Areas affected…Northern NE…Southern SD…Southwest MN
Concerning…Heavy rainfall…Flash flooding likely
Valid 120116Z – 120716Z
Summary…Repeating rounds of thunderstorms this evening over
saturated soils will pose a flash flood risk. 2-4 inches with
locally higher amounts is likely through 07Z.
Discussion…Fairly strong setup for locally heavy rainfall and
flash flooding this evening across portions of Nebraska and
southern South Dakota and southwest Minnesota. As of 01Z, a
stationary boundary was analyzed from northeast CO through NE into
NW Iowa. South of this boundary, rich/moist air exists with
dewpoints in the lower 70s. A recent mesoanalysis showed that the
best instability to be right along the SD/NE border and southward
where up to 3000 J/kg exists (MUCAPE of 1000 J/kg extends
northward into SD).
Convection has been blossoming in the last couple of hours across
northeast CO, NE, and SD with rapidly cooling cloud tops seen in
the latest GOES IR imagery. Radar estimates from hourly rates in
the 1-2″ range with local maxes in the 2-2.5″ range.
An increasing low-level jet will pump anomalously high moisture
over the stationary boundary that is draped west to east across
north-central Nebraska. This will be in combination with
increasing height falls with the approach of a strong shortwave
and the right entrance region of a jet streak north of the Great
Lakes will provide the large scale forcing for ascent. Mean flow
through the evening and overnight will be out of the southwest and
nearly parallel to the storm motions and this is likely to
contribute to training convection, particularly over northeast NE
and southeast SD. Convection is likely to congeal and form
multiple west to east oriented line segments. This could lead to
further rain totals with enhanced training.
The latest hi-res model guidance is performing poorly with the
current activity, and is generally displaced too far north into
the stable air. But the QPF does look somewhat reasonable and
shows swaths of 2-4″ with the potential for locally higher amounts
in the 5-6″ range. This is likely to fall on areas that have seen
recent heavy rainfall and thus have lowered FFG and saturated
soils (previous 7 day departures are 400-600 percent of normal).
As a result, flash flooding is likely through late this evening or
into the early overnight hours before thunderstorms move east or
weaken with loss of instability.
…Please see www.wpc.ncep.noaa.gov for graphic product…
LAT…LON 44409526 43769467 42899501 42239762 42210092
42730136 44010100 44209762