FOUS30 KWBC 220747
QPFERD

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 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

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