879
AWUS01 KWNH 260415
FFGMPD
CAZ000-261615-

Mesoscale Precipitation Discussion 1167
NWS Weather Prediction Center College Park MD
1114 PM EST Mon Nov 25 2024

Areas affected...Portions of Central and Southern CA

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

Valid 260415Z - 261615Z

SUMMARY...Moderately strong atmospheric river to continue
overnight and through Tuesday morning across portions of the
central and southern CA coastal ranges, San Joaquin Valley, and
southern Sierra Nevada. Some areal flooding concerns and a low-end
burn scar flash flood threat will exist.

DISCUSSION...The GOES-W Airmass RGB satellite imagery in
conjunction with CIRA-ALPW data shows a rather well-defined and
moderately strong atmospheric river advancing inland across
coastal areas of central and southern CA, the San Joaquin Valley
and into the higher terrain of the southern Sierra Nevada.

So far, the heaviest rainfall rates have been in the foothills of
Fresno and Tulare Counties where some occasional rainfall rates of
0.50"+/hour have already been observed. The moisture transport
over the coastal ranges and into the interior foothills is
relatively strong with IVT magnitudes reaching as high as 500 to
750 kg/m/s, and this is associated with 850/700 mb moisture flux
anomalies of 3 to 4 standard deviations above normal.

There is a rather substantial subtropical moisture connection to
this atmospheric river with PWs that are running anywhere from 2
to 4 standard deviations above normal, and CIRA-ALPW showing the
deeper layer moisture axis extending well offshore of the West
Coast all the way down to near 30N 140W where there is close
proximity of a deeper layer trough. Embedded within this moist
southern stream flow are multiple shortwave impulses interacting
with a frontal zone and there has been at least some weak surface
wave activity traversing this front, including one wave that has
advanced inland to the northeast of Monterey.

Some additional increase in IVT values are forecast overnight, and
especially in the 06Z to 12Z time frame as additional stronger
energy from offshore arrives, and this should promote some
additional localized increase in rainfall rates. The coastal
ranges of Monterey and San Luis Obispo Counties may see rainfall
rates increase into the 0.25" to 0.50"/hour range on occasion.
However, with the spillover of stronger moisture transport into
the foothills of the southern Sierra Nevada, and with favorably
orthogonal orientation of the flow into the terrain, the rainfall
rates here below snow level should occasionally reach into the
0.50" to 0.75"/hour range with possibly a couple instances of
rates higher than this which is supported by the 00Z HREF guidance.

Additional rainfall amounts going through Tuesday morning may
reach 1 to 3 inches for the coastal ranges, and as high as 3 to 6
inches for the foothills of the southern Sierra Nevada. Given the
persistence overnight and into early Tuesday morning of heavier
rainfall rates and the corresponding storm totals, there may be
some areal flooding concerns along with at least a low-end threat
for some burn scar flash flooding.

Orrison

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

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

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

LAT...LON   37841961 37471911 36941866 36351843 35881835
            35581837 35451868 35721934 35511987 35201997
            34761977 34532004 34582066 35352097 35722133
            36332183 36852153 37232057 37722011