MESOSCALE PRECIPITATION DISCUSSION 0061
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
144 PM EDT TUE MAR 13 2018

AREAS AFFECTED...UPSLOPE PORTIONS OF THE SIERRA NEVADA

CONCERNING...HEAVY RAINFALL

VALID 131743Z - 140330Z

SUMMARY...HEAVY RAIN SHOWERS AND THUNDERSTORMS WILL BE
INCREASINGLY LIKELY THIS AFTERNOON IN THE UPSLOPE REGIONS OF THE
SIERRA NEVADA. RAIN RATES MAY REACH 1 IN/HR...WHICH COULD LEAD TO
LOCALIZED FLASH FLOODING.

DISCUSSION...GOES-15 WATER VAPOR LOOPS SHOW A BROADLY DIFFLUENT
UPPER LEVEL FLOW PATTERN OVER THE STATE OF CALIFORNIA IN THE EXIT
REGION OF A CYCLONICALLY CURVED 100+ KNOT JET STREAK...AND
DOWNSTREAM OF A TROUGH AXIS IN THE NORTHEAST PACIFIC. A +PV
ANOMALY ROTATING AROUND THE TROUGH WAS ALSO NOTED BOTH IN RAP
ANALYSIS AND GOES-15 WATER VAPOR AS A LOBE OF WARMER BRIGHTNESS
TEMPERATURES AROUND 37N/125W AT 17 UTC. IN THE REGION OF STRONGEST
DIVERGENCE (IMMEDIATELY DOWNSTREAM FROM THE PV ANOMALY AND EASTERN
PERIPHERY OF THE UPPER LEVEL JET)...A WEDGE OF COLDER BRIGHTNESS
TEMPERATURES WAS OBSERVED ON SATELLITE...AND THIS CAN BE A
REASONABLY PROXY FOR THE LOCATION OF STRONGEST QG FORCING AT THE
CURRENT TIME.

A BROAD PLUME OF MOISTURE EXTENDED INTO CALIFORNIA...WITH
PRECIPITABLE WATER VALUES AS HIGH AS 1 INCH INTO THE SAN JOAQUIN
VALLEY AND A POSSIBLE SUBTROPICAL CONNECTION PER NESDIS SAB
ANALYSIS AND MIMIC-TPW LOOPS. THEREFORE...AVAILABLE MOISTURE IS
CERTAINLY SUFFICIENT FOR HEAVY RAINFALL RATES...AND THIS WILL BE
ENHANCED BY AVAILABLE INSTABILITY. OBJECTIVE RAP ANALYSIS AT 17Z
SHOWED AROUND 500 J/KG OF MUCAPE IN THE CENTRAL VALLEY...CENTERED
AROUND STOCKTON. VALUES THIS HIGH SUGGEST THAT RAIN RATES ON THE
ORDER OF THE PRECIPITABLE WATER VALUES (1 INCH) MAY BE ACHIEVABLE
AS AN HOURLY RAIN RATE. THE 12Z HREF AGREES WITH THIS NOTION...AS
ITS NEIGHBORHOOD PROBABILITY OF 1 IN/HR RAIN RATES EXCEEDS 30
PERCENT FROM 21-00Z IN THE CORRIDOR FROM KPVF (PLACERVILLE) TO
JUST SOUTH OF THE YOSEMITE VALLEY.

LIGHTNING HAS ALREADY BEEN OBSERVED AS OF 1730Z...A GOOD
INDICATION THAT CONVECTIVE RAIN BANDS AND THUNDERSTORMS WILL BE
LIKELY INTO THE AFTERNOON HOURS AS INSTABILITY IS LIKELY TO BE
MAINTAINED OR EVEN INCREASE SLIGHTLY (PER RAP AND HRRR 0-12HR
FORECASTS). IN FACT...A 15Z HRRR SOUNDING NEAR THE YOSEMITE VALLEY
VALID AT 23Z HAS AROUND 800 J/KG OF CAPE ROOTED VERY CLOSE TO THE
SURFACE....WITH STRONG LOW-MID LEVEL FLOW. THE COMBINATION OF
REASONABLY STRONG INSTABILITY FOR THIS REGION AND STRONG UPSLOPE
FLOW SHOULD LEAD TO HEAVY RAIN RATES IN UPSLOPE REGIONS OF THE
SIERRA NEVADAS. 1 IN/HR RAIN RATES HAVE AROUND A 10 PERCENT ANNUAL
EXCEEDANCE PROBABILITY...AND ARE CLOSE TO FLASH FLOOD GUIDANCE IN
THE REGION. THEREFORE...LOCALIZED FLASH FLOODING WILL ALSO BE
POSSIBLE. HOWEVER...THE TRANSIENT NATURE OF THE HEAVIEST RAIN
RATES SHOULD KEEP THE THREAT SOMEWHAT ISOLATED.

LAMERS

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

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

LAT...LON   40732222 40672176 40092150 39682098 39182057
            38482013 37791965 37361913 36811878 36451904
            36841972 37452047 38162107 39102155 39782205
            40332243