SPC MD 909


MD 0909 Image

Mesoscale Discussion 0909
NWS Storm Prediction Center Norman OK
0351 PM CDT Thu Jun 10 2021

Areas affected...Western South Dakota and western Nebraska

Concerning...Severe potential...Watch likely 

Valid 102051Z - 102245Z

Probability of Watch Issuance...95 percent

SUMMARY...Supercells are expected to develop along the lee trough
later this afternoon. Very large hail, severe wind gusts, and a
couple tornadoes are possible. With time, initial storms are likely
to grow upscale into intense lines/bowing segments capable of
significant wind gusts. A WW is likely within the next 2-3 hours.

DISCUSSION...Cumulus clouds have begun to form along the MT/SD/WY
border within the lee trough. Higher-based cumulus are also evident
farther south in eastern Wyoming as well. Continued boundary-layer
heating and mid-level height falls should continue to promote this
development and eventual storm initiation later this afternoon. The
19Z UNR sounding showed extreme surface-based buoyancy, though this
was only supported by a very shallow layer of low-level moisture.
Farther to the east, the 18Z LBF sounding showed a deeper moist
layer with similar instability/buoyancy. This richer moisture is
apparent on visible satellite where boundary-layer cumulus have
develop in central Nebraska/South Dakota.

Current visible satellite trends suggest that storm development is
still likely a 2-4 hours away, which has been the general consensus
of CAM guidance today. With 30-45 kts of effective shear orthogonal
to the lee trough, initial storms will likely be supercellular and
capable of very large hail (potentially 3+ inches) and severe wind
gusts. Given the lower boundary layer RH towards the west, the
tornado threat is not expected to be overly high, but a couple of
tornadoes will be possible. If a storm can remain discrete into the
evening, richer moisture to the east and a strengthening low-level
jet, a brief window of greater tornado potential could develop.

The main concern beyond the initial supercells is expected to be a
threat for significant wind gusts along with linear/bowing segments
that develop. Guidance has varied in the exact evolution of
convection this evening, but one or two of intense squall lines are
possible as storms move into central Nebraska/South Dakota.

A WW is likely within the next 2-3 hours.

..Wendt/Guyer.. 06/10/2021

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


LAT...LON   40950381 42540444 44400486 45650459 45890282 45160116
            42270068 41050084 40240165 40170278 40950381 

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