< Day 2 Outlook
WPC Day 3 Excessive Rainfall Outlook
Risk of 1 to 6 hour rainfall exceeding flash flood guidance at a point
Updated: 1944 UTC Tue Oct 26, 2021
Valid: 12 UTC Oct 28, 2021 - 12 UTC Oct 29, 2021
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
345 PM EDT Tue Oct 26 2021
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...
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.
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.
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt