Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
247 AM EST Fri Nov 22 2024
Day 1
Valid 12Z Fri Nov 22 2024 - 12Z Sat Nov 23 2024

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

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