Tornado Central

Several chases in the past couple of days. Below is the report from the most recent one (26 Apr 09).

Full pictures and video on my 26 April 2009 chase log (or on my Facebook profile).

Dan D., Robin T., Corey P., Mike F., Jen ., and I chased the lone tornadic supercell that tracked north of Roll, OK, this afternoon. We were originally hoping to initiation down near Childress since we liked the environment in southwestern OK and towards I40 best. On the way out west from OKC, we made a brief intercept on the cell that passed through Clinton in the early afternoon; that cell looked rather trashy after following it northeast of Clinton for 15-20 minutes, so we let it slide off and set our eyes back to the west. Noting initiation N of Wheeler, TX, we tredged towards Sweetwater, to the north of which we watched the cell take on some supercell characteristics. Road options aren't very good in that part of the state, so we had to trek all the way to Cheyenne to have an "ahead of storm" north option. At any rate, we made it through Roll, OK, after a very brief debate as to whether we should go east from Roll towards Leedey to get in better position of the storm or whether we should bite the bullet and drive northward to get a view of whatever was happening under the base. Fortunately, we were sick of being 10-20 miles SE of the storm, and we really wanted to get a good view of the base, so we chose the latter option. Several miles N of Roll, the tornado first came into view. The continued northward to get a bit closer to the tornado, which had the unfortunate side effect of not allowing me to take much in the way of (a) tripoded video or (B) decent still photography since I was driving (so holding the camcorder was the best I could do). This tornado (I guess it was #2 in previous reports -- we didn't see the first one) took on several different shapes, from a "dust cloud under a big bowl" to a nice stovepipe to an elephant trunk.









Since there were no east options until we got all the way up to Arnett, we opted to let the storm move away, setting our sites on renewed development N of Wheeler that was moving up towards Roll again. At one time, we saw a slender but easy-to-see funnel that extended nearly 1/2 of the way to the ground... On VNX and FDR, it looked like the funnel occurred where there was decent anticyclonic rotation -- the funnel did not appear to be related to the rain-free base or wall-cloud (where we'd expect cyclonically-rotating funnels). At any rate, we followed this storm up through Vici and N of Seiling, but it looked very wet and cold, particularly by the time it got to Vici. The Mesonet ob near Seiling was showing some cold northwest winds out ahead of the storm, which confirmed our visual obs of cold outflow pushing southeastward from the storm. Since the supercell seemed to be the southern anchor on the bow echo and squall line to its northeast, perhaps the cold outflow that pushed out ahead of the storm wasn't entirely from the supercell itself (i.e. perhaps it was actually outflow from the bow echo to the NE). At any rate, it was evident that the storm was not going to produce unless it got back into the warm sector, which didn't appear as though it was going to happen. At that point, we opted to call off the chase, essentially, though we kept our eyes open for the cells that were moving up S of Weatherford and El Reno.

Oh yeah, we also hit a calf / young cow that was standing in the middle of the Hwy 270 SE somewhere near Oakwood (between Watonga and Seiling). I saw a dark-colored animal (or person) walking on the right shoulder of Hwy 270... A northwest-moving truck passed me just before I caught a glimpse of a dark brown or black animal right on the yellow dividing line. With no time to avoid it, I hit it, at the full 65 mph. I thought it was a dog at first, but, after turning around to check it out, we saw one calf on the side of the road (where I saw it a minute before). A pick-up truck was pulled off on the shoulder, and the guy driving it said he hit a calf. We never found the calf that I (or we) must have time, since the calf that was on the shoulder was walking around. The only damage to my car was minor paint splintering on the bumper (presumably associated with the bumper flexing), which is quite remarkable given that we hit such an animal at 65 mph. We must have just nicked it with the extreme front-left side of my car... Night + occasional oncoming traffic (precluding me from using my high beams) + dark-colored animal in the middle of the road + passing a truck immediately before hitting the animal (always slightly blinding) = bad!

All in all, I'm certainly happy to have seen the "needle in the haystack" that was the Roll tornado. However, this has been a rather disappointing two-day chase outting considering >300 0-1km SRH both days. For whatever reason, we couldn't ever really avoid the myriad of cell interactions and mergers that we saw Sat and Sun, and the NE-SW orientation to the lines of convection the past two days probably didn't help the matter (in terms of cell interaction, splits, seeding, etc). If you told me that there'd be >1500 j/kg CAPE + 40-45 kt 0-6km shear + a dryline + >300 0-1km SRH, I would have expected much more.

Full pictures and video up at HERE!

We finally have moisture returning to the Plains! With a deep upper-level trough anchoring itself across the western U.S., it looks like we may enter an extended time of active weather in the central and southern Plains. Regarding chasing on Saturday... I think there is a conditional threat for strong to violent tornadoes after dark tomorrow across western Oklahoma. The forecast hodographs for some sites in western OK (such as Clinton, which was posted earlier) indicate 400-550 m2/s2 0-1km SRH, and the entire hodographs (through 6-8 km AGL) look very nice in the 00z-6z time frame. The 00z soundings from tonight are showing (finally!) deep rich moisture advecting into the southern Plains. For example, check out the BRO sounding from tonight. Heck, with what we've seen this year, I'd even be okay with LCH's moisture profile.

Of course, the threat is conditional since we first must have initiation. I'm not seeing much of any forcing aloft tomorrow that will help (e.g. very little seen on 500 mb Vort and UVV charts), so I think we'll need to rely heavily upon the dryline and, most likely, the front. I like how the front rather rapidly lifts northward after dark, when deep moisture should surge through northwestern OK and into southern KS. The 500 mb flow could be a bit stronger to bump up deep-layer shear, and we may see some wet structures (e.g. HP, perhaps), but I'll chase regardless. Right now, it looks like the area between Canadian and Shamrock, TX, would be a good starting target. I hope to be in western OK by sunset, since low-level shear will be highly supportive of strong tornadoes near and after dark.

