Mesoscale Precipitation Discussion 0766
NWS Weather Prediction Center College Park MD
654 PM EDT Tue Sep 04 2018

Areas affected...Southeast Minnesota; West-Central Wisconsin

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

Valid 042251Z - 050400Z

Summary...The threat of flash flooding should begin to increase
rapidly over the next several hours across southeast Minnesota and
west-central Wisconsin as bands of heavy rain and thunderstorms
repeatedly affect the same region from Albert Lea, to near
Rochester, to near Eau Claire. Rain rates to around 2 in/hr appear
likely, and 1-2 in/hr rain rates could be sustained almost
continuously for several hours in some locations. This would
likely lead to flash flooding, which could be relatively
widespread or severe in some counties in southeast Minnesota and
west-central Wisconsin.

Discussion...Convection continues to gradually increase and become
focused from north-central Iowa into southeast Minnesota and
west-central Wisconsin. As of 2230Z, regional radars generally
indicated convection was oriented in a fairly linear fashion, but
was not continuous. Instead, it was organized in small clusters or
supercells that were generally training along a similar path. This
may continue for another couple hours (through 00Z), with a
general trend toward a more continuous band of convection from
near AEL to near EAU. RAP analysis placed a low-level
frontogenesis maximum at 22Z around Winona County, MN, or just
east of Rochester, with a surrounding band of FGEN oriented in a
SW-NE fashion. The convection was generally organizing along and
just north of this low-level FGEN max, and the increasing trend of
the FGEN should favor an evolution to a more solid band of
convection. This will also lead to an associated increase in the
threat of flash flooding over the same area. Average storm motions
and the deep layer mean wind were quite close to the orientation
of the convective line, and thus continued training appears likely.

Also likely to increase the flash flood threat will be a slug of
low-mid level moisture now arriving in Iowa. GPS-PW observations
have shown a notable increase to around 2.1 to 2.2 inches in the
past several hours at most of the western Iowa observing sites.
These are near record values for the region based on the CFSR
climatology over the past 30 years. And the northward low-mid
level moisture flux will become increasingly focused into SC/SE MN
in the next couple hours. The KDMX radar VWP has shown an increase
in the low-level flow recently, with the 850mb winds now generally
35-40 knots.

Rain rates of around 2 in/hr have been observed at some local
mesonet sites in north-central Iowa, and shorter-duration rain
rates equivalent to that (e.g. 0.5 inches in 15 minutes) have been
observed northeast toward the Rochester area. This is consistent
with the environment in place across the region. If the increasing
frontogenesis can focus a strong band of convection on the
northern periphery of the pool of instability, it would be
possible for rain rates to locally exceed 2 in/hr.

Given the trends, flash flooding appears increasingly likely over
the next several hours across southeast Minnesota and west-central
Wisconsin. If a more continuous convective band can develop and be
sustained in a similar location for several hours, the possibility
of widespread and/or severe flash flooding cannot be ruled out,
particularly in the driftless region with sharper terrain
gradients and narrower basins from the greater Rochester area
northeast toward Buffalo and Trempealeau Counties in Wisconsin.

Lamers

ATTN...WFO...ARX...DMX...MPX...

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

LAT...LON   45069042 44379098 43429258 42929373 43199444
            43799410 44369311 45059143