FOUS30 KWBC 181827
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
227 PM EDT Wed Mar 18 2026
Day 1
Valid 16Z Wed Mar 18 2026 - 12Z Thu Mar 19 2026
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
WASHINGTON STATE...
...16Z update...
A long duration, multi-day, AR event will continue today/tonight
with a fairly steady axis of moisture transport into western
Washington. IVT values along the coast have been, and will continue
to be, between 500 and 600 kg/m/s through Thursday morning
(locally higher on occasion). Selected gauge observations within
the Washington Cascades showed peak rain values in the 0.4 to 0.5
in/hr range earlier this morning but these higher rates have been
localized within the Cascades east of the Puget Sound and
surrounding higher observations have been between 0.1 and 0.3
in/hr. With little change to the synoptic scale setup forecast
through Thursday morning, it will be mesoscale impulses within the southwesterly flow aloft that will bring about fluctuations in
rainfall intensity and location, but otherwise things look fairly
steady state today.
The latest 12Z HREF is in pretty good agreement with previous
cycles with minor mesoscale differences in location/magnitude,
highlighting the greatest 24 hour totals across the Olympic
Peninsula into the Cascades (north of I-90). 24 hour QPF values
should average in the 1 to 3 inch across these locations with
locally higher values near 4 inches expected, perhaps an isolated
spot near 5 inches through Thursday morning.
No changes were made to the Marginal Risk area and the previous
forecast discussion remains on track.
Otto
...Previous discussion follows...
Persistent AR across WA state will lead to bouts of moderate to
occasionally heavy rainfall across the Olympic Peninsula into the
Northern Cascades. Down-sloping will reflect a general min across
the Seattle area, but the terrain will be susceptible to ongoing
rain prospects and totals between 1-3" for the day. This is an ongoing
threat that will carry on beyond the period adding a compounding
factor to the hydrologic responses anticipated across the River
basins within northwest WA. High snow levels ~8000ft MSL will lead
to snow melt across the higher elevations within the Northern
Cascades and Olympic Mountain ranges to couple with the rainfall.
This excessive run off will be another factor in the additional
water sources that will drain into nearby streams and rivers that
will induce at least several action stage responses and some minor
to borderline moderate river flooding across the region. The
primary river of impact will continue to be the Chehalis River
where minor flooding is already occurring and additional elevated
stream flows and run off entering into the tributaries will
eventually cause the crests of the river to slowly rise. Flood
Watches are in effect across portions of northwest WA state,
including Mason county where the greatest focus of flooding will
occur around the Chehalis River. Considering the hydrologic factors
at play despite rates only topping out around 0.5"/hr at peak
intensity, a MRGL risk was carried over for the period with little
to no change from the previous risk issuance.
Kleebauer
Day 2
Valid 12Z Thu Mar 19 2026 - 12Z Fri Mar 20 2026
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
WASHINGTON STATE...
...1830Z update...
No changes to the Day 2 ERO based on the latest guidance,=20
including the 12Z HREF exceedance probabilities.
Hurley
...Previous discussion follows...
There are no changes from the prior forecast as the overall synoptic
pattern remains consistent in run-to-run continuity. Prevailing
onshore flow within a zone focused between a broad occluded cyclone
in the Gulf of Alaska and a stout ridge axis focused over the
Southwestern U.S. will lead to a prevailing moderate rain setup over
much of the same areas impacted the previous periods. Total rainfall
in the Northern Cascades over into the Olympic Peninsula will run
between 4-7" with local amounts to 9" possible in the 72 hour time
frame of the AR. The same impact scenarios apply with the focus
being a compounding factor of rainfall, snow melt, and prolonged run
off into local rivers and streams that could result in flooding
prospects for multiple days as river levels reach action stage and
minor to even moderate flood stages. It will be important to
maintain awareness on local river conditions as the additional rain
will exacerbate any receding potential within the river basins.
Highest impacted zone will continue to be the Chehalis River basin
where Flood Watches are in effect even into D3 (see below) due to
the prolonged AR and river levels continuing to run high after
several weeks of rainfall. Additional watches are in effect over the
Northern Cascades due to anticipated run off from rain and snow melt
leading to newer hydro concerns as the event wears on. The previous
MRGL risk inherited was unchanged given the continuity.
Kleebauer
Day 3
Valid 12Z Fri Mar 20 2026 - 12Z Sat Mar 21 2026
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
WASHINGTON STATE...
...1830Z update...
Minor tweaks to the Day 3 Marginal Risk was made to essentially
mirror the Day 2 ERO, since it's essentially the same 'event',
which as per the discussion below, will be be waning during the day
Friday as the plume of highest TPW (1.0+ inches) and IVT (600-700+
kg/m/s) push south of Washington and dissipate with time.
Hurley
...Previous discussion follows...
Final day of impact for the current AR event over WA state will
occur within the D3 period with much of any rainfall expected to occur
in the first half Friday before the event finally comes to a close
a strong shortwave trough exits the Pacific with a cold front
passage signaling the end to the threat. Additional 1-2 inches of
rainfall across northwest WA will bring final event totals to the
5-10 inch range over the span of 4 days which will ultimately
contribute to persistent river flood concerns thanks to the snow
melt coupled with rainfall to drain into the basins surrounding the
Olympic Peninsula and Northern Cascades. The last wave of precip
will be relatively heavy compared to the prior period as the
shortwave addition will add a bit more vigor in the ascent pattern
along with IVT and PWAT advection anomalies pushing 3-4 standard
deviations above normal, respectively. Thankfully, this last push
will materialize and end fairly quickly as the fast flow provided
by a stout 165kt upper jet max will advance the final
wave/disturbance eastward into southern British Columbia dragging
the cold front swiftly through the region between 18-00z Fri/Sat.
Flood Watches for the Cascades and portions of the Olympic
Peninsula, with a focus on the Chehalis River remain in effect
until Friday afternoon local time. The previous MRGL risk was
generally unchanged as the pattern remains locked-in across various
guidance.
Kleebauer
Day 1 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7ZRyDZuD7smE1kXYzV2y57c2U-nVj5kcPn46tNg7eMvf= H1XuWJtCRXqONkZAD-9K88MPfjlDQO-ix3JClRuA3UhRqFw$=20
Day 2 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7ZRyDZuD7smE1kXYzV2y57c2U-nVj5kcPn46tNg7eMvf= H1XuWJtCRXqONkZAD-9K88MPfjlDQO-ix3JClRuAdFFS_5Q$=20
Day 3 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7ZRyDZuD7smE1kXYzV2y57c2U-nVj5kcPn46tNg7eMvf= H1XuWJtCRXqONkZAD-9K88MPfjlDQO-ix3JClRuASX0Il3A$=20
=3D =3D =3D
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