Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 220306
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
804 PM PDT Mon May 21 2018


.SYNOPSIS...Warm and dry through Tueday, as region stays under high
pressure. But, as a low over Nevada drifts northeastward to the
northern Rockies late Tue and Wed, will see a chance of showers and
even a thunderstorm over the Oregon Cascades, mainly to south of
Santiam Pass. Another weather system will approach from the southwest
later in the week, with increasing southerly flow aloft. This will
bring cooler temperatures, along with increasing chance of showers
and perhaps a few thunderstorms. Though will see decreasing chance of
showers on Saturday and Sun under mostly cloudy skies, does appear
may be able to salvage the last part of holiday weekend with a dry
and mild Monday.

&&

.SHORT TERM...Tonight through Thursday...Minimal changes to current
forecast. Main weather features on evening water vapor include a 500
mb ridge axis extending from 35N 135W to near KUIL and a closed 500
mb low centered over Southern California. 02Z lightning data showed
some strikes over SE Oregon, but nothing near our warning area. Last
visible images showed minimal cloud cover along the coast or over the
coastal waters.

The GFS KTTD-KDLS gradient is forecast to drop from around 3 mb this
evening to neutral by Tue afternoon. Thus, not expecting a widespread
intrusion of marine stratus. The HI-RES ARW suggests stratus will be
generally confined to the beaches and also along the Columbia River
near KKLS. Have made adjustments to sky grids for tonight and Tue
morning using the National Blend, which produced a reasonable
solution. The 18Z GFS and 00Z NAM indicate 5-6C 850mb warming from
00Z Tue to 00Z Wed. This will give at least 8-10 degrees warming to
the inland areas, with 80s common in the interior valleys. Rest of
the short term discussion unchanged. Weishaar

Now, the pesky upper low that has been parked over the Great Basin
will drift slowly northeastward towards Idaho Tue and Wed. As it
does, will see moisture wrapping around the low into eastern Oregon,
and possibly up against the Oregon Cascades. Have kept a minor chance
of showers/thunderstorms over the high Cascades for Tue evening and
again on Wed. Elsewhere, onshore flow will gradually increase Tue
night and Wed, with marine stratus spreading back along most of the
coast. Will add patchy drizzle or fog at that time for the coast and
river valleys into the western slopes of coast mtns. With the
increasing onshore flow, think will see areas of low clouds form
inland later Tue night into Wed am, primarily from Eugene northward
to around Salem or Woodburn.

Max temps cool Wed, as deepening marine layer will keep highs only to
around 70 in south Willamette Valley such as Eugene and Corvallis.
But, may still get into the mid and upper 70s around
Portland/Vancouver metro where will start the day with less clouds.

Thursday could be more interesting. Another upper low will be
approaching California, with upper flow over Oregon becoming more
southerly Wed night into Thu. Start getting better divergence aloft
with getting more south to southeasterly 700 mb flow as the day
progresses. This will bring increasing chance of showers for Thu over
the Oregon Cascades and perhaps over good part of Lane County. But,
still some question as to how much mid-level moisture is around for
the developing showers/thunderstorms during the day. Either way, will
continue to evaluate this over next several shifts.  Rockey.

.LONG TERM...Thursday night through Sunday...Models starting to have
a bit more agreement, but still enough differences in run to run
consistency to make no major chances. Will see the California low
lift into the Great Basin Fri and Sat. Seems highest chance of
showers and even a few thunderstorms will be on Fri, when have better
flow aloft over region. But, as the low passes farther eastward on
Sat, will generally see decreasing chance of showers. Will be tad
cooler, with coastal areas in the 50s and 60s to possibly lower 70s
inland. Mainly due to plenty of clouds on the onshore flow. Think
will be between systems on Sunday as ridging starts. This will make
for a mostly dry day Sun with slow burn out of clouds. But, can not
rule out a few showers along the coast north of Lincoln City, and
over the foothills and Cascades. Seems more likely now that Mon will
be dry, with mild conditions over the region.      Rockey.

&&

.AVIATION...Onshore flow pattern continues this evening and
tonight under building high pressure aloft. Expect a return to
higher IFR to lower MVFR stratus along the south Washington and
at least the northern Oregon coasts overnight. Lower confidence
that persistent lower cigs return to the central coast (around
KONP/K6S2), but at least a chance of a enough onshore influence
to bring stratus in. Some stratus may attempt to push up the
Columbia River towards KPDX/KVUO, but expect that it is unlikely
to advance past KKLS per latest guidance. Some lower clouds
likely develop along the eastern portions of the Willamette
Valley between 12z and 17z, but elsewhere inland expect VFR
conditions through the period with only scattered clouds at best.

KPDX AND APPROACHES...A northwesterly onshore flow pattern under
building high pressure aloft should result in predominantly VFR
conditions through 00z Wednesday. There is a small chance of high
end IFR to low end MVFR cigs developing along the Columbia River
and east of the terminal between 13z and 16z. Cullen


&&

.MARINE...High pressure over the northeast Pacific and thermal
low pressure over northern California and/or the Great Basin will
more or less continue for much of the next week and result in a
summer-like northerly wind pattern. Northerly winds look to be
strongest during the afternoon and evening on Tuesday and
possibly peaking again late Friday. An incoming front may begin
to shift winds around to a more southerly direction next
weekend, but confidence in details are low at this point. Expect
seas to hover between 3 and 7 feet through the weekend. /Neuman

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory for winds until 3 AM PDT Tuesday 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 CWA or forecast area.


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