U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 211636 
Storm Prediction Center ac 211635 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1135 am CDT Tue Aug 21 2018 

Valid 211630z - 221200z 

..there is a slight risk of severe thunderstorms across the lower 
Great Lakes and central Appalachians... 

Scattered damaging winds and possibly a tornado are expected across 
the lower Great Lakes and central Appalachians vicinity mainly 
between 3-9 PM EDT. 

..lower Great Lakes to central Appalachians/mid-Atlantic states... 
Cloud cover has been slow to thin early today, but modest cloud 
breaks are noted at late morning across the upper Ohio Valley 
roughly coincident with the upper dry slot. A shortwave trough over 
the Midwest will continue to dampen as it moves east towards the 
lower Great Lakes while an upstream impulse over the 
upper Midwest accelerates east. A surface cyclone will track from 
Southern Lake Michigan into southern Ontario. However, convergence 
along the lead cold front should remain modest as an upstream front 
eventually overtakes this boundary early Wednesday. 

Cloud cover and 12z radiosonde observation-observed weak mid-level lapse rates are 
still expected to keep buoyancy modest overall, generally 
characterized by MLCAPE around 1000 j/kg amidst upper 60s to lower 
70s f boundary-layer dew points. 

With a broad swath of 30-40 kt mid-level southwesterlies, scattered 
storms should develop into organized clusters across the central 
Appalachians and upper Ohio Valley. A few supercells may also be 
favored in the western PA/northern WV/western Maryland region where 
stronger low-level southerlies should overlap the northern extent of 
the modest buoyancy plume with a risk for isolated severe gusts and 
a tornado. Otherwise, strong gusts producing scattered tree damage 
should be the primary hazard, especially with southeast extent where 
weaker low-level winds are expected. 

..southern Appalachians toward the Gulf Coast... 
While stronger forcing for ascent/vertical shear will be focused to 
the north of the region, thinning cloud cover and a moist 
environment (70s f dewpoints) will allow for pockets of moderate 
destabilization this afternoon. A few downbursts capable of 
localized wind damage may occur this afternoon/early evening with 
the strongest storms. 

..south-Central High plains... 
It still appears there will be a semi-focused potential for a couple 
of supercells along with a small multicell cluster with convection 
that forms off the Sangre de Cristos and Raton Mesa vicinity and 
moves eastward. A confined corridor of moderate buoyancy will 
overlap the fringe of modest mid-level westerlies. Isolated large 
hail and locally strong wind gusts will be possible late this 
afternoon through early/mid-evening. The need for a small slight 
risk upgrade will continue to be reevaluated through the afternoon. 

..parts of Utah/Colorado... 
Some strong thunderstorms are ongoing this morning across the 
region. A semi-strong belt of mid-level westerlies coincident with a 
relatively moist air mass and diurnal heating should allow for some 
additional storm intensification along with a greater areal coverage 
through the afternoon. While locally heavy rainfall may be the most 
common concern, a few storms could produce severe-caliber downburst 
winds and possibly small hail. 

.Guyer/Wendt.. 08/21/2018 


Mesoscale Discussion

acus11 kwns 211817 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 211816 

Mesoscale discussion 1334 
National Weather Service Storm Prediction Center Norman OK 
0116 PM CDT Tue Aug 21 2018 

Areas affected...parts of western and central PA...WV...and the 
western Maryland Panhandle 

Concerning...severe potential...watch possible 

Valid 211816z - 212015z 

Probability of watch issuance...60 percent 

Summary...a Severe Thunderstorm Watch is being considered for parts 
of western and central Pennsylvania into parts of West Virginia and 
the western Maryland Panhandle. Strong wind gusts will be the 
primary severe weather threat, though a tornado or two is possible. 

Discussion...surface streamline analysis indicated an area of 
confluence from eastern WV into western PA early this afternoon, 
which appears to be coincident to the leading edge of greater 
forcing for ascent spreading east across the discussion area ahead 
of the approaching upper trough. Breaks in cloudiness this 
afternoon have allowed for some surface heating within a moist 
environment contributing to moderate destabilization (mlcape of at 
least 1000 j/kg). 30-40 kt of southwesterly midlevel winds will 
maintain sufficient effective bulk shear for line segments/bowing 
structures, with damaging winds the primary severe hazard. However, 
a tornado threat does exist, especially in locations where surface 
and low-level winds have a southerly trajectory, enhancing low-level 

.Peters/guyer.. 08/21/2018 

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


Latitude...Lon 38288140 39778042 40797998 41337963 41437897 41397834 
40977810 39787820 38697910 37718022 37518103 37638133