U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 201938 
Storm Prediction Center ac 201937 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0237 PM CDT Wed Sep 20 2017 

Valid 202000z - 211200z 

..there is a slight risk of severe thunderstorms across portions of 

..there is a marginal risk of severe thunderstorms from northern MO 
to the u.P. Of Michigan... 

..there is a marginal risk of severe thunderstorms across portions 
of the Southern Plains... 

A few strong to severe storms with locally damaging wind and hail 
remain possible from the Upper Peninsula of Michigan to northern 
Missouri between about 4 to 11 PM CDT. A few strong storms are also 
possible from southwest through north central Texas. 


No changes warranted to 1630z outlook. 

.Darrow.. 09/20/2017 

Previous discussion... /issued 1129 am CDT Wed Sep 20 2017/ 

..Wisconsin through northern Missouri... 

Boundary layer dewpoints in the mid to upper 60s f are advecting 
northward through the pre-frontal warm sector, and this will result 
in moderate to strong instability with 1500-2500 j/kg MLCAPE this 
afternoon. However, mid-level lapse rates will remain modest due to 
relatively warm temperatures aloft with -6 to -7c at 500 mb. The 
stronger winds aloft will remain in Post frontal region but a belt 
of moderate flow with 40-50 kt in the 700-400 mb layer will exist in 
the frontal zone. Height will rise slightly during the day in wake 
of shortwave trough currently lifting into southern Manitoba. 
Destabilization of the boundary layer and low-level convergence 
should be sufficient to initiate storms along the cold front. Have 
introduced a 15% categorical risk area over WI where the best 
combination of stronger winds aloft and instability exists. 

..southwest through northwest Texas... 

Subsidence aloft associated with low-amplitude shortwave ridging 
remains a limiting factor for the development of storms today. 
However, strong diabatic heating and mixing in the vicinity of 
dryline should result in at least isolated storms from southwest 
through northwest Texas this afternoon. Temperatures aloft are warm due 
to mid-upper subsidence, and this will mitigate mid-level lapse 
rates and overall updraft strength despite 2000-3000 j/kg MLCAPE. 
Nevertheless, some storms could pose a risk for a few instances of 
downburst winds from late afternoon into early evening. 


Mesoscale Discussion

acus11 kwns 202017 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 202017 

Mesoscale discussion 1662 
National Weather Service Storm Prediction Center Norman OK 
0317 PM CDT Wed Sep 20 2017 

Areas affected...northeast Iowa through central Wisconsin 

Concerning...severe potential...watch possible 

Valid 202017z - 202215z 

Probability of watch issuance...40 percent 

Summary...storms may become capable of producing a few strong to 
locally damaging wind gusts and hail this afternoon into early 
evening from northeast Iowa into southern and central Wisconsin. 
Trends will continue to be monitored. 

Discussion...cold front extends from western WI through northeast 
and southwest Iowa. Upper 60s f dewpoints have advected through 
pre-frontal warm sector which, in conjunction with diabatic warming, 
have boosted surface temperatures to 85-90 resulting in 2000-3000 
j/kg MLCAPE. Despite weak forcing aloft and a gradual weakening of 
the low-level jet in wake of a shortwave trough lifting north 
through Manitoba and western Quebec, convergence and a destabilizing 
boundary layer are sufficient to initiate storms along the front 
this afternoon. While the stronger winds aloft remain Post frontal, 
moderate flow with 40-50 kt in the 700-400 mb layer resides in 
frontal zone, but vertical shear remains modest and supportive of 
mostly multicells and possibly some marginal supercell structures. 
Primary limiting factor for a more robust threat is the weak 
mid-level lapse rates with warm temperatures aloft (-6 to -7 c at 
500 mb). Nevertheless, overall parameter space appears sufficient 
for at least a modest risk for isolated strong to damaging wind 
gusts and some hail. 

.Dial.. 09/20/2017 

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


Latitude...Lon 43339151 44479055 44898969 44348877 43398889 42608981 
42059139 42529221 43339151