Friday April 29 Tornado Forecast

The 00Z NAM is out, tired of waiting on the GFS though so I haven’t seen that updated run. The NAM  seems to be pushing things a bit further south for tomorrow than previous runs, at least as far as the dryline is concerned. Not a major shift, but there is a notable SW shift with the dryline bulge location from the 12Z to 00Z. There is a lot of uncertainty surrounding tomorrow, especially with regards to morning storms over Oklahoma and how it will affect boundary locations and destabilization in it’s wake. The NAM 00Z shows a complex surface pattern that doesn’t make a whole lot of sense to me. I drew a map below with boundary locations/types based off the 00Z NAM surface chart. The GFS has a more traditional surface pattern and makes more sense to me. The 00Z GFS also has the dryline bulge located a little farther northeast (seems to be 3-6 hours faster than NAM) and is much more believable IMO. I was going to make a map for the GFS too, but unfortunately pivotal weather where I gank my maps from hasn’t finished updating the GFS so I couldn’t grab one. I can’t download images from COD any more for some reason, which has updated. So basically I’ve seen the 00Z GFS, but I can’t pull a map for it quite yet and I’m tired of waiting so you’ll have to do without lol. The surface pattern for tomorrow afternoon is so uncertain at the moment anyway that I’ll end up redrawing it in the morning anyway. Below is the NAM map.

Friday April 29 Day 2 MapThe GFS is more in line with the SPC outlook area and previous NAM runs, so I’m leaning that way. Basically shift the dryline bulge northeast a couple counties and make the cold front/warm front I drew on the map above all a warm front. That’s what the GFS looks like (has the warm front a tad farther south too).

Lots of uncertainty with convective evolution through the day tomorrow, both with morning storms and even more so with afternoon storms. The models have jumped around a lot with afternoon storms, showing convection developing over the warm sector, along the dryline and near the surface low/frontal boundary. Rather than go over the differences in the models and different runs, I’ll just throw my best guess out there. I think morning storms will start clearing the threat area by around 1pm. Additional storms will develop near the surface low and along the front as the mid level jet axis starts to nose into western Oklahoma during the afternoon. Dryline storms are a little more iffy, but if the dryline is bulging east as forecast then I think we’ll at least get a couple dryline storms in the vicinity of the bulge and potentially more scattered storms farther south along the dryline. Deep layer shear and instability should be quite favorable for supercells. I think we’ll see a mixed convective mode tomorrow with more clustered supercells farther north near/north of the surface low and along the warm front. Convection should trend towards more discrete supercells from the I40 area and south from there. The best tornado threat will likely occur with any tail end to more clustered storms up in central/north central Oklahoma and with any dryline storms. I’m skipping right over the possibility of any open warm sector storms for brevity here.

As if things weren’t tricky enough already with this forecast, things are about to get a little tricker with the low level shear situation. The NAM has consistently been much stronger with low level shear than the GFS. Tonights runs seem to have come into a little better agreement with the strength of low level wind fields, with both now showing 30-40kts at 850mb by 00Z across central Oklahoma, but there are differences with the extent of veering of low level winds, especially over southern Oklahoma and north Texas. The GFS starts to veer low level winds more down there, which puts a big dent in directional shear and lowers the tornado potential. The NAM keeps them more backed all the way down into north Texas. Because of that discrepancy I trimmed the southward extent of my peak tornado threat area box on the map below in this post.

Another complicating factor is the tendency for hodographs to exhibit an S shape with backing around 500mb or so (exact height of backing varies with time and location and I don’t care to pour through all of that right now lol). It doesn’t seem terribly pronounced in the area I intend to target northeast of the dryline bulge and it’s high enough (500mb or a little above that) that I’m not going to worry about it for now.

With the difference in the extent of backing at 850mb there is a major difference between the GFS and NAM with low level hodographs and hence tornado potential. The NAM has large looping hodographs in the low levels and are very favorable for not just tornadoes, but strong tornadoes. The GFS on the other hand veers low level winds a bit more, so despite the stronger wind speeds in the low levels compared to previous runs, the hodographs really aren’t enlarged much and it’s only favorable for a lower end tornado threat. To put a value to it, I pulled forecast soundings ahead of the dryline bulge in central Oklahoma for both models and 0-1km SRH was 150 with the GFS and 287 with the NAM. That is a huge difference and it would be the difference between a low end tornado threat and a higher end tornado threat. Who is telling the truth???? That’s the million dollar question right now. I’d hate to be working at SPC for this one. It’s a bastard to call the tornado threat at this point and there’s higher than usual stakes with the ass kicking SPC just took over Tuesday (I don’t agree with the public lambasting they took btw, but that’s a topic for another day).

Okay, despite the differences between the models, I think the best tornado potential falls over generally the same general area. Below is the area that I think will have the best tornado potential tomorrow.

Friday April 28th tornado threat mapThis should be the area that will see the best clearing in the wake of morning convection and strong instability. Additionally it is downstream of the dryline where more discrete storms should develop. It also encompasses the area I’d expect any sort of tail end from more clustered convection developing near the surface low or just north of it to track through. I have little confidence in the extent of any tornado threat for tomorrow right now, but I’m chasing so it doesn’t really matter. I think there is higher end potential if low level winds back strongly like the NAM is forecasting and we get good clearing/strong instability  in the wake of morning convection. I have fairly high confidence we’ll see torndaoes tomorrow, just uncertain of exactly what area will have a tornado threat and what the extent of any tornado threat will be. The most sure thing will be near the surface low and dryline bulge. We could get tornadoes farther north near the warm front and farther south along the dryline, but those areas are more conditional IMO. I will target northeast of the dryline bulge. My best guess at a starting target right now is some place down around Hobart, Oklahoma or maybe a little farther north from there closer to I40. I’ll pin it down in the morning when things are more clear. I do think there may be a strong tornado threat tomorrow if 1km SRH gets up closer to 200.

Alright, hopefully that’s enough rambling on details with tomorrow’s setup lol. That’s a long one with not many definitive answers on what’s going to happen. This is probably the most challenging forecast so far this year as far as SPC is concerned where you’re expected to quantify the threat. It should be a lot easier tomorrow, especially if they do an early afternoon OUN sounding, which I’d imagine they will. I plan on leaving town around 10:30 or so and heading towards OKC. I’ll pick a target on the way and likely move down towards the Hobart area or up towards Clinton. I’ll post from the road and try to update as much as I can so check back if you’re interested.

 

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