How do you pick a chase target?

Apr 22, 2004
Since this has come up in the site discussion forum under target area, I thought it might be nice for folks to share their basic strategy for selecting and refining their chase targets from several days out to the time convection begins. This could include meteorology analysis, forecast discussions from various sources, model products, topography, road networks, etc...
There is so much that goes into chase forecasting and planning that a paragraph or two here will not suffice. I'll give the short...

A few days out: Pattern recognition and model forecasts. A couple years ago the GFS was excellent at pegging waves in the plains up to a week in advance. Last year the lead time was less or my analysis was not as keen. Usually within 48-72 hours of a potential chase I really begin to examine the charts and model output. I'll make my own forecast and THEN compare notes with SPC or the NWS. If you do this in reverse you will have contaminated your thought process. Next question is can I chase that day at that time? If so then I start making arrangements like gathering chase partners and exploring potential chase day scenarios and routes.

Day 2: Refine chase target forecast based on latest obs and data. Take a peek at the SWODY2, HWO, and other forecasters for notes comparison. Storm mode and initiation time are more closely scutinized. Gas up the car, apply the RainEx, and wait for the 00z model output... or if target is a long drive than consider getting a head start (though I usually avoid chases more than 400 miles from home)

Day 1 (Chase Day): Depending on initiation time, distance, and confidence in target I'll either wait for the 12z model output or get going early. At the least, I'll take a quick peek at the latest surface obs, RUC output, satellite, and take one last look at other forecasts and make any adjustments to the target before heading out the door. I know my route so Im all set. All my maps and equipment are in an easy to grab bin that I can quickly throw in the car.

Picking the target? A combination of meteorology education and several years of experience. Usually a blend of real time data, model output, the GUT model (intuition), and a lesser extent climatology. Another consideration is the distance from home. Is there a good chance of a supercell 100 miles from home versus a great chance 500 miles away? In this case I'll lean heavily on the target closer to home. The road network for me is a low consideration. One thing I will do a little different this year is park about a half hour or so east of my target as I have a knack for overshooting.

Clues for a target? Surface boundaries are huge! In short, looking for the right combination of lift, shear, and CAPE. Boundaries are often easily observed on surface plots but not always. Others are seen on satellite images.

I have a chase planning page on my site under the "pre-chase" link which has similar discussion I wrote here.[/url]
This is lacking a ton of detail, so please take this with a grain of salt..

First of all, I've got to have a day in mind several days out before I start planning. Pending on what time of the working/school year it is for me, I begin this these plans...

3 or More Days Out
FORECAST: I start glancing at the models, discussing the forecast with others, and seeing how things may start to evolve.
PERSONAL: I begin to set plans in motion to figure out how much time I'd be allowed, how far I can travel, if I need to get time off work, etc.

2 Days Out
FORECAST: I see how things are beginning to trend and if things are making drastic changes. I continue my discussion with others as well as begin to look at SPC's analysis. This starts to give me a better, graphical idea, of where exactly I may have to travel.
PERSONAL: If things are looking very very nice, I'll bite early and request coverage for the day of the event. If I'm still unsure, I'll make mention of the time to my boss just to be safe.

1 Day Out
FORECAST: Normally I'm left with the midday analysis to base my decision off of if I have to travel beyond about 400 miles one way. Again, looking at trends in the forecast to see how things are changing. Discussions and looks at SPC continue thoughout this day.
PERSONAL: Normally at 75% sure, I'm preparing to leave. I get coverage for the day I'll be gone and shop the night before for road gear (snacks, drinks, batteries, tapes). If anything small needs to be done with the car, I'll get that knocked out during this time as well. If I need to save time, I'll drive a few hours out the night before and sleep in my car overnight 200 miles closer to my target.

Day Of
FORECAST: Whether I'm halfway there or leaving at 5am, I'll take a look at things to see where stuff is shaping up. Normally I have targeted a broad area as to where I am gonna head, but normally won't make a more precise target til later in the day when I see how things are evolving. If I am hurried, my broad target usually gets centered and I make adjustments from that point. By this point, I am in contact with other chasers in the field and am bouncing forecasts and targets back and forth among them. Usually, my final target is last second unless it's a pretty sure thing.
PERSONAL: I'm on the road early. If I am leaving Denver that morning, I'm usually on the road by 5am or earlier. If I've driven part way the night before, I'm up no later than 7:30a and on my way. Normally, I'll stop at a WiFi spot or library about 9/10am to get a look at things and see what SPC is saying, then grab a bite and continue my trek.

This season, there will be some changes with the addition of several key pieces of equipment, so I'm not sure how my enroute options will change, but that's basically how things set up. This is usually in the "marathon" chases where I normally am back in Denver within 36 hours of leaving after traveling well over 800/1000 miles.

Last season, I was able to chase days where I was able to camp out in the target area for the next day... this slows the pace down and rids of a lot of road time.
I guess I have an ingredients-based approach to forecasting. Not because I have conducted any sort of comparison of various methodologies, but because the way Charles Doswell and Tim Vasqeuz write and talk about operational meteorology makes sense to me as a hard-working but nevertheless amateur forecaster. There's nothing either of them have written about forecasting I don't agree with, so none of my ideas are new or original.

Up until 36 hours out, there's little else to do but watch models. You can check the GOM buoys if you're truly bored, but until recovery begins (given applicable situations), there isn't much to watch outside numerical progs. Because we love it and even though we know better, most of my chaser pals and I spend hours scrutinizing output, trying to apply our mesoscale analysis tools to low resolution model data. Of course it's not productive, but it's fun. It may not be good for mental health however.

What I love about the heart of chase season, on the other hand, is that your time to peruse even short-range model data is limited. If things are active, you have more than enough to worry about on Day 1 without getting too deeply into anything beyond. For a long period last season, I never looked more than 48 hours out. Many days last year, people would ask my thoughts about the next day and I had none. I had not even looked at the Day 2 in the morning because I wanted to devote every second to current, real data. When that happens, you know it's an active year. :)

On Day 1, it's a whole new approach. Over the years I have shaped my process to rely less and less on model output. I don't mean to criticize modelers whose work I respect, but because there is SO much current data to examine and parse, from surface observations, upper air charts, channel imagery, Doppler (even in the morning you're looking for those boundaries in clear air mode), profilers both permanent and VAD, and so on and so forth. At some point you realize that if you can ordinate what data are most applicable in a given scenario, you can run a model in your own head, incorporating what you know about current conditions as well as any pattern recognition skill you bring, climatology, geographic influences--the whole nine yards. If you only have to move the atmosphere forward six to ten hours, then it is vastly more efficient to ingest the data for your own diagnosis and prognosis than to spend critical hours watching the RUC without understanding the actual state of things. I know chasers who pretty much only use model data. They are not altogether unsuccessful. But I have witnessed what Vasquez writes about: that when the setup goes awry or things change unexpectedly, those folks are less prepared to adjust..

With our ever growing access and bandwidth, we can step ever more deeply into the raging data stream, for better or worse. It's hard to know when to stop. It's hard to tell yourself you've examined all the pertinent data, since there is always something you have neglected. If you wake up and discover the target is three hundred miles away, your forecast is so quick and dirty that blundering into a supercell is almost your only hope.

Lastly I examine the RUC and maybe a little ETA (NAM) on Day 1. And of course I read all SPC products, local office discussions, and thunderstorm outlook products. If there's time, I glance at emails from friends or Stormtrack's Target Area.

I try to imagine storms moving through space and time. Where will they initiate? Where will they mature? Where will these boundaries (if any) be at that time? There are many more questions than these. Dozens of questions to ask yourself. When I settle on an area (I try to draw a vaguely oval shape a la Tim Marshall), then I position myself more or less in the middle of that space. Here, I will prefer favorable road networks even if I think some intersection is not the absolute ideal spot. I like intersections a great deal. My final targets are almost always the intersection of major highways or interstates.

On drylines, I like to stay back as far as my curiosity will allow. There is some evidence that the maximum density of pre-init cumulus east of drylines occurs somewhere between 15 and 20 miles from the boundary (Ziegler and Rasmussen WAF, 1998). So I remain east of where I think point convergence might develop or where I can observe vigorous cu forming, perhaps due east or slightly northeast of a dryline bulge. I like room to maneuver. There is almost never a need to close on a storm immediately after initiation. In fact this is often a critical error as a young storm can leave you in the dust if you're parked directly underneath.. Predicting initiation along warm fronts isn't easy either. I suppose I like to stay south and east of where I imagine storms will fire, though in 2004 I repeatedly erred to the north. What we say on the internet in March is often not what we do in May.

