Mesoscale Precipitation Discussion 0800
NWS Weather Prediction Center College Park MD
359 AM EDT Sat Sep 08 2018

Areas affected...Southeast MO...Southern IL...Southwest
IND...extreme Western KY...

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

Valid 080800Z - 081400Z

SUMMARY...Very dangerous flooding expected ahead of the remnants
of Gordon.

DISCUSSION...The remnant center of Gordon appears to have melded
with the frontal zone across south central MO and the inner most
"band" of Gordon appears to be an effective warm front lifting
across the Mississippi River into S IL with very deep convection
with 10.3um cloud tops cooling to -65C.  These cells appear to be
along the leading edge of the deepest moisture with total PWATs
over 2.3" nearing 2.5" and mean mixing ratios over 16 kg/kg and
environmental instability is around 1000-1250 J/kg to maintain
these strong updrafts.   Given the deep moisture and flux
convergence along the band, cells will be capable of 2.5-3"/hr
rates and likely to induce flash flooding conditions.

RAP analysis depicts the mid-level wave is a bit north of the
surface low and a strong convergence trof extends ENE from just
south of BLV to SLO to LWV.  Between this boundary and the surface
front that is generally along the KY/TN border, very warm moist
and unstable air supports multiple bands of similar but slightly
reduced efficiency (slightly weaker winds/lower
moisture/instability) but with rates over 2"/hr are likely to
induce flash flooding as well.

Cells along these bands will rotate north on the mean flow into
the 850-7H deformation zone/convergence trof.  This trof is
particularly strong as it is fairly vertically stacked/steep north
to south further enhancing a deeper layer for
condensation/rainfall generation.  Due to surface/mid-level height
falls, winds will be steadily increasing with strong isallobaric
component.  30-35kt highly convergent 950mb winds will allow for
greater rainfall efficiency with rates on average of 1"/hr (higher
at times), along convergence trof even without solar
isolation/instability.  Given the orientation of the boundary is
generally parallel to the mean cloud barring flow and expected
track of the surface low (Gordon)...a narrow corridor generally
along/South of I-70 will produce 3-5" over the next 6hrs which has
lead WPC to introduce a High Risk of Excessive Rainfall for the
Day 1 period starting at 12z, where a large area is expected to
have flooding conditions and is a significant threat to life and
property.  Isolated totals over 6-8" are possible through the area
of concern, particularly near the intersection of the 850mb
front/surface front/surface low. 

Gallina

ATTN...WFO...ILX...IND...LMK...LSX...MEG...PAH...SGF...

ATTN...RFC...LMRFC...MBRFC...NCRFC...OHRFC...

LAT...LON   39768673 39478638 39088608 38488591 38088655
            37908700 37408802 36608891 36128971 36269031
            36409040 36709058 36999098 37249170 37459217
            37859230 38309201 38749145 38889133 39309004
            39598845 39728738