501
FXUS66 KPQR 160028 AAA
AFDPQR

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
527 PM PDT Thu Aug 15 2024

Updated short term forecast for Thursday evening.

.SYNOPSIS...An upper level low will most likely bring a widespread
soaking rain to northwest OR and southwest WA late Saturday through
Saturday night as showers and thunderstorms move into the area. Rain
rates look to be heaviest over the Cascades, posing a threat of
debris flows over/near recently burned areas associated with ongoing
wildfires as hourly rain rates may approach 0.75 inches/hr. There is
also a marginal risk of severe thunderstorms over the Cascades and
foothills Saturday into Saturday night, as isolated damaging wind
gusts up to 60 mph will be possible with any discrete cells that are
able to form. Cool and cloudy weather then continues into the middle
of next week an onshore flow maintains the pattern in place.

&&

.UPDATE...Thursday evening...Radar and surface weather observations
from 520 PM Thursday depicted isolated light rain showers developing
in the Salem, McMinnville, Forest Grove, Hillsboro and Beaverton
area. NBM PoPs are too low at <10%, so the forecast was adjusted to
manually increase PoPs to 15-30% in the aforementioned areas. Also
included this chance for the rest of the Portland metro and the south
WA/north OR Cascade foothills as these light showers slowly shift
east-southeastward. The latest iteration of the HRRR backs this up
well, as simulated reflectivity guidance is showing these showers
moving over parts of the Portland metro between 6-8pm Thursday
evening, approaching the Columbia River Gorge and Cascade foothills
after 8pm. It appears showery activity will diminish completely by
4-5am Friday before chances for showers and thunderstorms increase
significantly late Saturday. This is discussed below in the short
term discussion. -TK

&&

.SHORT TERM...Now through Saturday Night...The short term forecast is
highlighted by increasing chances for a widespread soaking rain
across all of western WA/OR (50-80% chance) as widespread showers and
scattered thunderstorms bring a threat of heavy rain and isolated
wind damage. These showers and storms are associated with an upper
level low that models and their ensembles show lingering over the
coastal waters off the south Oregon coast. This will result in
southeasterly flow aloft across southwest WA and northwest OR,
helping to advect moisture into the area and increase instability.
Ample QG forcing for ascent is evident as well as CVA and WAA
increases ahead of an incoming trough axis and associated vort max,
resulting in notable Q-vector convergence.

The threat for thunderstorms is currently highest in the Cascades
where instability will be maximized according to the latest model
guidance (CAPE values rising to 500-1500 J/kg according to the
GFS/EURO/NAM). Strong wind shear is also evident with impressive
0-6km bulk shear values between 40-55 knots. Admittedly, there is
some uncertain regarding the exact timing of this system and which
areas will undergo enough daytime heating for these higher CAPE
values to materialize. Nevertheless, the environment is favorable
enough to support at least a 20-30% chance of thunderstorms from the
Coast Range to the Cascades, with the best chance being over the
Cascades. In fact, the Storm Prediction Center (SPC) has introduced a
marginal risk for severe thunderstorms over the Cascades/foothills,
mainly for potential isolated convective wind gusts up to 60 mph with
any discrete cells that are able to form before cells congeal into an
MCS. Regardless of the storm mode, heavy rain will be a threat given
high PWAT values over 1", fairly deep warm cloud layer depths over
7kft, and somewhat slow storm motion with cloud layer winds <20
knots. The UWWRF 1km and the NAM Nest are both showing rain rates up
to 0.75 in/hr, which is not quite high enough for debris flow
concerns over old burn areas, but is high enough to warrant concern
for our new 2024 burn areas associated with ongoing active wildfires.
As such, a Flash Flood Watch has been issued for the south WA
Cascades and Oregon Cascades from Lane County northward to cover
this threat as this will pose a safety concern for crews working on
these fires. While confidence is high somewhere in the Cascades will
be hit hard from thunderstorms, confidence is low in regards to the
exact location and exact timing. Nevertheless, confidence has
reached the 50% threshold needed to issue a Watch, and zones may be
added or removed from the Watch as confidence increases. For now,
anyone near or within the watch area should ensure they have a
timely way to receive NWS warnings in case a Flash Flood Warning is
issued.

While the bulk of convective activity should shift northward into
northwestern WA Saturday night, forecast soundings show lingering low
and mid level moisture / weak instability over northwest OR and
especially over southwest WA. This will likely result in at least
some continued showery activity, however convection will become more
shallow and chances for thunder will decrease to 15-20% or less.
Despite convection becoming more shallow Saturday night, brief heavy
downpours will still be possible. Showers and storms should finally
begin tapering off Sunday morning, which is discussed below in the
long term discussion. -TK

&&

.LONG TERM...Sunday through Wednesday...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. Otherwise suspect the Pacific Northwest will remain
under the influence of broad upper level troughing centered offshore
through the middle of next week, which is well agreed upon by
ensemble guidance. As such, expect a continuation of seasonably cool
and cloudy weather through the end of the period with daily highs
remaining in the mid to upper 70s. The proximity to the upper level
trough will yield continued chances for sporadic shower
activity, but no significant rainfall or thunderstorm potential
is signaled beyond Sunday morning. /CB

&&

.AVIATION...Marine stratus has mostly cleared out of all areas,
with KONP being the last holdout. 40-50% chance of southern
coastal terminals remaining MVFR or lower throughout the day,
dropping back to IFR by 3z Fri. For other coastal terminals, MVFR
conditions redevelop around 3z Fri. Inland terminals should
generally remain VFR until 12-18z Fri, when the northern
Willamette Valley sees marine stratus pushing up the Columbia and
causing high-end MVFR cigs during that time period. 60% confidence
in MVFR cigs between 12-18z Fri. Winds at all terminals are
generally W/NW and under 8 kts throughout the entire period.

PDX AND APPROACHES...VFR expected until 12z Fri, with a 60% chance
of MVFR cigs between 12z-18z Fri due to marine stratus pushing up
the Columbia. Northwest winds under 8 kt throughout the period.
/JLiu

&&

.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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