2017-04-05 EVENT IL/IN/KY/TN/AL/GA

[Ethan Schisler]

Figured I would start a thread for this event tomorrow over the Ohio Valley and to some extend the Midwest region. There are (2) TWO potential target areas to look at....

1) The 12z 3km NAM shows a ~90kt H5 jet pushing into Southern Indiana/Southeast Illinois by 21z tomorrow. Storms should fire ahead of a 988mb surface low that will be positioned over SE Illinois and move northeast into Indiana. Most guidance is showing at least upper 50s to perhaps lower 60s dew points in this region, along with somewhat decent lapse rates, giving CAPE values of 1500-2000 J/KG on the triple point tomorrow. SRH should also be quite abundant as well with values exceeding 300 m2/s2 when storms approach the warm frontal boundary. Supercells would be the initial preferred storm mode given such such shear and curved hodographs. However storms should go upscale as they push east across the Ohio Valley with a higher threat of damaging winds. In my eyes right now, the highest threat of tornadoes in the best terrain appears to be early on from 18z thru 22z across SE IL into SW IN perhaps near the I-70 corridor. CAMS have been showing some decent UH tracks along that region as well.

2) The second target, and perhaps main target, for tomorrow will be further south across AL/GA/TN. There are concerns over morning convection and the extent to which the warm sector gets modified, however looking at morning data, a fairly impressive combination of CAPE/shear does exist over a large area of real estate in the south tomorrow ahead of a dry punch associated with this strong surface cyclone over IL/IN. The best threat in this region for tornadoes, some perhaps strong and long tracked appears to be in the 21z to past 00z time-frame....and the best threat being from Birmingham, AL northeastward where CAMs show several impressive UH tracks during this time frame, through a volatile environment. Surface winds along the actual front itself won't be as backed as up north, so it will be reliant somewhat on getting convection to develop a few counties east of that frontal position where winds are more significantly backed and SRH much higher.

As for chasing tomorrow, I plan to stick to the more "obvious" target in my eyes considering where I live, SE IL/IN. I think as long as storms don't interfere with each other too fast, it could be a somewhat decent day up here. A very impressive system for any-time of the year.

 

[Dan Robinson]

Agreed on Ethan's points. A classic-looking setup for a Midwest tornado outbreak. My main concern with the northern target is the wave ejecting too early, which the GFS and NAM have been hinting at. There is a ton of lift with that jet and its ascent overspreading the Midwest fairly early in the day. If that can hold back a few hours, I think a relatively high-end event may be possible east of the St. Louis metro (possibly IN the metro if things slow significantly).

 

[Jesse Risley]

Agreed on Ethan's points. A classic-looking setup for a Midwest tornado outbreak. My main concern with the northern target is the wave ejecting too early, which the GFS and NAM have been hinting at. There is a ton of lift with that jet and its ascent overspreading the Midwest fairly early in the day. If that can hold back a few hours, I think a relatively high-end event may be possible east of the St. Louis metro (possibly IN the metro if things slow significantly).

This is pretty well depicted on the 12z 3km NAM with seasonably scanty instability profiles across the northern part of the target sector, though it has also been fairly aggressive with the northward extend of ambient moisture return too.

All of the other ingredients are there, and the forecast soundings do show particularly ambient 0-1 km shear profiles east of the surface low and proximal to the boundary. The 12z NAM today is still trying to show an H7 dry punch nosing into the region between 15z and 21z, namely south of I-64 in the favored zone for initially discrete convective development; hence, the evolution of destabilization, or lack thereof, and the influence of ongoing forcing for ascent and timing may be the opiate of the chasing masses tomorrow.

It's really a tough call when the initial mid-lever perturbation and nose of the stronger mid-level jet core is already overspreading the region between 06z and 09z, but the late morning or early afternoon dry punch, if it reaches consummation, should make things interesting. The latest CAM guidance continues to depict mostly discrete activity in the northern target early on.

