Yesterday (May 19) in south-central Kansas was a great reminder that potentially strong tornadoes (see images above near Rago KS in the 6:30-6:45 pm CDT / 2330-2345 UTC time frame) can occur once in awhile without the typical supercell processes that involve large low-level wind shear or storm-relative helicity. I've written peer-reviewed papers on this subject for NWA Electronic Journal (2005, see here) and Weather and Forecasting (2006, see here). In particular, the setting shown in the 2005 NWA paper with Jim Caruso matched what happened yesterday (which was largely unexpected) quite well.
Non-supercell tornadoes that we tend to call "landspouts" (because of their seeming similarity to waterspouts, but over land) generally occur on sharp boundaries and get their "spin" from stretching of vorticity associated with the sharp wind shift, often seen as a fine line on radar or even satellite (see the frontal boundary in south-central KS on the satellite image above). While the boundary is the most important ingredient, the environment usually includes steep low-level lapse rates (a rapid drop in temperature above ground) in the lowest 1 or 2 km due to strong surface heating and no temperature inversion (little or no CIN or convective inhibition), along with well-mixed low-level moisture. Such tornadoes can occur even if the spread between temperature and dewpoint is large and cloud bases (LCLs or lifting condensation levels) are quite high, unlike supercell tornadoes. Notice that the Rapid Refresh sounding above, 30-40 minutes before yesterday's first tornadoes, had all these characteristics. Such a profile would promote rapid low-level upward stretching under storm updrafts, and if the stretching with thunderstorm updrafts were to occur directly over a wind shift boundary with vertical vorticity or "spin" along it, that would increase the chance of tornadoes.
The morning NAM and HRRR forecast panels above for mid to late afternoon on Saturday suggested that these ingredients (boundary, steep lapse rates, low-level moisture with little CIN, and convection on the boundary) might be in place for a "mesoscale accident". Short-term forecasting of such set ups is never easy and depends on everything coming together just right. But certainly, with so many tornado reports in the Kingman/Harper County area of south-central Kansas during the 90 minute period 5:30-7:00 pm CDT (2230-0000 UTC), including one tornado that significantly damaged wind farm turbines, everything came together in spades.
The surface map and SPC mesoanalysis panels above show these same ingredients coming together in real-time on the 2200 UTC panels before the tornadoes.
In contrast, the next SPC panels above include 0-1 km storm-relative helicity (SRH) at 2200 UTC, showing how poor the low-level shear environment was yesterday in regard to supercell processes. However, the surface vorticity panel by it shows how much vertical vorticity was focused and available with the boundary for non-supercell processes.
The 3 radar panels above also show some interesting mesoscale features. Along with the storms "back-building" to the south-southwest like a "zipper" on and directly over the boundary (common in prolific "landspout" events), notice the southwestward-moving outflow boundary (another radar fine line) visible behind the frontal boundary. As this intersected the frontal boundary progressively southward, it appeared to help in the generation of successive tornadoes, including the tornadoes near Rago KS tornado that occured in northern Harper County KS about 15 minutes after the last image above. I've seen this before in prolific landspout events when the radar is close enough to see it, and it could be an important issue in the production of stronger "landspout-type" tornadoes.
One other issue I will mention is the Non-Supercell Tornado parameter (NST, not shown) found on SPC's mesoanalysis site. It performed _horribly_ yesterday, showing no values at all during the outbreak. I'll have to look back at the ingredients that go into the NST parameter, but my impression is that it depends too much on the presence of low-level (0-3 km) CAPE, which may or may not be part of the environment in "landspout" settings (see the earlier sounding). What's probably more important is well-mixed moisture in the lowest 1-2 kilometers along with the steep low-level lapse rates, and a lack of CIN (convective inhibition) and absence of a temperature inversion that might otherwise slow stretching in low-levels. If I were writing the papers now that I referenced earlier, I would modify my description of these ingredients.
