Mesoscale Precipitation Discussion 0729
NWS Weather Prediction Center College Park MD
211 PM EDT Tue Aug 28 2018

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

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

Valid 281810Z - 282310Z

Summary...Again a swath of 2-plus inches of rain, with local
amounts approaching 5 inches, will occur this afternoon -
overlapping to a great extent with areas saturated by recent very
heavy rains. Flash flooding is likely, and some high-end flash
flooding is possible.

Discussion...Over the course of 2 days part of southwest Wisconsin
had experienced over 10 inches of rain, and 5 to 8 inches of rain
had generally fallen in a west to east stripe across
central/southern WI and ending at Lake Michigan to the north of
Milwaukee. In rare fashion, the repeating rounds of convection had
failed to stabilize the atmosphere over this region, owing to a
capped environment to the south and unseasonably strong wind
fields ahead of a broad 1.5 standard deviation upper trough.
Enhanced southwesterly flow had acted to restore lapse rates and
advect air from the unstable source region back up into Wisconsin.

At 1730Z the SPC mesoanalysis again depicted uncapped air with
mixed-layer CAPE values of 3000 J/kg over southern to eastern WI
and northeast IA. Radar and satellite loops gave a sense of
organized synoptic lift along the southern flank of a 70-110 kt
250-mb jet and in the vicinity of a surface cold front running
from northeast WI to southwest IA. An outflow boundary was evident
by tracing radar/satellite loops back in time - and ran from near
Chicago up to an intersection with the cold front north of Lone
Rock, WI. Subtle upglide north and east of this point will likely
further enhance precipitation potential this afternoon.

Hi-res model guidance was again nearly unanimous in depicting
another stripe of 1 to 3 inches of rainfall on average, and
isolated amounts of 4 to 5 inches through 23Z across southwest to
central and northeast Wisconsin. This will likely occur in
response to deep layer frontogenesis as the larger upper trough
slowly approaches, and the upper jet is forecast to further
intensify along the international border. Low level or 850-mb flow
was initially veered, but over time the RAP forecasts backing from
surface to 850-mb and increased low level convergence along the
frontal boundary and newly developing outflows. Seemingly the only
failure mode would be if ongoing scattered convection in southern
Iowa were to send out a stabilizing outflow. It appeared, however,
that there was already a vast pool of unstable air to be processed
in NE IA and WI...and the southern IA activity will likely be
unable to dominate, as deep layer ascent clearly favors areas
farther north per the RAP and the strong hi-res model QPF
consensus.

The effects of an additional 1 to local 5 inches of rain atop
saturated ground will almost certainly cause flash flooding, but
may result in particularly dangerous flooding in some locations.
WPC will be upgrading the Excessive Rainfall Outlook to High Risk
for this event.

Burke/Weiss

ATTN...WFO...ARX...DMX...GRB...MKX...MPX...MQT...

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

LAT...LON   45268844 45178693 44218735 43588770 43088948
            42719216 42639370 43529361 44569213