The first rule is there are no rules, right?
Beyond the obvious signs that draw the 200 chasers to that one great storm, I am particular about the 500mb windfields inside an already-potent supercell environment. Because I chase for tornadoes and not just supercells, I pay special attention to the flow/vent winds in a given area, looking for the place I believe will give a storm the best "balance," or the best environment that favors "classic" sup mode.

Beyond that, it's really hard to explain what I like or dislike about given areas without having an actual event to disect. I'm certainly not would you'd consider a "forecaster" but I do forecast my own chases to the best of my ability. One thing I have noticed over the years, I pick up on at least one major element of mesoscale forecasting each year, and apply that knowledge to the next year's chase forecasts. It's been a slow climb over the years, but it's moving in the right direction :wink:
There have been some excellent strategies posted so far, but I'd like to review the ideas shown and elaborate on some that may get lost, and add a few of my own.

The early stages I thought was well summarized by many - but it is particularly important to note the model trends and differences in model forecasts - basically how the model is responding to the new data. It is important to recognize that the model fields for the latest run start with the previous forecast state, and tries to push that forecast state toward the newest observations. This is a great method for saving data points sampled in earlier runs, but can wash out new data if it differs considerably from the previous forecast. So, if for instance a jet streak is stronger than forecast by the model, the newest model run may still underestimate the strength of the jet despite new observations showing the difference. Instead, you get a blend between the old and the new, and these small differences can make big differences on some forecasts. This is why it is important to always compare the latest model initial state with the observations, to make sure it isn't washing out an important feature. This also shows up in model trends, such as slowing down a feature, greater amplitude, lower moisture, etc.... As far as which model to use - beyond 36 hours the GFS has greater skill, between 12-36 hours the NAM and then the RUC for short term. Take model forecasts of precipitation very loosely - this is a serious weakness of model forecasts, but when the model does generate convection it changes the model fields strongly - so when wrong expect the model forecast to be poor.

The shift from model forecast dependence to observations is often going to be more successful - particularly in the short-term of selecting a target day of the event. It is assumed here that most folks are able to recognize a "synoptically favorable setup", but this might make another good FAQ topic. Of course, most events are missing one or more elements that requires you to look for something smaller scale to get the best storms. A few particular features to look for in selecting a target:

Find the low-level warm and moist axes - your target should be somewhere between them - as this is the axis of greatest instability.

Identify the edge of the capping inversion. 700 mb temps is a good tool for this - but the value is seasonally dependent. Regardless - you don't want to pick a target under the strongest cap - but preferably on the poleward gradient.

Ensure the low-level to mid-level winds are sufficiently strong to support supercell convection. This value can be as low as 30-35 knots with extreme CAPE, but 40 is better for modest CAPE values. Also find the low-level jet axis - most events occur on the west side of the low-level jet.

Look for high level shear as well to avoid chasing HP supercells, unless you like 'getting dirty'.

Look for jet streaks and short waves to help weaken the cap, increase shear and increase the instability parameters. The strength of these is hopefully proportional to the strength of the capping inversion.

Look at the satellite products to find features - water vapor for finding the short-wave (is it where the model forecast it to be?), clouds (will there be heating in my target area? Is there currently activity in the area of interest?).

Finally, look for surface features where convergence is enhanced, particularly where moisture pools such as outflow boundaries and warm fronts. With a dryline - local bulges are good - particularly the north side of the bulge, as well as the triple point where the dryline and warm front meet. Outflow boundaries intersecting with the dryline can prove golden if sufficient heating can take place. Warm front chases are a whole different monster - but staying south often at least gives you some visibility.

Moisture depth is important - as is noting the difference in the expected dewpoint temperature and the air temperature. The further west toward the high plains, the less of a concern is the latter.

Once you've found all these features, you can use animations and model forecasts to try and get a mental picture of where they are all moving and where things will be by later in the day. This is the hardest part, btw, to do well, but some have a gift for it.

Following Tim V.'s methodology, you should consider what the positive and negative factors are, and the trend toward improving to degrading conditions, and use these for guidance in making your choice.

It's usually worthwhile to then look at a road map and topoagraphy for your chosen target, as if the road choices are poor, lots of trees, hills, etc... may make choosing a less ideal target worthwhile.