Some Comments on Forecasting the Storm Chase Destination By: Dorian J. Burnette

Note: The following discussion becomes "meteorologically intense" in places. Basic knowledge of synoptic meteorology and severe thunderstorms and tornadoes is assumed.

Basics on severe thunderstorms and tornadoes can be found in Thunderstorms, Tornadoes and Hail! by Peter R. Chaston. Forecasting education (including the basics) can be found at Jeff Haby's The Weather Prediction.com. Advanced discussion about severe thunderstorms and tornadoes can be found in Severe Convective Storms edited by Charles A. Doswell III, which is available as a monograph from the American Meteorological Society.

Introduction:

During my tenure at WeatherData, Inc., I issued 1000s of storm warnings to clients across Canada, the United States, and Mexico. The number is no exaggeration, the programmers at WeatherData, Inc. told me what the actual number was...I do not know the exact number now, but these values were accurate at the time (late 2002). Of the 1000s of storm warnings, the number of tornado warnings I issued were well over 500. Many of the folks at WeatherData have issued more storm warnings than I have, and the discussions I have had with all them as well as other colleagues outside of WeatherData were an immense help in tweaking my warning methodology. I was very big on reducing my false alarm rate (FAR). Overwarning is a disservice to the folks who receive the warning. For example, a tornado warning from WeatherData, Inc. will shutdown plants costing them numerous dollars (the number can be in the millions...I am not kidding). The lower the FAR, the better, as long as it does not compromise safety of course. You do need to be cautious with how you use new research in an operational setting though. In other words, do not assume the latest research is the latest and greatest technique that works. It is quite clear to me though that a hook echo by itself is not necessarily enough to issue a tornado warning. Instead I used a checklist, and the more things I could check off, the more likely I was to issue a tornado warning. All of this has led to some of the ideas I have on forecasting severe thunderstorms and tornadoes that I will now present.

Use of Computer Models:

Computer modeling has definitely improved weather forecasting, but there is a very apparent over-dependence on the computer models nowadays (a.k.a. model cancer) in weather forecasting. Weather forecasting is not as simple as looking at the latest model run and calling the forecast done! ALL of the latest hard data (satellite, radar, upper air, surface) should always be analyzed first. I also recommend printing out the upper air and surface data and analyzing them by hand because this is the best way to really get a feel for what is going on. Thereafter, the computer models need to be critically evaluated based on ALL of these data. I often start by looking at the computer model's initialization and then I move into their individual forecasts. I compare the computer model forecasts with reality as I work through their individual forecasts, so certain elements that a particular model is not handling well can be noted. I am not partial to any comptuer model in particular because there are strengths and biases with each one that I will not go into here. Good science involves using the right tool for the right thing and being "biased" to one model all the time really will not cut it in terms of good weather forecasting. I do look at the Model Output Statistics (MOS), but I do not treat it any differently than any of the model panels. Do keep in mind that MOS has a climatology bias, and does not perform well in predicting extreme events. You should have no problem picking out problems in the model runs if you have analyzed ALL of the available non-model data thoroughly, and you will end up with a much better forecast.

There is an art to weather forecasting too. You cannot just jump into it and expect to do better than a veteran forecaster...experience and continued education are keys to improvement. I still have plenty of things to learn! See this essay by Dr. Chuck Doswell for further discussion of this model cancer problem.

The Sweet Spot Issue and The Too Dynamic Problem:

The discussion that follows has been on my mind for several years, and I am convinced that there is something here that needs to be fixed in our forecasting of severe thunderstorms and tornadoes. Since the atmosphere is run by non-linear equations, we have to set limits (both lower and upper) on the variables until someone comes along that can invent another math beyond calculus. I find it funny that we are only setting lower limits on what is needed for severe thunderstorms and tornadoes. Is there a sweet spot that when crossed on the side of too unstable or too much upper air support actually decreases the probability of organized severe thunderstorms and tornadoes? I believe the answer to this question is yes. Here are the specifics I look for that immediately draw flags and the reasoning behind them:

1) Synoptic-scale surface winds greater than 35 knots

• Strong surface winds mix out surface moisture instead of allowing it to pool well. Surface moisture convergence is frequently reduced as a result. This problem is magnified when general convergence is weak to begin with. No convergence = no storms. Do keep in mind that I am talking about synoptic-scale surface winds, and not the powerful surface winds that result when supercells create their own environment.

2) Storm system with a widespread area of lift and little cap and/or support yielding storm movements of greater than 50 mph

• It can be said that it is the "little" shortwave troughs that cause the most trouble. It is easy to become "wowed" by the amount of upper air support (strong jet, lots of vorticity, etc.) and not place much thought on the surface setup. Systems that are too dynamic in this way often have a widespread amount of lift. The result is lots of cloud cover and general rain and thunderstorms. I have become very big on the need for sunshine. No sunshine often means no organized severe thunderstorms and tornadoes. However, if there is a high amount of helicity to overcome the lack of sunshine, then weak tornadoes can occur. Another thing I look for that is related to this and flagged as a potential problem is a sharp shortwave trough (as opposed to a broad shortwave trough). Sharp shortwave troughs have their energy concentrated and their mode of forcing frequently overwhelms the environmental setup (e.g. 6 April 2001). Again, "little" shortwave troughs are better. I have seen numerous forecasts of severe thunderstorm/tornado outbreaks bust due to this issue.

