South Dakota Mesoscale Discussion

Areas affected…Eastern Montana, Western North Dakota

Concerning…Heavy rainfall…Flash flooding possible

Valid 120300Z – 120900Z

Summary…Thunderstorms blossoming in diffluent flow ahead of an
upper low will move quickly east-northeastward tonight. Despite
the rapid storm motions, rainfall rates may at times reach 2″/hr,
which could be sufficient to produce localized flash flooding on
the sensitive soils where FFG is as low as 0.75″/1hr.

Discussion…GOES-16 Water Vapor imagery clearly indicates an
upper level circulation moving out of southwest MT. Ahead of this
feature, cloud patterns suggest diffluence increasing across
eastern MT as 300mb winds split to the north and east. At the
surface, a wave of low pressure was analyzed near north central
Wyoming, with a warm front extending NE towards the SD/ND/MT
border from this surface low.

Ascent within this region is intensifying due to the
aforementioned features, as well as a piece of the LLJ angling NW
into eastern MT. Recent VWPs from KUDX and KBIS showed 15-20 kts
of inflow at 850mb, driving moist advection and subtle upslope
enhancement towards the discussion area. This deep layer ascent is
leading to convective growth, and reflectivity out of KBLX has
shown an increase in coverage of 40dBZ echoes. These enhanced
reflectivities are likely producing torrential rainfall as the
thermodynamic environment is extremely favorable for heavy rain.
MUCape remains above 1000 J/kg, and PWATs at 00Z were analyzed
above the 90th percentile at GGW and BIS, with recent GPS PW
observations indicating near daily record column moisture.

As the LLJ intensifies as progged by the RAP, the associated WAA
will advect better pooled moisture and instability from the S/SE
to drive continued convection through much of the night. The
consensus from the high-res CAMs indicates a high probability for
1-2″ of rainfall, with pockets to 3″ possible. While storm motions
will likely be fast on 850-300mb mean wind of 20-30 kts, storm
mergers or repeated rounds of thunderstorms are possible. Any
storm could have rainfall rates in excess of 1″/hr, and the HREF
neighborhood probabilities even indicate a modest chance for
greater than 2″/hr. It is because of these rates on top of
sensitive soils that HREF exceedance probabilities reach 60%.
These exceptional rainfall rates could produce flash flooding
anywhere across the region, but should generally remain localized
to where any storm mergers occur.


…Please see for graphic product…



LAT…LON 48810475 48730403 48540343 48170297 47880265
47620237 47110210 46540203 46250229 45800322
45560394 45350453 45170515 45120572 45240626
45590690 46190764 47370750 47820727 48260669
48540613 48780534

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