Mesoscale Precipitation Discussion 1064
NWS Weather Prediction Center College Park MD
928 AM EST Mon Dec 24 2018

Areas affected...Southwest OR...Northern CA

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

Valid 241430Z - 242030Z

Summary...Increasing instability ahead of a short wave and cold
front will feed low topped convection that could produce hourly
rainfall rates at or above 0.50 inches. These rates will foster a
flash flood threat, especially in areas with recent burn scars,
into the early afternoon hours.

Discussion...The GOES 15 IR loop showed a strong short wave near
42N 130 W, with an attending cold front well west of the northern
CA coast. Ahead of the front, the most recent blended TPW product
indicated precipitable water values near 1.10 inches (which is
about two standard deviations above the mean, supported by the
24/12z OAK sounding). In addition, the most recent RAP analysis
showed MUCAPE values of 500 J/KG  ahead of the short wave, and the
presence of instability is supported by the cellular appearance of
the convective looking clouds on the IR loop.

The combination of moisture and instability should be sufficient
to support embedded convection ahead of the front and short wave,
mainly after 24/17z. During this time, the most recent RAP/HRRR
showed the potential for MUCAPE values to peak near 500 J/KG.
There is a multi high resolution model signal (supported by the
most recent HRRR/experimental HRRR and 00z WRF ARW) for hourly
rainfall rates nearing 0.50 inches, mainly across northern CA,
during the 24/18z to 24/21z time frame, when the best instability
comes ashore.

Based on the depth of moisture in place, these rates appear
attainable, and there is some model support for hourly rainfall
rates near 0.75 inches as the front comes ashore. Total rainfall
amounts could approach 2.00 inches over the higher terrain of the
southern Klamath Mountains and northern CA Coastal Range before
245/21z.

Hourly rainfall rates of 0.50 inches could be enough to pose a
flash flood threat over portions of far Southwest OR and northern
CA, especially over recent burn scars. In these areas, isolated
mudslides and debris flows could occur, especially with higher
hourly rainfall rates.

Hayes

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

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

LAT...LON   42952354 42092326 39822301 39282302 39262374
            39852421 40462439 42172443 42872425