206
AWUS01 KWNH 160353
FFGMPD
CAZ000-160952-

Mesoscale Precipitation Discussion 1018
NWS Weather Prediction Center College Park MD
1152 PM EDT Sun Sep 15 2024

Areas affected...Portions of the Northern Sacramento Valley and
Foothills of the Northern Sierra Nevada

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

Valid 160352Z - 160952Z

SUMMARY...Slow-moving showers and thunderstorms with heavy
rainfall rates may cause some instances of flash flooding over the
next several hours, and especially with respect to the Park Fire
burn area.

DISCUSSION...The latest GOES-W WV satellite imagery shows a
deepening mid-level trough and associated closed low pivoting
south-southeastward across coastal areas of northern and central
CA. This energy has facilitated the development of an area of low
pressure in the northern Sacramento Valley which is oriented along
a frontal zone. A corridor of locally enhanced moisture
convergence and modest instability has pooled up across the region
and into the foothills of the northern Sierra Nevada.

This has led to the development and localized expansion of showers
and thunderstorms across Tehama County and this activity is
gradually settling down to the southeast as the stronger corridor
of height falls and deeper layer forcing pivots over the region.
MUCAPE values are only on the order of 250 to 500 J/kg, but the
latest MRMS data shows rainfall rates of 1.0 to 1.5 inches/hour
with the stronger storms.

Given the amplification of the mid-level closed low circulation
over northern CA that is expected overnight, an increase in deeper
layer ascent is expected, and this coupled with steepening
mid-level lapse rates and localized orographics should tend to
favor convection persisting in the near-term across portions of
the northern Sacramento Valley while also edging into the
foothills of the northern Sierra Nevada.

The 00Z HREF guidance suggests some spotty 2 to 3+ inch rainfall
totals may be possible over the next several hours, and this will
be supported by the aforementioned rainfall rates and slow
cell-motions. These rains may result in some localized urban
runoff/flooding concerns along the I-5 corridor. However, if these
rains can make it a bit farther east into at least the eastern
portions of Tehama County and the northern portions of Butte
County, there will be concerns for impacts involving the large
Park Fire burn area. Satellite and radar trends will need to be
very closely monitored overnight for the potential of heavy
rainfall impacting this burn scar, and especially since rainfall
rates are already quite heavy just to the west near I-5 involving
areas near and adjacent to Red Bluff vicinity. Instances of flash
flooding and debris flow activity cannot be ruled out.

Orrison

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

ATTN...WFO...STO...

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

LAT...LON   40972168 40742138 40172131 39642162 39472220
            39862261 40672238