568
AWUS01 KWNH 050727
FFGMPD
MNZ000-IAZ000-SDZ000-051300-

Mesoscale Precipitation Discussion 1207
NWS Weather Prediction Center College Park MD
326 AM EDT Sat Sep 05 2026

Areas affected...Eastern South Dakota...

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

Valid 050730Z - 051300Z

SUMMARY...Warm-air advection convection will be slow moving with
rates of 1.5"/hr .  Very dry, high FFG grounds may be able to
uptake much of the rainfall, but isolated exceedance remains
possible given localized totals of 2-3.5" are possible through
morning.

DISCUSSION...GOES-E WV suite shows broad warming across SD as
downstream ridging amplifies slowly, but also a broad area of
increasing divergence aloft exists as well, as sub-tropical jet
turns more easterly across KS and S NEB, as well as advancing
large scale trough enters the northwest providing diffluent flow
of the polar jet streak across the western Dakotas.  As such, the
LLJ has rapidly increased over the last few hours with 850mb winds
increasing to 40-50kts over the western Dakotas, with
confluent/convergent 25-30kts across eastern South Dakota.  This
convergent flow has increased deep layer moisture to over 1.5"
pooled along a stationary front that bisects SD from NW to SE
before turning east across N IA.  Surface Tds in the low to
mid-70s and increasing 850mb moisture but steepened lapse rates
over 700mb from mid-level dry air supports high MUCAPE over 4000
J/kg per RAP analysis.  Though highly capped, the strength of the
deep layer moisture confluence/convergence is supporting
isentropic ascent across the boundary resulting in convective
development across south-central SD with scattered TCu and
isolated CBs into north-central SD.

GOES-E 10.3um shows cluster cooling below -60C, and RADAR shows
numerous smaller updrafts starting to congeal into larger
clusters.  Rain-rates are starting to respond and increase locally
to 1.5"/hr.  This may occasionally uptick toward 1.75", but the
greater concern is residency time.  Deep layer steering in the
divergent flow aloft and building ridging (noted in 850-500mb
reduced flow east into central MN), suggests very slow north to
north-northeast motions.  However, given the strength of the
low-level inflow and 1000-500 thickness packing; propagation
vectors are toward the south-southeast at 5-10kts, resulting in
slow eastward drift at 5-10kts.  Upwind redevelopment in proximity
to the isentropic boundary may help with some
back-building/regeneration to further increase time at any given
location.  As such, widely scattered spots of 3-3.5"/hr are
possible within an axis of 2-3" east and north of the Missouri
River & frontal zone.

This increased residency will be required given naturally high FFG
values of 2-2.5"/hr and 2-3"/3hrs across much of the area of
concern.  Combined with long-period dry grounds as 0-40cm ratios
are dismally low less than 10%.  Rates can be high given hard
ground conditons but may not be excessive enough that slow
infiltation is more likely.  Still, the overall totals locally of
3-3.5" may still result in a low-end isolated to widely scattered
incident or two of flash flooding through the early morning and is
therefore considered possible.

Gallina

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

ATTN...WFO...ABR...FSD...

ATTN...RFC...KRF...MSR...NWC...

LAT...LON   45649842 45359757 44839674 44359639 43669620
            43149641 42929679 42939768 43219867 43999931
            44889962 45559925