Mesoscale Precipitation Discussion 1260
NWS Weather Prediction Center College Park MD
1128 AM EST Wed Dec 10 2025

Areas affected...Olympic and Cascade Ranges of Western
Washington...

Concerning...Heavy rainfall

Valid 101630Z - 110300Z

SUMMARY...Core of next atmospheric river surge through evening. 
Rates of .33-.75"/hr based on elevation likely to further compound
ongoing river flooding throughout the foothills of the Olympic and
Cascade Ranges.  Mud/Landslide potential continues to increase
with amount of deep soil moisture.

DISCUSSION...Recent regional RADAR mosaic shows core of solid
stratiform rainfall is expanding across much of western
Washington.  CIRA LPW shows solid sub-tropical moisture connection
tapping just northwest of Kaua'i only narrowing/concentrating
along and south of the stationary front 42N/142W to the Olympic
Range.  RAP analysis and recent VWP network observations denote a
surge of increased moisture/warm advection south of the front is
directed orthogonally to the Olympic Range and further downstream
to the western Washington Cascades with 30-40kts of boundary layer
orographic ascent and Tds in the mid 50s.  CIRA LPW also notes
that core of enhanced 850-700mb remains well displaced (east over
WA at this time) to the surface core indicative of the long,
gentle upslope of the AR moisture plume; so with limited unstable
air, orographic moisture flux convergence remains the primary
driver of intense rates, so stark rainshadows and much lower
rainfall totals/rates are expected in main, lower elevation
valleys.  

CIRA LPW analysis, extrapolation shows core of enhanced moisture
below 700mb will continue to be directed toward the Juan de Fuca
Strait/Northern Olympics into the Northern Washington Cascades
over the next 6-9hrs supporting rates of .25" in the valleys to up
to .75" in the highest terrain of the Cascades; and with the
strong warm sub-tropic air, all but the most extreme peaks will
continue to remain below the freezing levels.  RAP/HRRR along with
LPW extrapolation suggest core of enhanced moisture and winds will
slowly reduce from supporting over 800 kg/m/s IVT values below 600
kg/m/s toward 00z with the front sagging south toward the mouth of
the Columbia river by 06z.  

FFG values are static in the Pacific Northwest and are not likely
to be exceeded; however, NASA SPoRT shows 0-40cm saturation is
near 90%, so nearly all those rates will be fully run-off.  Flash
flooding/rapid inundation flooding is not expected except for the
highest reaches of watersheds/upper slope creeks, etc. but the
rainfall will definitely continue a steady rise/expansion of
ongoing river flooding across the area. Please refer to Northwest
River Forecast Center and National Water Center products for
details of these ongoing dangerous, life threatening conditions. 

Additionally, deep layer saturation of the upper soils suggests
mud/landslides may become increasingly possible through this
evening, especially given affects of strong winds acting upon the
trees and therefore their roots.  Remain aware of this potential
and refer to local emergency managers and USGS reports/products
regarding landslide concerns.

Gallina

ATTN...WFO...OTX...PDT...SEW...

ATTN...RFC...PTR...NWC...

LAT...LON   49092147 48632099 48032072 47612090 46942137
            46942189 47532210 47892184 48252199 48662228
            49002260 48782323 47892312 47002346 47192395
            47762440 48222461 48152365 48122326 49042328