U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 200508 
Storm Prediction Center ac 200507 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1107 PM CST sun Nov 19 2017 

Valid 201200z - 211200z 

..no severe thunderstorm areas forecast... 

the risk for severe thunderstorms appears negligible across the U.S. 
Today through tonight. 

Models indicate considerable amplification within the westerlies 
across the eastern Pacific into western North America during this 
period. By late tonight, this appears likely to include sharpening 
large-scale ridging centered near or just inland of the Pacific 
coast. Downstream of this ridging, the initially split belts of 
westerlies may remain more distinct, but several digging short wave 
troughs will contribute to a tendency for increased troughiness east 
of The Rockies, after a short-lived deamplification of the upper 
flow in the wake of large-scale troughing now rapidly shifting 
across and east of the Atlantic Seaboard. 

Perhaps the most prominent perturbation, a short wave within the 
northern stream, likely will be accompanied by continued significant 
surface cyclogenesis from the Canadian prairies through northwestern 
Ontario and adjacent portions of the upper Great Lakes region. 
Despite associated strengthening southwesterly low-level flow across 
parts of the Central Plains into the Great Lakes region, no 
appreciable moisture return is expected in the wake of the prior 
frontal passage, which has reached southern Florida and southern 
portions of the Gulf of Mexico. 

At least some boundary layer modification is possible across the 
northern Gulf of Mexico, as southern branch troughing digs southeast 
of the Texas Gulf Coast into the Gulf of Mexico, but large-scale 
ascent and destabilization near or inland of coastal areas is not 
expected to become supportive of an appreciable risk for 
thunderstorms. Ahead of the upper troughing, low-level flow may 
gradually turn from northeasterly/easterly to easterly/southeasterly 
across the Florida Peninsula. By late tonight (toward 12z tuesday), 
this may be accompanied by sufficient moistening to contribute to 
weak to moderate destabilization near southeast coastal areas, where 
pockets of enhanced low-level convergence may support a risk for 
isolated to widely scattered thunderstorm activity. 

Otherwise, just in advance of the building upper ridge, a mid-level 
cold pocket (associated with an increasingly deformed mid-level 
impulse migrating inland of the Pacific coast) may potentially 
become supportive of weak destabilization across the Puget Sound 
vicinity this morning into the mountains of northern Idaho/northwest 
Montana this afternoon. It might not be out of the question that 
this could be accompanied by some convection capable of generating 
lightning, but any such activity is generally expected to be 
short-lived, and too sparse in coverage (less than 10 percent 
probability) for a categorical thunder outlook. 

.Kerr.. 11/20/2017 


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