537
AWUS01 KWNH 211855
FFGMPD
WIZ000-MNZ000-IAZ000-220030-

Mesoscale Precipitation Discussion 0298
NWS Weather Prediction Center College Park MD
254 PM EDT Tue May 21 2024

Areas affected...Southern MN...Northern IA...Western WI

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

Valid 211855Z - 220030Z

SUMMARY...Blossoming strong thunderstorms containing efficient
hourly rainfall rates up to 2"/hr may causes areas of flash
flooding, particularly in areas with sensitive and saturated soils.

DISCUSSION...A dynamic synoptic scale pattern is fostering
excellent upper level ascent through two primary methods. The
first is a negatively tilted 250-500mb trough approaching from the
Central Plains and the second is the region`s placement beneath
two divergent 250mb quadrants (right entrance region via a jet
streak in southern Canada, left exit region via a jet streak in
the southern Plains). At the surface, a strengthening surface low
and its associated warm front are lifting north through IA and
heading for southern MN this afternoon. Visible satellite imagery
showed brief clearing over northern IA and southern MN in wake of
this morning`s rainfall, helping to increase MLCAPE in the region
by 200-600 J/kg over the past few hours. Meanwhile, the cold front
swinging east through eastern NE is responsible for sparking
storms that are quickly tracking northeast.

Adding to the excessive rainfall threat this afternoon is an
exceptional IVT topping 750 kg/m/s at 18Z, easily topping the 99.5
climatological percentile according to NAEFS. PWs will surpass
1.5" this afternoon within the highlighted region through 21Z,
which is also around the 99.5 climatological percentile. As peak
heating continues, lifting mechanisms via surface based heating
and mesoscale triggers (the warm and cold fronts) will ignite
numerous heavy thunderstorms over northern IA that are forecast to
race into southeast MN and western WI. Projected soundings via the
16Z HRRR this afternoon suggest MLCAPE is likely to surpass 500
J/kg and low-mid level RH values averaging around 90%. In
addition, warm cloud layers not only feature some of that
available instability but are also as deep as 10,000ft in most
cases. These storms are likely to generate exceptional downpours
with rainfall rates topping out around 2"/hr at their peak.

One deterring factor for flash flooding is that storm motions
should be progressive, but the areas that see excessive rainfall
longest will be closer to the developing triple point this
afternoon. These areas would receive heavy rainfall closer to the
warm front initially, then as storms along the cold front approach
from the southwest. The 12Z HREF depicts moderate chances (40-60%)
for 3-hr QPF > 3-hr FFGs in central MN between 21-00Z this
afternoon, which would include the Twin Cities metro area. NASA
SPoRT-LIS 0-10cm soil moisture is above normal (70-90%) in central
MN, making those areas particularly susceptible to flash flooding.
In summary, flash flooding is possible in parts of the Upper
Mississippi Valley this afternoon and evening with storms likely
impacting the afternoon rush hour.

Mullinax

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

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

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

LAT...LON   46029172 45179115 44349149 42439313 41579503
            41879601 42889647 44339584 45749369