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AWUS01 KWNH 180739
FFGMPD
WIZ000-ILZ000-MNZ000-IAZ000-181330-

Mesoscale Precipitation Discussion 0626
NWS Weather Prediction Center College Park MD
338 AM EDT Thu Jul 18 2019

Areas affected...SE Minnesota, NE Iowa, SW Wisconsin, NW Illinois

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

Valid 180730Z - 181330Z

Summary...Thunderstorms undergoing upscale growth will produce
heavy rain through the early morning. Rainfall rates of 1-2"/hr
will be likely, and while the complex may begin to move quickly to
the east-southeast, backbuilding of cells will have the potential
to prolong heavy rainfall over some areas. Rainfall of 2 to
locally 4 inches may produce flash flooding.

Discussion...Thunderstorms have developed along the nose of a 50
kt 850mb LLJ from the SW which is advecting moisture and
persisting instability into the region. These thunderstorms are
expanding in response to deep layer ascent within the broad RRQ of
an upper jet streak shifting through the Great Lakes, and
low-level convergence along a slowly lifting warm front ahead of
the LLJ. Additionally, bulk shear of 35-50 kts is supporting
organization of individual thunderstorms, and the recent runs of
the HRRR and experimental HRRR suggests upscale growth will
continue into an MCS early this morning.

As the MCS organizes, it will likely begin to turn east and then
southeast as the LLJ backs subtly more to the south into IL/WI.
The thermodynamic environment ahead of this MCS will remain
favorable for heavy rainfall, noted by RAP analyzed PWATs of
1.6-1.8 inches in the pre-convective environment, which are above
the 90th percentile for the date at KDVN and KILX. The persistence
of the LLJ overnight will likely further raise the PWATs towards 2
inches, which will also drive MUCape up towards 2000 J/kg,
producing a highly favorable environment for heavy rainfall. RAP
Corfidi vectors gradually become more angled to the SE against the
LLJ, which produces a more oblique angle to the right of the
relatively fast 0-6km mean wind. This suggests that until the
complex becomes outflow dominated and can race forward on cold
pool generation, training of echoes is likely as individual echoes
move NE along the boundary of the MCS.

Recent radar estimated rain rates from KDMX WSR-88D have reached
near 2"/hr, and latest HREF neighborhood probabilities for 1-hr
rainfall show a greater than 60% chance for 1 inch, and 20-30% for
2 inches. With rapid individual cell motion limiting total
rainfall, it will take backbuilding of cells to prolong these rain
rates enough to produce a flash flood risk since FFG across the
discussion area is generally 2-3"/3hrs. However, despite some
uncertainty into exactly how the MCS will evolve and where it will
turn to the southeast, there is enough high-res consensus among
recent CAMs that flash flooding is possible due to a strong signal
for a corridor of 2-3" of rainfall with locally higher amounts.

Weiss

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

ATTN...WFO...ARX...DMX...DVN...LOT...MKX...MPX...

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

LAT...LON   44429344 44339203 44259139 43999058 43698985
            43428926 43068871 42708845 42328851 41968875
            41598919 41468947 41468988 41529042 41689098
            41929161 42159205 42359231 42619271 42969323
            43019337 43299389 43489446 43709476 44139458



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