South Dakota Mesoscale Discussion

Areas affected…Northwestern Iowa…northwest Wisconsin…and a
large portion of Minnesota

Concerning…Severe potential…Watch possible

Valid 211833Z – 212000Z

Probability of Watch Issuance…40 percent

SUMMARY…A gradual increase in storm coverage and intensity is
expected through the afternoon. A watch is possible.

DISCUSSION…Temperatures have warmed into the low 80s with
dewpoints in the low 70s in the wake of morning convection across
northwest Iowa and southwest Minnesota. 18Z surface analysis shows a
cold front across western Minnesota which extends southward to just
west of Sioux Falls. Surface observations show enhanced convergence
along the front in west central Minnesota and northwest Iowa with
less convergence in between. Further evidence for these areas of
enhanced convergence can be seen on GOES 16 visible satellite
imagery where there is line of agitated cumulus in northwest Iowa
and southwest Minnesota with little development along the front
north of there. While 12Z CAM guidance was not overall supportive
for storm development in southern Minnesota and northwest Iowa,
current observations and a few lightning strikes support the threat
for at least a few severe thunderstorms this afternoon.

Farther north, a combination of better upper-level forcing, and a
pseudo warm front/differential heating boundary should support storm
development by mid to late afternoon. Effective shear around 40 to
50 knots and MLCAPE around 2500 to 3000 J/kg will support supercells
capable of mainly large hail and damaging winds. Mixing ahead of the
front has veered most surface flow ahead of the front which should
limit the overall tornado threat, but backed flow in the immediate
vicinity of the warm front/differential heating boundary could
support an isolated tornado threat.

Overall, a watch is possible, with storm coverage as the primary
limiting factor. The best potential for a watch would be in east
Central Minnesota into northeast Minnesota and northwest Wisconsin
where storm coverage should be greater with less likelihood of a
watch for the activity in southern Minnesota and northwest Iowa.

..Bentley/Hart.. 09/21/2019

…Please see for graphic product…


LAT…LON 43409602 44139572 44919551 45329537 45899503 46719464
47289411 47639377 47719320 47559243 47349203 47019167
46459145 45549150 44669219 43229298 42429441 42139532
42119604 42329651 42409678 43409602

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