3) Too much thermodynamics

• There has been discussion in the research community on whether there can be too much convectively available potential energy (CAPE) or not. This seems logical to me, but if true, it is probably not limited to just CAPE. I will be watching the stability values more closely for signs of this issue.

Prior Rain:

In 1996, Mike Smith suggested a correlation between violent tornadoes and rain prior to the arrival of the tornadic supercell. It is important to note that it is not the rain that causes this, which is where some folks who call this hogwash are coming from. Instead the issue here is that the prior rain will produce outflow boundaries with localized maxima of storm relative helicity and CAPE. It is well-known in basic severe thunderstorm theory that thunderstorms can become quite nasty when they run into boundaries. Another possible aspect of this is the change in stability of the atmosphere when it rains in the morning only to become sunny in the afternoon on a potential severe thunderstorm day...the rate of change in instability with respect to time dramatically increases. In other words, the atmosphere becomes unstable quickly and becomes more prone to explosive development. The prior rain aspect is important and is supported by additional peer-reviewed studies (e.g. Markowski et al. 1998; Rasmussen et al. 1998).

In the Field:

It is quite clear that only using radar data for storm warnings is not sufficient. There have been numerous accounts when strong rotation or a hook echo has been observed on radar but yet storm spotters and storm chasers report nothing of note with the storm. It also is important to note that the theory of tornadogenesis descending from a TVS aloft is breaking down because Project VORTEX recorded a F3-F4 tornado (developed via a rapidly intensifying updraft along the flanking line) that did not trigger a TVS on the NEXRAD Storm Attributes Table. See this discussion by Dr. Chuck Doswell for this and other intriguing thoughts. The storm attributes (TVS, MESO, etc.) is another thing that I feel some meteorologists have become way over-dependent on. I am not saying that these data are not valuable, but they are not 100% accurate and numerous case studies can make this clear. I only rarely look at such data.

The environmental conditions a particular thunderstorm is moving into seems to play a crucial role in how nasty it will become, and numerous peer-reviewed papers are making this increasing clear (e.g. Rasmussen et al. 2003; Davies 2004). Thus, I am always checking the latest surface observations and satellite observations in addition to the radar data. I mentioned having a checklist I used for tornado warnings. I continue to use this checklist when storm chasing in order to search for the storm(s) that is (are) either in or entering the best environment. For example, on 7 May 2002, a tornado warning was issued on a storm in Harper County, KS (and it was justified), but knowing that storm would soon be moving into a more stable environment in vicinity of Wichita, I had my chase team ignore the storm and continue farther west into southwestern KS where there had been plenty of afternoon sunshine. This was the correct move though I admit that it was tough to leave that nice looking storm behind. Here are the things I look for (some are part of the old tornado warning methodology others are not):

1) Strong gate-to-gate rotation on storm relative velocity data

2) Hook echo/kidney bean shape on base reflectivity data

3) Positive cloud-to-ground lightning bolts in vicinity of the "tornado area" of the storm

4) Decrease in storm foward speed or storm turning toward the right of the mean flow

5) Storm in vicinity of a boundary

6) Storm in or moving into an area of locally backed surface winds

7) Storm in or moving into an area where the level of free convection is low

8) Storm in or moving into an area where the lifted condensation level is low

9) Storm in or moving into an area with high amounts of instability (high CAPE, low lifted indices, etc.)

10) Storm in or moving into an area with high amounts of surface moisture convergence

See this discussion by Jon Davies for some specific details on the environmental values needed as well as some other useful tidbits regarding ingredients and patterns in tornado forecasting.

The Storm Prediction Center's Mesoanalysis Page (link on the currents section of the djburnette.com Weather Page) is useful in order to analyze the various indices and combinations there of. However, you do have to be careful because the values are the result of observations merged with the RUC computer model. Thus, it assumes the RUC model has a good handle on the situation. I also will look at satellite derived stability indices (links to which are also available via the link above). Although, they do not register values over areas that are cloudy and I have found that they tend to overestimate the amount of instability (CAPE will be too high, lifted indices will be too low). Of course, I also look at various surface plots.

Final Comments:

Storm chasing is not simple! Even if you make the perfect forecast, that is still no guarantee of anything because there are other problems that can occur (such as all tornadoes being rain-wrapped and obscured from view). Then there are the bust days, but let's not go there. Hopefully some of the comments here will help in the forecast process. I know this stuff has been a tremendous help to me, and it is my intent to update these comments over time. Any comments on this may be sent to Thanks to WeatherData, KSNW-TV, the Wichita National Weather Service Forecast Office, Jon Davies, Dr. Rich Sleezer, and many others for discussions on this stuff over the years.

Last Update: 17 January 2006