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 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt