764
FOUS30 KWBC 221546
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1046 AM EST Fri Nov 22 2024

Day 1
Valid 16Z Fri Nov 22 2024 - 12Z Sat Nov 23 2024

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

...16Z update...

Areas of heavy rain continue to affect north-central California
this morning with localized rainfall rates observed in the 0.5 to
0.7 in/hr range over the past 6 hours. 72 hour rainfall totals
across the Coastal Ranges into the northern Sierra Nevada have
resulted in double digit rainfall totals with at least one
localized 20+ inch report in Sonoma County.

Through today and tonight, the plume of IVT will continue to
gradually sink south with possible brief stalling of the moisture
axis in the vicinity of the San Francisco metro later this
evening. Adjustments to the 16Z ERO were very minor and while
there were small changes noted with the 12Z guidance, notably a
little faster to advance precipitation toward the south, forecast
reasoning below remains valid. Additional rainfall totals of 5-10
inches still look likely into the Sierra Nevada through 12Z
Saturday and localized 3-6 inch totals for the Coastal Ranges in
the vicinity of San Francisco Bay. See upcoming MPD #1165 for
additional short term details.

Otto

...previous discussion follows...


The long duration, multi-day atmospheric river event that has been
impacting northern California will finally begin to become more
progressive and start to wane in intensity as a cold front just
offshore of the West Coast advances inland and drives the axis of
deeper layer Pacific moisture transport south and eastward.

However, there will still be one more day of locally very heavy
rainfall totals which are expected to be excessive and capable of
driving locally significant flooding impacts. The 00Z HREF guidance
supports some additional 5 to 10 inch rainfall totals for the
24-hour period ending 12Z/Saturday, with the heaviest totals
expected to be over the windward slopes of the northern Sierra
Nevada, and perhaps locally a few of the coastal ranges just north
of the Bay Area where there has already been very heavy rainfall
totals over the last couple of days.

IVT magnitudes along and just south of the cold front will be rather
high at the start of the period this morning, with magnitudes over
coastal areas of northwest California upwards of 750+ kg/m/s, and
these values should decrease slowly and settle southward toward
00Z. However, as the front gets into the Bay Area and the Central
Valley by early this evening, there is some suggestion in the
guidance that the front will slow down again just a bit. Coinciding
with that will be arrival of a shortwave and associated upper-
level jet streak (130+ kts) rounding the base of the deep offshore
trough. This should foster a temporary resurgence/strengthening of
the IVT values in the 00Z to 06Z time frame across the Bay Area and
extending inland across portions of the Central Valley and
foothills of the Sierra Nevada. Thus expectations are that heavy
rains will overspread all of these areas along with some localized
increase in rainfall rates this evening. In general, the rainfall
rates will be capable of reaching 0.50" to 0.75"/hour across the
coastal ranges around the Bay Area and also into the upslope areas
of the northern Sierra Nevada. Given the additional rainfall
amounts, and considering the antecedent conditions at this point,
the Moderate Risk area is maintained across the northern Sierra
Nevada, with a separate Moderate Risk introduced just north of the
Bay Area involving Sonoma and Napa Counties. Other adjustments
include extending the Marginal and Slight Risk areas a bit farther
south to include the coastal ranges a bit south of the Bay Area and
also more areas of the Central Valley. Flooding impacts will again
be rather widespread and locally significant, with additional
concerns for debris flows, rock and landslide activity, and flash
flooding involving burn scar locations. In fact, the Park Fire burn
area involving parts of Tehama and Butte Counties will need to be
very closely monitored today for enhanced impacts given the amount
of rainfall that is forecast here.

Orrison

Day 2
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. By this time
rainfall intensity will be decreasing, and the axis of better
moisture will be dropping south of the areas hardest hit over the
past few days. Thus the flood risk should be pretty low Saturday
morning along this southward dropping front.

Some weak post frontal instability is forecast which could allow for
locally heavy showers in the wake of the front over the western
slopes of the central Sierra Nevada. Not expecting anything too
intense or widespread, however there is at least a chance that a few
cells could drop a quick 0.5" or so of rain. This would be falling
over increasingly saturated ground, and so a localized flood risk is
possible. Thus we will maintain the small Marginal risk where a few
heavier post frontal showers are possible Saturday.

Chenard

Day 3
Valid 12Z Sun Nov 24 2024 - 12Z Mon Nov 25 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
NORTHWEST CALIFORNIA INTO FAR SOUTHWEST OREGON...

Locally heavy rain will be possible Sunday into Sunday night over
northwest CA into far southwest OR. A front slowing near the coast
will provide plentiful low level moisture convergence, and mid level
shortwave energy should provide some additional lift as well.
Moisture is a limiting factor for heavy rainfall with the better
moisture plume situated well south and not really getting drawn into
this system. Thus this is not expected to be a significant
atmospheric river, and normally would not expect a system of this
nature to pose a flood risk. However given the significant rainfall
over the past few days, this additional rainfall may be enough to
cause some localized additional flooding as the area will be more
hydrologically sensitive than normal. Still some model differences
in the exact evolution of this system and thus rainfall amounts this
period are uncertain...but current indications suggest localized
amounts of 2-3" will be possible.

Chenard

Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt