U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 212004 
Storm Prediction Center ac 212003 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0303 PM CDT Tue Aug 21 2018 

Valid 212000z - 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 until 
9 PM EDT. 

Wind probabilities have been trimmed from the west across portions 
of WV/eastern Ohio/western PA, to account for ongoing convective 
trends. Some redevelopment remains possible along the cold front 
with a threat of isolated damaging wind, but the primary severe 
threat is expected to be with the convective band approaching 
central PA/eastern WV/western Maryland. The threat for a brief tornado or 
two remains along and ahead of this band, and also further southeast 
into portions of Maryland/northern Virginia along an outflow boundary (where 2% 
tornado probs have been extended), though the primary risk is still 
expected to be damaging wind. 

Minor adjustments have been made to the 5% wind probabilities across 
portions of Utah/co, based on ongoing convective trends. Severe wind 
gusts are expected to be the primary hazard in this region, though 
at least small hail will also be possible with the strongest cores. 

..south-Central High plains... 
No changes have been made to this area, see the previous discussion 
below for more information. 

.Dean/dial.. 08/21/2018 

Previous discussion... /issued 1135 am CDT Tue Aug 21 2018/ 

..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. 


Mesoscale Discussion

acus11 kwns 212104 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 212103 cor 

Mesoscale discussion 1338 
National Weather Service Storm Prediction Center Norman OK 
0403 PM CDT Tue Aug 21 2018 

Areas affected...VA...southern Maryland 

Concerning...severe potential...watch unlikely 

Valid 212103z - 212300z 

Corrected for mesoscale discussion delineation 

Probability of watch issuance...20 percent 

Summary...scattered strong thunderstorms will continue into early 
evening, with a primary threat of isolated damaging wind gusts. 
Watch issuance is not anticipated unless significant intensification 

Discussion...scattered thunderstorms are ongoing across much of Virginia 
into southern Maryland at 21z. The environment is characterized by rich 
low-level moisture, which is supporting MLCAPE of 1500-2500 j/kg, 
despite the presence of weak midlevel lapse rates. The southern 
fringe of strong midlevel flow is impinging upon this region, 
resulting in effective shear of 25-35 kt, which will support some 
storm organization with the strongest cells. The hail threat should 
remain minimal due to warm temperatures aloft, with isolated 
damaging wind gusts the primary threat. A brief tornado also cannot 
be ruled out with any storm interacting with a convective outflow 
boundary draped from northern Virginia into southern Maryland. The overall 
threat is currently expected to remain too marginal for watch 

.Dean/guyer.. 08/21/2018 

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


Latitude...Lon 37557975 38257931 38237873 38387771 38557670 37677593 
37347604 37137671 36877777 37207828 37337902 37357959