EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
252 AM EDT TUE MAY 21 2013

...VALID 06Z TUE MAY 21 2013 - 12Z WED MAY 22 2013...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
10 NNW ADH 30 WNW UNO 15 ENE BLV 10 S OLY 20 ESE PAH AWM
25 NNE BQP 25 NE GGG 15 NNW TKI 10 NNW ADH.


...EASTERN PORTIONS OF THE SOUTHERN PLAINS (NERN TX/ERN OK)...

MULTICELLULAR CONVECTION LATE MON NIGHT/EARLY TUE AM IS
PROGRESSING RATHER WELL...THIS ALONG THE MAINLY LINEAR ORIENTATION
WOULD LIKELY AMOUNT TO A LIMITED FLASH FLOOD THREAT WITHIN THE
CURRENT SLIGHT RISK REGION THROUGH TUE AM.  HOWEVER...BOTH THE
GLOBAL AND HIGH-RES MODEL SUITE CLUSTER FAVORABLY IN TERMS OF
DEPICTING AN ADDITIONAL ROUNDS OF HEAVY RAINFALL LATER TODAY IN
THE VICINITY OF THE THE SLOW MOVING LLVL FRONTAL BOUNDARY
STRETCHING THROUGH THE SOUTHERN PLAINS.  DURING THE DAY...THE
HIGHEST RISK FOR EXCESSIVE RAINFALL WILL BE ON EITHER SIDE OF THE
RED RIVER ACROSS ERN OK AND NERN TX...NORTHEASTWARD INTO NW ARK. 
THIS AS A SHORTWAVE CURRENTLY PIVOTING ACROSS SRN NM PUSHES EAST
INTO CENTRAL OK BY 00Z WED.  THE SHORTWAVE WILL FURTHER AMPLIFY
THE LONGWAVE TROUGH...
ENHANCING THE BROAD-SCALE DYNAMICAL FORCING (UPPER DIVERGENCE/LLVL
MOISTURE CONVERGENCE) ACROSS THE AFOREMENTIONED AREAS LATER
TODAY...PARTICULARLY BY LATE AFTN AND EVENING AS THE NAM/GFS/ECMWF
ALL SHOW ENHANCED RIGHT ENTRANCE REGION (FRONTOGENETIC) FORCING
FROM THE UPPER JET GRADIENT ACROSS THE CENTRAL PLAINS TO MID MS
VLY.  AMPLE LLVL INFLOW AND CONVERGENCE ALONG THE FRONTAL ZONE
WITH A MOISTURE RICH ENVIRONMENT IN PLACE (PW'S PEAKING ~1.75" OR
CLOSE TO 2 STD DEVS ABOVE NORMAL...WHILE K INDICES IN THE
MID-UPPER 30S)...WILL SUPPORT THE POTENTIAL FOR COPIOUS RAINFALL
WITH THE DEVELOPING CONVECTION LATER TODAY.  FOR NOW...CONTINUED
WITH A SLIGHT RISK OVER THIS REGION...
GIVEN THE STRONG (DEEP) INSTABILITY AND WITH THE DRY MID-LEVEL AIR
NOT FAR UPSTREAM...THUS THE
LIKELIHOOD THAT AS EXPERIENCED ON MON CELLS WILL BE MOVING AT A
FAIRLY PROGRESSIVE CLIP AND AS SUCH POSE A MORE LIMITED FLASH
FLOOD THREAT.  AT THE SAME TIME...THE MORE ORGANIZED CONVECTIVE
CLUSTERS...ESPECIALLY SUPERCELLS...WILL HAVE A MUCH GREATER RISK
FOR AT LEAST LOCALIZED FLASH FLOODING.


...PORTIONS OF THE LOWER-MID MISSISSIPPI VALLEY...

TUE NIGHT INTO WED MORNING...THE EXCESSIVE RAINFALL RISK SHIFTS
INTO THE LOWER-MID MS VLY...GENERALLY N OF THE LA/AR BORDER UP TO
THE STL AREA.  MODEST SWRLY NOCTURNAL LLJ (35-45 MPH) ALONG WITH
THE PW'S BTWN 1.5 AND 1.75" AHEAD OF THE AFOREMENTIONED SHORTWAVE
TROUGH AND SW-NE ORIENTED SFC COLD FRONT WILL GENERATE ADDITIONAL
AREAS OF HEAVY RAINFALL.  THE RISK IS HEIGHTENED GIVEN THE
NEGATIVE TILT OF THE TROUGH AXIS AND (THUS) SOME BACKING OF THE
FLOW ALOFT...MORE PARALLEL TO THE LLVL BNDRY THAN OTHERWISE...
WHICH WILL LEAD TO A SLOWER W-E PROGRESSION OF THE CONVECTION. 
ISOLATED 2-4+ INCH RAINFALL AMOUNTS WILL BE POSSIBLE IN A FEW
HOURS ACROSS THE THREAT AREA. 

HURLEY