Storm speed ?

The storms in s. Illinois yesterday (11-15) were moving at speeds up to 60 mph and were hard to chase and keep up with. Essentially a BUST chase for the most part.

Was curious what factors should you look at in your chase day analysis and planning stratagy to estimate the ground speed (storm motion) of potential thunder storms later on ? Is it the 250/300 mb wind speeds or lower level ? or something else ? Thanks for any helpfull info.

Jon Miller
 
500mb winds act as the steering winds for the system and will generally indicate storm speed/direction. Looking at forecast soundings taken in the area on the day of will give you a good idea of storm motion. Other factors will come into play, however, such as whether the environment is supportive of very large, isolated supercells, or if you are looking at an outbreak situation similar to yesterday with more compact storms moving swiftly along with the strong dynamics. With large, isolated supercells, the storms have the ability to modify their environment somewhat so that the speed and direction of the storm are both affected. You'll often hear chasers refer to a supercell "turning right," which is just another way of saying that the storm is now rotating so strongly that the rotation throws off the expected heading and moves the storm to the right. On outbreak days, it is more difficult for the storm to work against such a strong, dominant wind environment, so they will generally remain on their expected heading (even yesterday, while most storms were fast-moving, which is typical of outbreaks, there was a fairly wide variance in storm speeds likely owed to several factors, such as the local environment the storm was in and the individual storms themselves as they intensified). Each situation has to be looked at separately. Learning to read a hodograph is the best way to go to judge storm motion ahead of time.
 
Comet module on hodographs, storm motion, etc.
http://deved.comet.ucar.edu/mesoprim/hodog...ograf/print.htm

Also, there is a paper by Bunkers et al. (2000), that presents a method to include supercell dynamics in storm motion: Matthew J. Bunkers, Brian A. Klimowski, Jon W. Zeitler, Richard L. Thompson and Morris L. Weisman. 2000: Predicting Supercell Motion Using a New Hodograph Technique. Weather and Forecasting: Vol. 15, No. 1, pp. 61–79.
 
Agreed. Using forecast soundings and the storm motion/speed value is usually the easiest way to estimate speed and direction of storms. If the winds near the surface and on up are strong, then you can bet the storms will be moving very quickly too. That is, yesterday we saw wind speeds of at least 50kts+ from 2000 feet on up to 40,000 feet. Thus, you can bet storms were crusing.

I thought about hopping a last minute flight to INDY yesterday but decided against since the days are short now and the storms would be moving ridiculously fast to chase them easily. I'll save my free tickets until the late spring and summer.
 
Mike said...
"500mb winds act as the steering winds for the system and will generally indicate storm speed/direction. Looking at forecast soundings taken in the area on the day of will give you a good idea of storm motion. Other factors will come into play, however, such as whether the environment is supportive of very large, isolated supercells, or if you are looking at an outbreak situation similar to yesterday with more compact storms moving swiftly along with the strong dynamics. With large, isolated supercells, the storms have the ability to modify their environment somewhat so that the speed and direction of the storm are both affected. You'll often hear chasers refer to a supercell "turning right," which is just another way of saying that the storm is now rotating so strongly that the rotation throws off the expected heading and moves the storm to the right. On outbreak days, it is more difficult for the storm to work against such a strong, dominant wind environment, so they will generally remain on their expected heading (even yesterday, while most storms were fast-moving, which is typical of outbreaks, there was a fairly wide variance in storm speeds likely owed to several factors, such as the local environment the storm was in and the individual storms themselves as they intensified). Each situation has to be looked at separately. Learning to read a hodograph is the best way to go to judge storm motion ahead of time."


Correct me if I'm wrong, but don't you find storm motion by finding the mean wind throughout the portion of the troposhere that the storm occupies. IMO It is also misleading to say that strong flow throughout the troposhere means storms will move fast since directional shear will have a significant impact on storm motion? And don't storms deviate from the mean wind because of propogation? I have never heard anything about "rotation throwing off the expected heading".
 
