Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 281638
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
935 AM PDT Sun May 28 2017

.SYNOPSIS...The upper level ridge of high pressure is beginning
to shift east and will allow coastal clouds to penetrate furthers
inland today. Clouds will retreat mostly out of the valley today but
temperatures will be cooler than Saturday. The Cascades and Gorge,
however, will see their temperatures continue to climb as thermal
low pressure sits overhead. Thunderstorms are possible near
Willamette pass this evening with chances expanding northward
Memorial Day. Cascade thunderstorm chances continue Tuesday and
expand westward to the valley as well. Temperatures trend gradually
cooler with rain chances during the second half of the week as a
disturbance tracks across the Pacific Northwest.

&&

.SHORT TERM...(Today through Tuesday Night) Stratus has stopped it`s
northward push up the Willamette Valley near Silverton and the
stratus deck is already starting to quickly erode in many places.
This keeps conditions on track for a sunny afternoon for all of the
area except for the coast.

Have lowered the max temperature forecast from 3-6 degrees for the
valley since the thermal trough should remain further inland today
than was expected 24 hours ago. Did, however, increase temps along
the Cascades and especially the Central Gorge today since the peak
heating will occur under the thermal trough. Will still see a very
pleasant day across most of the area with mid-80s still expected for
Salem and north to the Metro and the upper 70s for the south valley.
The coast will struggle to get out from under the clouds today and
thus also struggle to warm much. The Gorge will not be the place to
escape the heat today as temperatures will hover in the low to mid
90s for the next couple days until the thermal trough gets truly
pushed east. Will also see a marine push through the coast range
gaps this evening, although not as strong as last Tuesday. Could
still get some rather gusty winds with localized 25-30 mph speeds on
occasion near the coast range passes.

The thermal trough will also begin to act as a focusing mechanism
for potential convection over the Sierra and south Cascades Today.
Still seems unlikely sufficient moisture will be in place to produce
storms over the central Cascades, but if it does materialize the
area around Willamette Pass will be the likely recipient of
lightning early this evening.

Thunder chances increase for the Cascades Monday, especially from
the NAM and GFS as they are more aggressive than the ECMWF in
developing an approaching upper level trough. Their solution would
bring more of a southerly flow to the region Monday and Tuesday
which would easily funnel moisture to the area. Monday`s storms would

likely fall apart upon leaving the higher terrain but a slight
southeasterly flow at 700 mb would likely have debris showers
drifting over the valley during the evening. The primary trough axis
then takes on a slight negative tilt fro Tuesday and would then act
as a forcing mechanism to bring a thunder threat for most of the
region Tuesday. Would still see storm initiation mainly over the
Cascades but increasing mid-level instability could reinforce the
cells as they drift of the Cascades toward the north-northwest.
Would not want to go canceling any Tuesday plans just yet, however,
as we still need to see just how sharp the approaching trough
becomes. The 00z run from the ECMWF keeps a much more broad upper
wave and likely just light shower activity vs. full thunderstorms.
Bentley/JBonk

.LONG TERM...(Wednesday through Saturday) No changes...previous
discussion follows. A cooler onshore flow pattern looks to take hold
during the second half of the week. Cooled temperatures closer to
model consensus for much of this period as a result. Rain chances
continue to vary wildly among different models so left a fairly
generic rain forecast at this point. However, PoPs were increased
most periods, particularly during the afternoon and evening hours
when daytime heating may trigger at least some showers across the
area, especially if the upper level trough lingers over the region.
/Neuman

&&

.AVIATION...Marine stratus this morning will persist through
today along the coast with MVFR/IFR conditions. Inland expect
stratus to burn off by 18Z-20Z for VFR conditions through the
rest of the afternoon and evening. Will see cigs sink back down
to IFR/LIFR tonight along the Coast, and Monday morning will turn
out fairly similar to this morning with marine stratus pushing
through coastal gaps and down the Columbia River. Better chance
that stratus makes it into the south and central Willamette
Valley than the north Willamette Valley, but can`t rule out
IFR/MVFR stratus impacting northern Willamette Valley terminals
Monday morning after sunrise.

KPDX AND APPROACHES...VFR through tonight. Slight chance of cigs
around 1000 ft impacting terminal Monday morning 15Z-18Z. -McCoy

&&

.MARINE...Quiet conditions continue with northerly winds below
20 kt through at least the middle of this week and likely
longer. Seas are down around 4 ft and will remain around 4 to 5
ft through at least the middle of the week. Weak system appears
likely later this week, with winds shifting to southerly on
Wednesday, but winds right now appear to remain below 20 kt with
this system as well. Seas may come up a foot or two with this
system, but should be little impact. Next chance for SCA
conditions may hold off until next weekend. -McCoy

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...None.
&&

$$

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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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