FOUS30 KWBC 180739
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
339 AM EDT Wed Mar 18 2026

Day 1
Valid 12Z Wed Mar 18 2026 - 12Z Thu Mar 19 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
WASHINGTON STATE...

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...

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...

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
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 * Origin: Capitol City Online (618:250/10)