A non-rotating storm will have motion that is a sum of two components -- advection and propagation. For the advection part, imagine putting a leaf in a river. The leaf moves with the river -- so if the river is moving quickly, the leaf moves quickly as well. Of course, this is a simplification, but that's largely the advection term. The propagation term is a little more complicated. Consider a stationary thunderstorm cluster (so say there is no advection of the storm, or the mean wind of the cloud-bearing layer is 0). As the downdraft develops, a cold pool will develop at the surface (rain falling through subsaturated air leads to evaporation cooling, etc). This cold pool will spread out from the center of the downdraft, with the extent of the spreading being a fucntion of the degreee of low-level shear. If the cold pool is sufficiently deep to lift parcels to their LFC, air parcels will be forced over the cold pool and lead to new convection. The original storm will lose it's inflow, so the updraft weakens and, soonafter, the original downdraft weakens as well. So, in this regard, the storm has appeared to have moved, though it's really move of a old-cell-dies, new-cell-develops.

Depending on the storm mode and environment, the propagation component can be quite significant, and thus the reason we hear (and look out for) "forward-propagating MCS"). For a forward-prop. MCS, the line is both being advected by the mean wind AND propagating downstream, and it's largely for that reason why signficant winds are often associated with them.

Now, supercells (courtesy of their inherent rotating updraft) add in another factor for forward motion -- a contribution from perturbation pressure gradients. The presence of rotation leads to perturbation pressures, which lead to perturbation pressure gradients. This is the reason why you often see storms make a hard right-turn ("a right-mover") when they develop signficant cyclonic rotation / mesocyclones -- ther vertical pressure perturbations align themselves such that the storm is "pulled" to the right of its previous motion. In addition, the perturbation gradients also act to pull the storm "backwards" a bit (in nonscientific language). So, the net result is that for a cyclonically-rotating supercell in the northern hemisphere, the rotation of a supercell leads to a movement that is slower than, and to the right of, the mean wind.

For what it's worth, the opposite is true of anticyclonic supercells ("left-splits"). In this case, the perturbation pressure gradients align the opposite of that of a cyclonic supercell. For anticyclonic supercells, the perturbtation pressure gradients result in a motion that is faster than, and to the left of, the mean flow. Case in point: the left-split from the supercell down near McAlester, OK, on May 4th, 2003. This storm was moving NNW at 90mph (yes, 90mph), largely as a result of these perturbation pressure gradients.

It should be noted that there is a little feedback cycle here. Remember what you know about hodographs and storm-relative helicity. For a cyclonically-curved hodograph, a storm motion to the right wil often enhance SRH. So, a supercell turns right, and ingests higher SRH air. With this ingestion, rotation can become more intense, so the perturbation pressure gradients can become larger as well, leading to further deviation form the mean flow. Of course, there are physically limitation to this.
 
Ok, on a more simplistic general storm motion speed I seem to recall on old formula based on 850mb and 500mb. I forget the exact formula and would have to look it up. It is something like (500mb speed + 850mb speed) / 2. Do any of you recall the exact formula without me having to go look it up in a book?
 
Thanks for all the great comments -I learned a lot - I wished I'd considered this in my chase planning on monday night - probably would have still gone as this was my only opportunity to chase this year at all . I got to s. Illinois (MT. Vernon to Marion area) in early afternoon just as the storms were firing up. Lot's of Doplar Tornado warnings but only 2 brief touchdowns reported by SPC. Only thing I saw was some lightning and a lot of localized flooding - storms were not descrete supercell types that you see in the plains but were very fast moving in a line - 50-60 mph in most cases. Stll an enjoyable chase on a High risk day with PDS watches.

Jon Miller
KT8NDO
 
We have had several "Fast mover" episodes around here the best thing to do is wait for storm to organize, plan intercept, wait and adjust position if necessary. All the storms I was on were very nice & very viewable into mesocyclone, just didnt last long lol. Then actually try to chase it. I gave up with such bad road conditions flooding and what not. If they didnt tornado within 15 min, ya wasnt going to see it again. Very exciting chase day for me.
Kevin
 
90 mph now that's a blazing fast storm, does anyone have any radar pics or photos on that cell or know where I can look at them? Very nice explanation on storm propogation I never knew it in that detail but I do now. It's amazing to me to watch the process of a multicell cluster go through a life cycle with each cell taking it's turn as the dominant updraft only to be replaced by a new towering cumulous in the cluster causing the once dominant mature cell to dissipate. Thunderstorms and even supercells are not solid systems floating in the air rather they are a constant changing process even in supercells. We see changes in the storms mesocyclone as one disolves a new meso has already formed and taken it's place. their are even times we see the entire supercell change from one type into another type. Perhaps it started out as an LP then it may go on to become a classic and may end up as an HP, you just got to love hybrids. At any rate I have just got to find some time lapse radar of this 90 mph supercell when was it May 4 2003?
 
