Areas affected…Southern Minnesota, Northern Iowa, Southeast
South Dakota
Concerning…Heavy rainfall…Flash flooding possible
Valid 110353Z – 110845Z
Summary…Thunderstorms will steadily move from South Dakota into
southern Minnesota and northern Iowa overnight. These
thunderstorms should produce locally heavy rainfall and will have
the potential for flash flooding.
Discussion…A strong squall line was advancing through parts of
SD and NE to the north of a quasi-stationary surface front. The
progression has been uniformly faster than hi-res models are
indicating, even runs that initialized at 00Z. This may be due to
a relatively strong cold pool, evidenced by the 4.6 millibar
pressure rise in one hour at Mitchell, SD (MHE). Therefore, the
line of thunderstorms should continue to make steady progress
through the overnight hours in the vicinity of the front. The
low-level theta-e gradient offers some preview of a future track
of the MCS, and it is situated very close to the MN-IA border.
Although the MCS is showing forward progression, a ribbon of heavy
rain has been focused along and just south of the track of a
well-defined MCV, evident in radar reflectivity loops and velocity
data very near MHE at 0330Z. KFSD dual pol estimates have shown
hourly rain rates exceeding 2 in/hr at times with KDP values
elevated (over 2 deg/km) and fairly uniform CC — indicating a
high likelihood of significant amounts of liquid water. MRMS rain
rates have not been nearly as high, but have approached 2 in/hr.
Either way, this is very close to FFG values across the region and
would be supportive of flash flood potential in the very near term
in SE SD.
The question is how these rain rates will be sustained in the next
several hours as the MCS advances into MN and IA. CIRA blended TPW
products and latest GPS observations show a considerable PW
gradient with values well under 1 inch near MSP, and values
approaching 1.8 inches near SUX. The higher PW values also
coincide with fairly strong instability. Models do show a
northward expansion of this warm, moist air mass overnight, and
thus focus most of the QPF signal in S MN. However, the models are
also 3-6hr too slow with the progression of the squall line, and
thus may be overestimating how far north the aforementioned air
mass can advect prior to the arrival of the storms. Therefore, the
discussion area was centered more on the MN-IA border region,
which may be at a greater risk of heavy rainfall and flash
flooding given these factors. However, the area was still fanned
out into parts of C MN given some uncertainties and the strength
of the low-level jet. The instability in the inflow region should
be sustained for at least another several hours, so maintenance of
a corridor of heavy rain rates just along and south of the path of
the MCV seems likely, and would be a favored region for additional
flash flooding. This would be especially true if an eastward
extension of convection develops, associated with enhanced WAA
ahead of the maturing MCS, and overlaps with the eventual corridor
of heaviest rainfall.
Lamers
…Please see www.wpc.ncep.noaa.gov for graphic product…
ATTN…WFO…ABR…DMX…FSD…MPX…OAX…
ATTN…RFC…MBRFC…NCRFC…
LAT…LON 44779555 44529348 43029302 42489653 43119819
44199786