Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 031049

Area Forecast Discussion
National Weather Service Portland OR
335 AM PDT Mon Aug 3 2020

.SYNOPSIS...Expect a mix of morning clouds and afternoon sunshine
with temperatures generally hovering near to slightly above average
for the next several days. Our first shot at a widespread light rain
event since late June/early July will arrive late Wednesday night
into Thursday.


.SHORT TERM...Today through Wednesday...Water vapor satellite imagery
early this morning reveals a shortwave trough lifting northeastward
across Washington. The main area of mid to upper level lift
associated with this wave has already shifted northeast of the region
and 500mb heights are beginning to build across northwest Oregon and
southwest Washington in its wake. Nighttime microphysics satellite
imagery reveals marine clouds have solidly filled in along the coast
and are pushing through the Coast Range gaps. Given current marine
cloud coverage and HREF guidance, we should see marine clouds spread
into both the far northern and southern ends of the Willamette
Valley this morning. More uncertainty exists in whether or not
clouds will fill into the central Willamette Valley this morning. In
addition, weak onshore flow coupled with the marine clouds along the
coast have already produced pockets of drizzle along the coast.
Expect this to continue this morning before precipitation odds
gradually dissipate as daytime heating helps to break up the marine
clouds towards midday.

Models are in decent agreement a vort max currently near 140W and
39N will slide eastward and maintain weak upper level troughing
across the Pacific Northwest Tuesday into Wednesday. Models are in
good agreement pressure gradients will remain onshore. This will
maintain cool conditions along the coast with temperatures warming
well into the 80s in the Willamette Valley and Columbia River Gorge
Tuesday and Wednesday. Onshore pressure gradients look to hold
relatively steady today into Tuesday. This should maintain the
breezy westerly winds through the Columbia River Gorge and the area
around the Fir Mountain Fire during this time. As the pattern
becomes more amplified Wednesday and thermally induced low pressure
strengthens east of the Cascades, pressure gradients appear likely
to increase across the Cascades. As a result, expect westerly winds
to pick up a notch in the aforementioned locations Wednesday.

.LONG TERM...Wednesday night through Sunday...Models and their
ensembles are in reasonable agreement a somewhat sharp and negatively
tilted trough will slide east-northeastward across the Pacific
Northwest Wednesday night into Thursday. There still remain some
differences among the ensembles in the timing and particularly the
amplitude of the shortwave trough, which results in uncertainty in
QPF amounts expected across the region. The WPC cluster analysis of
the 12z ensemble data suggests just 10% of the CMCE, GEFS and EPS
ensemble members produce a pattern where the main shortwave trough
just glances the region and produces little to no rain across the
CWA...perhaps just a couple hundredths in the most favored west
facing slopes in the Coast Range and Cascades north of a line
extending between Tillamook and Mt Adams. Meanwhile, 30% of the
ensemble members produce at least a tenth of an inch of rain across
almost the entire CWA with sizable areas of at least a quarter of an
inch across the typical wetter areas of the north Oregon Coast Range
and south WA Cascades. The remaining 60% of the ensembles produce
scenarios somewhere in the middle of these two extremes where at
least a tenth of an inch will fall across good portions of the Coast
Range and Cascades with lesser amounts in the Willamette Valley.
This is also reflected in the 07z NBM 1D viewer guidance, which
places odds of at least 0.05" falling in 24 hrs at KPDX at 55%,
0.10"/24hr at 39% and 0.25"/24hr at 14% Wednesday night into
Thursday. Given the uncertainty in the amplitude of the shortwave
trough these odds lower quite a bit farther south in the Willamette
Valley. For example, the odds of 0.05" in 24 hrs at KEUG is 24%,
0.10"/24 hrs is 16% and 0.25"/24 hrs is just 5%.

Beyond Thursday, models and their ensembles are in general agreement
weak upper level shortwave troughing and low level onshore flow will
persist across the region Friday into Saturday. This should allow
temperatures to moderate and return closer to seasonal averages.
Differences among models and their ensemble members then grows more
towards next Sunday. The majority of the EPS members suggest the
flow will turn more zonal Sunday, while more than half of the CMCE
system suggests shortwave ridging will push towards the region.
Meanwhile, nearly half of the GEFS members suggest another shortwave
trough will slide across the Pacific Northwest Sunday. Given the
range of possibilities, opted for a blend of model guidance and
deviated little, if any, from NBM guidance. /Neuman


.AVIATION...A weak front will continue to bring IFR stratus to
the coast this morning, with areas of drizzle over the north
coast. Expect IFR stratus to persist through the morning hours,
with conditions improving to VFR for a few hours this afternoon
as marine stratus retreats offshore. IFR conditions look to then
redevelop this evening and continue through tonight.

Inland areas could see a few hours of patchy MVFR stratus between
1500-2500 ft through around 18Z. The areas mostly likely to see
MVFR conditions are the Columbia River and the Cascade Foothills.
Otherwise expect VFR conditions inland with passing clouds this
morning giving way to mostly clear skies this afternoon.

KPDX and APPROACHES...Patchy MVFR stratus between 1500-2500 ft
is possible between 13-17Z. Otherwise expect VFR conditions with
passing clouds this morning giving way to mostly clear skies this
afternoon. /64


.MARINE...A weak front will continue to move across the waters
this morning resulting in a few southerly wind gusts to 15 kt.
High pressure then builds over the waters later today and hold
through Wednesday resulting in gusty northerly winds during the
afternoon/evening hours. A weak front looks to then move across
the water Wednesday night into Thursday resulting in a brief
period of southerly winds before high pressure brings the return
of northerly winds later this week.

Seas are currently around 3-5 ft this morning and will remain in
this range through Wednesday. Seas look to then build to around
5-7 ft by Thursday then to around 7-9 ft later this week as a
building northwesterly swell moves across the waters. /64





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