Areas affected…South Dakota and Nebraska Border Region
Concerning…Heavy rainfall…Flash flooding possible
Valid 070331Z – 070630Z
Summary…Flash flooding will be possible in the next few hours,
particularly over south-central South Dakota where a concentrated
area of thunderstorms may persist for a couple hours. These storms
are likely to build into northern Nebraska with time, but ground
conditions are less conducive for flash flooding there.
Discussion…KFSD radar showed training supercells in SC SD that
were beginning to congeal into a convective cluster. These
supercells were situated in an instability gradient on the
northeast periphery of a reservoir of strong instability.
Supercell motion vectors align nearly parallel with this gradient,
and so a corridor of heavy rainfall seems relatively likely along
at least a portion of the instability gradient from SD into C NE.
The flash flood threat associated with training thunderstorms
should be highest in the next couple hours in SD. Flash flood
guidance is much lower in these areas and ground conditions are
generally more conducive to producing flash flooding. In adjacent
portions of N NE, the Sand Hills region has much higher FFG due to
the soil type, and thus more significant backbuilding (longer
duration of heavy rain) may be required to produce flash flooding
further south. The duration of thunderstorms in SD will generally
be limited by an advancing cold front, which should focus
thunderstorms into Nebraska with time. Nevertheless, another hour
or two of heavy rainfall will be possible between K9V9 and KONL,
with an associated concentrated threat of flash flooding. Rain
rates of 1-2 inches per hour seem likely; these have been common
on MRMS which is attempting to correct somewhat for the presence
of hail. That is in line with the 1-hr FFG in that portion of SD,
but falls short of 1-hr FFG in N NE.
…Please see www.wpc.ncep.noaa.gov for graphic product…
LAT…LON 44169927 43599851 42759783 41929757 42069932