MESOSCALE PRECIPITATION DISCUSSION 0510
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
621 AM EDT WED SEP 23 2015
 
AREAS AFFECTED...EASTERN NE...WESTERN IA 
 
CONCERNING...HEAVY RAINFALL...FLASH FLOODING LIKELY 
 
VALID 231020Z - 231320Z
 
SUMMARY...EXTREME RADAR-BASED RAIN RATES WERE OBSERVED NEAR OMAHA
AT 10Z. UNSTABLE INFLOW WAS IN SHORT SUPPLY...BUT AN APPROACHING
OUTFLOW BOUNDARY WILL FOCUS ASCENT AND RESULT IN A QUICK DUMP OF
VERY HEAVY RAIN...LIKELY CAUSING FLASH FLOODING.

DISCUSSION...CONVECTION IN THE VICINITY OF OMAHA WAS IN THE MIDDLE
OF A WEAKNESS OF 700 MB WINDS...BUT THESE...AS WELL AS 850 MB
WINDS...WERE INCRASING ABOVE 15-20 KNOTS PER SPC MESOANALYSIS AND
HRRR FIELDS...WITH A BRIEF FURTHER BOOST TO INFLOW AHEAD OF AN
APPROACHING COLD POOL. AS THE COLD POOL OVERTAKES THIS REGION CELL
MERGERS WILL OCCUR...TAKING ADVANTAGE OF THE WEAK TO MODERATE
INSTABILITY THAT WAS IN PLACE...AND YIELDING A RAPID ACCUMULATION
OF RAINFALL.

THE RADAR CHARACTER OF THIS CONVECTION AT 10Z WAS CONCENING...WITH
HOURLY RATES NEAR THE RADAR ESTIMATED AS HIGH AS 4 INCHES PER
HOUR. INDIVIDUAL CELL MOTIONS WERE NORTH AT 5-10 KNOTS...AND WERE
COUPLED WITH CORFIDI VECTORS NEAR ZERO. NEW DEVELOPMENT WAS
FAVORED ON THE SOUTHWESTERN FLANK OF ONGOING CONVECTION IN THE LOW
TERRAIN NEAR THE MISSOURI RIVER WHERE CAPE MAY BE LOCALLY HIGHER.
NET MOTION WILL BE VERY SLOW TO NEAR ZERO UNTIL THE PRE-EXISTING
SUPPLY OF UNSTABLE AIR IS EXHAUSTED. THIS PROCESS MAY OCCUR RATHER
QUICKLY...WITH DECREASING TRENDS AFTER 13Z...BUT IN THE MEANTIME
RAIN RATES ARE LIKELY TO EXCEED THE 1.5 TO 2 INCH PER HOUR FFG BY
A WIDE MARGIN...RESULTING IN A PRONOUNCED LIKELIHOOD OF FLASH
FLOODING.

BURKE

ATTN...WFO...DMX...FSD...GID...OAX...

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

LAT...LON   42999657 42679568 42109514 41189502 40689546 
            40679637 41289738 41919779 42729746