So what are we looking at? Well in short a pretty widespread area from East Texas northward through Iowa is at risk for potentially significant storms. The SPC issued an "enhanced" risk (3/5) for severe weather 3 days out, which is notable alone, and it seems likely that tornado and severe thunderstorm watches will occupy many-a-mile tomorrow. You obviously can't just target East Texas to Iowa though so let's narrow it down a bit.
There are actually a few different options that a chaser could consider and they're all relatively reasonable for tornado potential. That's part of what makes this a little more complex, and in my opinion a little more interesting and fun. We'll dive into how things have evolved up to this point and I'll share my thoughts on how I think things may play out and how I'm going to chase this event as well as a couple other areas I think you could target.
Current Analysis and Forecast
All in all we've got an impressive system that has come together across the central US and this is putting a classic Plains setup on the table. A relatively high amplitude, though neutral-tilt, shortwave trough embedded within the Polar Jet has been swinging around the Rockies for the past 24-36hrs now and it has a powerful speed max associated with it. Winds aloft around 30,000ft are over 100kts and this cross-range flow has aided in deep lee-side cyclogenesis in the Colorado region. A strong surface-low (sub-990mb) spun up today and a significant organizing mid-latitude cyclone is evident. My morning observation chart analysis showed the effects of this low already ongoing and essentially giving us a head start on priming the environment for tomorrow. For one thing, a previous low-pressure system passed toward the Great Lakes earlier in the week and this helped reconnect us to rich Gulf moisture following a preceding cold front. The development of this more significant low has picked up right where we wanted it to and started pulling the approaching front back North as a warm front. At 14z this morning there were already mid-60's dew points in East Texas and a deep moisture axis extended well up to 850mb. A nocturnal low-level jet and further deepening of the surface cyclone will be key in establishing our pre-frontal moist sector and ramp up this moisture return even more so I fully expect to see mid-60's dew points all the way into Iowa. I always like to see the moist sector get established well before initiation is expected because it allows the moisture to build throughout the boundary-layer and help limit the chances of a shallow moist layer.
A dryline was analyzed on morning obs and has been drifting ENE throughout the day and this feature will be a key factor tomorrow. Observed soundings also show very very steep lapse rates over the SW US, approaching dry-adiabatic.
But that's just the large scale setup, to narrow down our possible chase areas we need to assess probable storm evolution/storm mode and mesoscale features. The axis of the upper-jet streak looks timed out to dive into the KS area by mid-afternoon so it appears that the stronger deep-layer shear will exist from eastern KS into MO and IA, something on the order of 50kts. Of great interest in severe storm forecasting is the orientation of shear vectors to the initiating boundary. In this case shear vectors should cross the dryline at about 45degrees and this is pretty good for the development of severe storms. This allows storms to roam relatively freely while still maintaining a sufficient "parcel residence time" along the region of ascent to sustain. Further South shear profiles will still support severe storms but with mid-level shear somewhat weaker and more parallel to the boundary I think it is very possible that storms will both ride the boundary and run into each other, both of which lead me to anticipate a more linear structure, somewhat less favorable for supercell tornadoes, although there is a realistic chance that pre-frontal storms develop and these would certainly pose a supercell threat. Forecast data shows long, looping hodos as well, particularly in the vicinity of the triple point where low-level winds will be a little more backed and the likelihood of mesocyclone/low-level rotation appears fairly high in this area. A strengthening of the low-level jet by late afternoon/early evening will only serve to enlarge hodographs even more and this would further increase the tornado threat.
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| HRRR Forecast Sounding (Via SHARPpy Software) for Kansas City, MO (Blue-ish Traces) and Longview, TX at 19z Friday 13 April 2018. You can compare the pre-storm environments in these two locations with this tool. Notice, in both locations the moisture extends well throughout the BL and both also show suffcient buoyancy. The TX profile however is already saturated woth strong vertical velocities suggesting ongoing convection and cloud cover. A veer-back-veer profile is evident in TX too and weaker mid-level winds could result in storm mergers early, inhibiting greater supercell potential, but the hodograph still suggests some tornado potential. I've overlayed the dryline motion across KS/MO on the hodograph to show boundary-relative winds in that region. |
How does this play out then? As mentioned, a sfc-low will be positioned in central KS. A strong cap will be in place and this could delay initiation but will also help maintain discrete cells if we get storms. Elevated convection will likely be ongoing through the morning but this will be North of the warm front and will lift out of the region early. Skies should be mostly clear as a clear slot works into the region and the depth of dryline mixing relative to the depth of the moist layer is more than sufficient to get parcels to their LFC (Level of Free Convection). Because of these factors I think it's reasonable to infer that CI (convective initiation) will occur in eastern KS/OK along the dryline by mid-afternoon and rapidly track northeastward. Storms will also fire to the South in East Texas around this same time but I suspect there will be issues with preferred storm mode. This is not to say that the risk is lower here but simply that it is a less favorable chase setup. All hazards are on the table across the entire risk area. Due to the factors we looked at I believe that storms in East KS/OK will become supercellular fairly quickly as they track into MO/IA. This is where I think the greatest threat for supercell tornadoes exists, including the possibility for a few strong tornadoes.
How Should I Chase This Setup?
Personally, I am going to target the triple point action. This seems like the most likely location to get supercell tornadoes given such a favorable temporal/spatial overlap of ingredients. There are some uncertainties regarding the exact location of initiation but I'm going to set up just northeast of where I think CI will occur and simply ride the gradients, picking off the cells as they move northeast. I think it's a good bet to give yourself a little room to play with since storm motion will be fast due to the strong wind fields and that's what I'm doing. The southern play in East Texas is another viable option as well. As I said, the only real limiting issues I see down here are possible messy storm evolution and a water-loaded profile that may result in HP cells. Regardless, embedded supercells will still be possible through the evening here and it's a solid target.
Lastly, I also think there's an outside play farther East along the warm front. While storms continue to merge to the South you could find yourself in a good spot if you caught an already organized supercell as it started to ride the WF. Once storms cross the WF, tornado potential will eventually wane but if you're looking for the outside chance or you live in the northern Ohio Valley and can't make the drive West then this is a play worth considering.
It's late and I have to get a little rest while I can, the next 2-3 days will be busy. Just remember, this post focused primarily on chasing tornado potential but anyone in the severe threat tomorrow should stay alert because dangerous severe storms are very possible. I'll update again tomorrow with a recap from the road of how the chase goes and we'll look at Saturday's setup. Make sure to follow my other online profiles (links to the right) to get real-time updates from the storms. Also follow my Twitch channel or my chase partner's Twitch channel for live video of our chase adventures. Stay cool, see ya soon.
DKK
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