Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
237 AM EST Thu Nov 21 2024
Day 1
Valid 12Z Thu Nov 21 2024 - 12Z Fri Nov 22 2024

...THERE IS A HIGH RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF
NORTHWEST CALIFORNIA...

...Northern California and Southwest Oregon...

Life-threatening flooding across coastal areas of northwest
California is expected due to the very strong and long duration
atmospheric river currently impacting the region which will
continue through Thursday night. Dangerous flooding and debris
flows are likely which will include rock and landslide activity
along with a threat for burn scar flash flooding.

The strong atmospheric river event into northern California will
continue as deep layered southwesterly flow continues to the south
of the northeast Pacific upper vortex. The anomalous deep layered
southwesterly flow will support 850 to 700 mb moisture flux
anomalies of 2 to 4+ standard deviations above the mean and IVT
values in the 500 to 800 kg/m/s range across northwest CA. This
will also be focused in close proximity to a quasi-stationary front
draped in a southwest to northeast fashion across the region.
However, the latest model guidance continues to advertise a strong
vort max/jet streak rotating around the base of the northeast
Pacific vortex by Thursday afternoon and this will support another
rapidly deepening surface low moving northeastward along the front
and to a position about 200 to 250 miles offshore of western Oregon
by very early Friday morning. While this second rapid cyclogenesis
event will not be as strong as its predecessor, it will help to
begin to push the axis of strongest onshore flow/moisture transport
back to the north. This will allow for heavy rains already
impacting northern California to edge back into southwest Oregon.
Rainfall rates are expected to reach 0.50" to 0.75"/hour at times
which is well supported by the 00Z HREF guidance. The persistence
of these rates are expected to support additional 24-hour rainfall
amounts of 3-6" and isolated additional 24 hour totals of 6-10"
across northwest California, and this will bring storm total
amounts to as much as 12-16"+ by early Friday morning. Somewhat
lower rainfall totals are expected for southwest Oregon, but an
additional 3-5" of rain is forecast here by early Friday morning.
Given the expected 2-day totals in the same region, the High Risk
area is maintained with very little change. Significant flood risks
will evolve today through Thursday night and early Friday morning
as soils become saturated and streams and rivers continue to rise
and overflow. Impacts will likely include debris flows along with
rock and landslide activity. Additionally, there may be some burn
scar flash flooding concerns as well given the relatively high
rainfall rates over these very sensitive areas. The Park Fire burn
area involving portions of Tehama and Butte Counties is one such
location that may see impacts.

Orrison

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