SPC MD 1483


MD 1483 Image

Mesoscale Discussion 1483
NWS Storm Prediction Center Norman OK
0231 PM CDT Thu Sep 20 2018

Areas affected...Portions of western Iowa...eastern Nebraska...
north-central and northeastern Kansas...and far northwestern

Concerning...Severe potential...Watch likely 

Valid 201931Z - 202130Z

Probability of Watch Issuance...80 percent

SUMMARY...Thunderstorms capable of severe wind gusts will slowly
increase along the front and move east throughout the
afternoon/early evening. A Severe Thunderstorm Watch may be needed
for at least portions of the area.

DISCUSSION...A cold front trailing into southeastern Colorado from a
deepening surface low near SUX will serve as the primary forcing
mechanism for thunderstorm development this afternoon over this
area. The strong frontal forcing, rather weak shear in the
cloud-bearing layer, and veered low-level winds will support a
rather quick evolution to a squall line mode, especially along the
sharper portion of the front in Nebraska and far northern Kansas, as
depicted in multiple convection-allowing model guidance through the
morning.  The consolidating cold front/narrow cold pool is expected
to undercut the squall line, which will limit the overall severe
threat.  However, a deeply-mixed boundary layer with mixed-layer
LCLs of 1600-2200 m and moderate mid-level lapse rates could support
damaging wind gusts with the stronger embedded downdrafts that can
remain close to the front.  

Farther west, thunderstorms have developed along and slightly behind
the front in eastern Colorado and far western Kansas aided by the
southern fringe of an elongated mid-level vorticity maximum.  These
storms may evolve more as cluster compared to the squall line to the
northeast, but rather weak low-to-mid-level shear magnitudes should
prevent any long-lived organization with these storms. However,
severe wind gusts are possible given the moderate lapse rates and
evaporation potential in the deeply-mixed boundary layer as they
push east into west-central Kansas through the mid-late afternoon.

Overall, a Severe Thunderstorm Watch may be needed for at least
portions of the MCD area to cover the severe wind threat.

..Coniglio/Hart.. 09/20/2018

...Please see www.spc.noaa.gov for graphic product...


LAT...LON   42339709 42659676 42979613 42949493 42869434 42589379
            41889388 40449476 39749536 39199627 38509775 38479883
            39559904 40329881 40609866 41089836 42339709 

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SPC MD 1474


MD 1474 Image

Mesoscale Discussion 1474
NWS Storm Prediction Center Norman OK
0248 AM CDT Tue Sep 18 2018

Areas affected...South-central...southeast and east-central SD

Concerning...Severe potential...Watch unlikely 

Valid 180748Z - 181015Z

Probability of Watch Issuance...20 percent

SUMMARY...An ongoing cluster of elevated storms moving to the east
across south-central South Dakota is expected to have a greater
potential to produce a periodic threat for large hail, as it tracks
into southeast and east-central South Dakota during the next few
hours.  If these storms are able to maintain their intensity, then
portions of far southwest Minnesota and adjacent northwest Iowa
could be affected, as well, toward 7 AM CDT.

DISCUSSION...IR imagery indicated cooling cloud tops again with the
ongoing eastward-moving SD cluster of storms, located in the
south-central part of this state at 0736Z.  A progressive midlevel
impulse shifting east from western SD/NE combined with warm
advection at the apex of the western branch of the central Plains
southerly low-level jet has been supporting this somewhat
disorganized cluster, as it moved through weaker instability. 
Although weak forcing aloft will be maintained as the impulse moves
east early this morning, this cluster will be advancing into the
eastern branch of the Great Plains low-level jet as it veers to
westerly by 12Z.  This latter evolution in the low-level wind field
will eventually limit convergence into this cluster of storms,
suggesting a diminishing trend in coverage of stronger storms and
potential decrease in activity.  Until this trend occurs, this
cluster of storms will begin to encounter stronger elevated
instability that is present across southeast SD into southwest MN
and northwest IA that should promote stronger updrafts in the short
term and larger hail at times.

..Peters/Edwards.. 09/18/2018

...Please see www.spc.noaa.gov for graphic product...


LAT...LON   43209889 43279946 43779986 44109981 44599938 44719857
            44829745 44749614 44319569 43399571 43139599 42989676
            43109825 43209889 

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