FOUS30 KWBC 210737
QPFERD

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 2
Valid 12Z Sat Nov 23 2024 - 12Z Sun Nov 24 2024

...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF NORTHERN CALIFORNIA...

The long duration atmospheric river begins to become more
progressive and wane in intensity Friday as a cold front finally
pushes the moisture axis southward. However still expecting rather
widespread 0.25" per hour rates with this last push of rainfall,
with localized 0.5" per hour amounts likely. These higher rates
should only last for a couple hours during the morning over coastal
areas, but should persist a bit longer in the Sierra Nevada.
Overall expecting 1-3" of additional rain over coastal areas, with
3-6" over the northern Sierra Nevada. Warmer temperatures will keep
snow levels pretty high such that even into the terrain areas,
most of the precipitation is expected to fall as rain (aside from
the highest peaks).

By Friday conditions over much of the area will be saturated, and
this last push of elevated rainfall rates on top of saturated ground
will likely result in a continued flood threat...with additional
landslides likely and a continued risk of burn scar flash flooding.
Only minor changes were made to the inherited ERO areas. A MDT risk
remains for the northern Sierra Nevada where the heavier rainfall
will persist longer into Friday resulting in the aforementioned
higher additional rainfall totals. A flooding risk is likely to be
ongoing over coastal areas as well Friday morning, but should be a
shorter lived threat with the higher rainfall rates coming to an end quicker...thus will continue to carry a Slight risk here.

Chenard


Day 3
Valid 12Z Sat Nov 23 2024 - 12Z Sun Nov 24 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SIERRA NEVADA...

At 12z Saturday the cold front and associated moisture plume will be
dropping south across the central Sierra Nevada. Rainfall intensity
will be dropping fairly quickly by this time and the prolonged
atmospheric river event will finally be coming to an end. We will
continue to carry a small Marginal risk over portions of the Sierra
Nevada where an additional 1-2" of rain is possible Saturday
morning. Typically rainfall of this magnitude would not be a
concern. However, much of this risk area will have seen at least 3-
7" of rain total from the event, so even the modest additional
rainfall Saturday morning could result in a flood threat, and thus
continuing with a Marginal risk for now seems best. Although the
risk is certainly lower than previous days, and quite possible this
risk could be removed on future updates if additional rain after 12z
Saturday trends down.

Chenard
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