Mesoscale Precipitation Discussion 1065
NWS Weather Prediction Center College Park MD
310 PM EST Mon Dec 24 2018

Areas affected...Northern California

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

Valid 242030Z - 250230Z

Summary...Developing convection with hourly rainfall rates near
0.50 inches could pose a flash flood threat through the afternoon
hours, especially over recent burn scars.

Discussion...Instability is increasing ahead of a short wave
located near 42N 127W, as evidenced by the cellular look to the
clouds off the northern CA coast in the GOES-15 IR loop. The most
recent RAP analysis showed MLCAPE values near 500 J/KG nearly
coincident with the cellular cloud structure, just ahead of a cold
front approaching ahead of the short wave. Thus far, the KBHX
radar has showed low topped convection with hourly rainfall rates
under 0.25 inches.

The short wave and cold front are expected to cross the northern
CA coast north of KSFO around 25/00z. Steepening lapse rates
associated with the short wave should produce MUCAPE values of
500-1000 J/KG (and HRRR soundings along the northern CA Coastal
Range showed some of the convection becomes surface based just
before 25/00z). As the convection develops due to dynamic lift
ahead of the short wave, the moisture in the column starts to
decrease, as precipitable water values drop below 0.75 inches
toward 25/00z.

There is still a multi high resolution model signal for hourly
rainfall rates near 0.50 inches, especially over the northern CA
Coastal Range (as best instability ahead of the front is expected
here). The 12z HREF probabilities of hourly rainfall rates peak
over this area before 24/23z, then drop off toward 25/02z, as the
instability wanes.

The greatest threat for flash flooding through 25/02z is expected
over recent burn scars across northern CA. In these areas, the
convectively enhanced rainfall rates could produce isolated
mudslides and debris flows. Elsewhere, the flash flood threat is
more limited, as the short wave and cold front remain progressive.

Hayes

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

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

LAT...LON   41932354 41192341 40962328 40922297 41042232
            40112148 39962125 39842106 39692095 39402104
            39522143 39702179 39752219 38892249 38272267
            38542336 40152429 41862410