999
AWUS01 KWNH 300139
FFGMPD
SDZ000-NDZ000-300730-

Mesoscale Precipitation Discussion 1147
NWS Weather Prediction Center College Park MD
939 PM EDT Sat Aug 29 2026

Areas affected...Northern South Dakota...ext Southern North
Dakota...

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

Valid 300140Z - 300730Z

SUMMARY...Slow moving/splitting supercells will increase in
coverage as shortwave approaches expanding into a larger west to
east complex with rates up to 1.75"/hr & localized totals of 2-4"
resulting in possible flash flooding.

DISCUSSION...01z surface analysis continues to show an elongated
stationary front from a surface low north of the Black Hills along
I-90 to another surface low south of PIR and continuing into SW
MN.  Moisture remains pooled along the boundary and through the
Missouri River Valley, but deeper low to mid-level moisture exists
across central MN and is advecting east to west under confluent
flow to pool into a broader area of 1.75+ Total PWat east of
surface pressure trof in the Missouri River Valley east of a
developing 850mb low.  This low will continue to strengthen as a
well defined but fast moving shortwave crosses out of E WY into
the Dakotas in the next few hours.  Deep layer convergence along
that north-south boundary north of the stationary front provided
sufficient ascent to break through weaker capping to utilize
modest 2500 J/kg conditionally unstable air north of the boundary
in the deeper moisture.   Proximity to the strong jet aloft but
with enhancing height-falls approaching supports strong effective
bulk shear into the 50-55kt range to support supercell structures
initially.  The low level confluence from the east, bent Bunker`s
Right moving cell propagation vectors to be well reduced and
therefore propagation has been limited.  This resulted in a
localized cell to slow accumulate rainfall totals in SE Walworth
county toward 2-3" and MRMS FLASH (while slightly hail
contaminated), still resulted in pocket of 200-400 cfs/sq. mi.
generally suggestive of a local flooding concern, but was very
small in scale.  However, that moisture/hail loading into the
updrafts has recently resulted in some splitting of cells, further
reducing local residency.

However, with the approach of the height-falls is a leading strong
shortwave across E WY that also has a favorable outflow channel to
the northeast in a jet streak denoted in GOES-E WV as cirrus
shield with transverse banding across SE MT into ND.  This will
further provide cyclogenesis and direct circulation enhancement to
the frontal zone and strengthen LLJ with SE to NE confluence along
the boundary bringing moisture flux up to bring PWat values over
2" supporting further rates up to 1.75-2"/hr as individual cell(s)
begin to cluster/grow upscale into a complex.  RAP forecast
suggests a strong downslope westerly wind burst will surge across
SW SD/NE NEB further aiding to cyclogenesis in the left rotor with
850-700mb flow strengthening to 15-25kts and expanding the deep
layer moisture flux convergence west to east.  While not generally
parallel to the strengthening deep layer steering flow, sufficient
bulk shear will continue to support rotating updrafts and reduce
forward motions even hinting at potential for repeating axis
across northern South Dakota.   Given this streak(s) of 2-3" with
isolated spot or two of 4" are becoming increasingly possible.

While the region has slightly drier than normal upper soil
conditions (0-40cm per NASA SPoRT LIS RSM product),  Hourly FFG
values of 1-1.5" are mixed in with 2-2.5" spots east of 100W;
becoming 1.5-2" and 2.5-3" for the 3 hour range.  Given the
potential for 2-3" and spots of 4", localized exceedance is
becoming increasingly possible within/along the 850-700mb shear
axis suggesting localized flash flooding will be possible through
the early overnight period.

Gallina

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

ATTN...WFO...ABR...BIS...UNR...

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

LAT...LON   46349900 45659812 44849831 44659947 44630131
            44870256 45610270 46240138