MESOSCALE PRECIPITATION DISCUSSION 0470
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
249 AM EDT TUE JUL 17 2018

AREAS AFFECTED...FAR NE NEBRASKA...FAR SRN SOUTH DAKOTA

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

VALID 170647Z - 171145Z

SUMMARY...THUNDERSTORMS WILL CONTINUE TO DEVELOP ALONG THE EASTERN
PORTION OF THE NEBRASKA - SOUTH DAKOTA BORDER REGION...NEAR WHERE
THE MISSOURI RIVER BECOMES THE STATE BORDER. THESE STORMS MAY BE
NEARLY STATIONARY AT TIMES AND COULD PRODUCE RAIN RATES AROUND OR
EXCEEDING 2 IN/HR...WHICH COULD LEAD TO LOCALIZED FLASH FLOODING
WHERE THE STORMS STALL.

DISCUSSION...GOES-16 IR SATELLITE CHANNELS SHOWED A PROMINENT MCS
WITH A LARGE REGION OF COLD CLOUD TOPS (-60C OR COLDER) OVER
WESTERN AND CENTRAL NEBRASKA. THIS WAS ASSOCIATED WITH A FORWARD
PROPAGATING SQUALL LINE THAT WAS BEGINNING TO DIVE IN AN
INCREASINGLY SOUTHWARD DIRECTION TOWARD NORTHWEST KANSAS. HOWEVER
THE IR SATELLITE CHANNELS ALSO SHOWED A BAND OF RAPIDLY COOLING
CLOUD TOPS TO THE NORTHEAST...NEAR THE SD-NE BORDER...WITH
INCREASED LIGHTNING ACTIVITY NOTED FROM A VARIETY OF LIGHTNING
NETWORKS. THIS APPEARED TO BE OCCURRING IN A ZONE OF ENHANCED
LOW-LEVEL CONVERGENCE BETWEEN PREVAILING EASTERLY FLOW AND
SOUTHWESTERLY FLOW FROM A MESO-HIGH THAT HAS DEVELOPED BEHIND THE
LARGER MCS. THIS AREA WAS NEAR A GRADIENT IN INSTABILITY AND
MOISTURE...BUT TUCKED JUST INTO THE REGION OF ELEVATED INSTABILITY
WITH MUCAPE VALUES AROUND 1000-2000 J/KG. FORECASTS FROM THE RAP
MODEL AND HI-RES MODELS SUGGEST THAT THIS BAND OF LOW-LEVEL
CONVERGENCE MAY PERSIST OVER THE NEXT SEVERAL HOURS AND PROVIDE A
NEARLY STATIONARY FOCUS FOR CONVECTION. THE AREA ALSO BEING IN THE
RIGHT ENTRANCE REGION OF AN UPPER LEVEL JET STREAK FAVORS
SUSTAINED VERTICAL MOTION AS WELL.

KLNX AND KFSD RADARS SHOWED VERY LITTLE STORM MOTION WITH THE
CONVECTION STRETCHING FROM NEAR ONEILL NE TO NEAR WINNER SD. IN
ADDITION TO THE LOW-LEVEL CONVERGENCE AXIS...THIS MAY BE DUE TO A
WEAKNESS IN THE MID-UPPER LEVEL FLOW CAUSED BY THE LARGE MCS TO
THE SOUTHWEST. KLNX VWP SHOWS A BACKING AND WEAKENING OF THE UPPER
LEVEL WINDS IN CENTRAL NEBRASKA. THESE MAY BE AFFECTED BY
AVERAGING OVER A LARGE MCS...BUT IT IS ALSO NOT UNCOMMON FOR
SIGNIFICANT CONVECTIVE SYSTEMS TO ALTER THE MID-UPPER LEVEL FLOW
IN WHICH THEY ARE EMBEDDED...AND THIS COULD PERHAPS LOCALLY REDUCE
THE MEAN WIND SUFFICIENTLY TO HELP ACCOUNT FOR THE NEARLY
STATIONARY STORM MOTION OBSERVED ON RADAR.

THEREFORE...CONDITIONS APPEAR FAVORABLE FOR SLOW-MOVING
CONVECTION...LOCALLY HEAVY RAINFALL...AND POSSIBLY FLASH FLOODING
IN A NARROW REGION NEAR THE NE-SD BORDER. THIS POTENTIAL MAY BEGIN
TO WANE IN SEVERAL HOURS AS THE MCS TO THE SOUTHWEST BECOMES
FURTHER REMOVED FROM THE AREA...AND ITS INFLUENCE ON THE MESOSCALE
ENVIRONMENT WILL WEAKEN SUBSTANTIALLY. UNTIL THEN...RAIN RATES
AROUND 2 IN/HR OR PERHAPS LOCALLY A LITTLE HIGHER WILL BE LIKELY
WITH THE STRONGEST CONVECTION (AND HAS ALREADY BEEN ESTIMATED BY
KLNX DUAL POL AND MRMS).

LAMERS

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

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

LAT...LON   43960007 43709902 43309807 42759743 41979726
            41879835 42709940 43220046 43740082