MESOSCALE PRECIPITATION DISCUSSION 0731
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
800 PM EDT FRI AUG 25 2017

AREAS AFFECTED...EASTERN SD...SOUTHWEST MN

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

VALID 260000Z - 260400Z

SUMMARY...CONVECTION TRAINING ALONG A FRONTAL BOUNDARY IN A HIGH
MOISTURE ENVIRONMENT COULD RESULT IN HOURLY RAINFALL RATES NEAR
2.00 INCHES...WHICH IN TURN COULD POSE A FLASH FLOOD RISK THROUGH
THIS EVENING.

DISCUSSION...REGIONAL RADARS SHOWED A LINE OF CONVECTION
STRENGTHENING NEAR A FRONTAL BOUNDARY EXTENDING ACROSS EASTERN SD.
THE CONVECTION WAS FOLLOWING AN AXIS OF 2000-3000 J/KG
MLCAPE...WHERE PRECIPITABLE WATER VALUES WERE CLOSE TO 1.50
INCHES. THE CONVECTION THUS FAR HAS BEEN MAINLY OUTFLOW
DRIVEN...AS DRY AIR IN THE MID LEVELS (SHOWN IN RECENT RAP
SOUNDINGS ACROSS EASTERN SD) HELPS A DEVELOPING COLD POOL PUSH THE
CONVECTION ALONG.  IN ADDITION... THE KABR RADAR SHOWED HAIL
SIGNATURE IN THE BASE DATA...WHICH IS SUPPORTED BY THE HCA
PRODUCT.

THE GOES-16 CLEAN IR LOOP SHOWED COOLING TOPS ACROSS CENTRAL
SD...AS CONVECTION ON THE SOUTHWEST SIDE OF THE LINE STRENGTHENS
IN THE PRESENCE OF THE BEST INSTABILITY. THE MOST RECENT HRRR RUNS
SUGGEST THAT WHILE THE CONVECTION SHOULD REMAIN MOSTLY OUTFLOW
DRIVEN...CELLS ALONG THE LINE COULD START TO DROP SOUTHEAST...AS
PROPAGATION VECTORS SUGGEST THAT STRONG CELLS COULD TURN
RIGHT...INTO THE MOTION OF THE LINE. THIS CELL MOTION WOULD
INCREASE THE POTENTIAL FOR SHORT TERM TRAINING...AND AN INCREASED
FLASH FLOOD THREAT.

THE MOST RECENT HRRR RUNS SUGGESTED LOCAL 2.50 TO 3.50 INCH
RAINFALL AMOUNTS WHERE SHORT TERM TRAINING OCCURS ALONG THE
LINE...MAINLY ACROSS EASTERN SD. THREE HOUR FLASH FLOOD GUIDANCE
VALUES ARE AS LOW AS 1.50 INCHES...WHICH SUPPORT A FLASH FLOOD
THREAT.

FURTHER NORTHEAST INTO SOUTHWEST MN... ELEVATED INSTABILITY NORTH
OF THE STATIONARY FRONT COULD SET UP TRAINING HERE...THOUGH THERE
IS MUCH LESS OF A FLASH FLOOD SIGNAL IN THE HIGH RESOLUTION
GUIDANCE. LOWER FLASH FLOOD GUIDANCE HERE ALSO SUPPORTS A FLASH
FLOOD THREAT...BUT TO A LESSER DEGREE THAN EASTERN SD.

HAYES

ATTN...WFO...ABR...FSD...LBF...MPX...OAX...UNR...

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

LAT...LON   45099678 45069539 44619465 43949488 43379636
            42819819 42659998 42850062 43040058 44749786