Mesoscale Precipitation Discussion 0667
NWS Weather Prediction Center College Park MD
1059 AM EDT Wed Aug 15 2018

Areas affected...Southwest & South Central South Dakota...

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

Valid 151500Z - 151930Z

SUMMARY...Training Cells through Lower FFG poses possible
localized flash flooding through early morning.

DISCUSSION...GOES-WV suite depicts a strong/compact vort center
rolling out of NE WY into the Black Hills of W SD at this time,
with a strong undercutting jet with AMVs up to 60-70kts across W
NEB.  This places the area of concern in very strong dynamic
ascent with strong DPVA and solid diffluent outflow on the
relatively flat (W-E) Jet.  CAA aloft also has been steepening
mid-level lapse rates with 6C Temps at 7H near the low, with winds
expected to strengthen from the south at 7H of 20-25kts aligned
with narrow moisture corridor with 4-6C Tds at 7H providing solid
moisture for cells.  While main threat as highlighted in SPC MCD
1293, supports moderate to large size hail, it will be accompanied
with modest rainfall, and with hail clogging
culverts/ditches...this may support backing up water outflows near
roads/intersection where they are more prevalent.

The greater concern is propagation vectors and mean cell motions
are fairly parallel to convective organization line at this time,
supporting likely training.  This has already been seen in Fall
River into the Oglala Lakota Resv. where FFG values are already
low (<1.5"/hr) and likely on par with hourly rain rates given
Total PWATs approaching 1.5" given the mass flux.  While hail is
overestimating QPE values from MRMS, the magnitudes used for OU
FLASH Unit stream flows and QPE/FFG ratios even reducing QPE
significantly because of hail still would suggest flash flooding
may be possible in S Oglala Lakota and Fall River counties, likely
to expand into Bennett county with time, given the training
environment.  While there is some expectation for convection to
propagate/develop into Northern Neb, the Sand Hills should not be
affected and are carved out of the area of concern. 

With slight veering expected in low levels after 17z...cells may
expand further north into Jones/Lyman/Stanley counties, as
suggested by a good portion of Hi-Res CAM guidance.  This guidance
is fairly agreeable in the 2-2.5" swath of forecast QPF for
continued possible flash flooding through the early morning
afternoon due to the training and lower FFG north of the NEB
border.

Gallina

ATTN...WFO...ABR...FSD...LBF...UNR...

ATTN...RFC...MBRFC...

LAT...LON   44469965 44169913 43139905 43230099 43060174
            42960218 42820240 43040283 43150341 43450320
            43700230 43920156 44170077