U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 210040 
Storm Prediction Center ac 210039 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0639 PM CST Mon Nov 20 2017 

Valid 210100z - 211200z 

..no severe thunderstorm areas forecast... 

there appears to be negligible risk for severe thunderstorms across 
the U.S. Through tonight. 

..01z outlook update... 
Ongoing convective development across far southeastern Florida 
coastal areas and the Keys is based within at least modestly moist 
low-level easterly flow. This regime is generally capped by 
relatively warm and dry air extending through a deep layer of the 
mid/upper troposphere, which seems likely to continue to suppress 
deepening/intensification of activity into thunderstorms. 

As winds just above the surface begin to turn to a southerly 
component across the southern into Central Peninsula, ahead of the 
short wave trough crossing the central Gulf of Mexico, northward 
moisture advection may occur beneath a mid-level environment at 
least trending more conducive to thunderstorm development (based on 
latest forecast soundings and water vapor imagery). Probabilities 
for thunderstorms still appear generally low, but may be maximized 
(and in excess of 10 percent) along the Atlantic coast between Miami 
and Vero Beach, mainly late tonight. 

.Kerr.. 11/21/2017 


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