Mesoscale Precipitation Discussion 0977
NWS Weather Prediction Center College Park MD
830 PM EST Sat Dec 07 2019

Areas affected...Portions of North Central CA

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

Valid 080130Z - 080400Z

SUMMARY...Showers and thunderstorms ahead of a cold front could
produce hourly rainfall rates of 0.50 inches or greater. Rainfall
rates this high could pose a flash flood threat through the
evening hours.

DISCUSSION...Regional radars across northern and central CA showed
showers and low topped thunderstorms maintaining their intensity
in an axis of 250/500 J/KG of MUCAPE extending from south of San
Francisco into the foothills of the Sierra Nevada Mountains
southwest of Lake Tahoe. The KDSAX radar showed hourly rainfall
rates nearing an inch over northwest Amador and western El Dorado
counties, which is supported by the most recent MRMS information.

As the next short wave in the flow ahead of the closed low off the
northern CA coast moves eastward, synoptic scale ascent with the
short wave could allow for the maintenance of the line of low
topped convection in place through about 08/03z or so. The low
level 30/40 knot southwest flow (per regional VWPs) is expected to
focus the 0.75 inch precipitable air in the instability plume
ahead of the front. This should support the threat for 0.50 inch
or greater hourly rainfall rates through the mid evening hours.

The most recent HRRR indicated the potential for local 2.00 inch
rainfall amounts in the foothills of the Sierra Nevada Mountains,
especially where short term training occurs. Given what has
occurred upstream, these amounts are plausible. There are many
recent burn scars in these locations, and rainfall rates over 0.50
inches could result in local landslides and debris flows, as well
as isolated flash flooding.

Instability is expected to slow wane during the evening hours, and
could end up being the limiting factor for additional flooding.
However, it is possible the threat for flash flooding to linger
after 08/04z.

Hayes

ATTN...WFO...MTR...REV...STO...

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

LAT...LON   39132031 38722006 38192030 37862063 36932157
            36822203 37062220 37202219 37232217 38142164
            38232157 38592131 38932093 39082066