Mesoscale Precipitation Discussion 0718
NWS Weather Prediction Center College Park MD
608 AM EDT Sat Aug 25 2018

Areas affected...Portions of Eastern IL and IN

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

Valid 251007Z - 251507Z

Summary...Thunderstorms with backbuilding character are starting
to fill in across northern IN and should develop southward with
time.  Hourly totals to 2" with local amounts to 4" are possible.

Discussion...An upper low is moving through Lake Michigan at this
time, which has led to a favorable environment aloft for
thunderstorms to grow in coverage to its south across northern IN
and east-central IL.  Inflow at 850 hPa is convergent out of the
west at 35-40 knots per VAD wind profiles.  Flow is fairly
unidirectional with height out of the west-southwest.  MU CAPE
upwind is 2000-3000 J/kg across north-central IL per SPC
mesoanalyses.  Since the instability pool is upstream, this is
leading to backbuilding of convection, particularly across
northern IN as of late.

The mesoscale guidance has a general idea of this convective
evolution, though recent HRRR runs are surprisingly low on
amounts.  A second batch of thunderstorms near the northern IL/IN
border are likely the back edge of the activity for northern IN. 
With time, cell collisions are likely to set up a cold pool with
forces an outflow boundary to shift south with time.  A bulk of
the guidance advertises local amounts in the 2-4" range over the
next several hours, with the 06z HREF probabilities of 1"+ an hour
indicating that heavy rain across the area should shift from north
to south with time.  It appears that the 00z ARW and 00z WRF NSSL
have the strongest signal for this rain -- advertising local
amounts in the 4" range, but at the moment they seem too far to
the south.  Available moisture and instability is capable of
producing hourly totals to 2".  This degree of rain could
challenge the three hourly flash flood guidance values.

Roth

ATTN...WFO...ILN...ILX...IND...IWX...LOT...

ATTN...RFC...NCRFC...OHRFC...

LAT...LON   41878592 41558484 40208496 39468567 38608800
            39738879 41158763 41658689