Day 1 Severe Weather Outlook

Valid 291200Z – 301200Z


Severe threat is expected to increase across the Plains this
afternoon and overnight.


Strong upper trough is forecast to shift slowly east across the
inter-mountain region during the day1 period, though a notable
short-wave trough will eject across the Great Basin into WY by
30/00z. In response to this feature, surface low is expected to
track into northeast WY by late afternoon. Strength of this
approaching system should encourage surface front currently draped
across southern KS to advance rapidly north into southern SD by peak
heating. Significantly more moist/buoyant warm sector should surge
north, then west toward the lee cyclone such that robust convection
is possible by 22-00z time frame near the Black Hills. Increasing
wind profiles and large-scale forcing ahead of 70-80kt 500mb speed
max suggest supercell storm mode. This activity should
spread/develop northeast along/north of a pronounced warm front that
should gradually lift toward the ND border. Convection is expected
to be more common north of the boundary and these elevated storms
should produce hail.

Of potentially more concern is the air mass recovery that will occur
over the central Plains into SD. 30/00z RAP/NAM aggressively moisten
this region with surface dew points rising to near 70F. If this
occurs, parameter space will increasingly support tornadic
supercells. However, it’s not entirely clear how storm initiation
will occur other than through warm advective processes, and thus
there is below normal confidence in storm coverage. Strongest
boundary-layer heating will be noted across the High Plains from
southwestern NE into eastern NM, and this is where convective
temperatures will at least be approached by 22-23z time frame.
Latest CAMs support this uncertainty with the primary corridor of
convection oriented along/north of the warm front, and more isolated
activity along the dry line extending into far west TX.

Have expanded severe probs west across SD to account for stronger
forcing acting on western plume of moisture. Strong LLJ should help
maintain this activity through the overnight hours. Otherwise, there
is concern, albeit low confidence, for isolated tornadic supercells
across the warm sector where forcing is notably weaker. Large hail
will also be noted with storms that develop across this Plains.

..Darrow/Cook.. 09/29/2019


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