U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 220106 
Storm Prediction Center ac 220105 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0805 PM CDT Thu Sep 21 2017 

Valid 220100z - 221200z 

..there is a marginal risk of severe thunderstorms from portions of 
southeast ND/northeast South Dakota eastward into the western upper Great 
Lakes area... 

A marginal risk for a few large hail events exists over a part of 
the upper Midwest tonight into Friday morning. 

..MN/northern WI/western Upper Michigan and vicinity... 
A weak short-wave trough moving northeast across the central U.S. 
This evening will combine with the beginnings of an observed 
increase in the southerly low-level jet to yield some enhancement of 
quasi-geostropic ascent across north-central portions of the country 
tonight. This increase in ascent -- given favorable elevated 
instability indicated by evening raobs -- may prove sufficient for 
development of isolated/elevated storms. Certainty regarding degree 
of convective coverage remains elusive, as sampling of a large 
number of cam runs reveals solutions ranging from very little if any 
convection to scattered coverage of intense updrafts. 

With cloud-layer shear sufficient for updraft organization and cape 
favorable for intense cells, will maintain 5% risk for hail across 
this region, but conditionality of the risk precludes slight risk 

.Goss.. 09/22/2017 


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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