Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 091020

Area Forecast Discussion
National Weather Service Portland OR
320 AM PDT Fri Jun 9 2023

.SYNOPSIS...The first measurable rain in a few weeks occurred
overnight across much of the Portland metro area, and more is
expected today as an upper level disturbance from the Great Basin
settles over SW Washington. The best chance for rain will be north of
Wilsonville, and wetting rains of over a quarter inch are possible in
the Cascades Mt Hood northward. The clouds and precipitation will
hold temperatures below normal north of Salem, with less impact to
the south. Showers will diminish as the disturbance weakens tonight,
with dry weather expected to prevail for the next few days outside of
isolated afternoon showers in the Cascades. Temperatures will rebound
to a few degrees above normal by Sunday, peaking on Monday before
moderating for the rest of the week.


.SHORT TERM...Today through Sunday...Well, so much for the dry
forecast. Areas of light rain associated with a shortwave disturbance
aloft crossed westward over the Cascades Thursday evening, affecting
the Portland metro area and the adjacent Cascade foothills. It is
tough to tell exactly what has reached the ground in the more remote
locations due to a RAWS network outage. But radar data from KRTX and
CWOP/utility/NWS observations seem to confirm a swath of measurable
rain north of roughly an Estacada-Hillsboro line. Looking upstream at
KPDT radar it appears more is on the way, and should lead to
occasional light rain throughout today across much of SW Washington
and much of the Portland metro area. Radar trends and
hi-res/convection-allowing models (CAMs) suggest the PDX metro will
be on the southern periphery of the rain, with areas south of
Wilsonville remaining mostly dry today. Latest HREF probs show about
a 20% chance of receiving at least 0.25 inch today in the PDX metro
area, increasing to 40-60% in the Cascades Mt Hood northward. Given a
ribbon of dry air between 850-700 mb, suspect these are a bit high
and we opted to stay closer to WPC`s QPF. That said, fuels in Fire
Weather Zones 663/660 and 607 stand to see more appreciable wetting
than was previously expected, which will mitigate the drying of fuels
for the next few days.

The current precipitation appears to be debris from earlier
convection over eastern Oregon that has been sustained by some weak
synoptic lift associated with the above-mentioned shortwave. This
support should continue as the shortwave parks somewhere near SW
Washington this afternoon. However the shortwave is shown by guidance
to move northward tonight and Saturday, taking the shower chances
with it. Assuming this track is accurate, most of the shower activity
should be north of the Cowlitz/Lewis County line by Saturday morning
and the air mass will begin a warming trend as 500 mb heights and 850
mb temps rise through the weekend. Lingering clouds Saturday morning
should clear out as the day progresses. Strengthening thermal low
pressure near the OR/CA border will draw drier northerly flow,
leading to much less cloud cover Sat night into Sunday.

Temperatures will be below normal today north of roughly Salem due to
the cloud cover and precipitation, as well as continued low-level
onshore flow. Much of the Portland metro area and SW Washington will
remain in the 60s for highs today, while areas south of Salem climb
closer to early June normals of lower to mid 70s. As mentioned above,
the air mass begins a warming trend Saturday, with the 00z GFS
showing Salem 850 mb temps of +7 deg C this afternoon, +10 deg C
Saturday, and +13 deg C Sunday. There will also be a decreasing
marine influence as the marine layer becomes shallower and low-level
flow turns more northerly. By Sunday, temps should be solidly above
normal as the latest NBM guidance shows an 80-85% chance of highs
above 80 deg F for the Willamette Valley. The one exception may be
along the immediate coast where the lack of offshore flow will keep
them cool with breezy N-NW winds and occasional stratus.  Weagle

.LONG TERM...Sunday night through Thursday: Quasi-cutoff low
pressure that had been meandering across central California is shown
to be getting displaced further south and east by multiple models
and their respective ensembles. The driving factor is high pressure
building over much of the northeast Pacific and, in some cases,
briefly extending northeastward covering B.C. and clipping NW

The general ridge axis moves little through the week and then de-
amplifies as lower pressure nodules pass eastward across northern
British Columbia. Given good agreement among the models and
respective ensembles, had no qualms with leaving the NBM temperatures
and PoPs as is for the period. This brings peak temperatures on
Monday with valley locations hovering around or just below 90
degrees. A cooler trend returns for the remainder of the period as
temperatures start in the upper 70s/lower 80s, but then settle to the
middle 70s on Thursday. Overnight lows will stay in the upper 50s
early, then trend toward the upper 40s and lower 50s later in the

The steady position of the ridge will then also continue somewhat of
a blocking pattern through much of next week as western Pacific
shortwaves ride northward along the westward edge of the ridge and
bring very little precipitation to our region as the shortwaves begin
dropping southward along the ridges eastward periphery. As such,
continue to expect very little, if any rain to affect the area
next week. Some thunderstorms may occur over the central Cascades,
but this is far from a certainty right now. /JBonk


.AVIATION...12z TAFs: An upper low over the Great Basin continues
to bring scattered light showers to northern Willamette Valley
terminals early Friday morning. Onshore flow pattern will persist
throughout the entire TAF period. As of 10z, coastal sites remain
predominately MVFR with marine stratus, but cigs could fall to IFR
thresholds after 12z Fri. Model guidance suggests a 60-70% chance
of IFR cigs at the coast 12-15z Fri. With potential intrusion of
marine stratus into the Valley plus moisture from the upper low,
inland sites have a 40-60% chance of MVFR cigs through 21z Fri.
Expect cigs to lift to predominately VFR thresholds for Valley
sites after 21z Friday.

For detailed Pac NW aviation weather information, go online to:

KPDX AND APPROACHES...Cigs will hover between high-end MVFR and
low-end VFR through 21z Fri. After 21z Fri, expect predominately
VFR thresholds with light winds. Scattered showers expected to be
light and not cause any vis issues.   -Alviz


.MARINE...Weak surface pressure gradients over the waters will
keep winds and seas light through Friday. High pressure then
strengthens offshore as thermal low pressure strengthens over NW
California. This will induce stronger N winds of 15 to 25 kt
across the waters this upcoming weekend, eventually whipping up
steep seas of up to 10 ft Sat night/Sunday. North to northwest
winds continue to remain elevated into early next week, though there
may be some easing as the axis of high pressure stretches across NW
Oregon Tuesday into midweek.  Weagle/DH






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