052
AWUS01 KWNH 230417
FFGMPD
CAZ000-231500-

Mesoscale Precipitation Discussion 1166
NWS Weather Prediction Center College Park MD
1115 PM EST Fri Nov 22 2024

Areas affected...Portions of Central and Northern CA

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

Valid 230415Z - 231500Z

SUMMARY...Strong multi-day atmospheric river event gradually
beginning to wane across central and northern CA, but flooding
concerns will continue into Saturday morning.

DISCUSSION...The long duration, multi-day atmospheric river
impacting central and northern CA is generally past peak at this
point and will be weakening in intensity going through Saturday
morning. However, there will continue to be sufficient levels of
additional rainfall for additional flooding concerns overnight.

GOES-W Airmass RGB satellite imagery along with CIRA-ALPW data
shows the atmospheric river axis along with an associated frontal
boundary aligned in a southwest to northeast fashion across the
Bay Area and extending inland across the Central Valley and the
adjacent windward slopes of the central and northern Sierra
Nevada. A wave of low pressure is noted riding northeast along the
front, and this coupled with stronger upper-level jet
dynamics/forcing advancing inland is still maintaining a corridor
of stronger IVT magnitudes and thus some enhanced rainfall rates.

IVT magnitudes are currently on the order of 700 to 800 kg/m/s
just south of the Bay Area and are in between 500 and 700 kg/m/s
across the Central Valley. Rainfall rates associated with this are
still reaching upwards of 0.50" to 0.75"/hour on occasion across
the coastal ranges and into the some of the orographically favored
upslope areas of the Sierra Nevada. As the cold front gradually
advances east and inland across the region, the high 850/700 mb
moisture flux anomalies and related IVT magnitudes will begin to
weaken, and this will eventually allow the rainfall rates to
subside.

The 00Z HREF guidance maintains high probabilities though of
seeing 0.50"+/hour rainfall rates going through about 09Z (1AM
PST) down across Santa Cruz, Santa Clara and Monterey Counties
before subsiding. Farther inland, with the enhanced IVT magnitudes
and upslope flow just ahead of the front still impacting the
foothills of the Sierra Nevada, these 0.50"+/hour rainfall rates
should persist through 12Z (4AM PST) across these areas and
including Butte and Yuba Counties southward down through Tuoloumne
and Mariposa Counties.

Locally an additional 2 to 4 inches of rain will be possible with
locally heavier amounts, and these additional rains are expected
to maintain areal concerns for flooding going into early Saturday
morning before the rainfall rates taper down. Some additional
debris flows, rock and landslide activity, and perhaps an isolated
concern for burn scar flash flooding will be possible overnight.
Conditions should improve substantially Saturday morning as the
front passes through the region and the atmospheric river impacts
gradually come to an end.

Orrison

...Please see www.wpc.ncep.noaa.gov for graphic product...

ATTN...WFO...HNX...LOX...MTR...REV...STO...

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

LAT...LON   40912135 40612096 40162058 39262043 38181971
            37651919 37031889 36661921 36302017 35812051
            35502107 35862167 36482218 37332260 38212285
            39342226 40452191 40832163