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Day 1 Outlook Convective Tornado
Hail Wind
Categorical Day1 1200Z Outlook
  
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 Forecast Discussion


000
ACUS01 KWNS 190558
SWODY1
SPC AC 190557

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
1257 AM CDT Tue Sep 19 2017

Valid 191200Z - 201200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER MUCH OF
EASTERN SOUTH DAKOTA...SOUTHEAST NORTH DAKOTA...AND WESTERN
MINNESOTA...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE EASTERN
DAKOTAS INTO WESTERN MINNESOTA...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM THE CENTRAL
DAKOTAS INTO WESTERN WISCONSIN...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS MUCH OF
WEST TEXAS...

...SUMMARY...
Significant severe thunderstorms capable of damaging wind, large
hail, and a few tornadoes are expected over parts of the eastern
Dakotas into western Minnesota this afternoon and evening. A few
severe storms are also expected over parts of west-central Texas.

...Synopsis...
An intense shortwave trough will develop across the northern Rockies
and spread into the northern Plains this afternoon and tonight,
enhancing shear profiles and lift over the region. Low pressure will
deepen across Nebraska and into the Dakotas during the afternoon,
with a cold front swinging northeastward into Minnesota overnight. 
The combination of a moistening air mass and cooling aloft will
create strongly unstable conditions, especially over the Enhanced
risk area, with ample wind shear beneath a developing 70-80 kt
midlevel speed max.

To the south, modest westerly flow aloft will exist over the
southern Plains, where strong instability will lead to at least
isolated severe storms during the late afternoon.

...Eastern Dakotas into western Minnesota and vicinity...
Conditions will destabilize rapidly during the afternoon as
convergence increases within the low pressure trough and ahead of
the developing cold front. Shear profiles will become quite strong,
and favorable for significant severe storms including supercells and
damaging bows. Supercells are most likely during the late afternoon
over eastern SD into southeast ND on the nose of the low-level lapse
rate plume, and before storms eventually merge into a linear
structure.  The strong mean boundary layer winds may mix out
moisture profiles through a deep layer, and this could decrease the
tornado threat. However, given very strong shear, including large
looping hodographs, a few tornadoes are possible prior to linear
mode. Cold temperatures aloft will favor very large hail. 

However, the most common threat is likely to be damaging winds,
perhaps widespread, with LEWPs/Bows developing within the main
squall line late in the day and into the evening. The 60 kt
low-level jet may result in embedded mesocyclones within the line,
which could produce enhanced swaths of wind damage. While there will
be capping overnight, it is not forecast to be strong, thus an
isolated/brief tornado cannot be ruled out. As the main shortwave
ejects northeastward, the squall line may lose some push, slowing
down over MN. Elevated storms capable of hail are possible as far
east as WI overnight.

...Western Texas...
A diffuse dryline will develop across western TX where daytime
heating will be strong. Temperatures aloft will still be relatively
cool, resulting in very steep lapse rate profiles. Dewpoints in the
60s will contribute to moderate to strong instability, but shear
will be weak. Still, a few slow-moving cells are expected to form,
with storm mode becoming complex as outflow is produced. Storm
initiation is expected over the higher terrain of southwest TX, and
extending northward along the dryline, perhaps into far southwestern
OK. A few reports of large hail and damaging winds are expected
across this relatively narrow zone.

..Jewell/Dean.. 09/19/2017

$$