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Area Forecast Discussion (AFD)

Issued by Portland, OR (PQR)

FXUS66 KPQR 190453 AAA

Area Forecast Discussion
Weather Service Portland OR
953 PM PDT Tue Jun 18 2019

Aviation discussion updated.

.SYNOPSIS...The pattern of areas of morning clouds and some 
afternoon sun continues through Wednesday, with seasonable 
temperatures. Thursday brings the coolest day of the week along with 
a chance for showers as an upper trough of low pressure moves across 
the Pacific Northwest. Dry weather and seasonable temperatures 
return Friday and Saturday.

.SHORT TERM...Tonight through Friday...Upper heights will continue 
to gradually lower tonight and Wednesday as another weak short wave 
moves through in the northwest flow aloft. This will continue the 
pattern of late night and morning clouds along the coast and in the 
northern interior. Daytime temperatures will cool to a bit below 
normal as 850 mb temperatures drop another 5 degrees Celsius.  

Wednesday night through Thursday night a pair of more substantial 
shortwaves carves out an upper trough of low pressure over the 
Pacific Northwest. Moisture deepens some with this system, although 
models still generally confined it to below 10K feet at its peak 
Thursday morning. The moist layer is marginally unstable, so given 
the dynamics associated with the shortwaves expect to see some 
showers with this system. While most of the forecast area will see 
some chance for showers, the best chances will be over the west 
facing slopes of the Cascades and Coast Range where orographic flow 
will enhance lift. Temperatures will be at their coolest of the 
week, with snow levels likely dropping down to the highers passes 
for the day before the summer solstice. Any accumulations would be 
likely a dusting at best, and would melt quickly during the day due 
to some of the strongest solar energy of the year.

Drier weather returns Friday as the upper trough moves east and 
ridging builds in from the west. Surface flow to remain onshore, but 
becomes more northerly with the surface ridge extending inland into 
Washington. This should result in a return to a pattern of morning 
clouds and afternoon clearing for the interior, with daytime 
temperatures returning to closer to normal after a cool morning. 

.LONG TERM...Friday night through Tuesday...The beginning of summer 
will not seem like it as a cool and somewhat unsettled pattern 
dominates the forecast area. A weak 500 mb ridge is forecast to be 
over the area Sat. However, onshore low-level flow will maintain the 
pattern of late night/morning low clouds and afternoon clearing. Max 
temps Sat will be close to normal. The long-wave pattern begins to 
show signs of change Sat night and Sun. A fairly deep 500 mb low is 
forecast to be near Haida Gwaii 12Z Sun. Expect a deeper marine 
layer Sat night and Sun, with the potential for some light precip in 
SW Washington and also the north Oregon coast Sun morning. 

By Mon the large upper level trough will be over the NE Pac. The 12Z 
operational GFS shows the upper low along 46N 130W at 00Z Tue, while 
the 12Z ECMWF is more to the west. The ECMWF ensemble mean suggests 
higher-than-average confidence with the upper low out near 135W. The 
GFS ensembles show decent continuity, but the mean 500 mb low 
position appears to be closer to 130W, similar to the 12Z 
operational run. All in all, the latter half of the extended period 
will feature below normal daytime temperatures. In fact, the CPC 6-
10 day temp/precip progs indicate well-above average probability of 
below normal temps. CPC also suggests above-average confidence for 
above normal precip, albeit at a slightly lower confidence level 
than temps. The WPC forecast valid Mon and Tue favors a GEFS/ ECMWF 
ensemble mean. The deep upper trough eventually swings into Western 
Washington and NW Oregon Tue night and Wed. Looking beyond day 7, 
the GFS and ECMWF continue the unsettled pattern the latter half of 
next week. Weishaar


.AVIATION...A disturbance is forecast to move across NW Washington 
late tonight, which may disturb the marine layer enough to limit 
MVFR or lower conditions, even along the coast. Still expect MVFR to 
continue along the S Washington and N Oregon coasts, but coastal 
areas south of KTMK may hold on to VFR much of the night. MVFR CIGs 
forecast to develop in the SW Washington interior overnight and 
extend up the Columbia River to KPDX around sunrise, but CIG may 
also end up being low VFR. 

KPDX AND APPROACHES...VFR conditions to prevail through at least
12Z Wed. Another upper level disturbance moves across NW Washington 
late tonight, which may induce mixing and make morning CIGs low VFR 
rather than MVFR. Highest chance for MVFR would be on east 
approaches. Bowen


.MARINE...Surface high pressure will reside over the NE Pacific 
through the week, while thermal low pressure remains over the 
northern California and southern Oregon coast. This will result 
in persistent northerly winds. The winds will be the strongest 
this afternoon through late evening, with gusts to 25 kt expected
over PZZ255/PZZ275. Wind speeds will be lighter from Wed through
Fri as an upper level trough moves in and weakens the NE Pacific
surface high. Gusts to 20 kt are expected during the afternoon 
and evening hours each day, but do not expect that additional 
advisories will be required. Will need to watch the area south of
Heceta Head late Wed afternoon for possible 25 kt gusts. Wind
speeds drop off even more this weekend and early next week as an
upper level low settles over the NE Pac. 

Seas mostly 4 to 6 ft for the next few days, although areas of
PZZ255 and PZZ275 likely to see brief periods of 6-8 ft at times
due to contributions from fresh swell. Weishaar


PZ...Small Craft Advisory until midnight PDT tonight for Coastal 
     Waters from Cascade Head OR to Florence OR out 60 NM.



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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 forecast area.