Mesoscale Precipitation Discussion 0705
NWS Weather Prediction Center College Park MD
1236 AM EDT Tue Aug 21 2018

Areas affected...Southern Wisconsin

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

Valid 210434Z - 211000Z

Summary...Areas of rain will persist through the night over parts
of Southern Wisconsin, and some of the rain bands could produce
localized heavy rainfall. Steadier rain is anticipated over
south-central Wisconsin, with the potential for higher rain rates
and flash flooding expected to be higher in southeast Wisconsin.
Rain rates in southeast Wisconsin could be as high as 2-3 in/hr.

Discussion...An environment capable of producing significant
rainfall should persist overnight across southern Wisconsin. Areas
just west of Madison recorded extreme rainfall totals in excess of
10 inches in some spots. Several mesovortices were persisting over
south-central Wisconsin, but the rain rates have generally
decreased. Steadier rain is expected in these areas, but
additional rain over already saturated soils could allow flooding
to linger. The greater risk for flash flooding is expected to be
focused in southeast Wisconsin for the remainder of the Night. A
secondary surface low was developing near the triple point of an
occluded cyclone, and this low was located over far northeast
Illinois. A focused region of low-level convergence was situated
just to the north of that in a roughly WSW-ENE fashion in the
general vicinity of I-94. To the north of I-94, surface
observations showed increasing winds out of the NE at around 10-15
knots, gusting to 20 knots in some cases. Models show the winds
south of Milwaukee increasingly becoming southerly in the next few
hours, and this should increase the low-level convergence.

KMKX radar shows the primary rain band gradually lifting north,
although there were some signs of the western extent of the band
stalling in Columbia and Dodge Counties, likely associated with a
slow-moving MCV. However, even if the rain band continues to move
north, ingredients would still favor the potential redevelopment
of convective rain bands closer to the Milwaukee metro area. By
06-08Z, the RAP forecasts the PW axis and CAPE axis to be focused
very close to MKE and the low-level convergence zone. A mid-upper
level low and +PV anomaly should continue to move ENE through
northern Illinois at the same time. To the north of that, there
should be a well of reduced mid-upper level wind speeds, and
slower effective storm motions. Therefore, any rain bands that do
redevelop could become anchored more easily along low-level
boundaries and stall. Several hi-res models, including a few runs
of the HRRR, show rain rates peaking around 3 in/hr in
redeveloping convection after 06Z. While this scenario is not
certain, it is at least plausible given the mesoscale setup, and
could lead to flash flooding in southeast Wisconsin given the
rainfall that has already occurred. If redevelopment does not
occur, flash flooding could still occur with the rain band
currently lifting north and gradually stalling, from Columbia
County east toward Ozaukee County.

Lamers

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

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

LAT...LON   44108754 42908751 42648876 42599019 43379063
            43888974