MESOSCALE PRECIPITATION DISCUSSION 0872
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
757 PM EDT SAT OCT 07 2017

AREAS AFFECTED...CENTRAL GULF COAST

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

VALID 072356Z - 080556Z

SUMMARY...CORE RAINFALL WITH HURRICANE NATE IS CAPABLE OF
PRODUCING HOURLY RAIN TOTALS TO 3" WITH LOCAL AMOUNTS TO 6".

DISCUSSION...HURRICANE NATE'S CDO COVERS THE EAST-NORTHEAST THIRD
TO HALF OF ITS CIRCULATION, WHICH COULD BE EITHER DUE TO SOME
WESTERLY VERTICAL WIND SHEAR OR THE INGESTION OF MORE STABLE AIR
FROM SOUTHEAST LA, WHERE CIN HAS DEVELOPED DUE TO CLOUDY
SKIES/LIGHT RAIN THIS AFTERNOON.  THERE HAS BEEN SOME RECENT
COOLING TO CLOUD TOPS PER INFRARED GOES-13 IMAGERY.  A TRAINING
BAND HAS BEEN CONCENTRATED IN SOUTHERN AL AS OF LATE DOWNWIND OF A
MAXIMUM OF INSTABILITY, WHICH RADAR HAS ESTIMATED HOURLY TOTALS TO
2".  PRECIPITABLE WATER VALUES OF 2.75-3" EXIST IN ITS CORE. 
INFLOW AT 850 HPA IS 30-50 KTS PER VAD WIND PROFILES, THOUGH WINDS
UP TO 85 KTS MIGHT EXIST BASED ON THE STORM'S CURRENT INTENSITY. 
UPSTREAM INSTABILITY OF 500-3000 J/KG MU CAPE VALUES EXISTS
OFFSHORE THE CENTRAL GULF COAST.  EFFECTIVE BULK SHEAR OF 25-45
KTS EXISTS PER SPC MESOANALYSES, WHICH WHEN COMBINED WITH FAIRLY
UNIDIRECTIONAL WINDS WITH HEIGHT HAVE LED SPIRAL BANDS AS NOTED ON
RADAR.

THE EXPECTATION IS FOR THE STORM TO MOVE NORTH OVER THE NEXT
SEVERAL HOURS, BRINGING ITS CENTER NEAR SOUTHERN MS.  THE
MESOSCALE GUIDANCE HAS A GOOD SIGNAL FOR LOCAL AMOUNTS OF 5" NEAR
AND NORTHEAST OF ITS CENTER.  THE AVAILABLE MOISTURE AND
INSTABILITY COULD LEAD TO OCCASIONS WHERE HOURLY RAIN TOTALS REACH
3".  THERE IS A CHANCE FOR CENTRAL CONVECTION TO DEVELOP GREATER
VIGOR DUE TO FRICTIONAL CONVERGENCE AROUND THE TIME OF LANDFALL
AND AS CIN SETS IN AT INCREASING DISTANCES FROM THE CIRCULATION
CENTER.  RAINFALL DURING THE PAST COUPLE WEEKS HAS BEEN WELL BELOW
NORMAL, SO ISSUES WITH HEAVY RAINFALL ARE EXPECTED MOSTLY WITHIN
URBAN AREAS.

ROTH

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

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

LAT...LON   32728872 32608690 30888510 29668496 29558530
            29838538 30078576 30308642 30358671 30138771
            30058858 29738867 29308893 29038903 28948938
            29499004 30619048 31998996