Originally posted by David Brookshier
90 mph now that's a blazing fast storm, does anyone have any radar pics or photos on that cell or know where I can look at them? Very nice explanation on storm propogation I never knew it in that detail but I do now. It's amazing to me to watch the process of a multicell cluster go through a life cycle with each cell taking it's turn as the dominant updraft only to be replaced by a new towering cumulous in the cluster causing the once dominant mature cell to dissipate. Thunderstorms and even supercells are not solid systems floating in the air rather they are a constant changing process even in supercells. We see changes in the storms mesocyclone as one disolves a new meso has already formed and taken it's place. their are even times we see the entire supercell change from one type into another type. Perhaps it started out as an LP then it may go on to become a classic and may end up as an HP, you just got to love hybrids. At any rate I have just got to find some time lapse radar of this 90 mph supercell when was it May 4 2003?

This reminds me of my high risk chase with Gene Moore in Ok Panhandle/ Kansas on April 6th 2001 with cells moving 70 to 95 mph. Anyone remember or chase this day?
 
Interesting discussion about storm movement. I have a few simple points to add, especially with all the use of hodograph talk, one paper is included in the stormtrack library: http://www.stormtrack.org/library/chasing/hodo.htm

The issue with using a hodo approach is not all storms obey the winds and turn right as the hodo indicates they may. Also, a cell may move for hours before assuming a right turning character. A quick look at a radar scan will show that some storms are deviating from the winds aloft and many are not. I think the basic question here is "can I chase this storm?" That would assume worst case, no right turn slow down in the cell motion. As stated earlier the use of an 850-500 mb wind average can be used. I'll add that I read in an aviation publication that for storm movement one should use the 12,000 - 14,000 ft AGL wind speeds as an average, this is generally what I use. You may have noticed storms move at different speeds in their lifecycle. On the average new storms on a dryline tend to move much faster until they load up with a big precip core. As you have discovered cold season and early spring storms move very fast and require the best of roads and visibility for a good day. So the question might be, will there be a tornado and will I be able to catch it.
 
For what it's worth - the Bunker's technique really isn't that complicated either from a graphical perspective. If you have a hodograph that includes height markers - you just draw a line from the mean 0-500 m wind to the 5.5-6km wind, then find the middle of the hodograph - and draw a line perpendicular to the first line you drew that crosses through the point on the middle of the hodograph. Finally, moving 7.5 m/s to the left or right of this crossing point along the second line gives the left or right mover estimated cell motion - with the point in the middle for the non-rotating cell motion. Of course - you rarely have a hodograph for the location and time you expect a storm to develop - but you can look at a forecast sounding for the target location and then apply this technique to estimate what cell motions to expect. For the graphically minded, I've included a sample below.

bunkers.jpg


So here, a right mover (labelled R) would be expected to track NNE at ~18 m/s.

Glen
 
I've used the Bunker's technique and found that supercells, especially if they are not in a strong dynamic environment, (strong winds aloft) go through a range of motions. Trying to pin down a supercell's motion (as in the example) to a given set of motions may lead to chase frustration. This is much like trying to catagorize a storm as LP, Classic and HP. Fact is many storms go through stages like LP to Classic and later HP. During these times the storm may initially move fast to the northeast, slowing during the classic phase and finally grinding to a stop or turning completely south during the HP mode. The causes of this were outlined in a earlier post and may result from increased convergence due to outflow, propagation, backbuilding or merging cells. All these factors may impact a supercell's motion, especially if it last 4-6 hours. As a chaser in the field I believe a storm's change in motion can be anticipated for both photography and safety's sake (large hail). The main clues are associated with the character of the inflow or flanking lines and the appearance of the core, or main precip mass. It's not uncommon for chasers to get overrun by the forward flank hail shaft as the cell makes its initial deviation to the right. I think schemes like Bunkers work best with situations like a strong pushing dryline where a storm's motion is most likely dominated by a strong synoptic feature.....as opposed to a lone supercell that may be enfluenced more by "storm scale" motions. Regardless, it's good to have all these techniques in your head so you can anticipate the storms changes before you get overrun, or worse yet, left behind.
 
