U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 221239 
Storm Prediction Center ac 221237 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0637 am CST Wed Nov 22 2017 

Valid 221300z - 231200z 

..no severe thunderstorm areas forecast... 

inland thunderstorm chances through tomorrow morning appear confined 
to parts of Florida. 

In the mean, the upper-air pattern will maintain a western ridge and 
eastern trough through the period. This will keep substantial 
low-level moisture and instability shunted out of the great majority 
of the U.S. Mainland, except for portions of Florida. A basal 
shortwave trough -- evident in moisture-channel imagery over parts 
of the northwestern Gulf and the lower Rio Grande Valley of Texas -- is 
forecast to dig southeastward across the western/central Gulf 
through the period and become less positively tilted. As this 
occurs, associated thunder risks over the Gulf should retreat 
farther away from the Lower Texas coast. By 12z, the perturbation 
should align closely with the southern part of larger-scale trough 
position, from near the Mississippi River mouth across the Eastern 
Bay of Campeche. Slightly confluent southwest flow aloft will 
result across FL, while a surface frontal-wave cyclone moves from 
the south-central to east-central Gulf along a residual baroclinic 

Meanwhile, a strong northern-stream shortwave trough -- now located 
from southern on across WV -- is expected to pivot east- 
northeastward to western New England and Long Island by 00z, then 
well east of Maine by 12z. Large-scale ascent preceding this 
feature will support a favorable environment for thunder well 
offshore from Delmarva and Long Island, where heat fluxes from ocean 
to boundary layer contribute the most low-level instability. 

Isolated to widely scattered thunderstorms may develop during the 
most strongly heated diurnal cycle across parts of the southern 
peninsula, where boundary-layer Theta-E will remain relatively 
maximized. Forecast soundings suggest peak MLCAPE 500-1000 j/kg but 
amidst low-level weaknesses of both low-level flow and shear. Given 
that factor and lack of more robust buoyancy, organized severe is 
not anticipated. Another round of thunderstorms may affect 
northwestern Florida and adjacent waters late in the overnight period -- 
largely after about 09z. This should occur as large-scale ascent 
and frontogenetic forcing ahead of the shortwave trough cross over 
suitably high-Theta-E low-level air, near and just north of the 
surface front. 

.Edwards.. 11/22/2017 


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