Area Forecast Discussion
Issued by NWS Portland, OR

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000
FXUS66 KPQR 211633
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
935 AM PDT Fri Jul 21 2017

.SYNOPSIS...High pressure will result in a warmer and drier day today
across southwest Washington and northwest Oregon. The warming trend
will continue for most of the area Saturday, though a weak frontal
system may brush the north coastal zones with some clouds and areas
of light rain Saturday morning. The increased moisture ahead of this
front will likely make it feel a bit more humid Saturday as
temperatures climb several degrees above normal. Onshore flow is
expected to return Sunday, though temps will remain near to slightly
above normal early next week. A gradual cooling trend is possible
later next week as an upper level trough tries to settle into the
region, but temps should remain within a few degrees of seasonal
normals.

&&

.SHORT TERM...Today through Sunday...The few patches of fog or
low clouds which formed in some of the more sheltered coastal valleys
have just about cleared out as of 930am this morning.
Today will remain mostly sunny across the district
with a several degree warmup as 850 mb temps warm up by 3-5 deg C
versus yesterday. Temps this afternoon should have no problem
climbing into the lower to mid 80s for the inland valleys. Onshore
flow will be a bit weakened today due to a couple shortwaves brushing
by to our W-NW, so some coastal spots could climb into the lower 70s
this afternoon.

The above-mentioned shortwave activity will slowly press a frontal
zone southeast toward the WA coast tonight and early Saturday. While
it appears our north coastal zones will be at the very tail end of
this frontal zone, the air mass just ahead of the front looks quite
moist according to model precipitable water (PW) fields. NAM/GFS
total PW approaches 1.5 inch along the coast early Sat, with amounts
in excess of 1 inch inland. This could be enough to wring out some
light rain along/near our coast late tonight/early Sat. Additionally,
model cross-sections show a decent amount of RH in the lower to mid
levels Sat afternoon, so there could be some clouds Saturday
especially north and west of Salem. Either way, the moist air mass
ahead of the front will probably make it noticeably muggy as
dewpoints attempt to climb into the 60s Saturday afternoon. With 850
mb temps climbing a few more degrees, expect inland valley temps to
climb further into the 80s and possibly reach 90...especially around
Eugene where there should be a bit more sunshine.

The remnants of the frontal zone are expected to fall apart by Sunday
morning, with high pressure reasserting itself over the NE Pacific.
As thermal low pressure strengthens to our south and east, this
should pull in a bit more onshore flow Sunday afternoon, especially
up the Columbia River. If this occurs as suggested by most model
guidance, Kelso may struggle to climb above the upper 70s and the PDX
metro may cool off a couple degrees Sunday. With a thermal trough
remaining strong along the coast near the OR/CA border, low-level
flow will be more northerly around Eugene Sunday. This will probably
delay any significant cooldown until early next week for the Eugene
area.

Another wrinkle to the forecast will be the potential for
thunderstorms developing along/near the Cascade crest starting this
weekend. Models are still struggling a bit with how to handle a weak
shortwave now near 33N/140W as it slowly approaches the northern CA
coast the next few days. Flow aloft will turn increasingly southerly
over SW Oregon as this system approaches this weekend. Most models
show sufficient instability for at least a couple late-day
thunderstorms around Willamette Pass Sunday, or at least the
potential for some thunder to develop around Crater Lake and advect
northward into our forecast area. Given drying fuel conditions in the
Cascades, will need to monitor this potential closely for any fire
weather concerns.  Weagle

.LONG TERM...Sunday night through Thursday...After gradually coming
into better agreement the last few runs, the ECMWF and GFS threw
their agreement out the proverbial window with their 00z operational
runs. The main struggle remains how to handle a weak and currently
very dry shortwave visible on satellite water vapor imagery near
33N/140W. The model trend had been toward phasing the shortwave with
a larger upper trough approaching from the Gulf of Alaska sometime
around Monday or Tuesday next week. The 00z ECMWF now leaves this
shortwave near the northern CA coast until Wednesday, before finally
kicking it northeastward ahead of the approaching upper trough Wed
afternoon and evening. This would result in a significant delay in
any cooling trend, and would allow for the threat of late-day thunder
in the Lane County Cascades to continue Sunday through Tuesday. We
already had a small area of slight chance thunder in the Lane County
Cascades Tuesday; we will leave this as-is and allow future shifts to
determine whether this area needs to be expanded.

Ensemble members continue to offer a wide variety of solutions with
this shortwave, from phasing or kicking out the shortwave as early as
Monday (thus resulting in a cooler and more stable onshore push
Tuesday), to leaving it near the coast for much of the week (would be
a warmer solution with a more prolonged thunder threat in the
Cascades). The majority of ensembles do bring some form of the larger
upper trough into the Pac NW Wed/Thu, which would result in a cooling
trend to near or slightly below normal toward the middle to latter
portion of next week.  Weagle

&&

.AVIATION...Mostly VFR conditions today with very light winds.
IFR cigs near KONP should clear in the next couple of hours to
VFR. IFR cigs for the coast this evening through Saturday morning
with Rain/Drizzle possible for KTMK and KAST. Inland areas will
remain VFR tonight with high clouds moving in from the west for
areas north of KSLE early Saturday morning.

KPDX AND APPROACHES...VFR conditions with light winds today
through Saturday. ~TJ

&&

.MARINE...A weak surface pressure gradient continues over the
waters through Saturday morning for light winds and low seas.
High pressure increases Saturday afternoon returning north winds
to the waters. Diurnal changes in a thermal trough over N. CA and
S. OR will result in gusty north winds Sunday and Monday
afternoons and evenings. The strongest winds will be south of
Lincoln City where models are showing the possibility for Gale
force gusts. The seas will increase in response to the winds for
steep and choppy conditions. Low pressure to the south weakens on
Tuesday for a slight decrease in winds,but fresh NW swell will
maintain steep seas. ~TJ

&&

.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. This area is
commonly referred to as the forecast area.


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