Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
345 PM EDT Tue Oct 26 2021
 
Day 3
Valid 12Z Thu Oct 28 2021 - 12Z Fri Oct 29 2021 

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF 
THE PACIFIC NORTHWEST COAST & BLUE RIDGE MOUNTAINS...

21Z Update...
Latest QPF guidance has increased through the Blue Ridge Mountains 
of NC/VA over the past 12 hours leading to an increase in the WPC 
forecast for the period. This area has been on the drier side over 
the past 2 weeks, but given the terrain isolated flash flooding is 
certainly possible. Have kept the Marginal Risk area in place to 
cover this threat. No changes were made to the Marginal Risk area 
for WA either as soils remain well above normal and additional 
rainfall may lead to isolated flash flooding.

Chiari

Previous Discussion...

Pacific Northwest...
Onshore flow of up to 50 kts, precipitable water values at or 
above 1.25", with a smidge of instability should weaken and shift 
south later in the period (Thursday afternoon onward) as a cold 
front shifts south and a low crosses southwest Canada.  
Considering the degree of inflow and modicum of instability, 
believe hourly rain totals of 0.5"+ are possible so included 
western WA and far northwest OR in a Marginal Risk area.  Two week 
precipitation anomalies are slowly increasing with some areas of 
200%+ of average as of Monday morning and rainfall from previous 
days (Tuesday and Wednesday) should continue that trend.

Blue Ridge Mountains of NC/VA...
Inflow at 850 hPa of 30-60 kts will come in perpendicular to the 
terrain for roughly 12 hours ahead of a near-record deep for 
October occluded cyclone moving across the Mid-South and Lower OH 
Valley.  Precipitable water values of 1.25"+ should be available, 
so when the instability is there (500+ J/kg), hourly rain totals 
to 1.5" are possible.  However, the guidance advertises this 
window to be quite short so overall totals are forecast to be 
modest.  Two week precipitation anomalies are well below average 
but the terrain is rugged.  Autumn leaf fall could cause clogging 
drainage as well.  Thought it prudent to raise a Marginal Risk due 
to the above. 

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