Mesoscale Precipitation Discussion 0774
NWS Weather Prediction Center College Park MD
233 PM EDT Wed Sep 05 2018

Areas affected...Southeast Iowa, Northwest Illinois, Northern
Missouri

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

Valid 051832Z - 052245Z

Summary...Scattered bands of heavy rain and embedded thunderstorms
should continue affecting parts of southeast Iowa, northwest
Illinois, and northern Missouri along and ahead of a surface front
this afternoon. Rain rates of 1-2 in/hr are expected, which could
cause flash flooding given relatively high soil moisture levels
across the region.

Discussion...Regional radar mosaic shows a ribbon of rain
stretching from the southern Plains into the Upper Midwest early
this afternoon, with embedded convective clusters. In general,
there was not much lightning, but the shallow convection was
producing heavy rainfall due to efficient warm rain processes. The
environment is characterized by deep moisture with precipitable
water values in excess of 2 inches (GPS-PW observations from
western Missouri into northwest Illinois are around 2.1 inches),
and tall, skinny CAPE profiles. The convection was somewhat
disorganized, however, with a lack of strong focused convergence
in the lowest levels of the troposphere. Recent RAP forecasts do
show an increase in low-level frontogenesis and convergence
through the afternoon hours in southeast Iowa and surrounding
areas, and the 17Z analysis did show the possible beginning of an
increase in 1000-925mb frontogenesis in far northeast Missouri.
This may aid in greater convective organization with time. If
convective bands can become oriented in a SW-NE fashion (parallel
to the deep layer mean wind), that would likely increase the flash
flood potential.

Despite the limited organization of convective clusters, the
rainfall efficiency and relatively high soil moisture levels (with
regional precip about 250-500 percent of average in the past two
weeks) are compensating to produce localized flash flooding
already, and this may continue through the remainder of the
afternoon. If a frontally-forced band of convection develops, a
greater threat of flash flooding could develop, and this appears
most likely from far northeast Missouri into the greater Quad
Cities area.

Lamers

ATTN...WFO...DMX...DVN...EAX...ILX...LOT...LSX...

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

LAT...LON   42209077 41738935 40529025 39479193 39349369
            40109455 41329334