Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
330 PM EST Thu Dec 17 2020
 
Day 3
Valid 12Z Sat Dec 19 2020 - 12Z Sun Dec 20 2020 

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE 
WASHINGTON AND OREGON COASTAL RANGES, INCLUDING PARTS OF THE 
CHEHALIS RIVER VALLEY AND THE OLYMPIC PENINSULA...

...21Z update...
The previous discussion below still looks on track. The latest 
models differ somewhat with the latitude of the strong onshore 
flow, perpendicular to the Washington/Oregon coast, confidence is 
high that a slow moving plume of anomalous moisture will impact 
the Pacific Northwest coast beginning late Saturday afternoon. The 
only change to the previous outlook was to extend the Marginal a 
little south in Oregon to cover the model spread in QPF. Once 
confidence increases, given snow levels will be high, a Marginal 
Risk may be needed for portions of the Cascade Range as well.

Otto

...previous discussion follows...

...Pacific Northwest...
An elongated Atmospheric River with subtropical origins over the 
central Pacific will quickly push into western WA/OR Saturday into 
Sunday, buoyed by strong, deep westerly flow. 850 mb flow 
increasing to 50-60 kts by 00-12Z Sunday 12/20 would be 3-4 
standard deviations above normal per the GEFS and SREF. The 
strengthening low-mid level onshore flow is expected to result in 
robust deep-layer moisture transport for several hours from late 
Saturday afternoon into Saturday night, especially along and west 
of the Cascades where 850-700 mb moisture flux anomalies peak at 
+3 per the GEFS/SREF. PW values of 1.00-1.25" inches along with 
strengthening 850-500 mb frontogenetic forcing and onshore flow 
(westerly upslope enhancement) will likely lead to 3-hourly 
rainfall rates of 1-1.5", despite the limited if not absence of 
instability, with much of the global guidance indicating pockets 
of 3-5" within the 12 hour period from 00-12Z Sunday 12/20. These 
rains could lead to localized short-term flooding issues, 
especially when combined with any melting snow and additional 
runoff along or near the slopes of the Olympic Mountains.

Hurley

 
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt