Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
738 PM EST Fri Dec 02 2022
 
Day 1
Valid 01Z Sat Dec 03 2022 - 12Z Sat Dec 03 2022 

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

...Coastal Ranges of Southern California...
Moisture will be gradually returning overnight along a 
quasi-stationary front draped over southern California and 
extending offshore down to the southwest. The moisture transport 
as driven by the low-level flow will be rather modest with only 
about 20 to 25 kts of southwest flow expected along the front and 
into the windward slopes of the coastal ranges. However, 
sufficient convergence/forcing along the front and the arrival of 
weak vort energy embedded within the deeper layer southwest flow 
aloft should favor an increase in shower activity overnight, and 
especially in the 03Z to 09Z time frame. 

The available moisture and forcing should favor some rainfall 
rates getting into the 0.25" to 0.50"/hour range at least 
occasionally, and the 18Z HREF shows some low-end probabilities 
over Santa Barbara and San Luis Obispo counties for some brief 
0.50"+/hour rates which is favored also by the latest HRRR 
guidance. The HRRR suggests the overall heaviest rainfall threat 
overnight tending to be over the Santa Ynez mountains in Santa 
Barbara County. As much as 1 to 1.5+ inches of rain is forecast 
over the terrain here, and there is a burn scar of note (Alisal, 
2021) that will need to be monitored should heavier rainfall rates 
materialize. So, while there is currently not a risk area 
highlighted for the coastal ranges of southern California, there 
is a non-zero threat of excessive rainfall, with a highly isolated 
and localized concern for runoff problems later tonight.

Orrison
 
Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt