U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 190558 
Storm Prediction Center ac 190557 

Day 1 convective outlook 
National Weather Service 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 

..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... 

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. 

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 South Dakota 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 Minnesota. Elevated storms capable of hail are possible as far 
east as WI overnight. 

..western Texas... 
A diffuse dryline will develop across western Texas 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 


Mesoscale Discussion

acus11 kwns 172244 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 172244 

Mesoscale discussion 1654 
National Weather Service Storm Prediction Center Norman OK 
0544 PM CDT sun Sep 17 2017 

Areas affected...east New Mexico...southeast Colorado...and the 
Texas and Oklahoma panhandles 

Concerning...Severe Thunderstorm Watch 486... 

Valid 172244z - 180015z 

The severe weather threat for Severe Thunderstorm Watch 486 

Summary...isolated to widely scattered storms capable of producing 
downburst winds and large hail will persist through at least 01z 
over the southern High Plains. However, a gradual weakening trend 
should commence with onset of nocturnal cooling. 

Discussion...storms continue spreading off the higher terrain of Colorado 
and nm and are also developing along southwestern extension of a 
front as well as in association with numerous outflow boundary 
interactions over the Texas Panhandle. Latest objective analysis shows 
the atmosphere to be moderately unstable in this region with 1500 
j/kg MLCAPE and 7-7.5 c/km mid-level lapse rates. Moreover, easterly 
low-level winds north of the front beneath modest southwesterly 
winds aloft is resulting in sufficient (35-40 kt) effective bulk 
shear for a few storms to develop mid-level updraft rotation. The 
stronger storms will continue to pose a severe risk next couple 
hours followed by an overall weakening trend as the boundary layer 
begins to stabilize. 

.Dial.. 09/17/2017 

..please see www.Spc.NOAA.Gov for graphic product... 


Latitude...Lon 38570361 38000247 35810095 34200042 33330115 32480305 
32770428 35480487 37970485 38570361