U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 191304 
Storm Prediction Center ac 191302 

Day 1 convective outlook resent 1 
National Weather Service Storm Prediction Center Norman OK 
0802 am CDT Sat Oct 19 2019 

Valid 191300z - 201200z 

..there is a slight risk of severe thunderstorms from the central 
Florida Peninsula to the Carolina coast... 

Thunderstorms associated with tropical storm Nestor may produce a 
few tornadoes, along with isolated damaging wind, from the central 
Florida Peninsula to the Carolina coast. 

In mid/upper levels, synoptic ridging will shift eastward across the 
remainder of the Great Lakes, Ohio Valley and mid-Atlantic regions, 
reaching central qc and New England by 12z. Upstream, broadly 
cyclonic flow will cover an increasing proportion of the Continental U.S.. a 
strong shortwave trough -- now evident in moisture-channel imagery 
over the Pacific northwest -- will dig east-southeastward and 
develop a closed 500-mb low around the end of the period, near the 
Black Hills. As this occurs, a corresponding surface low will 
deepen, with a cold front moving out of the central rockies to the 
adjoining High Plains late tonight. 

Another shortwave trough will eject from the central/Southern Plains 
toward the Ohio Valley and southern/central Appalachians. As this 
occurs, tropical storm Nestor is forecast by NHC to move 
northeastward across Georgia and SC to southern NC by the end of the 

..FL to coastal Carolinas... 
The potential for episodic supercells -- and related threat for a 
few tornadoes and isolated damaging gusts -- will continue over much 
of central/northern Florida through the remaining forenoon hours. See 
Tornado Watch 675 and related mesoscale discussions for the latest 
near-term guidance. 

This threat will be episodic and discontinuous spatially, and will 
exist both: 
1. Near the diffuse inland part of the surface warm front (where 
backed flow and srh are maximized, but instability will be on the 
marginal side due to extensive antecedent precip), and 
2. Eastward and southward across the warm sector, where srh is 
somewhat less (but still favorable), and lift will be maximized in 
convective/convergence bands arching through the middle-outer 
eastern semicircle of Nestor. Isolated, discrete to semi-discrete 
supercells also may occasionally develop in this regime, as has 
occurred during prior overnight hours. Though cloud cover and 
precip will remain extensive inland for several more hours, at 
least, this potential may be maximized anywhere that sustained 
insolation can boost cape and strip away whatever mlcinh remains. 

Supercell potential will wind down across Florida from southwest to 
northeast this afternoon into evening, as dry slotting aloft and 
veering low-level flow with time reduce thunderstorm coverage and 
low-level shear, respectively. Meanwhile, and continuing through 
the overnight hours, the risk will shift northeastward and spread 
slightly inland with the increasingly sheared warm sector across 
coastal sections of Georgia and the Carolinas. Uncertainty lingers as to 
how far, but more inland leeway has been drawn into the marginal/2% 
and slight/5% tornado lines in deference to the strong onshore/ 
inland Theta-E advection probable in this scenario. This is in 
keeping with most guidance regarding at least weak MLCAPE along and 
southeast of the warm front. This front will continue to be a 
buoyancy-limiting boundary, with favorable shear on both sides but 
supportive cape along and southeast of it. 

See NHC advisories for latest tropical watches/warnings and 
track/intensity forecasts regarding Nestor. 

.Edwards/Dean.. 10/19/2019 


Mesoscale Discussion

acus11 kwns 191518 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 191517 

Mesoscale discussion 2078 
National Weather Service Storm Prediction Center Norman OK 
1017 am CDT Sat Oct 19 2019 

Areas affected...much of Florida and adjacent coastal waters 

Concerning...Tornado Watch 675... 

Valid 191517z - 191615z 

The severe weather threat for Tornado Watch 675 continues. 

Summary...although a very low tornado threat will exist beyond 16z, 
Tornado Watch 675 will be allowed to expire at 16z as scheduled. 

Discussion...a brief lull in the tornado threat has occurred in and 
near the ww area over the past half hour or so. Most of the 
convection across Florida has remained sub-severe and only weak 
rotation has been observed via radar imagery. Ongoing convection 
still remains in a strongly sheared environment in the low-levels, 
however, with 200-300 m2/s2 0-1 km srh and weak MLCAPE supporting a 
very low tornado risk. However, as the center of Post-ts Nestor 
advances northeastward, low-level flow is expected to veer and 
lessen low-level shear. Furthermore, drying aloft across the 
eastern Gulf of Mexico will eventually overspread the discussion 
area from west to east throughout the day, which should ultimately 
limit convective coverage and also contribute to a lower tornado 
threat. The exception to this will be in far northeastern Florida 
and points farther northeast, where convection will be more 
widespread and a warm front will gradually lift northeastward, 
providing a sufficient cape/shear space for a brief tornado risk. 
This scenario will be addressed by a separate mesoscale discussion later today. 

.Cook/guyer.. 10/19/2019 

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


Latitude...Lon 29798470 30118458 30378378 30548301 30678212 30658151 
29948068 28438002 27507986 26468046 25668120 25728186 
26068242 26768295 27688314 28608334 29308392 29528442