So I was expecting to find a Jet stream or frontal boundary in the "north and central Kansas" tornadoes on Tuesday. But when I checked Weather Underground, no evidence of either a front nor overhead jet stream. Then there was this curious report: Source: Tornadoes strike north and central Kansas - KansasCity.com A near stationary tornado suggests there wasn't much shear around. Just curious, any thoughts from our weather folks on this outbreak? Bob Wilson
Actually, the 00z May 29, 2013 sounding from Topeka, KS (the closest upper air site) shows decent shear... (Unisys Weather - Topeka KS (TOP-72456) - -12 hr) Storm motion would be 252 degrees (which mean that it would move toward the east-northeast) at 26 knots according to the sounding. Helicity isn't off-the-charts at 191 m^2/sec^2, but you can see a good directional shear in the lowest 1500 meters of the surface (from the southeast at the surface to the southwest at ~750 mb). Jet stream-level winds are measured at ~75 knots (@250 mb). The thermodynamics are quite impressive (-8.1 lifted index; 2445 CAPE). The relatively low lifted condensation level (LCL) is favorable for tornado formation (<1000 meters). There's also a shallow warm layer between ~800 mb and 750 mb which tends to suppress convection (3.2 "cap"). Of course, these are the conditions at TOP, so they may not exactly represent the environmental conditions at Salina, KS. The shear is decent and the themodynamics are quite good for supercell development. I can't really explain the unusually slow motion of the tornado other than to speculate that it was associated with a right-moving supercell that deviated enough from the storm motion as to significantly slow down its forward progress.
Here's a short paper describing the mathematical relationship of shear and thermodynamics to tornado forecasting ("Significant Tornado Parameter").... http://www.spc.noaa.gov/publications/thompson/stp_scp.pdf By the way, here is an impressive radar loop of last week's Moore, OK, tornado.... http://i.minus.com/iluhrlDRbUSAQ.gif The tornado is located in the bottom of the "hook echo" and the debris ball from the tornado shows up as brightly-colored reflectivity a while into the loop.
Thanks! This is the type of posting that makes this forum so useful. After I get a chance to study, I may come back with some questions. So looking at the chart: ~80 degree shear, 26->947m (?) something, ~30 degree shear, 10690-12140m (?) close dew point and temperature spread up to 2501m (?) Am I reading it right? Bob Wilson ps. I see they have an iPhone app and the price is right . . . 'free.'
You're welcome, Bob. Glad to be able to contribute. There's a moderate risk of severe wx in the Central Plains again today (NOAA/NWS Storm Prediction Center). Looks like another active day!
Our long-time CBS affiliate weatherman here in Wichita, Merrill Teller, called this Bennington funnel one of the most unusual he has seen. I watched their coverage on TV as the thing just ravaged a couple of farms outside Bennington for a half hour. They stopped putting storm tracks on it since it was not moving. No EF rating yet, but it was wide (>1/2 mile).
Yes, that would be a reasonable interpretation of the wind barbs on the skew-T. I should point out that (if you look at the paper I linked in my previous post) the shear in the lowest 3000 meters (surface to about 700 mb) is the most important area of the skew-T for assessing tornado potential.
I'm taking the paper 'to lunch.' <grins> I'm also going to be interested in the air density as a function of altitude and the dew point. This ties back to my earlier training with steam tables. My thinking is the bulk of the thermal energy is embedded in the higher density, warmer air and as altitudes increase, the amount of energy falls off rapidly . . . of course this may be covered in the paper. Bob Wilson
Well the paper was good news and bad. First the bad It is an update to two earlier papers that apparently define a number of terms assumed to be known by the reader (i.e., MUCAPE, CAPE) The good news is the paper references the earlier work that hopefully defines these assumed terms in engineering units. But one curious thing was the sentence,"The effective shear term is set to zero for values less than 10 m/s, and becomes 1 for values greater than 20 m/s. " The reason it is interesting is this term is one of three scaled products. Anytime a value goes to zero in a product, it becomes the primary determinant having a strong, non-linear effect. As an update to the earlier papers, this is a pretty dramatic change, if I follow what is going on. As I was driving back from lunch, I was thinking about the effect of shear, especially when there is a layer of high humidity and cold. In effect, shear would work like a forced draft in ordinary combustion. It would increase the mass of humid air which driven into condensation would significantly release more heat. As the condensing, heated, rising air exits upward, it would draw more humid air into the 'mixing chamber.' The technical terms are interesting but it may take awhile before I get around to further study. Still, the paper has gotten my old man, brain cells to consider other factors. Thanks, Bob Wilson
Okay, maybe some terms should be defined. CAPE = convectively available potential energy MUCAPE = most unstable CAPE MLCAPE = maximum layer CAPE (i.e., layer in the sounding with the maximum CAPE) SRH - Storm Relative Helicity (m**2/s**2) - SRH (Storm Relative Helicity) is a measure of the potential for cyclonic updraft rotation in right-moving supercells, and is calculated for the lowest 1-km and 3-km layers above ground level. There is no clear threshold value for SRH when forecasting supercells, since the formation of supercells appears to be related more strongly to the deeper layer vertical shear. Larger values of 0-3-km SRH (greater than 250 m**2/s**2) and 0-1-km SRH (greater than 100 m**2/s**2), however, do suggest an increased threat of tornadoes with supercells. For SRH, larger values are generally better, but there are no clear "boundaries" between non-tornadic and significant tornadic supercells. The latter definition (SRH) was taken directly from SPC's mesoscale analysis page (SPC Hourly Mesoscale Analysis). That page has various parameter pull-down menus, e.g., thermodynamics, wind shear, which has the definition of the particular parameter in the pull-down menu (designated by a "?"), which is the easiest way I know to determine the definition and critical value of the parameter.
