MESOSCALE PRECIPITATION DISCUSSION 0769
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
217 PM EDT WED AUG 30 2017

AREAS AFFECTED...SOUTHEAST MS...SOUTHWEST AL...EXT WESTERN FL
PANHANDLE

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

VALID 301815Z - 302345Z

SUMMARY...NARROW MST AXIS WITH TRAINING PROFILE TO SLOWLY MOVE
EAST ALONG MS/AL COAST POSING FLASH FLOODING RISK THROUGH EVENING
HOURS.

DISCUSSION...NATIONAL RADAR MOSAIC AND GOES-16 DETECT PERIPHERAL
OUTER BANDS OF TS HARVEY STARTING TO ALIGN WITHIN THE MEAN
REMAINING MOISTURE AXIS OF THE WARM CONVEYOR BELT.  CIRA LAYERED
PW LOOP SHOWS THIS MOISTURE PLUME HAS BEEN RELATIVELY STATIONARY
AS HARVEY TRACKS FAIRLY PARALLEL TO IT WHILE ALSO SHOWING STEADY
DECREASE/NARROWING OF THE BAND AS A WHOLE.  A WEAK
SHORTWAVE/EFFECTIVE MID-LEVEL TRIPLE POINT WITHIN THE LARGER
PATTER HAS LIFTED NORTH INTO CENTRAL AL WITH TRAILING EDGE OF THE
TROF BEGINNING TO RETROGRADE (AS THE EFFECTIVE EASTERN EDGE OF THE
DEEPER MOISTURE).  WITHIN THIS CONFIGURATION...THE NARROW FLOW OF
THE WARM CONVEYOR BEGINS TO BECOME DIFFLUENT/DIVERGENT AS IT
ASCENDS WHILE ALSO FEEDING THE RIGHT ENTRANCE TO THE LARGER SCALE
POLAR JET.  AS SUCH CONVECTIVE BANDS WILL BE INCREASINGLY
SUPPORTED ALOFT FOR MAINTENANCE AS CELL MOVE NORTH.

THE MOISTURE AXIS IS ALSO COINCIDENT WITH THE INSTABILITY AXIS
WHICH IS RUNNING ABOUT 2000 J/KG MLCAPE PER RECENT RAP ANALYSIS. 
STRONG SURFACE FLOW 20-25KT SFC FLOW WITHIN THE AXIS BECOMES
CONVERGENT ALONG THE COAST TO CONVECT ALONG THE BOUNDARY. 
30-40KTS OF 850MB INFLOW WHILE FLOW ALOFT BECOMES SLIGHTLY WEAKER
(MEAN CELL MOTIONS OF 20-30KTS TO THE NNE) SUPPORT BACKBUILDING
PROPAGATION OF CELLS ALONG THE BANDS AS THEY VERY SLOWLY ADVANCE
EASTWARD (5KTS) ON OUTFLOW DUE TO MINOR DRY AIR MIXING. GIVEN
STRENGTH OF UPDRAFTS AND REMAINING AMPLE MOISTURE OF 2-2.25" TPW
SUGGEST RATES UP TO 2"/HR AND SLOW MOTIONS SUPPORT THE POTENTIAL
FOR 3-5" ALONG THE COASTAL COUNTIES OF MS TO AL OVER THE NEXT FEW
HOURS.  WITHOUT THE STRENGTH OF FORCING/FURTHER FROM MST/LL
HEATING SOURCE OF THE GULF...CELLS WILL TREND WEAKER AS THE MOVE
NNE ASHORE BUT STILL POSE FF RISK GIVEN 1-1.5"/HR AND 2-3" TOTALS
POSSIBLE 2-3 COUNTY ROWS NORTH OF THE COASTAL REGION.  

GALLINA

ATTN...WFO...BMX...JAN...LIX...MOB...

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

LAT...LON   32738695 32178616 30998645 30268676 30218746
            30108824 30158856 30088902 30258914 30838940
            32018905 32418825