SPC 1300Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0727 AM CDT Wed Sep 23 2020

Valid 231300Z - 241200Z


A few strong to severe storms with hail and gusty winds are possible
this evening from northern Minnesota into Wisconsin. A brief tornado
or two cannot be ruled out over parts of Louisiana and southwest
Mississippi with the remnants of Beta. A few strong storms may
affect coastal Washington this evening into tonight.

...Southern Louisiana and Mississippi...

Remnants of Beta are being absorbed within southern periphery of a
slow-moving shortwave trough centered over southwest MO. This will
result in northeast advancement of the system currently situated
over far southeast TX. By the end of the period, model consensus is
that Beta will be located over central MS. A coastal/warm front will
lift northward through southern LA into far southern MS today, and
the inland advection of mid 70s F dewpoints will result in weak
instability with 500-1000 J/kg MLCAPE in warm sector. Convection
will continue developing within several convergence bands east of
the center. The best threat for a brief tornado or two will be in a
narrow corridor this afternoon and evening as storms move northward
and interact with the warm front where 0-1 km hodograph size will be
sufficient for a threat of low-level mesocyclones.

...Northern Minnesota into western Wisconsin...

Steeper 7.5-8 C/km mid-level lapse rates will develop eastward
through the upper MS Valley in advance of a low-amplitude shortwave
trough. This will contribute to modest instability with 1000-1500
J/kg MUCAPE in vicinity of an east-west oriented baroclinic zone
despite somewhat limited low-level moisture. It is likely that
storms will develop this evening as deeper ascent accompanying the
shortwave trough interacts with the boundary. Effective bulk shear
might become sufficient for a few storms to develop modest mid-level
updraft rotation which, in conjunction with the thermodynamic
environment, will support some threat for hail. There may be some
tendency for storms to congeal into a small MCS capable of producing
a few strong wind gusts as well.

...Coastal Washington...

Stronger height falls will occur later today with the strongest
cooling aloft and destabilization after 00Z across western
Washington. A leading band of precipitation associated with warm
advection will give way to discrete cells moving rapidly east off
the ocean. While only a few hundred J/kg MLCAPE will exist, strong
large-scale lift as well as long hodographs may favor a few
long-lived cells, possibly with rotation which will contribute to a
threat for gusty winds and small hail.

..Dial.. 09/23/2020

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