U.S. Severe Weather Forecast (Convective Outlook)

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Storm Prediction Center ac 231252 


Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0752 am CDT Mon Oct 23 2017 


Valid 231300z - 241200z 


..there is a slight risk of severe thunderstorms over parts of the 
Carolinas and Virginia... 


..there is a marginal risk of severe thunderstorms elsewhere from 
northern Florida to the middle-Atlantic region... 


... 
Severe storms are possible from northern Florida to the mid-Atlantic 
this afternoon and evening. The primary threat will be damaging 
wind gusts, along with potential for a few tornadoes. 


... 
Mean mid/upper troughing over the eastern U.S. Will be reinforced 
with major cyclogenesis late in the period and into day 2. A 
strong, negatively tilted shortwave trough -- currently containing a 
500-mb low near the MO bootheel -- will eject northeastward then 
northward across the mid/upper Ohio Valley and Lake Erie today and 
tonight. Meanwhile, a northern-stream perturbation -- now evident 
in moisture-channel imagery over northwestern Minnesota and South Dakota -- will 
amplify greatly as it digs southeastward, reaching Iowa and MO by 00z 
and forming a closed low over Illinois tonight. By 12z, a large cyclone 
should cover much of the U.S. From the plains eastward, with a 
temporary double center -- one over eastern Upper Michigan and the other 
near the Illinois/in border. 


At the surface, 11z analysis showed a frontal-wave cyclone over 
western/southern in, with cold front extending to middle TN, eastern 
al, the western Florida Panhandle, and the central Gulf. A warm front 
was drawn from extreme southern coastal NC westward over central/ 
western SC, becoming diffuse in an area of convection and precip 
over northern Georgia and eastern Tennessee. The surface cyclone should deepen 
and occlude as it follows a curved path across eastern lower Michigan to 
near the eastern end of Lake Superior by 12z. The surface cold 
front is forecast to reach WV, the western Carolinas, central/ 
eastern Georgia and the eastern Gulf by 00z. By 12z, the cold front 
should reach central NY, eastern PA, eastern NC, and the northern/ 
western Florida Peninsula. The warm front should become more diffuse as 
it shifts rapidly northward through the Carolinas, Virginia and portions 
of the mid-Atlantic region through this evening. 


..northern Florida to mid-Atlantic region... 
A band of thunderstorms should develop near the surface cold front 
and sweep east-northeastward across the outlook area today, with 
damaging gusts and a few tornadoes possible. Isolated damaging 
gusts are possible with a more conditional tornado risk accompanying 
convection in an extensive prefrontal corridor. 


A roughly 150-nm-wide plume of prefrontal convection, precip and 
thick cloud cover are apparent at this time, from eastern Tennessee 
southward across much of Georgia and the central/eastern Florida Panhandle to 
the northeastern Gulf. This plume and associated/downstream 
mid/upper-level cloud cover will substantially impede insolation 
over much of the warm sector today, rendering weak boundary-layer 
lapse rates in many areas. Still, a combination of subtle/diffuse 
diabatic heating, low-level Theta-E advection and weak mlcinh should 
result in adequate buoyancy for surface-based convection, amidst 
northward expansion of rich low-level moisture. These factors will 
offset modest mid/upper-level lapse rate enough for 1000-1500 j/kg 
MLCAPE over parts of northern FL, 500-1000 j/kg MLCAPE as far north 
as the Carolinas, and 100-500 j/kg possible northward toward parts 
of central/eastern Virginia and the Delmarva region this evening. 
Strengthening deep-layer wind fields are expected over the warm 
sector today as the height gradient aloft tightens east of the 
deepening mean trough, and the Lower/Middle Mississippi Valley 
shortwave trough ejects northeastward. This will create a 
Low-Cape/high-shear scenario from the Carolinas northward, with 
lesser (but still adequate) shear and larger cape over parts of 
southern/eastern Georgia and northern Florida. 


The northern end of the main convective band likely will be manifest 
as a narrow, strongly forced ribbon of low-topped convection with 
little or no lightning -- already evident in non-severe form above a 
relatively stable surface air mass over portions of Kentucky and Tennessee. This 
band should develop further and accelerate northeastward across the 
central Appalachians today. Forecast soundings generally show only 
minor variations in lapse rates and low-level Theta-e; however, 
those variations occur across neutral stability. As often is true 
in near-neutral profile scenarios, very minor shifts -- often below 
model-predictability limits and spatially poorly sampled by 
upper-air thermodynamic observations -- may differentiate between 
profiles with unsupportive conditional stability and those with deep 
(albeit low-density) cape. As such, considerable uncertainty exists 
regarding how far north a truly convective wind-damage risk exists 
over the WV/PA/western Maryland region. Still, enough progs indicate 
neutral to very weakly unstable lapse rates that some northward 
expansion of the marginal convective-wind risk probably is 
warranted. Forecast mean-wind and deep-shear vectors are strong, 
but largely parallel to the boundary instead of orthogonal, as would 
represent a more optimal convective wind setup. 


.Edwards/Dean.. 10/23/2017 


$$ 

Mesoscale Discussion


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Storm Prediction Center mesoscale discussion 221900 
gaz000-flz000-alz000-222030- 


Mesoscale discussion 1736 
National Weather Service Storm Prediction Center Norman OK 
0200 PM CDT sun Oct 22 2017 


Areas affected...portions of southern Alabama and th western Florida 
Panhandle 


Concerning...severe potential...watch unlikely 


Valid 221900z - 222030z 


Probability of watch issuance...20 percent 


Summary...a few strong wind gusts and a brief/weak tornado or two 
may occur across southern Alabama into the western Florida Panhandle 
through the afternoon and early evening. A watch is not expected at 
this time. 


Discussion...heating ahead of a cluster of storms currently located 
over southwest Alabama has allowed for weak destabilization (less than 
1000 j/kg) across the Florida Panhandle into southern Alabama. While 
midlevel lapse rates are unremarkable, convection is likely being 
enhanced by an embedded mesoscale convective vortex (noted earlier in the morning over 
southeast ms). Near the apex of the bowing structure over Baldwin 
County, Alabama some stronger wind gusts will be possible as the storm 
cluster tracks east-northeast as low level lapse rates have 
steepened sufficiently. Additionally, a narrow zone of effective srh 
values around 200 m2/s2 will shift northeast across southern Alabama and 
the Florida Panhandle. A few briefly rotating cells have already been 
noted moving inland east of Mobile and this trend should continue as 
the system tracks northeast. Given the overall marginal nature of 
the thermodynamic and kinematic environment and transient/weak 
nature of any rotating cells, a watch is not expected at this time. 


.Leitman/Thompson.. 10/22/2017 


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


Attention...WFO...tae...bmx...mob... 


Latitude...Lon 30048707 30118764 30288797 30888803 31248785 31518752 
32018668 32128597 32108547 31988518 31588504 30878512 
30098554 29888573 30048707