Mesoscale Precipitation Discussion 0724
NWS Weather Prediction Center College Park MD
745 PM EDT Mon Aug 27 2018

Areas affected...NE IA...SE MN...SOUTHERN WI

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

Valid 272345Z - 280545Z

Summary...Convection training ahead of a slowing front in a high
moisture environment is expected to produce flash flooding into
the early morning hours.

Discussion...Regional radars showed convection ahead of a short
wave crossing  west central WI. The mode of the convection this
far as been strong winds/hail, as dry air in the mid levels is
allowing for cold pool development. The convection is moving
through a tongue of 2000-3000 J/KG of MUCAPE, which is largely
surface based, according to the most recent RAP soundings.

The southwest end of the convection line is beginning to slow, as
the short wave energy tracking from eastern MT int northern ND
allows the mid level flow to back. The mid level southwest flow
will cause a frontal boundary extending from northwest WI into
northwest IA to slow, as it becomes more parallel to the mid level
flow. The convection should become more west to east oriented
ahead of the slowing front, as the 850-300 mb mean wind becomes
better aligned with the propagation vectors. This alignment will
foster an environment where training becomes more likely.

An increasing low level southwest jet (reaching 40 to 50 knots
after 28/04z) will pump 2.00+ inch precipitable water air (which
is between two and three standard deviations above the mean) along
and ahead the front. This allows the column to moisten as the
convection evolves more of a flash flood threat. Strong low level
moisture transports occurs during this time, during which hourly
rainfall rates could exceed 2.00 inches over portions of central
and southern WI, as warm rain processes become more important
during the transition.

The greatest threat for flash flooding is expected to occur
between 28/02z and 28/06z (and possibly beyond, as the front
slows), over central and southern WI. Portions of this area
received more than 5.00 inches of rain early Monday morning
(mainly north of a Madison to Milwaukee line). There is a strong
high resolution model signal for 2.00/4.00 inches of rain central
and southern WI, though there is still some latitudinal spread on
the placement.

If the heaviest rainfall occurs over areas that received heavy
rainfall Monday morning, significant flash flooding is possible,
especially near and north of Madison and Milwaukee. The threat
could last beyond 28/06z, especially over southern WI, as the
front slows and moisture continues to focus on and south of the
front.

Weiss/Burke/Hayes

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

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

LAT...LON   45308952 45208812 44598740 43748752 43198822
            43249077 43189141 43319257 43999305 44429281
            44929190 45229077