MESOSCALE PRECIPITATION DISCUSSION 0071
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
450 PM EDT THU MAR 22 2018

AREAS AFFECTED...CENTRAL VALLEY OF CALIFORNIA...UPSLOPE OF SIERRAS

CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE

VALID 222049Z - 230249Z

SUMMARY...STRONG CONVECTIVE BANDS AND THUNDERSTORMS WILL BE LIKELY
TO PRODUCE HEAVY RAINFALL THIS AFTERNOON AND EVENING ACROSS THE
CENTRAL VALLEY OF CALIFORNIA...AND INTO THE UPSLOPE REGIONS OF THE
SIERRA NEVADA BELOW THE SNOW LEVEL. RAIN RATES NEAR OR OVER 1
IN/HR MAY CAUSE FLASH FLOODING AND DEBRIS FLOWS.

DISCUSSION...AN UNSTABLE AIR MASS OVER THE CENTRAL VALLEY OF
CALIFORNIA IS CREATING A FAVORABLE ENVIRONMENT FOR CONVECTIVE
DEVELOPMENT THIS AFTERNOON...AND SEVERAL BANDS OF STRONG
THUNDERSTORMS HAVE ALREADY DEVELOPED. AS OF 2030Z...ONE WAS
SITUATED FROM TUOLUMNE COUNTY TO NEAR MERCED ALONG A LOW-LEVEL
CONVERGENCE AXIS ENHANCED BY SOUTHEASTERLY FLOW UP THE VALLEY. THE
OTHER WAS SITUATED ALONG A PRE-FRONTAL CONVERGENCE AXIS FURTHER
NORTHWEST FROM NEAR PLACERVILLE TO JUST EAST OF STOCKTON. THESE
CONVECTIVE BANDS WERE MOVING SLOWLY WHICH IS LIKELY ENHANCING
RAINFALL. THE BAND FURTHER TO THE NORTH WAS LIKELY BEING
INFLUENCED MORE BY THE APPROACHING FRONT...AND THUS SHOWED MORE OF
AN EASTWARD DRIFT WITH TIME. HOWEVER...THE SOUTHERN ONE WAS NEARLY
STALLED...WITH NEW CELL GROWTH OCCURRING ON THE SOUTHWEST FLANK
AND CONTINUOUSLY MERGING INTO THE PRE-EXISTING BAND. GIVEN THE
SLOW MOTION...EITHER OF THESE COULD PRODUCE FLOODING...BUT THE
GREATEST CONCERN IS WITH THE SOUTHERN BAND AT THIS TIME GIVEN AN
OVERALL LACK OF MOTION AND FAVORABLE CELL TRAINING.

THE ENVIRONMENT WILL CONTINUE TO BE SUPPORTIVE OF ORGANIZED
THUNDERSTORMS. MODIFYING A 19Z RAP SOUNDING FOR MERCED WITH AN
AVERAGE OF SURFACE OBSERVATIONS FROM MODESTO TO MADERA YIELDS
AROUND 500 J/KG OF MLCAPE. THESE MODERATE INSTABILITY LEVELS
SHOULD BE MORE THAN SUFFICIENT FOR ORGANIZED STORMS IN THE
PRESENCE OF AROUND 40-50 KNOTS OF DEEP LAYER BULK SHEAR...AND
SHOULD ALSO BE SUFFICIENT TO REALIZE AN HOURLY RAIN RATE CLOSE TO
THE PRECIPITABLE WATER VALUES (CURRENTLY AROUND 1.1 INCHES).

INDEED...RADAR DUAL POL ESTIMATES FROM KHNX AND KDAX HAVE SHOWN
HOURLY RAIN RATES EXCEEDING 1 IN/HR AND EVEN APPROACHING 2 IN/HR
IN ISOLATED AREAS. THE MRMS ESTIMATES LARGELY AGREE WITH THIS.
SEVERAL PERSONAL WEATHER STATIONS IN THE HEAVIEST RAIN BAND NEAR
GROVELAND HAVE OBSERVED RAIN RATES AROUND 1.5 IN/HR...SUPPORTING
THE RADAR ESTIMATES QUITE WELL. RAIN RATES THIS
HIGH...PARTICULARLY IN THE PRESENCE OF TERRAIN AND FLASHIER
DRAINAGE BASINS...WOULD BE CAPABLE OF PRODUCING FLASH FLOODING AND
DEBRIS FLOWS. THESE TYPE OF RAIN RATES COULD CERTAINLY CONTINUE
OVER THE NEXT SEVERAL HOURS...AND INDEED THE 19Z HRRR EXPLICITLY
FORECASTS RAIN RATES OVER 1 IN/HR IN THIS REGION. ALTHOUGH HEAVY
RAIN AND FLASH FLOODING WOULD BE POSSIBLE THROUGHOUT THE OUTLINED
DISCUSSION AREA...THE GREATEST LIKELIHOOD IS EXPECTED BETWEEN
FRESNO AND SONORA...IN PORTIONS OF
TUOLUMNE...MARIPOSA...MERCED...AND MADERA COUNTIES.

LAMERS

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

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

LAT...LON   39772125 39142064 38132001 37411945 36851888
            36221859 35761864 35721908 36111949 36551992
            37032046 37462086 38032134 38772152 39492157