At any rate, as I've heard Chuck Doswell say many times, there are many ways to get a tornado. Mother Nature doesn't care if the vorticity to be stretched is from SRH (tilted horizontal vorticity) or a sharp wind shift boundary (localized vorticity already oriented vertically). A prolific non-supercell tornado setting like yesterday doesn't happen very often, but for tornado forecasters it is definitely one to get familiar with and watch for.
- Jon Davies 5/20/12
Update: Wichita NWS (see here) has rated one of Saturday's tornadoes that struck a farm northwest of Harper as EF3 in intensity. That's pretty impressive for an event driven primarily by non-supercell tornado (NST) processes. It seems that the more prolific an NST event and the longer it goes on, the more intense and "supercellular" in nature the tornadoes can become, particularly if deep layer wind shear is significant (around 35 knots in this setting).
9 comments:
Jon, so you think all of these were NSTs? Some accounts on CFDG seem to imply that one of them was a "normal" supercell induced tornado wit a clear slot etc, but cannot stronger landspouts develop these types of features after they form and become established?
Matt:
Yes, the longer an NST event goes on, the associated tornadic storm(s) can develop features like clear slots, broader rotation on radar, etc. to behave like tornadic supercells. But the tornadoes wouldn't get going without the boundary vorticity ("spin" source) and steep low-level lapse rates (enhanced stretching) in the first place, which are non-supercell processes. Yesterday's setting looked pretty benign from a purely supercell tornado perspective (SPC had the largest tornado threat at 5% over south-central Nebraska). So from a forecasting viewpoint, it's very important to recognize the "non-supercell" aspects/ingredients of the setting, without which there would likely have been no tornadoes over south-central Kansas. - Jon
Jon,
Was that a forecast sounding or a sounding from research experiment in progress?
Mike
It was an operational Rapid Refresh 1 hr forecast sounding, which as you know, has replaced the RUC.
Enjoyed reading your writeup as always. I've never really paid attention to NST parameters before, but I definitely learned allot from reading this!
Jon, I've looked closely at the Level II radar data for this event. The EF-3 tornado did indeed begin its life cycle as a non-mesocyclone tornado, not under the main updraft. It's clear that the low level shear along the flanking line was augmented by the boundaries you mention.
But the evidence shows that as the tornado propagated northeastward along the flank it moved under the backbulding updraft area. Once there, it became juxtaposed with the weak rotation already present in the updraft because of the deeper layer shear, and synergy commenced.
At that point, the low level inflow was very strong, as I can attest standing along Highway 14 experiencing it. So I think there was also low level shear involved....and then the tornado transitioned into something that looked like a mesocyclone-tornado, including clear slot and even a more conventional looking lowering with a horseshoe shaped base.
I think this is a hybrid case, and in the last instance, this properly could be classified as a "mesocyclone-associated" tornado.
Jon, Great write up of the event. I witnessed from start to finish of what is being called the Rago tornado. It started at ~6:15pm and roped out at 6:48pm. I started out due North of this tornado which appeared to be a land spout looking tornado under the flanking line (no visible wall cloud) with no precip close to it. This tornado however had a much stronger circulation at the ground and therefore exceeded the 2 to 5 minute tornadoes we witnessed earlier in the day further north. This was evident even before it reached the wind turbine farm south of Spivey, KS.
At this same time to my East and Northeast appeared to be the main mesocyclone updraft tucked up close to the main precip core. This main meso circulation was very persistent and always had a visible funnel 1/3 to 1/2 way to the ground.
As the Rago tornado moved to the East-Northeast it got stronger and stronger. Once the main circulation with funnel roped out to my East-Northeast the Rago tornado took over once reaching the main precip area. It obtained maximum size as a medium sized elephant trunk tornado as it approached Hwy 14.
I was able to obtain some great high contrast images with time stamps from being on the backside, (West) of the storms most of the day. I just kept dropping South and East. Turned out to be a rare event and great chase day for me.
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I was there that day and filmed 7 of the 8 tornados that I saw. I filmed the entire 30 minute life span of the EF-3. I was driving the closest vehicle on the south side of the tornado, as it crossed highway 14. Quite an extraordinary weather event to witness, considering the fact that there was only a severe thunderstorm "watch" issued at the time the first tornado touched down.
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