U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus02 kwns 200540 
Storm Prediction Center ac 200539 

Day 2 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1239 am CDT Fri Jul 20 2018 

Valid 211200z - 221200z 

..there is a slight risk of severe thunderstorms Saturday afternoon 
and evening across parts of the Tennessee Valley through southern 
Appalachians and adjacent Piedmont... 

Severe thunderstorms are possible Saturday, mainly during the 
afternoon and evening, across parts of the Tennessee Valley through 
the southern Appalachians and adjacent portions of the Piedmont. 

Generally south of the stronger mid-latitude westerlies, and to the 
east of a prominent subtropical high centered over the southern High 
Plains, models indicate that large-scale troughing will continue to 
evolve across areas east of the Mississippi Valley through the 
Atlantic Seaboard. A closed low becoming increasingly detached from 
the westerlies appears likely to remain the most prominent embedded 
feature, gradually digging with an associated surface low south of 
the Great Lakes into the Ohio Valley by 12z Sunday. A more modest 
impulse may progress northward through the eastern periphery of the 
larger-scale troughing, accompanied by another more compact surface 
cyclone. While it appears that this latter feature may migrate 
mostly offshore to the east of the mid Atlantic coast, the spread 
among model output remains sizable. 

Upstream, to the north of the subtropical high, another significant 
short wave trough and embedded closed low are forecast to accelerate 
east/northeast of the Canadian/northern U.S. Rockies through the 
Canadian prairies. The main surface low associated with this 
feature appears likely to develop well to the north of the 
international border, but a weak low may develop within surface 
troughing ahead of the associated southeastward advancing surface 
front, across the northern High Plains. 

While perhaps highest moisture content will remain focused near/east 
of the wave off the mid Atlantic coast, seasonably high moisture 
content appears likely to persist at least one more day across the 
eastern Gulf into South Atlantic coast states, before at least some 
drying associated with the upper trough commences. Otherwise, at 
least a pocket of relatively moist air may persist in closer 
proximity to the digging closed low, while more modest moisture 
lingers or develops northward through the northern plains and across 
much of the southwest. This is all expected to contribute to 
seasonable diurnal destabilization and areas of scattered 
thunderstorm activity. Some of this may pose a risk for severe 

Considerable uncertainty lingers, particularly concerning 
sub-synoptic details which could impact convective potential for 
Saturday. There is a signal within at least some model output that 
remnant convection and/or convective outflow from Friday night may 
be in the process of spreading across parts of the central/eastern 
Gulf states at 12z Saturday. It is possible that this activity 
could re-intensify and pose a severe wind risk as it advances toward 
Alabama/Florida Panhandle coastal areas, where Cape May become 
moderate to large by midday. 

If this initial development does indeed occur, it remains unclear 
how this may impact the environment to the north. However, guidance 
is generally suggestive that the lingering moisture, daytime surface 
heating, and weak to modest mid-level cooling probably will 
contribute to moderately large cape across at least parts of the 
Tennessee Valley, southern Appalachians and adjacent Piedmont by 
late afternoon, if not before. It appears that this may be 
favorably timed with forcing for ascent accompanying a 30-50 kt 
mid-level jet streak, which would provide sufficient vertical shear 
for organizing storm clusters. Isolated supercell structures might 
be possible initially, perhaps accompanied by a risk for hail and a 
tornado or two, but the risk for potentially damaging wind gusts 
appears the primary potential hazard at the present time. 

..northern plains... 
With stronger forcing for ascent expected to generally pass well to 
the north of the international border Saturday, late afternoon and 
evening thunderstorm development near the pre-frontal surface trough 
may remain fairly isolated in nature. However, given the 
development of moderate cape in the presence of steep lapse rates 
and shear associated with the westerlies, a couple of supercells 
appear possible. 

..maximum risk by hazard... 
Tornado: <2% - none 
wind: 15% - slight 
hail: 15% - slight 

.Kerr.. 07/20/2018