South Dakota Mesoscale Discussion

Areas affected…Northern Nebraska

Concerning…Heavy rainfall…Flash flooding possible

Valid 110657Z – 111030Z

Summary…Thunderstorms should repeatedly affect some areas of
northern Nebraska during the early morning hours, and are likely
to produce a swath of heavy rainfall with localized totals in
excess of 4 inches. This may lead to some flash flooding.

Discussion…KLNX radar shows a training band of thunderstorms
that has persisted for a couple hours over northern Nebraska.
These storms were situated on the southern extent of a boundary
layer cold pool, and were likely somewhat elevated. A strong
southerly low-level jet was providing adequate ascent over the
stable layer to sustain convection for multiple hours in nearly
the same location. The heaviest rain has thus far fallen over the
Sand Hills region of northern Nebraska, or just to the north in
parts of South Dakota.

The potential for flash flooding is complicated by uncertainties
in ground conditions and the potential hydrologic response. The
Sand Hills are not typically a favored location for flash
flooding, as the sandy soils tend to allow for much better
infiltration and FFG is much higher on average. However, the
region has experienced an extended period of wetter-than-average
conditions, with 6-month precipitation departures over 150 percent
of normal in some areas. Therefore, the region may be more
susceptible to runoff issues than it would normally be.

However, regardless of these uncertainties, it does seem as though
training convection will persist for at least another hour or two.
Radar estimated rain rates (dual pol and MRMS) have reached 2
in/hr in some of the stronger convection, and these rain rates are
likely to intersect a portion of the heavier rainfall footprint
from earlier in the event. This may lead to localized rainfall
totals in excess of 4-5 inches in a corridor from southern Cherry
County to the O’Neill area of Holt County. Amounts that high in
six hours or less could conceivably lead to flash flooding,
particularly with the wet pattern so far already this year.
Convection may begin to wane in intensity and coverage around
08-09Z. Recent radar trends show a slight decrease in coverage of
40 dBZ reflectivity and models show a weakening of instability
and LLJ intensity in the next few hours.


…Please see for graphic product…



LAT…LON 42919944 42839760 42569665 41949670 41719806
41729956 42050063 42580040

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