773
AWUS01 KWNH 142321
FFGMPD
INZ000-WIZ000-ILZ000-MNZ000-IAZ000-150430-

Mesoscale Precipitation Discussion 0619
NWS Weather Prediction Center College Park MD
721 PM EDT Sun Jul 14 2024

Areas affected...Southern WI...Northern IL...Eastern IA...

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

Valid 142320Z - 150430Z

SUMMARY...Increasing convective development in proximity to
mid-level shortwave, deep moisture and highly unstable airmass,
supportive of intense, efficient rainfall production with rates of
2-2.5"/hr along leading edge of MCS.  Combined with areas of
recently saturated soil conditions across S WI, NE IL;  flash
flooding will become increasingly likely into the overnight hours.

DISCUSSION...GOES-E WV denotes a shortwave trough across WI
centered in the vicinity of LaCrosse moving east, generally
over-topping the northeast side of the synoptic ridge axis moving
into greater diffluent flow.   This has spurred a surface wave to
develop along the SW WI/NE IA border with an effective warm front
extending east and then southeast parallel to the SW Lake Michigan
coast toward Chicago with broad southwesterly flow behind it with
increasing Tds from low 70s at the front back to low to even some
mid-80s Tds along a trailing pre-frontal convergence trough that
bisects Iowa from SW to NE.  CIRA LPW denotes this slug of
enhanced sfc to 850mb moisture overlapping some modest moisture
values generally associated with the mid-level trof through 500mb
resulting in totals of 1.75 to 2" from central IA into S WI.
Strong surface heating and the moisture and strong capping has
lead to very high CAPE values with MLCAPE reaching as high at 4500
J/kg in the capped region further southwest though values over
3000 J/kg will support strong updrafts.

Given the upper-level DPVA and increasing diffluence; convective
initiation has started in earnest in proximity to the convergence
along the pre-frontal trough into and downstream of the core of
the upper-level low.  Expectation is from rapid development and
increased moisture flux into the cells.  Combined with
strengthening LLJ toward dusk of 25-30kts of southwest flow;
upscale growth into an organized MCS is expected (See SPC MCDs for
additional details on severe concerns).  Strong convergence along
the leading edge will maintain this flux and support very high
instantaneous rain rates with 2-2.5"/hr totals perhaps falling in
as short as 30 minutes with the strongest convergence.

These rates/totals alone are likely to result in localized flash
flooding conditions given most of the area of concern has 1hr FFG
values between 1-1.5".  Recent heavy rainfall last evening from
two systems, resulted in swaths of 2-5" totals from W WI to
south-central WI to north-central/northeast IL reducing longer 3hr
FFG values to below 2" given 0-40cm soil saturation over 70% per
NASA SPoRT LIS product; further increasing the likelihood of flash
flooding across these areas.  Urban centers of Madison, Rockville
and the Chicagoland area will also be at increased risk given this
overlap and flash flooding is considered likely given if overall
totals remain more in the 2-3" range with widely scattered 4"
totals.

Further west into eastern Iowa...
The grounds are less saturated and 3hr FFG is between 2-2.5", so
still prone, but perhaps not rising to the higher risk/certainty
level.  Additionally, the line (discrete cells) may be more
scattered in nature due to stronger capping.  Cell motion will
mitigate extreme totals, but if cell rotation is enhanced...so
will flux and reduce forward propagation (especially in the apex
of the 500-1000 thickness ridge), more localized totals up to 3-4"
may be possible; and so flash flooding may be more
localized/scattered in nature.

Gallina

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

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

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

LAT...LON   43959048 43688914 43178800 42688774 41918743
            41408700 40908720 40468799 40508971 40929117
            41739298 42059329 42449338 42919299 43449171
            43859108