Clifton, Virginia Weather

398
FOUS30 KWBC 210045
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
745 PM EST Wed Nov 20 2024

Day 1
Valid 01Z Thu Nov 21 2024 - 12Z Thu Nov 21 2024

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

01Z Update:
Latest MRMS QPE and observed rainfall reports are starting to
indicate more .50+ in/hr rates along the NorCal coastal ranges,
especially northwest of Santa Rosa. Expect these areas of heavier
rainfall rates to expand northward overnight, including the King
Range south of Eureka, especially after 06Z per the latest (18Z)
HREF probabilities. Given the latest trends, there is no need to
change the configuration of the current Moderate, Slight, and
Marginal Risk areas.

Hurley

----Prior Discussion----
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...

Life-threatening flooding across coastal Northwest California is
expected due to the very strong and long duration atmospheric
river Thursday into Thursday night. Dangerous flooding, rock
slides, and debris flows are likely.

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 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 max
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 Wednesday.
Hourly rates of 0.50"+ are likely at times supporting additional
24 hour rainfall amounts of 3-6" and isolated additional 24 hour
totals of 6-8" across northwest CA into southwest OR. Storm total
amounts of 12-16"+ possible across portions of northwest
California. Given the 2 day 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.

Taylor

Day 3
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 atmospheric river does begin to wane throughout
this period but at least for the first 6-12 hours, heavier
precipitation with the final push of higher moisture is expected
across coastal Northern California into southwest Oregon as well as
portions of 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). This factor along with the rainfall totals
expected over the course of the next 3 days will heighten the
flooding risk, particularly for sensitive areas including burn
scars. The Moderate Risk remains generally unchanged given the
consistent model guidance and lack of significant changes in the
QPF. In the northern Sierra Nevada, max amounts of 2-4" are
expected - the latest PQPF shows high chances for at least 2"
(above 80%) and moderate values for 4" (30-40%) in the terrain. For
the lower elevation areas and the coastal ranges, additional
amounts of 1-2" are expected.

Taylor

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