AWUS01 KWNH 210601
FFGMPD
CAZ000-211800-

Mesoscale Precipitation Discussion 1162
NWS Weather Prediction Center College Park MD
100 AM EST Thu Nov 21 2024

Areas affected...Northern California

Concerning...Heavy rainfall...Flash flooding possible

Valid 210600Z - 211800Z

SUMMARY...Nearly stationary atmospheric river will maintain heavy
rainfall across northern CA through Thursday morning with
increasing concerns for widespread areal flooding and possibly
some burn scar flash flooding.

DISCUSSION...The latest GOES-W Airmass RGB satellite imagery in
conjunction with CIRA-ALPW data shows a well-defined atmospheric
river and an associated front currently stalled over northern CA
and the adjacent offshore waters extending southwestward off the
West Coast. Strong warm air advection and moisture transport
focused in close proximity to this front continues to overrun
northern CA with rather widespread moderate to heavy rain
impacting the region.

Strong low to mid-level southwest flow around the south side of a
powerful deep layer cyclone west of Vancouver Island will be
locked in place overnight and through Thursday morning which will
maintain enhanced atmospheric river activity over northern CA.
Multiple weak waves of low pressure are expected to ride
northeastward along the front over the next 6 to 12 hours which
will take aim on the northern CA coastal ranges, and this will be
facilitated by some backing of the mid to upper-level flow as
additional shortwave/jet energy digs through the base of the
larger scale trough offshore of the West Coast.

Enhanced 850/700 mb moisture flux coupled with the deeper layer
warm air advection pattern and orographic ascent/upslope flow over
the higher terrain of northern CA will focus a persistence of
moderate to heavy rain, with IVT magnitudes north of the Bay Area
reaching 500 to 750+ kg/m/s. This will support elevated rainfall
rates which based off the 00Z HREF guidance should occasionally
reach well into the 0.50" to 0.75"/hour range. Some isolated rates
potentially exceeding 0.75"/hour will be possible and especially
over southern parts of Humboldt County where the southwest-facing
slopes will see stronger orographic ascent coupled with very close
proximity of the aforementioned front.

The persistence of these elevated rainfall rates over the next 6
to 12 hours will likely result in additional rainfall amounts by
18Z (10AM PST) Thursday morning of 3 to 5 inches over the coastal
ranges and the foothills of the northern Sierra Nevada, with some
isolated 6+ inch amounts possible. These rains will be in addition
to the already several inches of rain that have fallen for the
event, and thus isolated some storm total amounts by late Thursday
morning may well be 10+ inches. Lesser amounts of as much as 1 to
3 inches will generally be expected for the interior valleys
including the northern Sacramento Valley.

Expect increasing concerns for widespread areal flooding impacts
from the additional rainfall going through Thursday morning, and
this may include some concerns for burn scar flash flooding at
least locally. Debris flow and landslide activity will be a
notable concern as well from these heavy rains.

Orrison

...Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!=
DZ3fjg!9BjNe-FW4HxpKd13oWUWc_JNgylrROGd6XTEv4pWKbm3bvcktYTNiMqJ079UYAML0jex=
TIjq5wZE6gguTaoK9-EWBdE$  for graphic product...

ATTN...WFO...EKA...MFR...MTR...STO...

ATTN...RFC...CNRFC...NWRFC...NWC...

LAT...LON   41972347 41602315 41142289 41142210 40672151=20
            40082122 39472046 39052031 38842061 38542159=20
            38122193 37882259 38322329 39262410 40662456=20
            41842417=20

=3D =3D =3D
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