FOUS30 KWBC 200809
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
309 AM EST Wed Nov 20 2024

Day 1
Valid 12Z Wed Nov 20 2024 - 12Z Thu Nov 21 2024

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

The latest model guidance remains consistent with the overall
evolution of the strong multi day atmospheric river event into
northwest California. The low that deepened rapidly over the
northeast Pacific Tuesday, will continue to anchor deep layered  
west southwest flow on the south side of this system, impacting  
northwest California day 1 with IVT values of 500-800 km/m-1 s-2. 
and 850-700 mb moisture flux anomalies of 2-4+ standard deviations 
above the mean. There is fairly good model agreement with heavy 
precip totals of 3-5" primarily over northwest California and the
northern Sierra, with isolated totals of 6-8" possible. HREF
neighborhood probabilities are very high for 5"+ amounts across
northern California, with most areas showing 80-90%+, while 8"+
probabilities are 40-80% along the northwest CA coast and 40-60%+
over the northern Sierra. Hourly rainfall amounts of .25-.50"+ 
likely through the day 1 time period across northwest CA with 
increasing flooding risks with time as soils become saturated and 
streams and river levels rise. The HREF 1 hour probabilities for 
.50"+ totals remain high in the max inflow band across northwest 
CA for all of day 1. This band does continue to sink south for the
first few hours of day 1, but then become nearly stationary for the
remainder of the day 1 time period. No significant change to the
previous outlook. The slight risk was extended approximately 75
miles farther south in the lee of the northern Sierra where 2-4"
rainfall totals are possible.

Oravec


Day 2
Valid 12Z Thu Nov 21 2024 - 12Z Fri Nov 22 2024

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

The strong Atmospheric river event into northwest California will
continue into day 2 as deep layered southwesterly flow continues 
to the south of the northeast Pacific upper vortex. The anomalous 
deep layered southwesterly flow will continue to 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 km/m-1 s-2 range across 
northwest CA. During the second half of day 2, a strong vort 
rotating around the base of the northeast Pacific vortex will 
support another rapidly deepening low moving northeast 
approximately 200 miles off the Pacific Northwest coast. While this
second rapid cyclogenesis event will not be as strong as the 
Tuesday event, it will help to begin to push the best onshore flow 
axis back to the north during the second half of day 2, returning 
heavy precip into southwest Oregon. There is a likelihood of very 
heavy rains again falling day 2 over similar regions from the day 1
period. Hourly rates of .25-.50"+ likely to continue at times 
supporting additional 24 hour rainfall amounts of 3 to 6 inches and
isolated additional 24 hour totals of 6-8" across northwest CA 
into southwest OR. Storm total amounts of 12 to 16+ inches possible
across portions of northwest California. Given the 2 days of very 
heavy totals in the same region, no significant changes were made 
to the previous high risk area. Significant flood risks will 
continue to increase day 2 along with rock and land slides as soils
become saturated and streams and rivers continue to rise and 
overflow. Ahead of the next deepening low, rising heights over 
northern California will keep much of the precipitation in the form
of rain. This will increase the runoff threats for areas in the 
lee of the northern Sierra. Across this region, the risk area was 
increased from slight to moderate from the previous issuance.


Day 3
Valid 12Z Fri Nov 22 2024 - 12Z Sat Nov 23 2024

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

The multi day atmospheric rive event will begin to weaken on day 3
as the onshore southwesterly flow weakens and the best onshore flow
axis shifts southward. The heaviest totals day 3 likely into the
upslope of the northern Sierra where high heights will again favor
a majority of the precip falling as rain. A moderate risk was added
for this region where additional totals day 3 of 2-4 inches
possible across areas that received 5-8+ inches days 1 and 2. Along
the northwest California coast, additional heavy preip totals 
likely day 3, but amounts will be much less than the day 1 and day
2 amounts, generally 1 to 1.50"+, with localized 2"+ totals. A 
slight risk was maintained here given the effects from the day 1 
and 2 amounts, which will support additional runoff issues for 
lesser than normal amounts to produce runoff concerns.

Oravec
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