MESOSCALE PRECIPITATION DISCUSSION 0844
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
811 AM EDT SAT SEP 30 2017

AREAS AFFECTED...NORTHEAST FL COAST

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

VALID 301211Z - 301811Z

SUMMARY...A STALLING AREA OF LOW PRESSURE WITH INCREASING
INSTABILITY AND INFLOW IN ITS NORTHERN QUADRANT SHOULD LEAD TO
HOURLY RAIN RATES TO 2.5" AND LOCAL AMOUNTS TO 4".

DISCUSSION...AN AREA OF LOW PRESSURE HAS BEEN DRIFTING NORTHEAST
ACROSS THE FL PENINSULA, AND IS NEAR SANFORD AT THIS TIME.  THE
REGION IS WITHIN THE WESTERN PORTION OF AN UPPER LEVEL HIGH, TO
THE SOUTH OF A DEPARTING WEAK JET STREAK AND EAST OF AN UPPER
LOW/TUTT CELL RETROGRADING FARTHER INTO THE GULF OF MEXICO. 
RECENT RADAR TRENDS SHOW INCREASING BANDS OF SHOWERS AND
THUNDERSTORMS WITH CLOUD TOP TEMPERATURES NEAR -60C FORMING
BETWEEN THIS LOW AND A WEAK FRONT/BAROCLINIC TROUGH BETWEEN
JACKSONVILLE AND DAYTONA BEACH.  PRECIPITABLE WATER VALUES OF
2.1-2.3" EXIST HERE PER RECENT GPS VALUES.  INFLOW AT 850 HPA IS
20-30 KTS PER VAD WIND PROFILES, WHICH IS DRAWING IN ML CAPE
VALUES OF 500-1000 J/KG.  EFFECTIVE BULK SHEAR OF 20-25 KTS EXISTS
NEAR THE COAST.

THE LOW IS NOT EXPECTED TO MOVE MUCH OVER THE NEXT SEVERAL HOURS,
THOUGH A NORTH TO NORTHEAST DRIFT IS POSSIBLE.  EXTENSIVE
CLOUDINESS SHOULD KEEP THE INSTABILITY GRADIENT NEAR THE COAST FOR
THE NEXT SEVERAL HOURS, WITH SHOWERS/ THUNDERSTORMS MOVING IN FROM
THE ATLANTIC REINFORCING THE GRADIENT.  THE MESOSCALE GUIDANCE
SHOWED A SCATTERED SIGNAL FOR LOCAL AMOUNTS OF 4", AND HINTS THAT
THE NEXT 2-4 HOURS MAY BE THE PEAK OF THE SHORT TERM RAINFALL. 
HOWEVER, RAP MASS FIELDS ARE UNDERESTIMATING THE INSTABILITY
AVAILABLE AND SHOW LESS CONCENTRATION TO THE CONVECTION THAN SEEN
ON RECENT RADAR IMAGERY, SO HAVE USED A SIX HOUR WINDOW.  GIVEN
THE AVAILABLE MOISTURE, HOURLY RAIN TOTALS TO 2.5" ARE POSSIBLE. 
THIS COULD BE AN ISSUE IN URBAN ENVIRONMENTS AND AREAS OF POOR
DRAINAGE.

ROTH

ATTN...WFO...JAX...MLB...

ATTN...RFC...SERFC...

LAT...LON   30738148 30008129 29288106 29048155 29668179
            30688205