Friday April 24 Forecast

GFS Comparison

042415 Forecast Day 2 map

Well it’s kind of a tale of two models tonight as there are significant differences between the NAM and GFS. The spread with the location of surface features is bad enough, with the NAM being farther SW than the GFS, but the bigger difference is the quality of moisture in the warm sector and CAPE that is really driving the uncertainty for tomorrow.

I tend to trust the NAM more than the GFS when it comes to placement of surface features. The NAM seems to do an alright job with the location of boundaries and the GFS has botched several this year in the short-mid range forecasting window. Not a huge deal on the discrepancy with the surface pattern since that will be easy to discern tomorrow afternoon and we are only talking about making minor east to west adjustments to the threat area and chase targets based off that difference between the models. The big difference between CAPE and quality of the warm sector tomorrow makes things much trickier. We pretty much know we are going to have good shear profiles tomorrow ahead of the dryline (less know near the surface low, as the really good shear profiles will be ahead of the dryline where the 850mb and 500mb jets couple, also there is 130kt upper level jet axis crossing the dryline in Oklahoma). There are differences in the amount of directional shear between the NAM and GFS, namely in the degree of turning between 850-500. The GFS is showing around 35 degrees or so of turning in the target area, while the NAM has more like 75. That’s a pretty substantial difference, but even the GFS directional shear should be enough for a tornado threat if we get good moisture and instability to work with. The quality of moisture over the warm sector, the amount of insolation/instability and the moisture gradient along the dryline are all the things I’m more concerned about and will be watching closely tomorrow morning. If you look at the above maps you’ll see the NAM maintains a fairly tight moisture gradient along the dryline, packing the better quality 60 degree dewpoints in fairly close to the boundary. Meanwhile the GFS has a much wider moisture gradient, which can make it tougher for storms to becomes sustained as they come off the boundary and also mitigates the severe weather threat as they have to work farther out into the warm sector before they can realize better moisture. I briefly looked at why the moisture gradient was wider with the GFS and the only thing I noticed was that the GFS wants to veer 925mb winds tomorrow morning, which dries out the air a little more than the NAM solution immediately ahead of the dryline.

By late afternoon the difference in CAPE across the warm sector is pretty substantial between the two models, with the NAM showing around 1500-2000J/kg and the GFS only showing 500-1000J/kg. That’s a big difference. This also plays a big role in how high the composite indices are going, which is readily apparent if you check supercell composite or sig tornado across the two models. I do think the NAM tends to juice things a bit much, but I still trust it more than the GFS. The key tomorrow is going to be moisture quality and heating over the warm sector. That will probably drive the extent of the severe weather/tornado threat tomorrow more than any other factor.

Not surprising the GFS is stingier with convection tomorrow too, while the NAM blows up storms near the surface low by early afternoon and then lights up the dryline around 20Z.

All that being said, my best guess is that the truth is somewhere in the middle tomorrow. I do think the NAM likely has a better handle on the surface pattern. Seems like SPC was hedging their bet that way too scooting the higher probs a little farther west in the latest outlook. The tornado potential does look as good if not better farther south with this setup, but I didn’t pay much attention to it since I will be chasing Kansas tomorrow. I’m not quite sure why SPC dropped the enhanced risk across Oklahoma. I pulled forecast soundings just east of ICT and OKC and while OKC probably has better hodographs it also has a bit more of a cap. I imagine the reason for the lower risk in Oklahoma is due to morning storms that are forecast to move through Oklahoma, which may limit the instability there and turn the air over more than the warm sector farther north where it should be largely undisturbed.

Anyway, enough rambling. The tornado threat tomorrow is going to be tough to call until morning. The shear profiles will be good enough to get the job done, we just need good instability which is going to be driven by moisture quality and the amount of daytime heating. Storms should fire first near the surface low in west central Kansas and then pop along the dryline after that. Storms could fire near the surface low fairly early in the afternoon. I wouldn’t put much stock in the timing of convective initiation until we see the HRRR and how surface heating is going tomorrow morning, but right now my best guess is 4-5pm for dryline storms to fire in Kansas. Storm motions don’t appear to be as fast as I thought they were before, but with strong winds aloft they will be moving at a pretty good clip. How fast exactly will depend on where you are at. Storm motions will be slower near the surface low and areas farther north along the dryline, but as you get closer to the Oklahoma border and the mid/upper jet axis in Oklahoma storm motions will speed up. I think in southeast Kansas they will be towards the upper end of what you can realistically hang with. I’ll error on the side of caution and probably setup downstream a ways from the boundary just to make sure I don’t get my pants pulled down. Nothing sucks worse than falling behind a storm and struggling to catch up to it as it produces tornadoes. I’m a tornado chaser so on days with quick storm motions I’ll hang out downstream and not worry about being on the storm until it develops a tornado threat.

I will update again the morning and things should be much more clear then. If the NAM can come close to verifying then I think we could see a strong tornado or two with dryline storms. If the GFS verifies I still think we will see a few tornadoes tomorrow, but they’ll be lower end (at least up in Kansas). I am taking off work at 11am tomorrow so I should be ready to hit the road anytime after lunch. I plan on hanging out in Wichita for a while though. I probably won’t have to go too far to start since storms should fire near or just west of the 135 corridor. Anyway, check back in the morning for a better idea of the extent and placement of the tornado threat for tomorrow. It will be close as to whether or not Wichita is in the threat area, so should have a much better feel for that in the morning too.

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