Mesoscale Precipitation Discussion 0767
NWS Weather Prediction Center College Park MD
952 PM EDT Tue Sep 04 2018

Areas affected...Southern Nebraska and North Central Kansas

Concerning...Heavy rainfall...Flash flooding possible

Valid 050150Z - 050700Z

Summary...Thunderstorms developing ahead of a cold front will
become more widespread during the next few hours with training
likely. Rainfall rates of 1-2 inches per hour are expected, with
rainfall totals of 2-4 inches possible. This rainfall will have
the potential to produce flash flooding.

Discussion...Regional radar mosaic and GOES-16 IR products this
evening depict thunderstorms blossoming across NW Kansas and S
Central Nebraska. Rainfall rates within these storms have reached
1.5-2 inches per hour according to latest KUEX hourly rainfall
estimates, and this activity is forecast to increase through the
next several hours.

Analysis of the latest METAR data shows a cold front draped from
near Sioux City, IA to Kearney, NE, west towards Akron, CO. This
front is being driven southward into an environment characterized
by very high moisture content with PWATs of 1.6 to 1.8 inches
analyzed by latest RAP as well as the GFS blended TPW product.
While the deeper moisture plume lies just east of the threat area,
these values are still above the 90th percentile for the date,
suggestive of the potential for very heavy rainfall. Additionally,
warm advection on 20-30 kt 850mb winds are forecast to increase
the warm cloud depth above 4000m, which combined with moderate
MLCape around 1000 J/kg support increasingly efficient warm rain
processes in thunderstorms.

Convergence of flow between 850-700mb is likely as the front sags
southward, which combined with increasing diffluence within the
right rear quadrant of a 300mb jet should allow convection to
increase in coverage. These storms will develop across NW KS and S
Central NE before spreading E/NE on 0-6km storm motion of 10-15
kts from the S/SW according to latest RAP proximity soundings.
This is nearly parallel to latest cell motions and suggests
training of thunderstorms is likely, so despite relatively lower
HRRR soil moisture flash flooding will be possible.

Weiss

ATTN...WFO...DDC...GID...GLD...ICT...LBF...OAX...TOP...

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

LAT...LON   41559704 41079640 40019721 39249834 38799931
            38630053 39000140 39860144 40230078 40520006
            40889922 41359810