121
FXUS66 KPQR 161109
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
409 AM PDT Fri Aug 16 2024

.SYNOPSIS...One more day of benign onshore flow today with
highs mainly in the upper 70s. A strong upper low developing
offshore will then bring widespread wetting rain to the area
Saturday and Saturday night, with concerns for localized flash
flooding near ongoing wildfire burn areas. There is also a
chance of thunderstorms across the area on Saturday, a few of
which could produce severe wind gusts from the Cascades west to
the I-5 corridor. Showers taper off on Sunday, yielding a
return of mild onshore flow through much of next week.

&&

.SHORT TERM...Now through Sunday Night...Satellite imagery shows
broken stratus filling in across the inland valleys of
northwest Oregon and southwest Washington early Friday morning
as the region remains under the influence of stable onshore
flow. In the upper levels, a trough beginning to dig south from
the British Columbia coast along 130W will feature prominently
in the potential for widespread wetting rain and scattered
thunderstorms across the area this weekend. In the meantime,
expect one more day of benign weather as morning clouds give way
to mostly sunny skies this afternoon with temperatures near or
slightly below seasonal norms in the upper 70s to low 80s in the
inland valleys.

The primary short term forecast concern remains rain and
thunderstorm potential Saturday and Saturday night. The upper
trough will continue to deepen as it digs southward over the
offshore waters today, with models depicting an unseasonably
strong upper low centered approximately 300-400 miles off of the
southern Oregon Coast by Saturday morning. This will place our
area in south to southeasterly flow aloft, helping to advect
rich mid level moisture and enhanced instability into the
region while a 90 kt jet rounding the base of the trough
provides strong forcing for ascent. These factors will create a
favorable environment for thunderstorms Saturday afternoon and
evening as a shortwave trough swings northward across the area.
One notable trend in the guidance over the past 12-24 hours has
been to delay the arrival of this feature, which could hold off
the onset of precipitation and thunderstorm potential farther
north towards Salem and Portland until late afternoon or early
evening. The trade off of this delay is that this will allow
temperatures to warm into the low 80s and contribute to
increased instability, thus enhancing the thunderstorm
potential. This is reflected in a slight uptick in thunderstorm
probabilities to 20-30 across the Willamette Valley and Coast
Range and 30-40 percent along the Cascades. WPC continues to
highlight the Cascades for a marginal risk (5-15 percent chance)
of severe thunderstorms, but has also extended this area
slightly farther westward to include the I-5 corridor. The
primary severe threat from these storms would be potential for
a few outflow gusts of 60 mph as the strong wind field aloft
mixes down to the surface.

Shifting gears to heavy rain potential, the combination of
strong forcing and ample moisture characterized by model
precipitable water values as high as 1.25-1.40", or the 90-95th
percentile of mid August climatology, suggests any showers or
thunderstorms will be capable of producing heavy downpours, with
hi-res guidance continuing to depict maximum hourly rainfall
rates around 0.75". While this remains below debris flow
thresholds for older burn areas, it does continue to present a
threat for newer burned areas associated with ongoing
wildfires. The Flash Flood Watch therefore remains in effect
along the Cascades to capture this threat. Given the
aforementioned delay in timing of the best forcing, some hi-res
guidance is beginning to suggest the greatest potential for
these rainfall rates may ultimately be across northern portions
of the Oregon Cascades into southern Washington, but given the
lingering uncertainty have opted to leave the watch in place
across all Cascade zones at this time. In terms of a wetting
rain of a quarter inch, guidance continues to depict a 50-70
percent chance area wide through Saturday night, with higher end
outcomes remaining closer to 0.75-1.00" for most locations.

Rain will be starting to taper off by Sunday morning as most of
the energy associated with the upper trough shifts north of the
region, but certainly could see a few stray showers linger into
the day on Sunday and bring minimal additional rainfall amounts
to the area, especially over southwest WA. Temperatures will
continue to hover right around or slightly below seasonal norms,
with Sunday afternoon highs in the upper 70s to around 80. /CB

.LONG TERM...Monday through Thursday...Little change in the long
term forecast as ensemble guidance continues to agree on upper
level troughing over the NE Pacific through much of next week.
This will keep western Oregon and Washington in a stable onshore
flow regime through the period, yielding seasonably mild
temperatures with morning cloudiness and afternoon highs
generally in the upper 70s. The proximity to the upper level
trough will also yield continued chances for sporadic shower
activity, but no significant rainfall or thunderstorm potential
is expected beyond this weekend. /CB

&&

.AVIATION...Currently low stratus clouds along the coast, lower
Columbia River Valley, and Cascade foothills as of 10Z Friday.
General onshore flow continues across the area as we maintain
westerly to southwesterly flow aloft and an onshore surface
pressure gradient. Expect MVFR CIGs along the north OR coast (KAST
area) and LIFR/IFR CIGs at KTMK and southward (KONP) between
10-19Z Friday. Specifically, there is a 20-40% chance of LIFR
conditions for KTMK and KONP, with chances around 40-60% of
lowering to at least IFR. KAST has a 40-60% chance of MVFR CIGs.
Inland will see VFR conditions through most of the TAF period,
with the exception of a 30-50% chance of MVFR CIGs until 18-19z
Friday. Will improve to VFR thereafter. Otherwise, winds will be
out of the northwest and remain under 10 kt.

PDX AND APPROACHES...Predominately VFR thresholds through the TAF
period, with a 20-40% chance of MVFR cigs between 13-17z Friday.
Expect improving CIGs with increasing cloud breaks towards clear
skies by Friday afternoon. /Sala

&&

.MARINE...Expect benign conditions over the waters through early
next week. Winds remain N/NW through Saturday before turning SW
as surface low pressure develops. Winds are expected to remain at
10 kts or less with gusts up to 15 kts; wind-driven seas of 2 of
4 feet. -Batz

&&

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

&&

$$

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