Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
331 PM EST Thu Dec 08 2022

Day 1
Valid 16Z Thu Dec 08 2022 - 12Z Fri Dec 09 2022

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Churchill


Day 2
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...

...2030z Update...

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).

Churchill


...Previous Discussion...

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
this period.

Oravec


Day 3
Valid 12Z Sat Dec 10 2022 - 12Z Sun Dec 11 2022

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE
NORTHERN AND CENTRAL CALIFORNIA COASTAL REGIONS...

...2030z Update...

The only substantial change for this update was to extend the
Marginal Risk northward to encompass nearly the entirety of the
northern CA coast (overlapping much of the Marginal on Day 2), as
most of the model guidance consensus indicates an addition 1-3
inches of precipitation on Day 3. In fact, the 12z GFS and GEFS
indicate more precipitation here than in central CA, whereas the
ECMWF and ECENS indicate just as much precipitation in northern
CA, but more precipitation (up to 2-4 inches locally) in coastal
portions of central CA. Some of the precipitation will be changing
to snow in lower elevations (as snow levels begin to rapidly fall
on the backside of the system) late in the Day 3 period, but there
is enough of a threat during the first half of Day 3 to maintain
the overlapping Marginal Risk areas (as the bulk of the heavier
precipitation is also during the first half of Day 3).

Churchill


...Previous Discussion...

The amplifying mid to upper level trof off the Pac NW coast day 2
will continue to press southeastward day 3 across the coastal Pac
NW and northern California.  The axis of anomalous 850-700 mb
moisture flux on the southeast side of this amplified trof will
continue to press steadily southeastward, with values 3 to 4+
standard deviations above the mean affecting areas of central to
southern California.  This will support a period of heavy
precipitation, especially through the Sierra and along the central
California coastal range, with hourly rates topping near
.50"+/hour. Much of these regions have seen above average precip
over the past few weeks.  A marginal risk was confined to the
coastal sections from the coast range north of San Francisco,
south to approximately Santa Barbara where stream flow anomalies
from the National Water Model indicate regions of much above
normal to high flow from the above mentioned recent heavy
precipitation.  Additional heavy rains may result in isolated
runoff issues, especially across recent burn scars.   While heavy
precip is also likely in the low elevation upslope of the Sierra,
stream flows are mostly at or below average levels.  For this
reason and after collaboration with WFO STO, no risk area was
denoted here.

Oravec


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