FOUS30 KWBC 170741
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
341 AM EDT Tue Mar 17 2026

Day 1
Valid 12Z Tue Mar 17 2026 - 12Z Wed Mar 18 2026

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

Active pattern across the Gulf of Alaska will lead to another AR
contribution into portions of the Pacific Northwest as the main
IVT surge will adjust south into WA state after impacting
neighboring British Columbia the past 24-48 hours. IVT forecast
remains consistent within guidance as values between 500-700 kg/ms
are expected across northwest WA state with the surge propagating 
far enough inland to impact the Northern Cascades. Rainfall amounts
between 2-3" will be common in the hardest hit areas, mainly into 
the Olympic Peninsula, Chehalis River valley, as well as the 
Cascades to the east of Seattle. Latest 00z HREF neighborhood
probabilities for >2" of rainfall over the course D1 are running
between 50-70% across the Olympic Peninsula into the Chehalis River
basin where Flood Watches have been issued for the upcoming AR.

Probabilities for the Northern Cascades are more robust for the
same 2" threshold, running between 70-90+% across the entire area
from Mount Rainier up to the Canadian border. Greater than 3" 
probs are modest between 50-70% in those interior terrain areas, 
including the highest elevations of the Olympics. Snow levels are 
forecast to be around 8000ft MSL leading to a large portion of the 
mountain ranges to see ample rainfall and snow melt that will 
contribute to higher run off prospects. Rainfall rates will lack 
vigor to the point for a more substantial impact, so the threat is 
mainly due to prolonged moderate to locally heavy rain prospects 
falling over relatively moist soil layer present after being 
impacted for weeks leading in coupled with the expected snow melt. 
The previous MRGL risk was generally unchanged with the threat 
leaning towards the low to mid level of the risk threshold. Areal 
hydrologic concerns are still viable with river and small stream 
flooding the most notable potential impacts as forecasts around 
some of the major river basins, including the Chehalis River, are 
expected to reach into Action stage and even some minor river flood forecasts.

Be sure to check into the latest forecasts from the local Seattle 
WFO and the National Water Center for the latest on the river 
flooding threat.

Kleebauer


Day 2
Valid 12Z Wed Mar 18 2026 - 12Z Thu Mar 19 2026

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

Occluded low over the Gulf of Alaska and a strong mid-level ridge 
positioned across the Southwest U.S. will lead to a continued 
transport of modest IVT into WA state for the D2 period. AR 
dominated pattern will enhance rainfall prospects for the northern 
half of WA leading to cumulative rainfall totals breaching 4" in 
spots within the Northern Cascades, Olympics, and adjacent coastal 
areas of northwest WA. Real concern is the growing hydrologic 
factors with river flooding coupled with moderate rainfall posing a
problem for areas around the main rivers and tributaries for the
aforementioned areas. Flash flood threat remains lower end by
definition due to the rates anticipated with the rainfall as 
hourly rainfall will remain <0.5"/hr for a majority of Wednesday. 
However, the coupling of the previous days rain and the already 
saturated soils will likely lead to some hydro concerns mid-week,
especially for the Chehalis River basin and the rivers/stream
connected with the Northern Cascades as they encounter the tandem
of rainfall and snow melt. Maintained the previous MRGL risk 
inherited with no modifications necessary as the forecast remains on track.

Kleebauer


Day 3
Valid 12Z Thu Mar 19 2026 - 12Z Fri Mar 20 2026

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

Persistent 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 for multiple days due to the prolonged AR and river levels
continue 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. 

Kleebauer
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--- SBBSecho 3.28-Linux
 * Origin: Capitol City Online (618:250/10)