Severe Weather Threat 4/12-14/12

4/14: So, today’s the big outbreak day that everyone’s been talking about. As usual, I am skeptical, especially for areas south of I-70. NMM WRF initiates nothing before 01Z, and no widespread Plains outbreak has ever had initiation that late. The cap looks *so far* not prohibitive. Shortwave ridging builds behind the current cluster of storms in OK, allowing destabilization to occur during the late morning-early afternoon hours. By 21-0Z, CAA at 700mb look to rapidly erode the cap from W-E. Really the problem is the lack of lift, due to the positive tilt of the longwave trough and the position of the risk area in the right-entrance region of a powerful upper jet. The power of the system argues that storms will still develop, just in more isolated fashion, in srn KS and OK. But the WRF NMM has a very, very good track record. From my recollections, 9.9 times out of 10 it does not forecast initiation, it really doesn’t. It’s a tough forecast. Any remaining convection that hangs on in OK/KS, or any leftover cloudcover, will only hurt initiation chances. OTOH. if the cloudcover leaves too fast, LCLs will rise dramatically.

That’s all I have to say. Everything else has been hashed and rehashed. Parameters look incredible for strong tornadoes. All we need is for storms to develop and tap that.

4/13: I’ve decided to clump all these together into one post.
(Result: Initiation early due to an unforeseen lead disturbance. The main show did not produce as prolifically as anticipated due to the LLJ not coming back as strongly as anticipated, and resulting weak low-level flow effecting weak, wet, nontornadic RFD’s.)

The threat today is conditional. I was originally thinking the cap would not break due to rising heights, unfavorable jet orientation, and shortwave ridging, but I no longer think this will be the case necessarily. Nevertheless, this is still something to watch.

The LLJ is forecast to veer significantly this morning and afternoon and then back just in time around 0Z in advance of the main system out west. This is one of the weirder setups I’ve seen in awhile. A front/windshift boundary will be draped from NE-KS from the initial system, and will begin retreating as the LLJ backs and intensifies later this evening.

If deep convective initiation takes place, the triple point between this pseudo-front and the dryline might be the focus, where winds are locally backed SE’ly. Depending on the strength of the front and some other subtle features, the triple point might lie anywhere from SW OK to near Woodward. The location will dictate the threat – further south, and the cap will hold off initiation. Further north, and any supercell that develops will be in a very favorable environment overnight for tornadogenesis.

Right now I have no specifically target, but I think the greatest tornado threat (if any) lies within a box bounded by Woodward-Elk City to the west, AR City (KS) – Stillwater to the east (all cities are OK unless specified otherwise).

The main event still looks on track for Saturday. I still think capping will be an issue south of the KS-OK border on Sat, but north of there conditions look ripe. Still, given model disagreements on the trough evolution and the aforementioned cap problem, I would’ve held off on the HIGH. Depending on which model solution you use, I could see an outcome similar to either 3/12/06 (displaced west) or 5/29/04.

Post for 4/12 below: (Result: initiation held off everywhere except for NE CO/NW KS due to cloudcover – bust for just about everyone who attempted to forecast this system.)

Normally, I’d sleep now; it’s 5 AM and I just grinded through a problem set. But we are in for a special severe weather treat over the next few days, and I promised myself I would “journal” about it. However, since it’s late, I’ll keep it short. Cliffs notes:

1) Today is a classic “day before the day” setup. Such setups have yielded prolifically (see 5/4/07, 5/3/99) — there is no competing convection to impede inflow. From this pattern recognition, the potential is there for a few prolific cyclic supercells, if everything goes right.

2) What can go wrong: cap, moisture return, deep-layer shear. Shortwave ridging will delay surface-based convective initiation to 22Z at the earliest. South of ~I-40 (and if I was more bullish, lat of TX-OK Panhandles border), I think might remain capped to surface-based convection. Meanwhile, there are questions about moisture return, which models typically have trouble with. I wouldn’t be surprised to see 60F Tds extend only to DDC. If the “sweet spot” between “capped” and “not enough moisture” is too small, we might not have a large enough spatial area for a more significant event. Finally, there is little delta-V between H5, H7, and H25. Don’t know how that would impact storms and updraft ventilation.

3) Dprog/dt: The RUC is showing weaker H7 winds, weaker moisture return north of DDC, and a lesser eastward progression of the dryline. Watch this in subsequent RUC runs and while nowcasting. These trends are not favorable for the tornado setup.

Upshot: I’d pick Englewood, KS as my chase target, hopefully in the sweet spot between weaker moisture return and stronger capping. I agree with the SPC 10% risk area, but confined about 50 mi W-E of DDC, from lat DDC to lat OK-TX border. I’d expect tornado threat to be maximized between 0Z-3Z as LCLs drop, moisture continues to stream in, and low-level shear picks up — but before CINH increases sufficiently to choke off storms.

Looking ahead, we have several days of severe weather coming up. Friday’s setup looks to be tempered by weak low-level flow, especially north towards KS, and relatively weak mid-upper level flow in the equatorial side of the jet stream.

Saturday looks interesting, but there are some kinks in the setup — quite literally, with a possible back-veer-back profile. The bigger issue is that the main longwave trough is still in the Four Corners region and the setup area is in the subsident right-entrance region of the upper jet. That, combined with progged H7 temps aoa 10C south of the Red River (usually underdone), makes me skeptical about storm coverage south of the OK-KS line. I would do a 30% hatched SLGT at this juncture, and only north of the OK-KS line.

Usual disclaimer: Use SPC/NWS/etc. products for official guidance. I am just a college student. Don’t burn me on the stake for being wrong (as I was on 3/2).


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