Mesoscale Precipitation Discussion 0704
NWS Weather Prediction Center College Park MD
931 PM EDT Mon Aug 20 2018

Areas affected...South-central Wisconsin

Concerning...Heavy rainfall...Flash flooding likely

Valid 210131Z - 210500Z

Summary...A significant flash flood event appears to be unfolding
in parts of Dane County, Wisconsin, with a persistent heavy rain
band that has been stationary for several hours. The heavy rain is
likely to continue for at least another hour. Additional heavy
rain bands were developing to the east and may begin to fill in
across south-central Wisconsin. This may cause the flash flood
threat to expand tonight. Rain rates of 1-2 in/hr will be more
common, and may exceed 2 in/hr in the strongest rain bands.

Discussion...A nearly stationary rain band over western Dane
County, WI was producing very heavy rain. Estimates of total
rainfall thus far from MRMS and KMKX dual pol are in excess of 5
inches from Cross Plains to Mount Horeb, and as high as 8-9 inches
in some areas on the dual pol estimates. The hourly rain rates
have been estimated to be as high as 3 in/hr. This rain band has
been sustained by very focused low-level convergence, and that was
likely helping to keep the rain rates high over several hours. An
additional band of convection has developed to the S/SE, extending
through Janesville and into the Chicago Metro Area. On its
southward extent, this band was showing more eastward motion,
likely pushing out as the synoptic low moves ENE. However, on the
north side of the low track, into southern Wisconsin, the
convective rain bands were showing reduced motion near the
deformation zone north of the low, and beginning to curl back to
the northwest. Additional smaller rain bands were beginning to
increase in coverage from Madison east to Lake Michigan over
southern Wisconsin as well. All this may lead to an expansion of
heavy rain and the flash flood threat this evening.

In the immediate future, through around 11 PM CDT, additional
heavy rainfall focused in western Dane County could lead to severe
flash flooding given the very heavy totals already observed. CREST
hydrologic model already shows maximum unit streamflow values
above 1000 cfs/mi, which is more consistent with significant flash
flood event. Furthermore, the flash flooding may expand more into
the city of Madison proper, as the convective rain band over Rock
County merges in from the southeast, leading to an eastward
expansion of the very heavy rain rates.

Lamers

ATTN...WFO...ARX...MKX...

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

LAT...LON   43768945 43338837 42838800 42598870 42678926
            42908993 43409013