Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1035 AM EST Sun Dec 03 2023
 
Day 1
Valid 12Z Sun Dec 03 2023 - 12Z Mon Dec 04 2023 

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR ALL OF THE 
OREGON COAST RANGES AND MUCH OF THE OREGON CASCADES...

16z update...
Timing and magnitude of moisture flux orientation into the Pacific 
Northwest Ranges appears to be on target and aligned with previous 
discussion(s).  The cold front is about to progress through the 
far NW portion of the PacNW in the next hour or so; and this will 
likely diminish the lower-end risk for much of the remainder of 
the forecast time period across portions.  However, since it has 
not fully arrived at 16z and there will be a narrow window prior 
to 04.12z for additional .10-.25"/hr rates across W WA into the 
Olympic Range...the thought is to retain much of the Marginal Risk 
areas in WA and NW Oregon.  

Further south, the central Oregon coastal Range and downstream 
Oregon Cascades will remain within the core of the stronger 
southwesterly onshore flow for greater than six more hours with 
similar 400 to 600 kg/m/s IVT values.  One-quarter to one-third 
inch per hour will remain on exposed southwestern facing orography 
with highest peaks perhaps reaching near .5" occasionally.  Given 
strong warm advection, freezing levels rise above all but the most 
extreme peaks, so there may be some rain on snow melt to help with 
increased run-off and rapid rise responses on some creeks/streams 
in the vicinity.  So no changes were made to the Slight Risk areas 
across the coastal Ranges and Oregon Cascades. 

Gallina


A moderately-strong atmospheric river, centered on OR, will 
continue to push into the coast today through this evening. A 
brief reprieve from the rain is expected tonight between impulses 
of energy. On top of the rain that's already fallen, expect 
another 2 to 4 inches of rain through this evening, with local 
amounts as high as 6 inches for the day. More widely scattered 
flash flooding impacts are likely today due to so much rain 
falling in a short amount of time. NASA Sport imagery shows soil 
moisture profiles about normal for this time of year across 
western OR, or about 75% saturated. The additional rainfall today 
should take less time than yesterday to cause flash flooding 
impacts since there's rain ongoing and soil moisture levels 
continue to increase. 

The ERO risk areas today remain largely the same, with the only 
change of note to extend the Slight Risk into the northwest corner 
of CA, from  Crescent City northward as the coastal ranges in that 
area may see 2 to 4 inches of rain through this evening. The 
Slight Risk remains in effect for much of the OR Cascades, as 
rising snow levels will allow for snowmelt to contribute to rising 
river levels on the west side of the range, along with most to all 
of the precipitation on the foothills of the Cascades falling as 
rain. Much lighter amounts of rain are expected from central WA 
north today as the focus says largely over OR, with only 1 to 2 
inches of rain expected. Since this too will be added to previous' 
days rains, an isolated flash flooding instance can't be ruled 
out, and the Marginal remains unchanged.

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