534
AWUS01 KWNH 160107
FFGMPD
WIZ000-MNZ000-IAZ000-160700-

Mesoscale Precipitation Discussion 0444
NWS Weather Prediction Center College Park MD
906 PM EDT Sat Jun 15 2024

Areas affected...southern Minnesota through western Iowa

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

Valid 160105Z - 160700Z

Summary...Showers and thunderstorms will gradually expand in
coverage ahead of a warm front through early tonight. Rainfall
rates of 1-2"/hr are likely, which through multiple rounds could
produce 2-3" of rain with locally higher amounts. Flash flooding
is possible.

Discussion...The regional radar mosaic early this evening depicts
increasing coverage of showers and isolated thunderstorms from
western Iowa into much of southern Minnesota. These storms are
firing in response to increasing ascent along a surface trough
analyzed by WPC resulting in increased low-level convergence, and
in an area of modest but sufficient upper level diffluence. This
ascent is acting upon favorable thermodynamics, with the core of
the most intense environment impinging into the surface trough
thanks to advection on an 850mb LLJ that is arcing out of the S/SW
at 30-40 kts according to regional VWPs. This is helping to draw
northward PWs that are over 1.75 inches, collocated with MLCAPE
above 2000 J/kg, resupplying the environment to enhance rainfall
coverage and intensity. Recent radar-estimated rainfall rates are
still modest at 0.5-1"/hr, but the coverage of these has steadily
increased in the past 1-2 hours.

As the evening progresses, the impressive LLJ is progged via the
RAP to reach 40-50 kts, which when combined with a ribbon of PWs
surging northward towards 2 inches, will result in 850-700mb
moisture flux that could exceed +4 sigma into northern IA and
southern MN. Wind speeds within this LLJ will be 1.5-2 times the
mean 0-6km wind as well, suggesting even further enhancement of
ascent, and where this collision occurs in the vicinity of the
surface trough, repeated development of thunderstorms is likely as
supported by simulated reflectivity from the recent CAMs. Not only
will thunderstorms regenerate and repeatedly lift northeast, but
the strong ascent into the enhanced PW/instability will drive
rainfall rates which have a 10-20% chance of exceeding 2"/hr
according to the HREF neighborhood probabilities. The greatest
potential for the intense rates will be south of this surface
front where instability is greater, but more modest rates may be
more persistent to the north through regeneration. Although cells
may move quickly to the northeast on the 25-35 kts 0-6km mean
wind, the increasing and slowly veering LLJ will cause Corfidi
vectors to veer and collapse to around 10 kts, further supporting
the idea of regenerating and repeating cells. Where this occurs,
more than 3 inches of rain is possible (20-30% chance) with
locally higher amounts.

FFG across the region is as low as 1.5"/3hrs in southern MN where
7-day rainfall has been more than 100% of normal in some areas.
Otherwise, FFG is generally 2-2.5"/3hrs due to a relative lack of
recent rainfall. This is reflected additionally by NASA SPoRT
0-40cm soil moisture that is near normal in IA, but around the
80th percentile into MN. Despite these differences, the HREF FFG
exceedance probabilities are uniformly around 20-25%, suggesting
instances of flash flooding are possible anywhere within the area
into tonight.

Weiss

...Please see www.wpc.ncep.noaa.gov for graphic product...

ATTN...WFO...ARX...DMX...FSD...MPX...OAX...

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

LAT...LON   45669382 45629275 45049250 43889236 42599254
            41659313 41019375 40699453 41239516 42379598
            43389617 43989612 45049542