U.S. Severe Weather Forecast (Convective Outlook)

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Day Three

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acus03 kwns 210938 
swody3 
Storm Prediction Center ac 210937 


Day 3 convective outlook resent 1 
National Weather Service Storm Prediction Center Norman OK 
0437 am CDT Tue Aug 21 2018 


Valid 231200z - 241200z 


..there is a slight risk of severe thunderstorms across eastern South Dakota 
and far southwest Minnesota... 


... 
Severe thunderstorms are possible across eastern South Dakota and 
far southwest Minnesota on Thursday afternoon and evening. 


... 
A shortwave trough is expected to progress across the 
northern/Central Plains on Thursday while downstream shortwave 
ridging moves across the upper Great Lakes. Another shortwave trough 
will likely move into the Pacific northwest late Thursday 
night/early Friday morning while largely stationary ridging across 
the southern third of the Continental U.S. Gradually begins to dampen. 


At the surface, a low attendant to the northern/Central Plains 
shortwave trough will move just ahead of its parent upper system. As 
it does, surface troughing will also pushing eastward, with 
resulting dryline separating the warm and drier air emanating from 
the High Plains with the more moist air from the Southern Plains and 
southeast. 


..northern/Central Plains...Upper/Middle MO valley... 
moderate moisture return is anticipated across the region ahead of 
the approaching shortwave trough and its attendant surface low. 
There is some chance this moist advection is impeded by ongoing 
showers and thunderstorms across Kansas but the consensus amongst the 
most recent guidance is that mid 60s dewpoints will extend across 
far north as the NE/South Dakota border by Thursday afternoon. Combination of 
this increased low-level moisture, surface convergence, and ascent 
from the approaching shortwave will likely lead to thunderstorm 
development over central South Dakota Thursday afternoon. Expectation would be 
for these thunderstorms to then track southeastward towards 
southeast South Dakota and vicinity along the instability gradient. Steep 
mid-level lapse rates and moderate vertical shear result in the 
potential for these storms to be organized and capable of severe 
hail and/or strong wind gusts. Severe coverage is expected to be 
high enough across eastern South Dakota to merit a slight risk. 


For areas farther south (i.E. Central/eastern ne), despite favorable 
low-level moisture, warm mid-level temperatures and displacement 
south of the stronger forcing for ascent will likely keep the warm 
sector free of convection. An isolated storm or two is possible in 
area of strong surface convergence and the airmass is supportive of 
a severe threat if an updraft can persist. 


..north-Central/Northeast Kansas...Lower MO valley... 
on Thursday evening, a strengthening low-level jet is expected to 
lead to increased warm-air advection atop the outflow left by 
antecedent thunderstorms. Current guidance suggests this warm-air 
advection will be enough to lift parcels to their lfcs, leading to 
the development of showers and thunderstorms. Isolated severe hail 
is possible within the strongest cells. 


.Mosier.. 08/21/2018 


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