 

[Jesse Risley]

Morning forecast data still showing a potentially significant severe weather day across a broad corridor, with some changes to the northern target, as expected.

I still think there is a plenteous window for some tornadic activity in the northern target, more proximal to the surface cyclone and warm frontal boundary, as large parts of S IN and N KY are undergoing destabilization already as nocturnal and foreday convective debris undergoes some necrosis, or at least has remained to the west of the eastern warm sector. If some of the short-term CAMs come to fruition, this looks to be a 3-4 hour window this afternoon before activity amalgamates into more QLCS or disheveled storm modes. This a bit further east of what earlier models were depicting 24-48 hours ago, as I doubt S IL or SW IN will get in on as much of this action, however, but not unexpected based on juxtaposition of the H5 energy and surface low along with ongoing precip to the immediate east and south the low.

Some of the forecast skew-T profiles further south into parts of TN, AL and GA are classic supercellular soundings, with ambient streamwise vorticity, seasonably generous CAPE profiles, classic LFC-LCL differentiation, and adequate deep layer shear suggest a potentially active afternoon and evening. Taking a glance at some of the forecast skew-T profiles for east central Dixie Alley in particular, after 20z, given the favorable PWAT, freezing levels, shear profiles and hydrolapse presence owed to differential advection across the region, I'd expect some significant hail reports too.

Given an otherwise less than optimal chase terrain, it does appear that there could be some mixed morphology of supercell types today, though probably favoring more HPish modes; nonetheless, I'd expect to see perhaps a few photogenic opportunities that are not entirely reflective of a monolithic HP mess. 12z skew-T forecasts across the areas with the optimal dynamics, largely the southern zone, do forecast low cloud base heights, with some soundings depicting CAPE below the freezing levels, though 4-6km SR winds of 17-20kts in some areas will coincide with increasing 0-1km SRH indices >250 m**2/s**2 after 20z in areas with otherwise favorable storm motion.

Good luck to those biting today.

 

[John Farley]

For anyone not keeping track, there is a tornado emergency now in a couple of SW Georgia counties. EDIT: Reports of a damage path a half-mile to a mile wide, and heading for some fairly densely populated areas (Americus, Plains, among other locations).

 

[Royce Sheibal]

Based on the VAD, it looks like the hodographs in the ATL area and western GA / east AL are recovering. They had gone VBV as the MCS passed through. That storm about to cross into GA looks nasty and is head due east into the metro. I've never chased in the Southeast, so I don't know much about how bases recover post MCS down there, but it looks like bases didn't rise very much post passage.

 

[Jeff House]

High Risk is justified especially for cells crossing the outflow boundary of the morning cluster in Georgia. OFB is lifting into SC as well. While the area should feature the most concentrated tornadoes, it is a mess. Separately, MD 444 covers continuing Midwest concerns noted above.

In between, back end of Georgia outflow in North Alabama is lifting into southeast Tennessee. As of mid-afternoon, believe OFB intercepts the hybrid CF/DL in southern Middle Tennessee. Appears that the short-wave coming out of Arkansas is enough to recover the LLJ in East Tennessee and adjacent areas. Still some cool air in southeast Tennessee associated with North Georgia precip. Despite just-in-time everything (LLJ, dews, temps) isolated tornadoes may form in East Tennessee. Odds are lower than in the High Risk area, but visibility should be better in Tennessee.

 

[Jesse Risley]

It looks like the show is starting across the norther target area as well, as storms are beginning to far from far S IN to SC TN as the stronger H5 jet core approaches from the MS River valley. Two 18z soundings are available, one from BNA and one from ILN. Better moisture still remains further south, likely partially owed to ongoing deeper convection there, but should nose into N KY and E TN this evening. Shear profiles up north still look decent and activity is trying to remain somewhat discrete attm. In the immediate term, a couple of cells headed towards the I 64 corridor will be flirting with the surface boundary and are worth watching, despite Tds in the low to mid 50s in most locales towards LVX.