MESOSCALE PRECIPITATION DISCUSSION 0691
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1042 PM EDT THU SEP 22 2016
 
AREAS AFFECTED...PORTIONS OF CENTRAL & EASTERN IA 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE 
 
VALID 230241Z - 230641Z
 
SUMMARY...THUNDERSTORMS ARE BEGINNING TO EXPAND IN COVERAGE ACROSS
CENTRAL AND EASTERN IA.  CONSIDERING THE LOW FLASH FLOOD GUIDANCE
IN THE AREA, FLASH FLOODING IS CONSIDERED POSSIBLE.

DISCUSSION...CONVECTION IS BEGINNING TO FIRE NEAR THE SURFACE AND
850 HPA INFLECTIONS OF OF A STATIONARY FRONT ACROSS IA, WHICH
APPEARS TO BE SAGGING SLOWLY SOUTHWARD WITHIN SURFACE OBSERVATIONS
AND RADAR IMAGERY FROM DES MOINES IA.  MUCAPES OF 1000-2000 J/KG
LIE ACROSS THE REGION, WITH PRECIPITABLE WATER VALUES OF 1.75". 
LOW-LEVEL INFLOW IS FAIRLY WEAK, 15-20 KTS FROM THE SOUTHWEST. 
HOWEVER, 0-6 KM BULK SHEAR VALUES ARE 30-50 KTS, WHICH COULD ALLOW
FOR ORGANIZED CONVECTION.  TEMPERATURES AT 700 HPA ARE ROUGHLY 9C,
WOULD ROUGHLY APPROXIMATES A CAP EDGE IN SEPTEMBER.

THE MESOSCALE GUIDANCE SHOWS AN INCONSISTENT SIGNAL FOR 2-3"
AMOUNTS LOCALLY, WITH HRRR AND ARW RUNS THE MOST AGAINST HEAVY
RAINFALL.  THE 18Z SSEO NEIGHBORHOOD PROBABILITIES SUGGEST AN
INCREASING HEAVY RAIN THREAT WITH TIME OVERNIGHT INTO THE EARLY
MORNING HOURS.  THE INFLOW AT 850 HPA BEGINS TO OVERTAKE THE MEAN
850-400 HPA WINDS WITH TIME OVERNIGHT, WHICH COULD INCREASE
RAINFALL EFFICIENCY.  AVAILABLE MOISTURE AND INSTABILITY SUPPORTS
HOURLY RAIN RATES TO 2".  WITH THE FORWARD PROPAGATION VECTORS
(SUPPORTING EAST-SOUTHEAST AT 15-20 KT MOTION), INSTABILITY
GRADIENT NORTH OF THE FRONT, AND THE 850-400 HPA MEAN FLOW
PARALLEL TO EACH OTHER, CELL TRAINING IS CONSIDERED THE MAIN
THREAT.  WITH RELATIVELY LOW FLASH FLOOD GUIDANCE VALUES IN THE
AREA FROM RECENT RAINS, THE RELATIVELY LOW EXPECTED AMOUNTS COULD
LEAD TO ISSUES.  FLASH FLOODING IS CONSIDERED POSSIBLE.  DUE TO
THE UNCERTAINTY, USED A FOUR HOUR WINDOW TO SEE HOW THE CONVECTIVE
COVERAGE/ORGANIZATION IMPROVES.

ROTH

ATTN...WFO...ARX...DMX...DVN...FSD...

ATTN...RFC...MBRFC...NCRFC...

LAT...LON   43049350 42899173 42789124 42289054 41819109 
            41359237 41709498 42609518 42909459