SPC 1630Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
1130 AM CDT Wed Sep 23 2020

Valid 231630Z - 241200Z


A tornado or two may occur this afternoon and evening across
southern portions of Louisiana/Mississippi. A few strong to severe
storms with hail and gusty winds are possible this evening from
northern Minnesota into Wisconsin and western Upper Michigan. A few
strong storms may also affect coastal Washington this

...Southern Louisiana/southern Mississippi...
The post-tropical remnants of Beta will continue to move
northeastward and gradually phase with a slow-moving shortwave
trough over the Ozarks/middle Mississippi Valley. The surface low
near the southeast Texas/southwest Louisiana border at late morning
will move northeast and reach western/central Mississippi tonight,
while a low-preceding warm front and moisture-rich warm sector
(low/mid 70s F surface dewpoints) will gradually develop northward
across southeast Louisiana/southern Mississippi through the
afternoon and tonight.

Some cloud breaks have been noted and convection will continue
developing within several convergence bands east of the surface
low/remnant center. The main risk for a couple of tornadoes will be
in a relatively narrow corridor this afternoon and evening as storms
move northward and interact with the warm front where 0-1 km
hodograph size will be maximized in conjunction with an easterly
component to the near-surface winds.

...Northern Minnesota/western Wisconsin/western Upper Michigan...
Prior forecast reasoning remains largely unchanged. Steep 7.5-8 C/km
mid-level lapse rates will develop eastward through the upper
Mississippi 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, even
with 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 severe 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 expected after 00Z across western
Washington. A leading band of precipitation associated with warm
advection will give way to discrete cells moving rapidly east/inland
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.

..Guyer/Leitman.. 09/23/2020

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