Looking at the image below now, this depicts the surface as of this morning. We see the same cold front has progressed to about a couple hundred miles from the ocean in most locations and the warm front is just south of Delaware. We still see a few storms initiating in the small area between the cold front and the ocean and off of the Carolina coast, but these will remain fairly isolated in nature with only a couple of these storms having reached severe limits today.
The major story will begin tonight when this cold front begins to lift back to the north (at which time it actually turns into a warm front, got to love the Norwegian Cyclone Model). You can actually see this beginning to occur now in the image below. The system from Wednesday has begun to drift back north over the Mississippi, Alabama, and Arkansas areas and has combined with a new system located over the Plains and back toward the Rockies and desert Southwest.
This will continue to be the case throughout the evening and overnight hours and as that warm front pushes north, the warm and moist air will return with it. Check out the current gradient in dew point temperatures from one side of that cold front to the other. The initialization of the 18Z NAM run shows this gradient quite well (shown below), with dew points increasing by about 30°F over just a space of a little over one hundred miles.

So where will the threat be tomorrow? The current Day 2 Convective Outlook from the SPC shows a large slight risk area (the 15% and 30% areas on this map) with an embedded moderate risk area (the 45% region in this map) which covers many of the portions of the U.S. which were hit hard on Wednesday. Additionally, the SPC has hatched an area (this means they believe there is a 10% or greater chance of significant severe weather within this area) which includes all of the moderate risk area as well as some of the nearby slight risk area.
The main threats are quoted below from the SPC with shorthand spelled out so everyone can understand what they are saying:
“THE STORM DEVELOPMENT SHOULD BE FOCUSED ON A ZONE FROM THE MID OHIO VALLEY SOUTHWESTWARD ACROSS KENTUCKY AND TENNESSEE INTO PARTS OF MISSISSIPPI AND NORTHERN ALABAMA…WHERE RAPID INTENSIFICATION IS EXPECTED GIVEN THE FAVORABLE THERMODYNAMICS ALONG WITH SHEAR PROFILES VERY SUPPORTIVE OF UPDRAFT ROTATION.
WITH STORM MODE LIKELY TO BECOME A MIX OF SUPERCELLS AND COMPLEX LINE SEGMENTS…THREATS FOR BOTH DAMAGING WINDS AND SEVERAL TORNADOES APPEARS EVIDENT…ACCOMPANYING THE LIKELIHOOD FOR LARGE HAIL. A COUPLE OF STRONG TORNADOES WILL ALSO BE POSSIBLE — MAINLY DURING THE MID TO LATE AFTERNOON HOURS IN AND NEAR THE MODERATE RISK AREA.”
With this information, I will take a look at a few variables to show you the threat they speak of. The first thing, which I mentioned previously, is the return of warm and moist air to this region. The 18Z NAM run shows what it expects the dew points to be tomorrow afternoon. A wide area of 60°F plus readings stretches all the way into southern Indiana and along to the Ohio/Kentucky border. This will be plenty of moisture for storms to initiate as normally about the 55°F line is considered the cut off point for severe weather to occur, though it can occur below this reading. With that in mind though, it appears the northern boundary of the SPC Outlook follows this 55°F reading pretty well to its northernmost extents throughout the afternoon right into the overnight.

The next thing to consider is the amount of warm air advection reaching these areas. I will not show this image here but it appears most areas in the warm sector will be receiving at least some marginal warm air advection at 850 mb which should help with uplift. Additionally, a strong low level jet will cross from Arkansas and Louisiana northeastward to Ohio and Pennsylvania throughout the day and into the night resulting in some added shear as it passes above these areas. Looking even higher into the jet stream, a strong 100 knot jet streak will be in place over most of the threat area, extending more towards the northeast towards nightfall. This is seen in the image below by what the 18Z NAM depicts at around midnight Friday night.

I will take a look at one last factor that could aid in this event, helicity from 0 to 3 km as depicted by the NAM model. This is a measure of the amount of rotation that is found in the updraft of a storm from the surface to 700 mb, the more there is, the more likely the storm can produce a tornado given that other factors also look conducive for storm and tornado formation. By midday, we see a large area of helicity in excess of 300 over most of Kentucky and Tennessee as well as parts of Illinois and Indiana. This area was a little farther to the south and west in the morning hours and will continue to track to the north and east heading towards nightfall. The good news about this is it appears most of the tornadoes should occur during the day, however it is still never good news to see a tornado threat. Additionally, helicity will be supportive and above 300 in many areas with the passing of the cold front which may raise the likelihood of tornadoes inside the squall line which should form. This is where the risk of night tornadoes would potentially be the best.
One last thing to look at now is how this compares to the surface to 6 km shear. It appears the best values during the day will be towards the north but then also good right in front of the passing of the cold front which I will not show here.
Below is where I think the biggest threat lies from Friday morning through Friday night. My threat map depicts all of the severe weather parameters I have been talking about in this post to pinpoint where severe weather is most likely to occur. The green line shows anything to the south and east of that should be in the warm sector and have dew points at or above 55°F at some point during the day. The brown line boxes in the area inside this warm sector that will benefit from the low level jet being over them at some point during the day. To the left of the blue line, but to the right of the green line, shows anywhere where a 100 knot plus jet streak will be above them at some point during the day. To the left of the orange line is the area where 0 to 3 km helicity appears favorable. I then shaded in an area based on these parameters that I consider to have the best threat in red. I left out areas to the east and a little on the western edge because I felt daytime heating may hurt these areas, for the west not enough and for the east the storms would come through to late at night after the air had cooled down some already.
It is important to note that my threat map is solely based off just a few indicators of severe weather as depicted by the 18Z model run of the NAM. Therefore, it is possible that things could change between now and the threat tomorrow and also possible that other models are depicting the evolution of the threat slightly differently. The important thing is that if you are in or around the threat area I highlighted or the area the SPC has highlighted for tomorrow, to remain alert. Storms will be moving very fast with this system and therefore you may not have much time to prepare. Prepare now, know your plan in case of an emergency, just in case. Watch for supercells and small line segments tomorrow during the day and then a large line of storms to form tomorrow evening and into tomorrow night as the cold front passes through. All threats are possible with this system, from tornadoes, some potentially large and long tracked, large hail, and strong damaging winds. This should be a very interesting setup to watch unfold tomorrow.