MESOSCALE PRECIPITATION DISCUSSION 0948
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1228 PM EST FRI DEC 22 2017

AREAS AFFECTED...CENTRAL TN...NORTHERN AL...EXT SOUTHEAST/CENTRAL
KY

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

VALID 221730Z - 222230Z

SUMMARY...THOUGH WEAKENING CONVECTION MOVING INTO AREAS OF LOWER
FF THRESHOLDS IN THE CUMBERLAND PLATEAU.  POSES ISOLATED THREAT.

DISCUSSION...CURRENT MRMS RALA MOSAIC DEPICTS LEADING CONVECTIVE
LINE MOVING INTO N AL AND CENTRAL TN TOWARD THE CUMBERLAND PLATEAU
WHERE COMPLEX TERRAIN ALLOWS FOR LOWER FFG (LESS THAN 2"/3HRS OR
1.5"/1HR) AND AREAS MORE SUSCEPTIBLE OF FLASH FLOODING. 
CONVECTION IS SUPPORTED MAINLY AS ELONGATED SWLY FLOW S/W
ORIGINATING OUT OF THE EASTERN PACIFIC ADVANCES THROUGH WESTERN TN
WITH SUPPORTING RIGHT ENTRANCE TO JET STREAK OVER KY PROVIDING
CONTINUED LARGE SCALE ASCENT.  RAP MESOANALYSIS CONTINUES TO SHOW
SOME SHALLOW INSTABILITY REMAINING ACROSS N AL INTO S CENTRAL TN
WITH MAGNITUDES OF 100-200 J/KG.  ADDITIONALLY WITH THE S/W THE
5-85MB MOISTURE FLUX PER CIRA LPW ALLOWS FOR FAIRLY SATURATED
RAINFALL PRODUCTION ENVIRONMENT...THOUGH A TIGHT GRADIENT AT 7H
HAS SOME MID-LEVEL DRY AIR TO HELP MIX OUT AND SUPPORT COLD POOL
GENERATION ACROSS S CENTRAL TN/N AL. STILL THE ORIENTATION OF THE
FLOW THROUGH MUCH OF THE CLOUD BARING FLOW IS UNIDIRECTIONAL TO
SUPPORT TRAINING AS MOST OF THE PROPAGATION VECTOR FROM THE COLD
POOL IS ORIENTATED FAVORABLY WITH THE MEAN FLOW. THESE
ENVIRONMENTAL PARAMETERS SHOULD HELP MAINTAIN RAINFALL EFFICIENCY
FOR A FEW HOURS ESPECIALLY SOUTH AND WEST BEFORE INSTABILITY IS
EXHAUSTED. THESE LOCATIONS WERE WETTED BY RAINFALL EARLIER THIS
WEEK MAKING GROUND POTENTIALLY MORE HYDROPHOBIC THAN NORMAL.  THE
TIMING OF WEAKENING SHOULD BE AROUND 21-23Z BUT GENERALLY EAST OF
THE LOWEST FFG/SPINE OF THE PLATEAU FROM MARION TO CLAIBORNE
COUNTIES...SO THERE ARE MIXED SIGNALS FOR FLOODING BUT NOT
CONSIDERED OUT OF THE REALM OF POSSIBILITY.

HI-RES GUIDANCE...PARTICULARLY THE 12Z ARW AND RECENT HRRR RUNS
SHOW GOOD AGREEMENT TO ORIENTATION BUT STILL GENERALLY LAG ABOUT
1HR IN TIMING.  BOTH SUGGEST 1.5-2.5" MAXIMUM TOTALS WITH THE BULK
IN THE LEADING BURST OF DEEPER CONVECTION.  THIS MAY LEAD TO
ISOLATED EXCEEDANCE OF 1-3HR FFG VALUES AND POSE AT LEAST A
POSSIBLE THREAT FOR FF TO OCCUR.      

GALLINA

ATTN...WFO...HUN...JKL...LMK...MRX...OHX...

ATTN...RFC...LMRFC...OHRFC...

LAT...LON   37018381 36858331 36448349 35848454 35048583
            34598766 34868800 35968781 36488689 36778518