U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 280055 
Storm Prediction Center ac 280054 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0754 PM CDT Tue Jun 27 2017 

Valid 280100z - 281200z 

..there is an enhanced risk of severe thunderstorms across parts of 
central and eastern Nebraska into far southern South Dakota... 

..there is a slight risk of severe thunderstorms outside of the 
enhanced risk across parts of the central and northern plains... 

..there is a marginal risk of severe thunderstorms outside of the 
slight risk area across parts of the north-central states... 

Severe thunderstorms capable of large hail and damaging winds are 
expected over much of Nebraska and South Dakota this evening, with 
the potential for hailstones greater than 2 inches in diameter and 
damaging wind gusts in excess of 65 knots. 

..central and northern plains/mid Missouri Valley... 
The latest water vapor imagery shows a shortwave ridge over eastern 
parts of the Dakotas extending southward into the mid Missouri 
Valley with a shortwave trough in the northern rockies. At the 
surface a cold front is moving eastward across the western Dakotas 
and far western Nebraska. Ahead of the front, a corridor of 
maximized low-level moisture is located across the central and 
northern plains. The rap is analyzing moderate to strong instability 
along this corridor with MLCAPE in the 2000 to 4000 j/kg range. 
Thunderstorms are ongoing along the western edge of the stronger 
instability in southwestern South Dakota and west-central Nebraska. 
This activity should continue to grow upscale, moving eastward into 
higher quality moisture and stronger instability. As the shortwave 
trough moves into the region from the west and low-level flow 
strengthens across the plains, mesoscale convective system development will be possible. 

In addition to moderate instability, the WSR-88D vwp at North 
Platte, NE shows a supercell wind profile with strong directional 
shear in the lowest 2 km above ground level and 30 to 35 kt of westerly flow in the 
mid-levels. The 00z sounding from North Platte also shows a very 
steep mid-level lapse rate exceeding 8.0 c/km. This environment 
should support supercells with large hail and the more dominant 
storms may be capable of producing hailstones greater than 2 inches 
in diameter. Supercells will be most likely over the next 2 to 3 
hours but a transition to linear Mode is expected to take place 
during the mid to late evening. This should result in an increasing 
wind damage threat especially if a linear mesoscale convective system can organize across 
the region. Due to the strong instabililty and steep low-level lapse 
rates, any persistent bowing line of storms may be able to produce 
damaging wind gusts exceeding 65 knots. The severe threat should 
eventually reach the mid Missouri Valley by during the late evening 
and overnight period. See mesoscale discussion 1165 for more information on the South 
Dakota part of the enhanced risk area. 

Isolated severe storms will be possible further to the south across 
western Kansas and the Oklahoma Panhandle where a narrow corridor of 
moderate instability is analyzed by the rap. Farther west and north 
across parts of the northern High Plains and North Dakota, 
convective coverage should remain more isolated but a marginal 
severe threat will be possible due to an adequate combination of 
instability and deep-layer shear. 

.Broyles.. 06/28/2017 


Mesoscale Discussion

acus11 kwns 280113 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 280112 

Mesoscale discussion 1166 
National Weather Service Storm Prediction Center Norman OK 
0812 PM CDT Tue Jun 27 2017 

Areas affected...southwestern South Dakota...western/central 

Concerning...Severe Thunderstorm Watch 372... 

Valid 280112z - 280245z 

The severe weather threat for Severe Thunderstorm Watch 372 

Summary...thunderstorm clusters capable of producing severe wind 
gusts are progressing eastward across southwestern South Dakota and 
west-central Nebraska. The risk for severe winds from these 
clusters will continue into the evening hours. 

Discussion...thunderstorms have grown upscale across southwestern 
South Dakota producing several measured wind gusts. This organized 
cluster of storms will move out of watch 372 over the next hour and 
into downstream watch 373, where low-level moisture and instability 
is maximized. 

There is a relative minimum in thunderstorm coverage across northern 
Nebraska with a second cluster of storms across west-central 
Nebraska. This cluster of storms appears less organized on radar 
imagery and has not yet produced measured severe wind gusts. As 
this cluster moves into an area of better moisture and instability, 
an increase in organization and intensity is expected over the next 
few hours. There is uncertainty regarding the eastward extent of 
damaging wind gust potential as the boundary layer cools and 
surface-based convective inhibition increases. 

.Jirak/Hart.. 06/28/2017 

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


Latitude...Lon 41160176 41690150 42470132 43310167 43780215 44060210 
44220141 44359988 43479907 42669846 42159795 41329702 
40749734 40539885 40620026 40790147 41160176