
capping inversion. Pyle/Weishaar

.LONG TERM...Saturday night through Wednesday. The models are in
general agreement through about Mon or Mon night, then start to
diverge at the end of the extended period. The ECMWF maintains a
strong 500 mb ridge over the area Sat night and Sun, while the GFS
pushes the ridge axis east of the Cascades by Sun afternoon.
Above-normal daytime temperatures can be expected inland Sun and
Memorial Day. The ECMWF hints at a slim threat of convection over
Willamette and McKenzie Passes Sun afternoon. Will leave this mention
in for now, but believe a mid-level cap will remain in place. Mon may
have a slightly better risk of Cascade convection as the mid-level
flow shows a little more south component. The ECMWF would actually
suggest a slight SE-S 700 mb flow, which is more favorable for
convection.

Models start to diverge Mon night and Tue. The GFS shows a short-wave
disturbance reaching the area late Mon night and Tue. This would
bring a deeper marine layer and result in a more stable air mass. the
ECMWF holds the 500 mb trough axis over the area Tue, for a continued
slim threat of Cascade thunderstorms. For now have shown a threat of
thunderstorms south of Mt. Jefferson Tue. Big differences in the
operational runs of the GFS and ECMWF Wed, with the GFS depicting a
cooler and stable air mass. The ECMWF shows much stronger ridging.
Weishaar
&&

.AVIATION...Widespread stratocumulus over the waters and the
coast from near KAST to KONP as well as the coastal mountains.
Also residual moisture in the onshore flow maintaining
stratocumulus over the Cascades north of Mt. Jefferson. Areas
south of KSLE are mostly clear. Cloud deck is around 4500-6000
ft. The onshore flow weakens overnight as high pressure builds.
Also subsidence from building 500 mb heights should lower the
cloud deck. So late tonight and Thu morning should see cigs
closer to 2500 ft, and the more pessimistic model (NAM) sounding
shows cigs near 1500 ft. As far as coverage inland, areas near
the lower Columbia river look most vulnerable to the MVFR cigs.
The north Oregon coast will probably see MVFR cigs develop
overnight. Where MVFR cigs develop expect clouds to
lift/dissipate around midday Thu.

KPDX AND APPROACHES...VFR through around 12Z-14Z Thu then 70
percent chance of MVFR cigs around 2500 ft through 18Z. Mostly
clear skies thereafter. /mh

&&

.MARINE...Pattern continues with high pressure over the NE
Pacific and low pressure inland combining to continue SCA wind
gusts through early Thursday. Will first see winds within 10 nm
of the Coast drop below SCA criteria later tonight, and then
across the entire waters Thursday morning. Benign conditions look
to reign Thursday afternoon through this weekend as the pressure
gradient remains relatively week across the waters.

Steep seas continue at 8-10 ft with dominant wave periods around
7 to 9 seconds. Expect seas to back down a bit this morning
before building back up near 10 ft this evening. Extended the SCA
for Seas as short periods under wind wave driven/fresh swell
conditions dominate. Longer period seas will start prevailing
Thursday as the winds ease with little threat of notable winds or
seas
affecting the waters until perhaps early next week. /JBonk

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory for winds until 8 AM PDT Thursday for
     Waters from Cape Shoalwater WA to Florence OR from 10 to
     60 nm.

     Small Craft Advisory for hazardous seas until 5 AM PDT Thursday
     for Coastal Waters from Cape Shoalwater WA to Florence OR
     out 60 nm.

     Small Craft Advisory for winds until 2 AM PDT Thursday for
     Coastal Waters from Cape Shoalwater WA to Florence OR out
     10 nm.

&&

$$

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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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