AWUS01 KWNH 301211
FFGMPD
MNZ000-IAZ000-NEZ000-SDZ000-301700-

Mesoscale Precipitation Discussion 0758
NWS Weather Prediction Center College Park MD
809 AM EDT Tue Jul 30 2024

Areas affected...Far southeast South Dakota, western and central
Iowa

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

Valid 301208Z - 301700Z

Summary...Training and showers and thunderstorms will move
southeast through the morning. Rainfall rates may exceed 2"/hr at
times, leading to a corridor of 2-3" of rainfall with locally
higher amounts. Flash flooding is possible.

Discussion...The regional radar mosaic early this morning
indicates an expansion of convection from from NW IA back into
central SD. These thunderstorms are forming along the convergent
nose of a LLJ which is observed via regional VWPs to be out of the
S/SW at 25-30 kts, drawing higher MUCAPE of 2000-3000 J/kg
northward. At the same time, a weak mid-level impulse noted in WV
imagery is pivoting eastward across SD to work in tandem with the
LFQ of a modest upper jet streak to drive additional ascent. PWs
across the area as measured by GPS are around 1.1-1.3 inches,
around the 75th percentile for the date, which when combined with
the elevated MUCAPE is providing a favorable environment for heavy
rain rates. Convection has expanded rapidly in the past hour
across SD, with some backbuilding of radar-estimated rain rates
over 1"/hr.

The CAMs are struggling to initialize the current activity, and
feature a wide variety of solutions through the morning. This is
lowering confidence in the evolution the activity today, but the
ingredients suggest an increasing flash flood risk the next
several hours. As the LLJ only slowly veers more to the W/SW, it
will continue to surge elevated instability northward into the
region of greatest ascent. Mean 0-6km winds of 15-20 kts suggest
progressive storms which will limit the duration of heavy rates
within any cell, but Corfidi vectors becoming increasingly right
of this mean wind suggest an enhanced training potential as cells
build back into SD and then train into IA. The HREF neighborhood
probabilities suggest a 30-40% (5-10%) chance for 1"/hr (2"/hr)
rates, which through training could produce 2-3" of rain with
locally higher amounts. Exactly where the heaviest rain axis sets
up is still very uncertain, but the ingredients suggest it will
occur somewhere in the vicinity of the convergence of the LLJ,
which is supported primarily by the ARW2 and RRFSp1 members.

NASA SPoRT 0-40cm soil moisture percentiles are above 80-90% from
far eastern SD into much of IA due to recent rain, and this is
reflected by locally compromised 3-hr FFG as low as 1.5-2"/hr. Due
to the spread of the CAMs, the HREF exceedance probabilities are
modest at just 10-20%, but it is still possible that any enhanced
training of these intense rain rates could result in isolated
instances of flash flooding.


Weiss


ATTN...WFO...DMX...DVN...FSD...OAX...

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

LAT...LON 43749740 43619624 43369547 43009425 42429280
41819220 41309197 40829226 40729287 40709428
40999533 41849623 42679699 43229774 43519770

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