U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 211253 
Storm Prediction Center ac 211251 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0751 am CDT Mon may 21 2018 

Valid 211300z - 221200z 

..there is a slight risk of severe thunderstorms across portions of 
Indiana and Ohio... 

Isolated to widely scattered strong to severe thunderstorms are 
possible across the Ohio Valley as well as portions of New Mexico 
and the central and southern High Plains. Damaging wind and large 
hail will likely be the main severe threats. 

..Ohio Valley... 
Scattered showers and thunderstorms remain prevalent early this 
morning from near the confluence of the Mississippi and Ohio rivers 
northward across Illinois toward the Southern Lake Michigan 
vicinity. Part of this convection continues to muddle a warm sector 
to the east of a weak northeastward-transitioning surface wave and 
near/south of an eastward-extending warm front that will continue to 
develop slowly northward across Illinois, Indiana, and Ohio. 

The ongoing precipitation and related cloud cover casts some 
uncertainty on the exact degree/locations of more appreciable 
destabilization later today, although cloud breaks are noted in 
early-morning visible satellite imagery from Kentucky into southern 
portions of Indiana and Ohio. While the overall magnitude/extent of 
the severe risk remains uncertain, at least some severe 
thunderstorms can be expected this afternoon into evening. The most 
supercell-favorable deep-layer and low-level shear later today will 
exist just ahead of the aforementioned weak surface wave and the 
eastward extending warm front, mostly across central portions of 
Indiana into Ohio in areas along/north of I-70. Severe hail and 
possibly a tornado could occur within this general corridor, while 
isolated bouts of damaging winds will be the main hazard across a 
broader part of the region. 

..New Mexico and west/southwest Texas... 
Moist Post-frontal low-level upslope flow will continue to yield an 
increase in moisture with west-northwestward extent across New 
Mexico during the day. The region will not be overtly influenced by 
the slow-moving/digging closed low over the southwest states, but 
relatively plentiful moisture and orographic ascent will lead to 
scattered thunderstorms this afternoon. Initial thunderstorm 
development is expected this afternoon across interior New Mexico 
and by late afternoon/early evening farther east across eastern New 
Mexico and far West Texas. 30-35 kt of effective shear, steep lapse 
rates, and ample moisture for the region will support multicells and 
possibly a few supercells capable of severe-caliber hail, along with 
gusty winds. Areas such as far West Texas and southeast/ 
south-central New Mexico will continue to be reevaluated for a 
possible categorical slight risk upgrade given a somewhat more 
severe-favorable environment. 

..Central High plains... 
Gradual Lee-side cyclogenesis will influence weak upslope 
trajectories with a modest influx of low-level moisture through the 
day. While upper heights will generally rise, weak low-level upslope 
flow and differential heating along the mountains/higher terrain 
should yield isolated east/southeastward-drifting thunderstorms this 
afternoon. Sufficient low-level moisture and effective shear (30-40 
kt) coincident with very steep lapse rates could support some strong 
multicells/brief supercells capable of severe hail/gusty winds on a 
very isolated basis. 

.Guyer/Goss.. 05/21/2018 


Mesoscale Discussion

acus11 kwns 202326 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 202326 

Mesoscale discussion 0479 
National Weather Service Storm Prediction Center Norman OK 
0626 PM CDT sun may 20 2018 

Areas affected...western Kentucky...western Tennessee...eastern 
Arkansas...and northern Mississippi 

Concerning...Severe Thunderstorm Watch 110... 

Valid 202326z - 210030z 

The severe weather threat for Severe Thunderstorm Watch 110 

Summary...an isolated damaging wind threat continues across 
remaining portions of ww 110, although much of the region has 
stabilized recently due to convective overturning. Portions of ww 
110 may be cancelled before the 01z expiration time. 

Discussion...storms continue to propagate slowly/erratically in and 
near the remaining valid portions of ww 110 currently. These storms 
are in a weakly sheared environment, with widespread convective 
overturning and expanding rain-cooled air/outflow limiting the 
magnitude of surface-based instability needed to maintain a robust 
severe threat. Storms propagating through the warm, moderately 
unstable pre-convective airmass (in eastern Arkansas and near middle 
tennessee) may continue to pose a threat for isolated damaging wind 
gusts in the short term, although ongoing trends suggest that the 
overall severe threat will continue to wane through 01z. Some 
portions of ww 110 may be cancelled early as a result. 

.Cook.. 05/20/2018 

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


Latitude...Lon 36129127 36169085 36289008 36498937 36948847 37438791 
37858766 37918731 37748715 37058733 36088769 35388799 
34438825 33998832 33738853 33698929 33809036 33999111 
34119139 34579138 35409127 36129127