The 00z NAM has the mid-level and surface lows farther south compared to the 12z NAM. The vort max associated with the 500mb trough/low moves from the eastern TX PH and far W OK at 18z, to far E OK by 00z. As such, the strongest DPVA is moving into western AR and nearing southwestern MO by 00z. With the southward shift, it looks like the warm front play may stay along the OK/KS border (or perhaps clip the southeastern-most Kansas county) in a region of only 25-30 kt southwesterly or southerly 500mb flow on the southeast and east side of the mid-level low. The better mid-level flow is forecast to be quite a bit farther south and southeast; I'm awaiting forecast soundings from the 00z NAM to help visualize the forecast vertical wind profiles / hodographs... At least the very backed low-level flow will act to increase deep-layer beyond what I would normally expect for only 25-35 kt 500 mb flow.

Meteorologically, I'd like to be in the Tahlequah, OK, to Bentonville, AR, by 00z. Unfortunately, from a look at Google Maps terrain and satellite imagery, that area looks pretty much unchaseable. As such, I'll play the area next to the sfc low as long as the 50F isodrosotherm can get up that far to the NW. If the 00z NAM is right, I'll probably start somewhere between Pawhuska, OK, and Caney, KS, in the "bent back" region. I'm a little concerned about the width of the instability corridor, as it looks like the surface temps warm from <60 F to the 60-70 F range between 18z and 00z in this area. In addition, current observations show 60-62 F tds along the TX Gulf coast. As such, with only 10-20 kt sfc winds through the night (and veered 850 mb flow), I really can't buy the NAM's forecast of >60 F Tds pretty much everywhere east of the dryline as far north as extreme southeastern KS by 00z... The cooler sfc temps will help keep LCLs relatively low up there, and the cool mid-level temps help ensure relatively steep mid-level lapse rates.

Well, this one is sort of creeping up on me. The 12z NAM is forecasting a relatively decent chance of chaseable storms across parts of the southern Plains Thursday afternoon. As a shortwave trough swings into the area overnight Wednesday into Thursday, both the GFS and NAM forecast the trough to cut-off through at least the mid-levels (e.g. 500 mb). The orientation of the upper-level jet looks nice, with northeastern OK and adjacent portions of KS, AR, and MO located in the left-exit region, which will result in a strengthing (per NAM) or quasi-steady-state (in a system-relative standpoint per GFS) low pressure system that moves eastward from the panhandles to northeastern OK by evening Thursday. Associated with the trough and mid-level low is very cold temperatures; 500 mb temps of -18 to -22 C are forecast to move atop the warm sector by afternoon.

Closer to the sfc, the NAM is a bit more aggressive in returning more substantial moisture to the area NE through SE of the sfc low, indicating sfc Tds >60 F possible east of the dryline by afternoon; the GFS primarily is forecasting mid-upper 50s Tds N of I40, with >60F Tds S of I40. The warm front associated with the sfc low looks like it'll be located (and moving slowly Nward with time) along the KS/OK border into southern MO. The GFS is forecasting more in the way of QPF during the day compared to the NAM, which, I assume, is why the GFS 2 m Ts are lower compared to the NAM (thereby impacting CAPE forecasts). Even so, both models are indicating >1500 j/kg E of the dryline Thursday afternoon.

This upcoming system is presenting a couple of target area opportunities. First, there's the usual warm-sector play E of the dryline. If we can get upper-50 Tds up here, the cold mid-level temps should result in workable instability. Another target would be ENE of the sfc low along the warm front, where one would have the benefit of backed sfc winds and a good baroclinic boundary. Surface moisture may not be quite as good (given advection is occuring from the south), but mid-level temps are likely to be cooler. The NAM is currently forecasting the best deep-layer shear along the dryline S of I40, as weaker mid-level flow resides in NE Oklahoma.

Of course, I think the biggest source of caution right now, for me at least, is the realized degree of moisture return. Current analyses indicate northerly winds of 30-40 mph across the western Gulf of Mexico behind the cold front that is current located from near a Tampa Bay, FL, to Yucatan peninsula line, and it will continue it's march southeastward into, at least, the northwestern Caribbean. In other words, the GoM is going to be completely scoured (heck, even part of the Caribbean will!).However, as Rich T points out this time of year, the temperature of the air behind the cold front is very important in determining the amount of airmass modification that occurs. In this case, the air behind this CF is actually rather cold With such cool air moving over the GoM, there should be more in the way of vertical moisture flux off the GoM owing to more a substantial difference between the sea surface the overlaying air. We are going to need everything we can get, I think, since current Tds along the upper Texas coast are in the 20s. Yes, the 20s, with temperatures in the upper 50s! *edit: Actually, Tds in deep south TX are even lower -- e.g. CRP currently has a Td of 19F. I remain quite skeptical of >58F Tds returning by Thursday, but there is a chance that we will see more rapid airmass modification than we saw ahead of previous systems this year for the reasons mentioned above.

Right now, I'm hoping this slows down a bit, since I'd rather be be targetting far eastern OK. Given the bias in the NAM/GFS to be a bit fast (after least they seem to be too fast more often than being too slow), I'm hoping we can back this up to the I35 corridor. The area along the OK/KS border immediately E/NE of the sfc low looks tasty to me.