U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 211244 
Storm Prediction Center ac 211242 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0742 am CDT Thu Sep 21 2017 

Valid 211300z - 221200z 

..there is a marginal risk of severe thunderstorms upper Midwest... 

..There is a marginal risk of severe thunderstorms northwest Texas to 
central Kansas... 

Isolated large hail events are possible across parts of the upper 
Midwest tonight into Friday morning. A few severe wind gusts are 
possible from central Kansas to the southern Texas Panhandle between 
5 to 9 PM CDT. 

..upper Midwest... 
Mid-level heights should slowly rise today as an anticyclone becomes 
established over the Midwest while a longwave trough edges slightly 
east across the west. This should mitigate surface-based storm 
development as an eml shifts north-northeast in tandem with an 
advancing warm front from the Iowa/MO border. As the low-level jet 
intensifies this evening and likely reaches 50-60 kt at 850-mb 
across the central Great Plains, pronounced warm Theta-E advection 
should yield isolated to scattered elevated storms during the late 
evening to early morning within a substantial gradient of MUCAPE. 
Around 30-35 kt effective shear should Foster a risk for at least a 
supercell or two producing large hail. An upgrade to slight was 
considered, but confidence is too low regarding greater coverage of 
supercells and location where they are likely to form. 

..central/southern Great Plains... 
A subtle mid-level impulse over eastern nm should progress into 
central NE/Kansas by this evening. This may aid in isolated storm 
development during the early evening along the dryline from central 
Kansas to northwest Texas. Hot temperatures in the middle to upper 90s will 
support large dcape values of 1400-1800 j/kg. On the fringe of 
modest deep-layer shear, a few severe wind gusts may occur until 
storms decay during the late evening. 

.Grams/Kerr.. 09/21/2017 


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... 


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