856
FXUS66 KPQR 171758
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
1056 AM PDT Sat Aug 17 2024

Updated aviation discussion

.SYNOPSIS...A strong low pressure system offshore will bring
widespread wetting rain to the area this afternoon into tonight,
with concerns for localized flash flooding near ongoing
wildfire burn areas. There is also a chance of thunderstorms
across the area today, a few of which could produce severe wind
gusts and large hail 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...Water vapor satellite
imagery reveals a large and well defined upper level low
offshore of the Pacific Northwest this morning, as has been
depicted by models for the past several days. Diffluent
southerly flow aloft is present across western Oregon and
Washington on the periphery of this feature, with numerous
showers already being picked up by regional radars over the
offshore waters as of 3 AM Saturday. Southerly flow will pivot
a bit more southeasterly over the course of the day, helping to
advect rich mid level moisture and enhanced instability into the
region to set the stage for widespread wetting rain showers
with embedded thunderstorms across the area this afternoon and
evening. The trigger for this activity will be a potent,
negative-tilt shortwave trough which will lift northward across
the area this afternoon, coincident with peak heating.
Temperatures will warm into the low to mid 80s in the interior
valleys and into the 70s in the higher terrain today prior to
the arrival of this feature, contributing to destabilization
characterized by MUCAPE values of 500-1500 J/kg across much of
the area away from the coast this afternoon. Additionally,
strong mid and upper level wind fields will provide ample shear
values to support organized updrafts for any thunderstorms that
develop. Finally, all of this will occur within the left exit
region of a 90 kt jet streak rounding the base of the large
scale trough, providing additional forcing for ascent across
northwest Oregon and southwest Washington.

Given all of the favorable factors described above, SPC
maintains a rare slight risk for severe thunderstorms across
the Cascades today, with a marginal risk extending westward to
the I-5 corridor. For some historical context, we have to go
back nearly four years to September 2020 to find the last time
any portion of our forecast area was included in an SPC Day 1
slight risk. The primary severe threat will likely be damaging
thunderstorm wind gusts of 60+ mph given a well mixed sub cloud
layer contributing to DCAPE values of 700-1100 J/kg. That said,
can`t rule out the possibility of some large hail to the size of
quarters, especially farther south over Lane County where cells
may tend to be more discreet early in the event. SPC maintains
a 15 percent chance of both severe modes for areas in the slight
risk, and a 5-15 percent chance in the marginal areas. This
means that any point in the Cascades slight risk area will have
a 15 percent chance of a severe thunderstorm occurring within 25
miles today, with a corresponding 5-15 percent chance within 25
miles of any point in the marginal risk area. In terms of
timing, hi-res guidance has been fairly consistent over the past
several runs in depicting showers and thunderstorms popping up
across Lane County around 1-2 PM this afternoon and then
spreading northward and reaching the Mt Hood and Portland areas
approximately 4-5 PM. Activity then continues into the evening
and starts to ramp down in intensity after sunset through about
midnight.

Shifting gears to the heavy rain/potential flooding aspect of
today`s activity, the forecast remains largely on track with
what has been depicted in previous model runs. Yesterday`s
sounding from SLE showed a precipitable water value over 1.1",
with additional moisture streaming into the area today looking
to send PWATs above 1.25" and perhaps as high as 1.50" according
to the higher end of guidance, which would be right around the
maximum values of mid August climatology. Given the ample
moisture and forcing, hi-res guidance continues to depict
rainfall rates mostly topping out around 0.75" per hour, though
latest HREF guidance does show a 5-10 percent chance of rates
locally as high as 1 inch per hour starting to show up over some
areas this afternoon. As such, will keep the Flash Flood Watch
intact along the length of the Oregon and south Washington
Cascades this afternoon into tonight. The primary areas of
concern remain the recently burned areas associated with ongoing
wildfires, but will have to also keep an eye on burn scars from
past years give the slight uptick in probabilistic guidance for
rainfall rates. Flood concerns in the inland valleys should
remain more limited in scope to some localized ponding of water
in areas that happen to see heavier rainfall rates over a
prolonged period of time, but widespread flooding concerns are
not expected in the lowlands.

Showers will ramp down in intensity and eventually taper off
from south to north through late this evening as the bulk of
the energy associated with the upper level trough shifts north
of the area into Washington. Could certainly see some lingering
shower activity into the day on Sunday and perhaps some isolated
thunderstorms into Sunday morning across southwest Washington,
but any additional rainfall will be minimal. The region will
start to settle back into a benign onshore flow regime by later
Sunday afternoon, with highs remaining in the upper 70s to
around 80 in the interior valleys. /CB

.LONG TERM...Monday through Friday...Little change in the
long term forecast as ensemble guidance continues to favor
mean 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, bringing
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 and perhaps some minimal thunderstorm
potential at times, but no significant rainfall or strong
thunderstorm potential is expected beyond this weekend. /CB

&&

.AVIATION...VFR conditions prevail at 18Z Sat, but isolated
thunderstorms that started around 14Z over parts of the Willamette
Valley to the coast are expected to depart the area to the
northwest shortly. Next shortwave to trigger more robust
thunderstorms this afternoon into the evening hours. Hires models
show most active thunderstorms over the Cascades during this time.
But valley locations too have a chance for thunderstorms.
Approximate timing for stronger batch of thunderstorms at TAF sites:
KEUG 21Z Sat-01Z Sun, KSLE 22Z Sat-02Z Sun, KPDX 23Z Sat-03Z Sun.
After that thunderstorm chances drop but not quite end until about
09Z Sun. Predominantly VFR conditions throughout, though passing
thunderstorms could briefly reduce visibilities below 6SM due to
heavy rainfall. Gusty erratic winds will be possible after 00Z as
thunderstorms pass through, with a 5-14% chance of severe convective
wind gusts up to 60 mph. Frequent lightning will also be possible
through this time range.


PDX AND APPROACHES...Thunderstorms and showers reach KPDX around
00z Sun through about 03Z Sun, then chances drop but not quite end
until about 09Z Sun. Predominantly VFR conditions throughout,
though passing thunderstorms could briefly reduce visibilities
below 6SM due to heavy rainfall. Other impacts of thunderstorms
covered above. /mh

&&

.MARINE...Expect benign conditions over the waters through mid next
week. Winds remain N/NW this morning but will turn SW late
tonight/early Sunday as surface low pressure moves through the
waters. Showers pass through all waters on Saturday, with around a
20% chance of isolated thunderstorms possible. Low pressure
remains over the NE Pacific through mid-week, maintaining SW
winds. Winds are expected to remain at 10 kts or less with gusts
up to 15 kts; wind-driven seas of 2 of 4 feet.
-Alviz/Sala

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...None.
WA...None.
PZ...None.

&&

$$

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