693
AWUS01 KWNH 201831
FFGMPD
WIZ000-ILZ000-202330-

Mesoscale Precipitation Discussion 0291
NWS Weather Prediction Center College Park MD
231 PM EDT Mon May 20 2024

Areas affected...Southern Wisconsin, extreme northern Illinois

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

Valid 201830Z - 202330Z

Summary...Showers and thunderstorms increasing downstream of a
potent vorticity impulse will train to the N/NE through the
afternoon. Rainfall rates of 1"/hr or more are likely, which could
produce 1-3" of rainfall. This could result in at least isolated
instances of flash flooding.

Discussion...A potent shortwave noted on GOES-E WV imagery this
afternoon is pivoting northeast across IA/IL/WI, producing
downstream ascent through divergence and height falls towards Lake
Michigan. This impressive lift is being aided by the RRQ of an
upper jet streak which is gradually traversing to the east, and
robust isentropic ascent as low-level flow traverses atop a warm
front to the south. Together, these forcing mechanisms are driving
robust lift into thermodynamics favorable for heavy rainfall
characterized by GPS measured PWs as high as 1.4 inches, above the
90th percentile for the date, and SPC RAP analyzed MUCAPE of
1000-2000 J/kg. During the past 1-2 hours, convective coverage as
rapidly expanded as noted via the regional radar mosaic, with
radar estimated rainfall rates reaching above 1"/hr on KMKX
WSR-88D.

The high-res CAMs are generally under-initializing the current
reflectivity observed via the radar, suggesting that the
environment is more favorable than progged by models. As the
potent shortwave advects northeast into the evening, pronounced
S/SW flow downstream of it, with 850-300mb winds reaching 30-40
kts, will surge PWs as high as 1.6 inches and SBCAPE potentially
climbing towards 2000 J/kg along and just south of the slow
northward advance of the warm front. With ascent likely to remain
impressive across the area, convective development is expected to
continue to expand and intensify, with organization of cells
likely within bulk shear reaching 35-45 kts. This suggests that
rainfall rates will reach at least 1"/hr as noted by HREF
exceedance probabilities reaching 30-50%, and may at times
approach 2"/hr as reflected by HRRR 15-min accumulations of around
0.5". As storms continue to develop and organize, they will
encounter 0-6km mean winds that are progressive around 40 kts, but
aligned to the layer Corfidi vectors to support likely training
training of echoes from SW to NE. Where the longest duration of
training occurs, 2-3" of rainfall is possible.

This portion of the Upper Midwest has been dry recently with AHPS
7-day rainfall departures generally just 25-50% of normal which
has resulted in 40cm soil moisture below the 30th percentile
according to NASA SPoRT. Despite that, FFG is in some places as
low as 1"/1hr and 1.5"/3hrs, especially around urban corridors,
which could be exceeded by these training heavy rain rates. This
could result in areas of rapid runoff, with training over any
urban areas producing the greatest chance for any instances of
flash flooding.

Weiss

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

ATTN...WFO...ARX...DVN...GRB...LOT...MKX...

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

LAT...LON   44188865 44128810 43978771 43638762 43008768
            42438773 42038786 41948836 42058894 42408951
            42578987 43019011 43898957