U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 201957 
Storm Prediction Center ac 201956 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0256 PM CDT sun may 20 2018 

Valid 202000z - 211200z 

..there is a slight risk of severe thunderstorms from extreme 
northeast Oklahoma into southwest MO and over a portion of the lower 
Mississippi and Tennessee valleys... 

Strong to severe storms will persist through early evening from 
extreme southeast Missouri into northeast Arkansas and the Tennessee 
Valley. Other strong to severe storms might occur from extreme 
northeast Oklahoma into northwest Arkansas and southwest Missouri. 
Isolated strong storms also remain possible over south central and 
southeast Texas. Downburst wind and large hail will remain the 
primary threats. 


Primary change to previous outlook has been to introduce a small 
slight risk area from northeast OK into southwest MO. Storms are 
developing over northeast OK in association with what appears to be 
a convectively enhanced vorticity maximum (mcv). This activity might 
intensify as the boundary layer destabilizes downstream. A belt of 
stronger winds aloft accompanying the mesoscale convective vortex and surface winds backed 
to southeasterly north of an outflow boundary retreating northward 
through southern MO are contributing to vertical shear profiles 
sufficient for at least marginal supercell structures, especially if 
storms can interact with this modifying boundary as they 
move/develop northeast. See sels mesoscale discussion 475 for additional information. 

.Dial.. 05/20/2018 

Previous discussion... /issued 1128 am CDT sun may 20 2018/ 

..mid Mississippi Valley into the mid-south... 
Several low-level convective boundaries are analyzed extending from 
southwest Indiana across far southeast Missouri, with an additional 
boundary arcing southward from ongoing convection over east central 
Missouri then extending westward across northern Arkansas. The 
latter boundary is moving southeastward across southeast Missouri 
into a very moist low-level air mass where surface dewpoints are in 
the upper 60s to lower 70s. Moderately steep mid-level lapse rates 
sampled by the 12z lzk sounding extend into the mid-south and are 
contributing to MLCAPE of 1500-2500 j/kg. The area is along the 
southern edge of stronger westerly mid-level winds which may aid in 
storm organization with primarily multicell storms likely this 

Model guidance including 12z cams/href indicate storms will increase 
in coverage this afternoon as continued heating weakens the cap and 
convergence is focused along the primary outflow boundary. The 
strong thermodynamic support suggest a few storms will become severe 
capable of producing large hail and strong wind gusts. Activity 
will continue into the evening before storms begin to diminish by 

..parts of eastern Oklahoma into western Arkansas and southwest 
visible imagery indicates clouds have thinned over this area in 
advance of a mid-level short-wave trough lifting northeastward from 
the plains toward the lower Missouri Valley. This will promote air 
mass destabilization in the wake of overnight convection. Model 
guidance indicates a few storms will redevelop this afternoon from 
eastern Oklahoma into southwest Missouri along/immediately ahead of 
an advancing cold front and/or convective outflow. Although winds 
aloft may slowly weaken into tonight, sufficient vertical shear may 
persist to aid development of a few strong to severe storms this 
afternoon and evening. A marginal risk has been added across parts 
of this area. 

..southern into eastern Texas... 
An extensive band of convection is moving southward/southeastward 
extending from east of the dfw metroplex into central Texas. The 
air mass across the region in advance of the convection remains 
moist and unstable with MLCAPE of 1500-2500 j/kg expected this 
afternoon. The convection is likely to continue this afternoon with 
a few strong to severe storms possible. Overall severe threat will 
be limited by marginal vertical shear, but stronger cells will be 
capable of producing hail and strong wind gusts. 

Other strong to isolated severe storms are expected to develop this 
afternoon within the moist upslope low-level flow over southwest 

..Ohio Valley... 
A few strong storms will be possible this afternoon and evening over 
parts of the Ohio Valley near and south of a frontal boundary 
extending from southern Indiana into southern Ohio. Forcing for 
large scale ascent appears to be weak which may limit the coverage 
of storms. 


Mesoscale Discussion

acus11 kwns 202325 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 202325 

Mesoscale discussion 0478 
National Weather Service Storm Prediction Center Norman OK 
0625 PM CDT sun may 20 2018 

Areas affected...southwest and central MO 

Concerning...Severe Thunderstorm Watch 111... 

Valid 202325z - 210100z 

The severe weather threat for Severe Thunderstorm Watch 111 

Summary...isolated damaging wind gusts are the primary 
severe-weather threat during the early evening across southwest and 
west-central Missouri, and counties east of ww 111. 

Discussion...water vapor imagery showed a progressive shortwave 
trough tracking from eastern Kansas into the lower Missouri Valley, with 
the southern portion of this trough glancing the northern extent of 
greater instability across southern into central MO. Forcing for 
ascent with the trough and an increase in low-level warm advection 
into central/eastern MO through tonight (per a strengthening 
southerly 850-mb jet) should maintain the potential for 
showers/thunderstorms from west to east. Steep low-level lapse 
rates located downstream of the ongoing storms moving into 
south-central and central MO should aid in the potential for locally 
strong wind gusts. This threat should continue for the next 2-3 
hours (until 9-10 pm), prior to boundary-layer stabilization. 
Despite the likelihood for convection to persist this evening into 
tonight, a stabilizing boundary layer with the loss of daytime 
heating and weak midlevel lapse rates suggest storm intensities 
should wane with the onset of increasing surface-based inhibition. 

.Peters.. 05/20/2018 

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


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