MESOSCALE PRECIPITATION DISCUSSION 0947
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
726 AM EST FRI DEC 22 2017

AREAS AFFECTED...NE TX...SRN AR...WRN TN...NRN MS

CONCERNING...HEAVY RAINFALL...FLASH FLOODING LIKELY

VALID 221225Z - 221755Z

SUMMARY...SWATH OF 1 TO 3 INCH RAINS THROUGH 18Z WITH FLASH FLOOD
RISK INCREASING BY LATE MORNING AND AFTERNOON.

DISCUSSION...THE MUCH ANTICIPATED EXCESSIVE RAINFALL EVENT WAS
INITIATING THIS MORNING AS OF 12Z...WITH LIGHTNING DETECTIONS FROM
TX TO TN... DUAL-POL HYDROCLASS TAGS OF HEAVY RAIN... AND INFRARED
CLOUD TOPS PUSHING -60 DEGREES C NEAR THE ARKLATEX.

SOME UPGLIDE WAS PRESENT...BUT FOR THE MOST PART A UNIFORM WARM
SECTOR HAD DEVELOPED SOUTH OF A WEAKLY DEFINED WARM FRONT WHICH
RAN FROM LITTLE ROCK TO NORTH OF JACKSON MS...AND WHERE SFC
DEWPOINTS WERE IN THE 60S. FORCING FOR THE EARLY MORNING DEEP
CONVECTION PRIMARILY OWED TO THE SYNOPTIC CIRCULATION IN A
STRENGTHENING 130-140 KNOT UPPER JET AND ASSOCIATED
FRONTOGENETICAL FORCING...WITH THE SFC COLD FRONT SETTLING TOWARD
THE OUTLINED DISCUSSION AREA.

THE EVENT WILL WAIT FOR AN UPSTREAM EJECTING SOUTHERN STREAM
SHORTWAVE BEFORE PROGRESSING...SUCH THAT A MULTI-HOUR PERIOD OF
WSW TO ENE TRAINING IS ANTICIPATED... LEADING TO A MODERATE RISK
OF RAINFALL EXCEEDING FLASH FLOOD GUIDANCE. STRONG FORCING COUPLED
WITH WEAK BUT SUFFICIENT CAPE OVER 500 J/KG MAY YIELD NARROW FOCUS
FOR AMOUNTS EXCEEDING 3 INCHES THROUGH 18Z...PARTICULARLY FROM FAR
NE TX THROUGH SOUTHERN ARKANSAS INTO ADJACENT MS/TN. THE
PERSISTENCE OF HEAVY RAIN FARTHER EAST TOWARD NASHVILLE MAY
INCREASE BY EARLY AFTERNOON.

MODEL AGREEMENT IS STRONG...THOUGH THE 06Z NAM CONUS NEST AND 00Z
WRF-ARW2 / PARALLEL ARW FROM NCEP / WERE PERHAPS HANDLING THINGS
BEST. DUAL POL ESTIMATES ALREADY INDICATED HOURLY RAIN RATES OVER
1.30 INCHES...AND FLASH FLOODING WILL BECOME INCREASINGLY LIKELY
THROUGH 18Z AS BROADER 1-3 INCH AMOUNTS SATURATE A LARGE AREA.
EXPECT MULTIPLE ROUNDS...WITH THE BACK EDGE NOT SWINGING THROUGH
UNTIL OVERNIGHT.

BURKE

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

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

LAT...LON   36128696 35568693 34488974 33569225 32839517
            33819430 34819275 36048903