MESOSCALE PRECIPITATION DISCUSSION 0720
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
954 PM EDT THU AUG 24 2017

AREAS AFFECTED...NORTHERN NE INTO WESTERN IA

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

VALID 250200Z - 250600Z

SUMMARY...CONVECTION TRAINING ALONG A FRONTAL BOUNDARY EXTENDING
FROM NORTHERN NE INTO WESTERN IA COULD POSE A FLASH FLOOD THREAT
INTO THE EARLY MORNING HOURS.

DISCUSSION...REGIONAL RADARS SHOWED AN INCREASING AREA OF
CONVECTION ALONG A STATIONARY FRONT EXTENDING FROM NORTH CENTRAL
NE INTO WESTERN IA THIS EVENING. THE CONVECTION IS DEVELOPING IN
AN AXIS OF 500-1000 J/KG OF MUCAPE ALONG AND JUST SOUTH OF THE
FRONT...AND THESE VALUES WERE SUPPORTED BY 00Z RAOBS IN THE
REGION. 

AN AXIS OF 1.50 INCHES OF PRECIPITABLE WATER AIR HAS POOLED ALONG
THE FRONT...AS AN INCREASING LOW LEVEL JET (THE LATEST RAP
INDICATES LOW LEVEL FLOW INCREASES TO 35 KNOTS AFTER 04Z) SUPPLIES
MOISTURE ALONG AND NORTH OF THE FRONT IS A BROAD WARM AIR
ADVECTION PATTERN.

THE MID LEVEL FLOW BACKS OVER WESTERN AND CENTRAL NE AHEAD OF A
SHORT WAVE OVER WY/CO...AND WEAKENS TO UNDER 10 KNOTS. THIS COULD
ALLOW THE CONVECTION THAT DEVELOPS TO SPREAD ALONG THE FRONT TO
MOVE SLOWLY SOUTHEAST (BASED ON PROPAGATION VECTORS). THE TRAINING
CONVECTION INCREASES THE POTENTIAL FOR FLASH FLOODING.

THE MOST RECENT HRRR AND 12Z WRF ARW HAD THE FOCUS FOR CONVECTION
FURTHER WEST ACROSS WESTERN AND CENTRAL NE...WHERE THREE HOUR
FLASH FLOOD VALUES ARE FAIRLY HIGH (OVER THE SAND HILLS). THE BEST
LOW LEVEL INFLOW SUPPORTS CONVECTION OVER EASTERN NE INTO WESTERN
IA...WHERE FLASH FLOOD GUIDANCE VALUES ARE LOW 1.00 TO 1.50 INCHES
OVER NORTHEAST NE AND FAR WESTERN IA. MOST OF THE AVAILABLE HIGH
RESOLUTION GUIDANCE IS NOT CAPTURING THIS ACTIVITY WELL.

GIVEN THE HOURLY RAINFALL RATES SEEN ON THE KFSD RADAR...2.00 TO
3.00 INCHES OF RAINFALL CAN BE EXPECTED ALONG THE FRONT... WITH
THE HIGHEST AMOUNTS OVER EASTERN NE/WESTERN IA. GIVEN THE TRAINING
OVER AREAS WITH LOWER FLASH FLOOD GUIDANCE VALUES... FLASH
FLOODING IS POSSIBLE INTO THE EARLY MORNING HOURS.

HAYES

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

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

LAT...LON   42940064 42889525 41469486 41209540 41730012
            42520136 42860122