U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 210551 
Storm Prediction Center ac 210550 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1250 am CDT Thu Jun 21 2018 

Valid 211200z - 221200z 

..there is a marginal risk of severe thunderstorms for portions of 
the southern High Plains...northern High Plains into the Colorado 
Front Range...and Ohio Valley southward into Mississippi... 

Isolated severe gusts and hail will be possible today across parts 
of the High Plains and north-central rockies. Strong gusts capable 
of wind damage are possible from parts of the lower Mississippi 
Valley and lower Ohio Valley and in the North Carolina/Virginia 

..Mt/WY/co Front Range... 
A mid-level shortwave trough over Oregon early this morning will 
move east into southern Montana/northern Wyoming by late tonight. Scattered 
thunderstorms are forecast to develop near the higher terrain over 
southwest Montana/Wyoming by mid afternoon. Weak to moderate buoyancy and 
very steep 0-2 km lapse rates will support an isolated risk for 
severe gusts with the stronger downdrafts. Isolated large hail may 
accompany the more intense updrafts with the organized multicellular 
convection. Farther southeast, models suggest isolated storm 
coverage from the Laramie range into the Colorado Front Range. The 
upslope low-level flow beneath 30-40 kt 500mb flow will support a 
wind profile for widely spaced supercells capable of an isolated 
severe risk. This activity will likely diminish during the evening. 

..southern High Plains... 
Moderate westerly mid-level flow to the northeast of a mid-level 
anticyclone centered over southern Arizona will overspread the southern 
High Plains. Although large-scale forcing will likely remain 
negligible, orographic forcing with moist upslope flow with 50s f 
dewpoints near Raton Mesa may result in a couple of thunderstorms 
developing during the late afternoon/early evening. The NAM/GFS 
show a seemingly robust convective signal with just a few cams 
generating storms. Strong 0-6 km shear (50 kt) will support any 
sustained updraft to evolve into a supercell given the moderate 
buoyancy on the eastern periphery of steep mid-level lapse rates. 
Guidance shows some variability in the placement/timing of the 
thunderstorms. A portion of this region may warrant an upgrade in 
severe probabilities if confidence increases on convective coverage. 

..lower Ohio Valley into AR/MS/al... 
A mid-level low over the lower MO valley will meander southeast as a 
belt of stronger cyclonically curved flow noses into the MO bootheel 
vicinity during the afternoon. A cold front will likely serve as a 
focus for thunderstorm development as A Reservoir of upper 60s to 
lower 70s degree f dewpoints contribute to moderate buoyancy by mid 
afternoon. Models show the strongest deep-layer shear (30-40 kt) 
generally from near the Kentucky/Tennessee border southward. Strong gusts 
capable of damage and marginally severe hail may accompany the 
stronger storms. 

Strong heating east of the higher terrain will destabilize the 
boundary layer during the morning through the mid afternoon. Models 
indicate storm development may be delayed over the Piedmont and 
coastal plain until later in the afternoon thereby allowing steep 
0-2 km lapse rates to develop (7-8 degrees c/km). Storms will 
likely develop initially near the higher terrain as a mid-level 
vorticity lobe moves from the Tennessee Valley at daybreak to the Piedmont 
by dusk. Deep westerly flow over this area will favor storm motions 
to the east in an increasingly moisture rich environment over the 
coastal plain. Water-loading multicells capable of mainly localized 
strong gusts may result in pockets of wind damage with the more 
intense cores. 

.Smith/squitieri.. 06/21/2018 


Mesoscale Discussion

acus11 kwns 202342 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 202341 

Mesoscale discussion 0808 
National Weather Service Storm Prediction Center Norman OK 
0641 PM CDT Wed Jun 20 2018 

Areas affected...northeast la...southwest Arkansas 

Concerning...severe potential...watch unlikely 

Valid 202341z - 210045z 

Probability of watch issuance...5 percent 

Summary...a brief tornado or two is possible over the next hour or 
so across northeast la and southwest Arkansas. 

Discussion...despite calm surface winds, high Theta-E airmass 
(characterized by temperatures in the low 80s and dewpoints in the 
mid 70s) is contributing to strong low-level cape (i.E. 0-3 km 
MLCAPE over 75 j per kg based on the latest mesoanalysis). 
Additionally, a narrow corridor of relatively confluent, slightly 
stronger low-level likely exist across northeast la and southwest 
Arkansas. This slightly enhanced low-level flow is likely contributing to 
a maximum in low-level shear and storm-relative helicity. Latest 
mesoanalysis estimates 0-1 km bulk shear is near 25 kt and 0-1 km 
srh is over 150 m2/s2. This favorable low-level shear coupled with 
the very moist, tropical airmass and strong low-level instability 
may result in a brief tornado or two. Marginal and isolated nature 
of the threat precludes the need for a watch. 

.Mosier/grams.. 06/20/2018 

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


Latitude...Lon 32989406 33479295 33279247 32839255 32369342 32429426