Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
330 PM EST Thu Dec 08 2022
Valid 16Z Thu Dec 08 2022 - 12Z Fri Dec 09 2022
The probability of rainfall exceeding flash flood guidance is less
than 5 percent.
Valid 12Z Fri Dec 09 2022 - 12Z Sat Dec 10 2022
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM FAR
SOUTHWEST OREGON INTO NORTHWESTERN CALIFORNIA...
Little change to the inherited Marginal Risk area, with some minor
expansion inland to reflect relatively high snow levels (rising to
5000-6000 feet) during the primary period of precipitation (00-12z
Saturday). Models remain in good agreement concerning this period
of interest, with the 12z HRRR indicating localized hourly rates
of 0.50-0.75"/hr. The occasionally repeating nature of these rates
may result in localized totals over 6-12 hours as high as 3-4
inches, which may result in isolated instances of flash flooding
(with particular vulnerability over burn scars and low-lying areas
with poor drainage).
There is fairly good agreement in the latest model guidance with
the amplification of the upper trof off the Pac NW coast late
Friday afternoon into the early hours of Saturday. This will
strengthen the low level southwesterly flow into coastal sections
from northwestern California into Oregon, with 850-700 mb moisture
flux anomalies reaching 2 to 2.5+ standard deviations above the
mean from far southwest Oregon into northwestern California. This
will support heavy precipitation during the 0000-1200 UTC Saturday
time period across the marginal risk area, with isolated hourly
rates of .50"+ possible. The axis of anomalous onshore flow will,
however, be fairly progressive to the southeast. This and only
slightly above average precip values over the past week for
northwest California will keep the risk level at marginal at best.
There were no significant changes from the previous outlook for
The Day 3 outlook will be updated by 2030Z.
Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt