Mesoscale Precipitation Discussion 0600
NWS Weather Prediction Center College Park MD
246 AM EDT Sun Aug 05 2018

Areas affected...Southeast SD...Ext Southwest MN...Northwest IA...

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

Valid 050646Z - 051030Z

SUMMARY...Growing cluster with internal cell mergers pose flash
flooding risk in the next few hours.

DISCUSSION...06z Surface analysis depicts a shallow well of very
rich moisture with Tds in the low 70s. Given a pocket of 1.5" TPWs
and steep mid-level lapse rates for MUCAPEs to 5000 J/kg, hail
generation will be dominant but still heavy rainfall up to 1.5"/hr
is likely.  VWP depicts modest WAA/moisture flux along 20-25kts SW
flow supported sufficient convergence/isentropic ascent to develop
a few cells precursory to the main line of cells.  These cells,
while not as deep/strong as the main cells upstream are slow
moving ahead of the main forcing allowing for some increased
totals.  It is the favorable orientation to the main cells that
will support mergers as well as training across McCook to
Minnehaha county and possibly into Sioux Falls proper (increasing
the threat for flash flooding due to urban runoff factor).  As
cells move eastward, convection should wane with a bit less
instability and reducing inflow as an 850mb low develops further
west.

While the upstream cells are moving fast, continued SWly low level
flow/WAA northeast of the developing 850mb low poses the potential
for additional development along the flanking line/outflow
boundary laid out by the main cells.  While there is a higher
probability that these WAA cells may be further south across the
southern counties of SD, recent HRRR runs suggest they could be
further north and track along the area to be affected in the next
hour or so further leading to factors for possible flash flooding
in SE SD through until morning.  

Gallina

ATTN...WFO...FSD...

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

LAT...LON   44339754 44279634 44009503 43379497 42789530
            42979669 43089843 43569877