Day 1 Severe Weather Outlook

Valid 122000Z – 131200Z


Scattered strong-severe thunderstorms are expected across portions
of the mid/upper Mississippi Valley, southwestward to the
south-central Plains.

…20Z Update…

Tornado Watch 648 was recently issued for southeast MN, southwest
WI, and northeast IA to cover the threat of a couple tornadoes near
the warm front across the region. Surface winds have veered for
areas farther south across central/southeast IA, reducing the
tornado potential. Strong to severe storms are still possible in
this area and trends are being monitored for possible Severe
Thunderstorm Watch issuance.

Thunderstorms have developed amidst the along the front stretching
from far northeast MO southwestward into northwest OK. As mentioned
in MCD 1967, moderate buoyancy will likely support a few stronger
storms but limited vertical shear should mitigate a more widespread
severe threat. Even so, convective trends will be monitored closely
for possible watch issuance.

..Mosier.. 09/12/2019

.PREV DISCUSSION… /ISSUED 1123 AM CDT Thu Sep 12 2019/

Water vapor imagery shows a strong upper trough moving across
NE/SD/ND today, with a 70 knot mid level speed max nosing into
MN/IA. The primary surface low is currently over southwest MN and
will drift slowly northward this afternoon, with the warm sector of
the low becoming established across much of IA and southeast MN.
Morning clouds and precipitation have slowed daytime heating, but it
appears likely that sufficient destabilization will occur this
afternoon to weaken the cap and allow scattered thunderstorm
development. Forecast soundings across southern MN and northern IA
show favorable low-level and deep-layer vertical shear profiles to
promote supercell structures capable of hail, damaging winds, and a
few tornadoes – especially with storms within the warm frontal zone
as it lifts northward across area and into southwest WI. Storms are
also expected to develop along the trailing cold front over
western/central IA during the late afternoon, where steep low-level
lapse rates and ample CAPE will promote a greater risk of damaging
wind gusts. This activity may persist much of the evening, tracking
across southern WI and northern IL.

Model guidance is consistent in considerable coverage of
thunderstorms this afternoon along the trailing cold front from
northern MO into OK. Midlevel winds are relatively weak, but
congealing outflows along the front and MLCAPE values around 2000
J/kg will pose a risk of damaging wind gusts.

…PA/NJ into Chesapeake Bay region…
The southern fringe of stronger west-northwesterly flow aloft has
sagged into this region today, where weak midlevel height falls are
forecast this afternoon. Visible satellite shows mostly clear
skies, allowing temperatures to warm through the 80s and lower 90s
in a very moist regime. 12z CAM solutions agree on the development
of scattered thunderstorms over southeast PA and southern NJ,
building southward through the late afternoon into DE/eastern MD and
coastal VA. Forecast soundings indicate 30-35 knot flow in the
2-6km layer, which should promote faster storm motions and greater
potential for momentum transfer of strong/damaging winds to the
surface in stronger storm clusters.


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