MESOSCALE PRECIPITATION DISCUSSION 0069
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1154 AM EDT THU MAR 22 2018

AREAS AFFECTED...SACRAMENTO VALLEY...UPSLOPE REGION OF SIERRAS

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

VALID 221553Z - 222145Z

SUMMARY...CONVECTIVE RAIN BANDS AND THUNDERSTORMS SHOULD CONTINUE
DEVELOP OVER THE CENTRAL VALLEY AND AND FOOTHILLS OF THE SIERRA
NEVADA RANGE...PARTICULARLY IN THE AFTERNOON HOURS. SOME
CONVECTION MAY TRAIN ENOUGH TO PRODUCE 1 IN/HR RAIN RATES WITH A
THREAT OF FLASH FLOODING.

DISCUSSION...AS OF 15Z A STRONG BAND OF CONVECTIVE RAIN WAS
LOCATED FROM NEAR COLFAX ALONG INTERSTATE-80 TO NEAR STOCKTON IN
THE CENTRAL VALLEY. THIS IS DEVELOPING ALONG A PRE-FRONTAL
CONVERGENCE AXIS IN THE PRESENCE OF PRECIPITABLE WATER VALUES
AROUND 1.0 TO 1.2 INCHES (PER GPS-PW OBS AND RAP ANALYSIS) AND
SEVERAL HUNDRED J/KG OF MUCAPE (PER RAP ANALYSIS). THE MOIST AND
MARGINALLY UNSTABLE ENVIRONMENT SHOULD CONTINUE TO ALLOW FOR
ORGANIZATION TO THE CONVECTIVE BAND AS WELL AS HEAVY RAIN RATES.
ALREADY KDAX AND KBBX DUAL POL ESTIMATES HAVE REACHED OR BARELY
EXCEEDED 1 IN/HR IN SMALL AREAS...AS HAS MRMS. THE CONVECTIVE BAND
WAS NOT EXACTLY TRAINING...BUT INDIVIDUAL CELL MOTION WAS AT A
VERY SMALL ANGLE RELATIVE TO THE ORIENTATION OF THE BAND.
THEREFORE...HEAVY RAIN WAS LINGERING OVER INDIVIDUAL LOCATIONS FOR
CLOSE TO AN HOUR. KDAX SPECIFIC DIFFERENTIAL PHASE (KDP) VALUES IN
THE BAND HAVE REACHED 2-3 DEG/KM WHICH ARE ALSO USUALLY INDICATIVE
OF VERY HEAVY RAIN RATES. THIS BAND COULD POSE A RISK OF FLASH
FLOODING...AS RADAR TRENDS CONTINUE TO SHOW HINTS OF NEW CELL
GROWTH AT THE SOUTHERN END OF THE LINE IN THE MOIST AND MARGINALLY
UNSTABLE AIR MASS OVER THE VALLEY. HOWEVER...THIS MAY REQUIRE THE
BAND TO GROW WIDER OR REORIENT TO LENGTHEN THE AMOUNT OF TIME
HEAVY RAIN OCCURS AT A GIVEN LOCATION.

THE ONGOING HEAVY RAIN IS OCCURRING EARLIER THAN MOST HI-RES MODEL
GUIDANCE HAD SUGGESTED...WITH MANY SHOWING A MARKED INCREASE IN
CONVECTIVE ACTIVITY AROUND 21-00Z IN THIS REGION AS THE SYNOPTIC
COLD FRONT APPROACHES. THE ASSOCIATED +PV LOBE SHOWS UP CLEARLY IN
GOES-WEST WATER VAPOR LOOPS...AND IS STILL ABOUT 200KM OFFSHORE.
THE EXPECTATION IS STILL THAT THIS WILL OCCUR. MODELS SHOW A
SLIGHT DECREASE IN PRECIPITABLE WATER VALUES INTO THE AFTERNOON
HOURS...BUT A CORRESPONDING INCREASE IN CONVECTIVE INSTABILITY.
THIS SHOULD CONTINUE TO ENABLE ANY CONVECTIVE BANDS AND
THUNDERSTORMS TO BE FAIRLY ORGANIZED. 12Z HREF AND RECENT HRRR-TLE
PROBABILITIES OF 1 IN/HR RAIN RATES BECOME PARTICULARLY ELEVATED
IN THE 21-03Z TIME FRAME.

GIVEN THE COMBINATION OF ONGOING HEAVY RAIN (KAUN MEASURED 0.96
INCHES IN AN HOUR) AND THE POTENTIAL FOR ADDITIONAL HEAVY RAIN
LATER...A SLIGHT RISK WAS ADDED TO THE DAY 1 EXCESSIVE RAINFALL
OUTLOOK...AN UPGRADE FROM THE EXISTING MARGINAL RISK. FLASH
FLOODING WILL BE POSSIBLE THIS MORNING AND INTO THE AFTERNOON
HOURS. IT IS POSSIBLE THE CURRENT CONVECTIVE BAND WILL DIMINISH
SOMEWHAT LATER THIS MORNING...BUT ADDITIONAL CONVECTION WOULD BE
EXPECTED TOWARD THE END OF THE VALID TIME OF THIS DISCUSSION
(21-22Z).

LAMERS

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

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

LAT...LON   39922154 39632110 39282076 38812052 38402030
            37951998 37401956 37061943 36881974 37122034
            37422068 37822102 38352123 38822159 39172192
            39802200