U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 190041 
Storm Prediction Center ac 190040 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0640 PM CST Sat Nov 18 2017 

Valid 190100z - 191200z 

..there is a marginal risk of severe thunderstorms through middle to 
late evening across parts of the upper Tennessee Valley into the 
central and southern Appalachians... 

A remnant band of thunderstorms approaching the central and southern 
Appalachians region may continue to be accompanied by locally 
damaging wind gusts through about 8-9 PM EST. 

..01z outlook update... 
Strong mid/upper forcing for ascent, associated with a vigorous 
short wave trough progressing east of the Mississippi Valley, is 
maintaining convective development across eastern Kentucky and 
Tennessee into portions of northern Alabama. As activity advances 
into a progressively cooler and drier low-level environment near the 
central and southern Appalachians, thermodynamic profiles are 
becoming less conducive for the development of charge separation and 
lightning. However, remnant convection may continue to aid the 
downward Transfer of high momentum air to the surface, resulting in 
a continuing risk for localized damaging surface gusts. 
As the cold front reaches the crest of the southern Appalachians 
toward 03-04z, the risk for deeper convection enhanced wind gusts 
seems likely to diminish. 

The risk for pre-frontal thunderstorm activity east of the 
Appalachians later tonight is more unclear. However, latest 
forecast soundings from the rapid refresh continue to suggest this 
potential is generally low, but perhaps maximized on the 
southeastern fringe of the mid-level trough as it pivots toward the 
Atlantic coast. This is where a modest influx of moisture could 
contribute to weak cape through the favorable mixed phase layer for 
charge separation. Any such activity is expected to be elevated 
above a neutral to stable near-surface layer, which may tend to 
limit potential for convectively enhanced gusts to reach the 
surface. Given the apparent low probabilities for thunderstorms, 
severe wind probabilities still seem less than 5 percent. 

.Kerr.. 11/19/2017 


Mesoscale Discussion

acus11 kwns 182324 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 182324 

Mesoscale discussion 1773 
National Weather Service Storm Prediction Center Norman OK 
0524 PM CST Sat Nov 18 2017 

Areas affected...portions of Tennessee and northern Alabama 

Concerning...Severe Thunderstorm Watch 517... 

Valid 182324z - 190130z 

The severe weather threat for Severe Thunderstorm Watch 517 

Summary...a squall line will continue east within and near watch 517 
this evening, with an attendant threat of a few damaging gusts and 
perhaps a tornado. The line should gradually weaken with eastward 
extent this evening, such that downstream watch issuance is not 
currently anticipated. 

Discussion...a nearly continuous line of thunderstorms is advancing 
across Tennessee and northern Alabama early this evening. Over the 
last hour, there was a localized uptick in tornado potential 
south/east of the Nashville area, with three distinct tornadic 
debris signatures in Rutherford, Wilson, and Trousdale counties. 
While cells will continue to outpace remaining low-level buoyancy, 
impressive low-level shear may still yield a quick tornado across 
middle Tennessee over the next hour. Otherwise, an isolated damaging 
wind threat will likely persist through the 01z expiration time of 
watch 517. 

Farther south, convection has at times consolidated into a few 
stronger segments over northern Alabama. Local observations have yet 
to indicate severe wind gusts, but considering the strength of 
low-level flow and presence of weak mixed-layer cape, isolated 
damage will remain possible for the next couple hours, before 
surface-based buoyancy is shunted farther south. 

.Picca.. 11/18/2017 

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


Latitude...Lon 34048819 36588602 36588440 36278421 34978545 34268621 
33988719 33918800 34048819