Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
323 PM EST Sun Dec 09 2018
 
Day 3
Valid 12Z Tue Dec 11 2018 - 12Z Wed Dec 12 2018 

...Pacific Northwest...

A trough will approach the region at the start of Day 3 advecting 
an Atmospheric River toward Washington and Oregon.  Precipitable 
water of 1 inch transported by a low level 35 to 45 knot jet will 
be directed eastward into the Olympic Mountains, Coastal Range and 
Cascades for a brief period of time.  This combined with 
sufficient upslope, anticipate efficient rain processes with this 
system. Weak instability will try to move onshore, but with less 
than 100 J/kg expect rain rates to stay well below 0.50 inches per 
hour. Given the heaviest QPF will fall across the higher terrain 
where snow will likely occur and the progressive nature of this 
atmospheric river, do not expect flash flooding will be of 
concern.  Therefore, a Marginal Risk was not issued for this area. 
 If QPF and rain rates increase, a Marginal Risk may be needed 
across this region.  

With that caveat stated, the probability of rainfall exceeding 
flash flood guidance is less than 5 percent.  



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