099
AWUS01 KWNH 131914
FFGMPD
WIZ000-ILZ000-MOZ000-IAZ000-140100-

Mesoscale Precipitation Discussion 0266
NWS Weather Prediction Center College Park MD
313 PM EDT Mon May 13 2024

Areas affected...Portions of the Midwest

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

Valid 131912Z - 140100Z

Summary...Thunderstorms will increase in coverage and then move
slowly in the vicinity of a mid-level deformation axis draped
across the Upper Midwest through this evening. Rainfall rates
within this convection will reach 1-2"/hr, which through the slow
motions could accumulation to more than 3" in isolated locations.
Flash flooding is possible.

Discussion...The regional radar mosaic this afternoon shows
rapidly expanding coverage of showers and thunderstorms across the
Upper Midwest from Missouri through lower Michigan. These
thunderstorms are blossoming downstream of a prominent upper low
noted in GOES WV imagery, with ascent being provided through
downstream divergence and weak upper diffluence along the LFQ of a
jet streak positioned to the south. South and east of this low,
thermodynamics are gradually increasing noted by GPS observed TPW
of around 1.2 to 1.3 inches, which is right around the 90th
percentile for the date according to the SPC sounding climatology.
This elevated moisture is combining with SBCAPE analyzed by the
SPC RAP of 1000-1500 J/kg to produce a favorable environment for
rapid convective growth with heavy rainfall.

As the upper low evolves and tracks slowly east will result in a
sharpening deformation axis pivoting N/NE of the upper low, into
which the impressive theta-e ridge will impinge on the downstream
south/southeast flow. 850mb winds are modest at 10-20 kts, but as
this lifts the rich theta-e air into the deformation axis, it will
result in enhanced convergence, driving additional convective
growth from SW to NE along the boundary. This will cause Corfidi
vectors to collapse to less than 5 kts while at the same time
turning anti-parallel to the weak mean 0-6km winds, suggesting
nearly stationary storms with repeated rounds of heavy rainfall.
Both the HREF and HRRR sub-hourly fields indicate a high
probability for 1-2"/hr rain rates within this convection, and due
to the slow storm motions, HREF probabilities for 3"/6hrs reach as
high as 30% through this evening.

Recent rainfall across the Upper Midwest has been highly variable,
reflected by 7-day anomalies varying between 50% and 200% of
normal. This has left 40cm soil moisture anomalies generally near
normal according to NASA SPoRT, and 3-hr FFG around 1.5-2"/3hrs.
While the greatest flash flood risk may exist across urban and
less permeable areas, HREF exceedance probabilities for this 3-hr
FFG do peak at 40-50% later today, further indicating the
potential for at least isolated flash flooding into the evening.


Weiss

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

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

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

LAT...LON   42758832 42428776 41948770 41518807 41258857
            40659013 40019122 39639218 39589314 40079378
            40819380 41479333 42039206 42529029