Mesoscale Precipitation Discussion 0601
NWS Weather Prediction Center College Park MD
1030 PM EDT Sun Aug 05 2018

Areas affected...Eastern Nebraska...Iowa...Ext Southwest
Wisconsin...

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

Valid 060230Z - 060830Z

SUMMARY...Increasing isentropic ascent, moderate moisture flux and
some mean cell motions aligning for some training pose possible
scattered flash flooding conditions over night.

DISCUSSION...02z surface analysis depicts a well defined wind
shift/convergence zone along an effective front from the
MCS/squall line along the WI/IL border and crosses the southern
third of IA and just north of Nebraska City and back toward
McCook, NEB.  VWP network across NEB and Kansas shows steady
southerly surface to 850mb increase up to 25kts at 850mb
currently.  GOES-WV suite and AMV denote broad westerly flow
across the Dakotas into Nebraska with a subtle shortwave spin near
the Sand Hills providing broad scale ascent and focusing the LLJ. 
AMV and RAP analysis suggest a weak diffluent pattern over E NEB
into IA with a potential subtle split forming in the vicinity of
the frontal boundary.  As such, this will amplify low level
cyclogenesis across N Nebraska and accelerate the LLJ to 30-35kts
with slow veering. 

Current, regional RADAR mosaic and 10.3um IR channel show
convective expansion with new development upstream in response to
the LLJ/isentropic ascent focused near the MO River into central
IA. Mid-level lapse rates in the 7-7.5C/km range allow for strong
instability across E NEB, into IA with MUCAPE values of 2500-3000
J/kg.  The combination of these factors will further enhance
updraft/moisture flux with the strengthening of the LLJ which will
support reduced difference of  mean flow vectors to the LLJ
supporting some back-building into the best forcing across W IA. 
Tds in the upper 60s will ascend into the developing cells and
with low 70s Tds north of the boundary, elevated convection
rainfall will remain strong/efficient supporting rain rates in
excess of 2"/hr (as total PWATs increase toward 2" by 06z).   As
the wave advances eastward in the strong westerly flow, the LLJ
will veer further and support mean cell motions to align for some
training in the mid-overnight period.  This suggests 2-4" totals
will become increasingly likely within the area of concern.  Very
dry recent pattern is likely to help withstand some of these
totals, but the training/high rates still pose a possible flash
flooding risk, particularly across N Sac to Hamilton and
Butler/Black Hawk to Dubuque county given 1-3" totals from rounds
earlier today.

Hi-res CAMs are very supportive of this evolution to provide some
confidence.  The 18z and 00z NAM-Conest depicts a very convincing
evolution to the MCS but like many other guidance members shows a
distinctly slow evolution and north and west bias compared to
overall reality.  This bears some reduction in overall latitude of
the best QPF signal and suggest the best corridor is likely to be
in the vicinity of DNS to ALO/ULZ rather than across Northern IA. 
Also have included area across Northeast IA into SW WI, where
isentropic ascent across outflow boundary from Squall Line is
producing 1-2" totals currently which are likely to maintain over
the next hour or so before moving eastward into a more stable
environment and reduced isentropic ascent.

Gallina

ATTN...WFO...ARX...DMX...DVN...FSD...OAX...

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

LAT...LON   43369087 42389067 41989200 41789331 41459477
            41059647 42019687 43019448 43339302