U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 200554 
Storm Prediction Center ac 200552 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1252 am CDT Mon Aug 20 2018 

Valid 201200z - 211200z 

..there is a slight risk of severe thunderstorms this afternoon and 
evening across parts of the middle Mississippi Valley and middle south 

A few severe thunderstorms are possible this afternoon and evening 
across parts of the middle Mississippi Valley and mid south region. 
Damaging wind gusts appear the primary hazard, but there may be some 
risk for an isolated tornado or two. 

A modest lower/mid tropospheric cyclone, now evolving within a 
southern branch of mid-latitude westerlies across the lower Missouri 
Valley, appears likely to remain progressive through this period. 
Models suggest that the mid-level circulation will gradually turn 
eastward into the middle Mississippi Valley during the day, as a 
significant speed maximum propagates around its southern through 
southeastern periphery. It is then forecast to accelerate 
northeastward toward the Great Lakes region, weakening and 
undergoing deformation to the southeast of troughing digging within 
the stronger westerlies across the central Canadian provinces and 
adjacent international border area. 

Downstream, a weaker southern branch impulse is forecast to progress 
east of the northern mid Atlantic coast. Upstream, larger-scale 
troughing now developing inland of the Pacific northwest coast 
appears likely to undergo considerable deformation between ridging 
to the northwest and southeast, but at least one fairly significant 
elongating remnant mid-level circulation may slowly shift across the 
northern intermountain region and northern rockies. 

..Mississippi Valley/mid south... 
The lack of colder mid-level air and generally modest to weak 
low/mid-level lapse rates probably will continue to limit severe 
weather potential with the remnant cyclone, particularly given its 
modest strength. The lingering remnants of ongoing convection now 
overspreading the middle Mississippi Valley, and associated 
rain-cooled air, will also inhibit or complicate warm sector 
boundary layer destabilization. However, guidance is suggestive 
that seasonably high boundary layer moisture content may contribute 
to moderately large cape (2000+ j/kg) with insolation beneath the 
dry slot associated with the mid-level low. This appears most 
probable across parts of the mid south, where a zone of stronger 
differential heating may evolve and provide the focus for strongest 
thunderstorm development this afternoon. It is possible that this 
may include supercells, and an upscale growing convective cluster by 
early evening, aided by favorable deep layer shear beneath 30-50 kt 
southwesterly 700-500 mb flow. More uncertainty exists concerning 
the strength of the low-level hodographs, but at least some guidance 
suggests strengthening of southwesterly 850 mb flow to 30-40+ kt is 
possible across the mid south into parts of the lower Ohio Valley 
before boundary layer instability wanes this evening. This may 
augment the risk for potentially damaging wind gusts, and contribute 
to some potential for a tornado or two. 

Otherwise, a warm frontal zone across northern Illinois, in closer 
proximity to the mid-level circulation, could also become a focus 
for convection with a risk for at least brief/weak tornadoes, given 
sufficient boundary layer destabilization. The extent of this 
potential, however, remains unclear at the present time. 

..southern Virginia/North Carolina... 
In the wake of the departing short wave impulse, models suggest that 
a 30 kt west northwesterly mid-level speed maximum migrating east of 
the southern Appalachians through the southern mid Atlantic coast, 
could enhance convection this afternoon. Thunderstorms are expected 
to form in the presence of moderately large cape, associated with 
daytime heating of a seasonably moist boundary layer. 

..northern intermountain region/rockies... 
A relatively cool mid-level air mass and at least modest vertical 
shear on the northern periphery of a 30-50 kt mid-level speed 
maximum may support an isolated storm or two with some risk for 
severe hail and strong wind gusts along the higher terrain of 
central Idaho and southwest Montana into northwestern Wyoming. 

.Kerr/squitieri.. 08/20/2018 


Mesoscale Discussion

acus11 kwns 192248 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 192247 

Mesoscale discussion 1324 
National Weather Service Storm Prediction Center Norman OK 
0547 PM CDT sun Aug 19 2018 

Areas affected...southwest Missouri into northern Arkansas 

Concerning...Tornado Watch 343... 

Valid 192247z - 200045z 

The severe weather threat for Tornado Watch 343 continues. 

Summary...the threat of damaging winds and a few brief tornadoes 
continues across the watch area. 

Discussion...storms continued to develop across southwest MO and 
northwest AR, ahead of a weak surface low and near a diffuse warm 
front. The latest surface analysis indicates 2 hr pressure falls in 
excess of 2 mb, associated primarily with low-level warm air 
advection. Hodograph size on the sgf VAD wind profile has gradually 
been increasing, now with 0-1 srh in excess of 200 m2/s2. This trend 
may persist over the next couple hours, coincident with an increase 
in the 850 mb flow. As such, the north-south oriented line of storms 
that moved out of Kansas into MO is becoming more of a qlcs with swaths 
of damaging wind and/or brief tornadoes possible. 

To the south and closer to the warm front, cells over Arkansas are more 
robust overall in terms of being instability driven with hail cores 
evident on radar. Low-level shear is not as strong as farther north, 
but a brief tornado cannot be ruled out with this activity. 

.Jewell.. 08/19/2018 

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


Latitude...Lon 35269249 35139403 35339419 35719416 36199421 36599432 
37009432 37419435 37839434 38139256 38129247 37799186 
37389149 36779125 36089124 35589154 35389189 35269249