U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 210559 
Storm Prediction Center ac 210558 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1258 am CDT Thu Sep 21 2017 

Valid 211200z - 221200z 

..there is a marginal risk of severe thunderstorms across portions 
of the north-central u... 

Hail-producing storms are possible from the eastern Dakotas into the 
Upper Peninsula of Michigan Thursday night. 

Amplification of the upper flow field over the U.S. Is expected 
today, as positive-tilt troughing over the western states acquires a 
more neutral tilt with time. This will manifest as short-wave 
energy digs southeastward across California and the Great Basin. In 
response, ridging will increase across the eastern states. 

At the surface, Tropical Storm Jose is forecast to linger off the 
southeast New England coast, while weakening with time. Meanwhile, 
a cold front is forecast to progress across the intermountain west 
in conjunction with amplification of upper troughing. Ahead of the 
main frontal advance, weak cyclogenesis is expected over the 
Nebraska/South Dakota area overnight as a weak lead short-wave 
trough aloft moves northeastward across the plains. 

..the north-central states... 
Weak short-wave troughing shifting northeast across the plains 
through the second half of the period will combine with development 
of a southerly nocturnal low-level jet to yield some increase in 
quasi-geostrophic ascent across the north-central U.S. Overnight 
Thursday and into Friday morning. With relatively steep mid-level 
lapse rates contributing to fairly substantial elevated cape, ascent 
should prove sufficient for isolated/elevated storm development 
during the late evening and overnight hours. With sufficient 
veering of the wind field with height expected within the 
cloud-bearing layer, a couple of organized -- possibly rotating -- 
storms could evolve, which would be accompanied by a risk for large 
hail. With that said, questions persist regarding potential for -- 
and coverage of -- convective development, and thus at this time 
will maintain only 5%/marginal risk area. 

.Goss/Dean.. 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... 


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