MESOSCALE PRECIPITATION DISCUSSION 0218
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
402 PM EDT SUN AUG 18 2013
 
AREAS AFFECTED...WESTERN FL PANHANDLE...SOUTHEAST AL...SOUTHWEST
GA 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 182001Z - 190101Z
 
SUMMARY...A NARROW BAND OF CONVECTION HAS FORMED, WHICH EXTENDS
FROM WELL WITHIN THE GULF OF MEXICO INTO THE FL PANHANDLE INTO
SOUTHEAST AL.  REGIONAL FLASH FLOODING IS POSSIBLE FROM HEAVY
RAINFALL WITHIN THIS BAND OF THUNDERSTORMS.

DISCUSSION...BASED ON 19Z DATA, LOW-LEVEL INFLOW FROM THE GULF OF
MEXICO IS OVERRIDING A FRONTAL BOUNDARY CURRENTLY ORIENTED
WEST-EAST TO THE SOUTH OF THE WESTERN FL PANHANDLE AND THE COASTS
OF AL/MS, CLOSE TO A SURFACE TROUGH TO THE NORTH OF THE STATIONARY
FRONT ACROSS THE FL PANHANDLE AND SOUTHEASTERN AL.  THE NARROW
BAND OF INFLOW IS ON THE ORDER OF 30 KNOTS FROM THE SOUTH,
MAXIMIZED NEAR THE 850 HPA LEVEL. AN OUTFLOW BOUNDARY FROM EARLIER
CONVECTION NEAR SOUTHERN AL HAS HELPED TO INCREASE LOW-LEVEL
CONVERGENCE INTO THIS REGION.   WINDS CLOSER TO THE SURFACE ARE
OUT OF THE EAST AT 10-15 KNOTS, WHICH BRINGS THE LOW-LEVEL WIND
SHEAR CLOSE TO 40 KNOTS.  CAPE VALUES IN THE REGION ARE EXPECTED
TO REMAIN IN THE 1000-2000 J/KG RANGE UNTIL SUNSET. 
CURRENT/EXPECTED CAPE VALUES AND 0-3 KM VERTICAL WIND SHEAR
SUPPORT ORGANIZED CONVECTION, MOST LIKELY IN THE FORM OF WET
MACROBURST MULTICELLS OR SUPERCELLS.  PRECIPITABLE WATER VALUES OF
2-2.25 INCHES ARE AIDING PRECIPITATION EFFICIENCY WITHIN THE
THUNDERSTORM BAND.

CORFIDI VECTORS ALONG WITH THE RECENT OUTFLOW BOUNDARY PROGRESSION
ACROSS THE NORTHERN GULF OF MEXICO SUGGEST SOME EASTWARD MOVEMENT
TO THE BAND IS POSSIBLE WITH TIME.  RAINFALL RATES WITHIN THE BAND
ARE ON THE ORDER OF 1-2 INCHES PER HOUR IN THE STRONGER CELLS. 
THE RECENT MESOSCALE GUIDANCE HAS A HINT OF THIS BAND'S EXISTENCE
DURING THE 18-00Z PERIOD, THOUGH THEY VARY A BIT CONCERNING HOW
FAR NORTH THE BAND WILL DEVELOP/PROPAGATE.  LOCAL RAINFALL AMOUNTS
OF 5 INCHES ARE POSSIBLE DUE TO CELL TRAINING ALONG THIS
CONVECTIVE BAND, USING RECENT RAINFALL TOTALS NEAR GULFPORT MS AS
A GUIDE.  THE FLASH FLOOD GUIDANCE REMAINS RELATIVELY HIGH, ON THE
ORDER OF 2.5 INCHES/HOUR OR THREE INCHES WITHIN THREE HOURS.  DUE
TO THIS, CONFIDENCE IN REGIONAL FLASH FLOODING IS AVERAGE/NOT
OVERWHELMING, SO USED THE POSSIBLE CATEGORY OVER THE LIKELY
CATEGORY.  USED A FIVE HOUR TIME WINDOW FOR THE MPD, AS MASS
FIELDS FROM RECENT RAP/ARW RUNS SUGGEST ACTIVITY SHOULD CONTINUE
UNTIL 01Z.

ROTH

ATTN...WFO...FFC...TAE...BMX...MOB...

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

LAT...LON   31418622 31558610 32098556 32518497 32488465 31988448
            31158483 30218522 29798550 29658562 29848568 30018586
            30268633 31418622