For those who are interested:

Edwards, R., R.L. Thompson and J.A. Hart, 2002: Verification of Supercell Motion Forecasting Techniques. Preprints, 21st Conf. Severe Local Storms, San Antonio. [125K PDF]
http://www.spc.noaa.gov/publications/edwar...ards/motion.pdf

Corfidi, S.F., 1998: Forecasting MCS Mode and Motion. Preprints, 19th Conf. Severe Local Storms, Minneapolis MN [39K HTML]
http://www.spc.noaa.gov/publications/corfi...di/mcs_mode.htm

Corfidi, S.F., 2003: Cold Pools and MCS Propagation: Forecasting the Motion of Downwind-Developing MCSs. Published in Weather and Forecasting. [3156K PDF]
http://www.spc.noaa.gov/publications/corfi...idi/mcs2003.pdf


As others have mentioned, forecasting supercell motion is difficult in many instances. Unfortunately, one of the primary supercell and tornado forecast parameters -- Storm-Relative Helicity -- is extremely dependent upon supercell motion. Thus, one can see how model forecasts or SPC/RUC mesoanalysis can be in large error if the storm motion is hightly deviant. In other words, just because you see the NAM only forecasting 100 m2/s2 0-3km SRH does not mean that there is no risk of tornadoes... If there's still sufficient deep-layer shear for supercells, there may be highly deviant storm motion which may markedly increase the SRH for that storm.
 
Originally posted by Gene Moore
Trying to pin down a supercell's motion (as in the example) to a given set of motions may lead to chase frustration.

I wouldn't disagree with you there - but of course we usually are only concerned with particularly cases where keeping up with a storm simply may not be practical. Storm environments yeilding slow storm motions are really not much trouble all even if they deviate greatly from what you expect in terms of direction from the perspective of chase positioning. It's the cases where the expected storm direction deviates greatly from the available road network when combined with fast motions (e.g., E-W roads, ne @ 45 knt storm motion is bad - but with a more easterly (or northerly) cell motion you might have a better chance of sticking with a storm). I think in these cases, Bunker's offers a decent ballpark estimation of what you might encounter - and really it's much better than having no clue at all - and can be a tool for deciding if an event is 'chaseable'. As for storm type - I've generally found the storm-relative mid-level winds to be a pretty reasonable predictor. Of course not perfect - but HP's are often well predicted with forecasts of very weak mid-level storm-relative winds - which you can't get without a storm motion estimate to substract from the midlevel winds. As you suggested though - in the field once you have a storm you should forget the forecast storm motion and pay attention to what the real thing is doing to avoid costly positioning errors.

Glen
 
We are in agreement, although I've seen HP's develop in some strong wind environments, that is ....with over 100 knots aloft. One such case was May 25, 97. We had a slow moving tornadic supercell tied to the warm front. And, that might be part of the key, the source of convergence and a boundary. I've seen lumbering supercells with wind profiles that should have yielded 30-40 knot movements especially along warm fronts and OFB's. Again, these slower situations are least likely to occur where storms form on a pushing boundary like the recent dryline bulge in Kansas.
 
Getting a bit off topic, but has anyone ever used the Stensrud et al. (1997) BRN shear as a storm type predictor? The bonus to using this is supposed to be independence from needing storm motion - which is needed to find storm type following the Thompson (1998) storm-relative mid-level winds. The former suggested BRN shear values of 40-100 m^2/s^2 were supposed to be most favorable for tornadic storms (when all other conditions favor storms to develop). I've generally trusted the storm-relative wind forecasts - but maybe I should be conferring with the BRN shear values as well.

Glen
 
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