Battleground, Washington Weather Forecast Discussion

502
FXUS66 KPQR 081003
AFDPQR
Area Forecast Discussion
National Weather Service Portland OR
300 AM PDT Mon Sep 8 2025
.SYNOPSIS...A closed upper level low pressure system will bring cool
and showery weather to the region Monday and Tuesday before the
area gradually dries out late in the week. There is 80% chance
another Fall-like storm system impacts the region late Saturday into
Sunday, bringing cool temperatures and widespread rain to all of
western WA and western OR.
&&
.SHORT TERM...Now through Wednesday Night...The short term forecast
remains highlighted by cool and showery weather. Water vapor imagery
from early Monday morning depicted increasing moisture over southwest
Oregon, extending across the northeast quadrant of a closed upper
level low centered over the northeast Pacific around 40N/130W. Radar
observations showed a few rain showers beginning to develop between
Medford and Roseburg. These showers will move northward into
northwest Oregon this morning and afternoon as the upper level low
moves closer to the coast and moist southerly flow aloft remains in
place inland. Models are in good agreement regarding the timing of
rain showers, suggesting showers will begin between 7-9am in Lane
County before spreading up the Willamette Valley, Coast Range,
Cascades and foothills between 9am-5pm, reaching southwest WA last.
While most locations should see at least some showery activity by
Monday evening, it still appears the south WA/north OR coast and
Willapa Hills will see little to no showers today. Some showers will
have the potential to produce locally heavy rainfall, and possibly a
few flashes of lightning (15-20% chance). The reason probabilities
for thunderstorms are low is due to limited instability. 700-500 mb
lapse rates are expected to be around 5.6-6.0 degrees Celsius/km,
with SBCAPE values generally under 500 J/kg. Model soundings suggest
equilibrium levels will be below the hail growth zone, meaning
hail is unlikely to occur, and charge separation/lightning will be
difficult to obtain for most showers. The main exception to this is
in the Lane County Cascades. This is where instability and
equilibrium levels look highest, especially in the early to mid
afternoon. As such, this area has a 35-40% chance of thunderstorms;
there is also a 20-40% chance for hourly rain rates over
0.5 in/hr in this area between 4-8pm today. This could potentially
result in debris flow concerns over recently burned areas, however
storms are not expected to train over the burn areas, which will help
mitigate this concern to some degree. In addition, hourly rain rates
would likely need to reach 1.0 in/hr or higher for debris flows to
materialize, which only has a 5-10% chance of occurring according to
the latest HREF guidance.
Showers will continue Monday night into Tuesday, but will be less
frequent. There is a 15-25% chance of thunder Tuesday afternoon as
limited instability remains in place. Stronger showers or
thunderstorms are expected to produce locally heavy rain. Due to the
hit or miss nature of showery precipitation, rain amounts will be
quite variable across the area, with some locations receiving as
little as 0.05-0.10 inches, while other locations receive 0.25-0.75
inches or more. Forecast 48-hr rain amounts from 5am Monday through
5am Wednesday are generally at or over 0.25 inches in the Willamette
Valley and over 0.5-0.75 inches in the Cascades.
Come Wednesday, most locations will be dry as showers begin to
diminish in coverage. The main exception is over the Cascades where
there is a 50-70% chance of showers Wednesday afternoon/evening along
with a 10-20% chance of thunderstorms. Temperatures will remain
cooler than average with highs most likely in the low to mid 70s,
except 60s at the coast. -TK
.LONG TERM...Thursday through Sunday night...Ensembles remain in
general agreement the closed upper level low will slowly meander
eastward over the northwestern Great Basin Thursday into Friday
before settling over Montana and Wyoming this upcoming weekend. As
the upper low exits the region, shortwave ridging will shift across
the area and allow the area to temporarily dry out late in the week.
Nonetheless, there are still a subset of ensemble members that keep
the upper level low pressure close enough to the region that embedded
shortwave troughs rotating around the parent low keep shower chances
going on Thursday, particularly across the Cascades.
Global ensembles have shown a marked uptick in the number of
members depicting a scenario where a shortwave trough will
push into the Pacific Northwest late Saturday into Sunday. This would
bring another round of cooler temperatures and wet weather to the
region, which would be good news for further subduing the 2025
fire season. Confidence for widespread rain this upcoming weekend has
increased, as nearly every ensemble member from the ENS/GEFS/GEPS is
now showing at least some QPF. Only a small handful of members show
no rain at all. NBM 24-hr PoPs reflect this well and peak close to
80%. Although confidence is high that widespread rain will occur over
the weekend, confidence is low for exact rain amounts. There is
currently a 55-65% chance for at least 0.25" of rain between 5pm
Saturday and 5pm Sunday according to the NBM, which seems reasonable
based on LREF QPF guidance. -TK
&&
.AVIATION...Satellite imagery and terminal observations as of
early Monday morning shows mid to high level cloud cover as a low
pressure system approaches the region. There is a 40-60% chance
for MVFR CIGs along the coast between 12-16z Mon. Chances for
LIFR/IFR conditions have decreased, though can`t rule out
ceilings dropping for a few hours. If CIGs drop along the coast,
it should improve to VFR by 17-18z Mon.
Scattered showers are forecast to move from south to north across
the area after 12-15z Mon. General thinking is showers will reach
KEUG by around 15-17z and KPDX by 18-20z. Additionally, there`s a
15-25% chance of isolated thunderstorms developing after 21z Mon.
Conditions could temporarily lower to IFR/MVFR in heavier showers
or thunderstorms. Any passing thunderstorms could produce heavy
rain, gusty winds, lightning, and small hail. Otherwise, winds
today are west-northwest along the coast and variable across the
Willamette Valley, under 10 kt.
PDX AND APPROACHES...Predominately VFR through the TAF period.
Winds generally west-northwest around 5-6 kt or less. Rain showers
may arrive around 18-20z Mon with thunderstorm chances after 00z
Tue. Brief IFR/MVFR conditions possible in heavy showers or
passing thunderstorms. -Alviz/HEC
&&
.MARINE...As of early Monday morning, a weak low pressure center
over the northeast Pacific is bringing light and variable winds
across the waters. Buoy 46029 observations as of 2 AM Monday have
been showing east/southeast winds as the center of the low
continues to spin offshore. Tonight into Tuesday, this low
weakens and high pressure will re-build, returning northerly to
northwesterly winds. Northerly winds persist through Thursday.
Winds today through Thursday remain under 10 kt with minimal
impacts. Seas remain around 2-3 ft at 12-14 sec through Tuesday,
building to 4-5 ft at 11-12 sec Wednesday to Thursday with a
west-northwesterly swell.
By Friday into the weekend, guidance is suggesting another low
pressure system approaching the Pacific Northwest. This would
shift winds more southerly and build seas to around 5-7 ft at
11-12 sec as the west-northwesterly swell increases. However,
there is still uncertainty with the exact timing and track of this
system. For now, guidance suggests only a 10-15% chance for
frequent southerly wind gusts of 21 kt or greater (small craft
conditions) across the waters on Saturday. -Alviz
&&
.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...None.
&&
$$
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NWS PQR Office Area Forecast Discussion