MESOSCALE PRECIPITATION DISCUSSION 0950
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
549 PM EST FRI DEC 22 2017

AREAS AFFECTED...SOUTHERN AND EASTERN AR...FAR NORTHWEST
MS...WESTERN TN

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

VALID 222245Z - 230330Z

SUMMARY...CONTINUED BROAD ISENTROPIC ASCENT OVER SAME LOCATIONS
WILL KEEP PERSISTENT MODERATE SHOWERS WITH OCCASIONAL EMBEDDED
CONVECTIVE ELEMENTS. 

DISCUSSION...OVERALL ENVIRONMENTAL SETUP HAS NOT VARIED MUCH OVER
THE LAST FEW HOURS AND IS EXPECTED TO MAINTAIN FOR THE NEXT 3-6
HOURS BEFORE MORE SIGNIFICANT HEIGHT FALLS AND DPVA BREAKS OUT
CONVECTION ALONG THE COLD FRONT AROUND 03-04Z ORIENTING MORE N-S
AND PRESSES THROUGH THE MS VALLEY OVERNIGHT.  IN THE MEAN
TIME...THE SURFACE COLD FRON CONTINUES TO PRESS SOUTH ACROSS
CENTRAL AND SOUTHEASTERN TX BEFORE ANGLING NORTH AS A STATIONARY
FRONT TO A WEAK SFC WAVE NW OF SHV. AT THIS POINT THE FRONT THEN
ANGLES BETWEEN LIT/SGT TO THE SE TIP OF THE MO BOOTHEEL.  A WEAK
OUTFLOW BOUNDARY REMAINS ABOUT 20-30 MILES SE OF THE FRONT.  THIS
SMALL SEPARATION HAS LIMITED A FOCUSED CONVERGENCE ESPECIALLY
GIVEN ITS OBLIQUE ANGLE TO THE FRONT FROM SSWLY INFLOW.  AS SUCH
ASCENT HAS BEEN ISENTROPIC IN NATURE AND MORE GRADUAL LEADING TO
WEAK/SCATTERED CONVECTIVE ELEMENTS THAT EXTENDING FROM CLARK TO
LEE COUNTIES IN AR. LOW LEVEL MOISTURE FLOW CONTINUES TO BE
CONFLUENT THROUGH NE TX AND UNIDIRECTIONAL ACROSS THE FRONTAL
BOUNDARY/OUTFLOW ALLOWING FOR GOOD MOISTURE AVAILABILITY IF ANY
CELLS CAN TAP GREATER INSTABILITY THAT IS AVAILABLE FURTHER SOUTH
REPRESENTED BY TEMPS NEAR 80 AND TDS IN THE LOW 70S...ALLOWING FOR
CAPPED CAPES TO 1000 J/KG.  MUCAPES AT THE ELEVATED CONVECTIVE
INITIATION LEVEL NEAR THE VICINITY OF THE  FRONT REMAIN AROUND 500
J/KG AND NOT EXPECTED TO VARY MUCH THROUGH 03Z.  AS SUCH MORE OF
THE SAME EVOLUTION/TRAINING OF MODERATE SHOWERS WITH EMBEDDED
CONVECTIVE ELEMENTS IS EXPECTED. 

SOME HI-RES CAMS THAT HAVE PERFORMED WELL TODAY SUCH AS THE 12Z
ARW AND HRRR RUNS SUGGEST ADDITIONAL CONVECTION TO BREAK OUT ALONG
THE OUTFLOW BOUNDARY JUST SOUTH OF THE AXIS OF 2-5" RAINFALL FROM
EARLIER TODAY IN S AR TOWARD NW MS.  THE GUIDANCE SUGGESTS THIS
CONVECTION MAY PRODUCE AN ADDITIONAL 1-3" IN E CENTRAL AR TO SW TN
PROLONGING FLOODING THREAT THERE MAKING FLASH FLOODING AT LEAST
POSSIBLE BEFORE 03Z.

THE 18Z NAM-CONEST DOES PRESENT A WORST CASE SCENARIO THAT
REQUIRES AN INCREASE IN SOUTHERLY FLOW TO BREAK OUT DEEP
CONVECTION FROM TKX TO MEM AND HAS SOME SUPPORT FROM THE 12Z
NSSL-WRF AND NMMB. IF THIS WERE TO UNFOLD CONVECTION WOULD BREAK
OUT IN THE 00Z TIME FRAME WITH AN ADDITIONAL 2-4" OVER ALREADY
FLOODED AREAS MAKING FLASH FLOODING LIKELY.  HOWEVER...INCREASED
CLOUD COVER STEADY NOT INCREASING FLOW AND FORCING STILL WELL
UPSTREAM LEADS TO THIS SCENARIO CONTINUING TO FADE AS A
POSSIBILITY.  STILL EVEN MODERATE SHIELD PRECIPITATION GIVEN TPWS
UP TO 1.7" WILL SUPPORT A MORE PROLONGED INUNDATION FLOODING SET
UP FOR THIS AREA AT LEAST. 

GALLINA

ATTN...WFO...LZK...MEG...OHX...SHV...

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

LAT...LON   36248834 36098700 35518706 34708916 34069119
            33609238 33409340 33679393 34279357 34919235
            35978972