What do you look for to determine your chase target area or the storm initiation point?
Day of forcasting:
Knowing WHEN and WHERE on the DAY OF. Once the storms first fire, its all easy. But how do you know storms are going to fire at 4pm near Ardmore OK, and know that at 8am? Are you just using models, or are you working this out on your own? Are you using models to get a basic idea, and then refining on your own? Specifically, which models do you use? The cap breaking when and where is a tough one for me.
There's a lot that goes into picking a target and forecasting initiation. I'll try and hit the basics here and go into a little more detail on when and where to expect initiation with regards to capping, and perhaps explain the other facets in more detail later. I take an ingredients based approach when forecasting for chases. I'll throw out some good starter values here with an example for TwisterData so you can follow along, and we'll assume its the morning of the event. Keep in mind these are rough estimates and broad rules of thumb. There are always exceptions to the rules and special circumstances. Hopefully, you're in the ballpark for a classic spring time setup though.
Supercells require four things:
- Moisture - Check the surface dewpoint plots
http://www.twisterdata.com/index.ph...12¶meter=DPTF&level=2&unit=M_ABOVE_GROUND
We're looking for values greater than 55-60 F. You'll want a little more later on in the season (at least 65 come June). For high plains setups (Texas panhandle, Colorado) or cold core/air setups (meaning cold air aloft, not just cold at the surface) greater than 45 F can work. Moisture is the limiting factor in late winter early spring. If I don't see some yellows edging into the plains, I don't look further.
- Instability - Check the CAPE plots
http://www.twisterdata.com/index.ph...our=12¶meter=CAPE&level=SURFACE&unit=none
We want at least 1500 J/Kg for a shot at decent supercells and tornadoes. You can squeek by with 750 J/Kg in early season setups and even less in setups with an extreme amount of shear. The more instability we have, the larger and more robust your supercells will be. Instability is in part determined by moisture, so you'll notice that if we don't have sufficient moisture, we probably don't have any instability.
- Shear - Check the 0-6km Bulk Shear plots
http://www.twisterdata.com/index.ph...=12&fhour=12¶meter=SHRM&level=500&unit=MB
We need at least 30 knots to sustain supercells, and we're looking great at 50-60 knots. Too little and your storms may remain linear, or quickly transition to HP and gust out. Other shear plots to check out are the 0-3km storm relative helicity, for supercell potential and 0-1km storm relative helicity for tornadoes. Values over 150 are good for each corresponding level/threat. If you have a site that plots Effective Shear, or Effective Storm Relative Helicity such as Earl Barker's site or SPC Mesoanalysis, use these plots instead as they are superior.
- Lift - Check out the temperature and dewpoint plots
http://www.twisterdata.com/index.ph...12¶meter=TMPF&level=2&unit=M_ABOVE_GROUND
We're not looking for specific values here, we're looking for where there are sharp gradients (ie fronts and boundaries). Where is the warm front, the cold front, the dryline? These are your sources of lift.
Check out the 500 mb wind plots
http://www.twisterdata.com/index.ph...=12&fhour=12¶meter=WSPD&level=500&unit=MB
These winds will indicate where the longwave or shortwave trough and the upper level energy. Where the jet hits your frontal boundaries, are areas that will have enhanced lift. So line up where the streaks of higher values on this plot, bisect the temperature/dewpoint gradients on the other plot. If you don't see a well defined front or trough, you probably don't have enough lift for storm initiation. Texas panhandle and Colorado chasers, note that the rise in terrain is often your source of lift, so just look for an east wind at the surface.
Alright, so we've found a spot where all the best ingredients overlap, but when and where do the storms actually go up. Forecasting initiation can be tricky and there is a lot involved. Of the above ingredients, we want to focus most on lift. When and where the lift is, determines when the storms go up. Go through the forecast hours and watch where the fronts and troughs are going. Remember, if its the morning of the event, our 12z model run and 0 forecast hour corresponds to 6 or 7am. Go through the hours to 15 or so (which would be 9 or 10pm). Your ingredients will probably peak somewhere in the 9 to 12 hour time frame which is afternoon/evening. Where are the fronts and troughs then? That's where we want to look for storm initiation. If you have a trough over a front, you've got your lift. There is a crucial player here that will make or break your chase forecast, however, and that is the cap:
- Cap - Check the CINH plots
http://www.twisterdata.com/index.ph...hour=12¶meter=CIN&level=SURFACE&unit=none
This plot measures the strength of the capping inversion or lid on the atmosphere. Values above 50 will suppress thunderstorm initiation unless you've got a well defined trough bisecting your front. Values above 150 are probably always going to suppress thunderstorm initiation. The cap is double edged sword and should not always be loathed. If there is no CINH being plotted in the morning hours, and you've already got a trough overhead, your storms may very well initiate before the rest of your ingredients are at their peak. The cap plays a huge role on how your chase day pans out. If it erodes too soon, you'll wind up with a large number of junky storms in the morning. Too late and you'll be chasing well after dark, or not at all.
Some prefer the 700 mb temperatures to determine the cap's strength:
http://www.twisterdata.com/index.ph...=12&fhour=12¶meter=TMPC&level=700&unit=MB
This can be a good estimate of cap strength, but varies depending on the time of year and location.
The 700 mb level may not be the actual location of the capping inversion though, so I prefer lid strength index or cap index from Earl Barker's site:
http://68.226.77.253/models/eta/central/CENTRAL_ETA212_ATMOS_LSI_12HR.gif
This shows the width of the cap as plotted on a sounding, as opposed to the area of the cap which the CINH plots. I believe this to be a better indicator fo the cap's strength and friendlier plot to look at. You'll notice it plots values where the CINH plots do not. 0 indicates no cap, and 1-2 a modest cap that can be overcome with some lift. 3 requires a strong trough bisecting a sharp front. Surface based thunderstorms will probably not initiate with values higher than that. What you want to see here is the cap eroding down below by early to mid afternoon with a hole that is larger than a county, and you want that hole to stay open for several hours.
"But Skip," you say, "its 9am and there are already storms over my target, and the CINH and Cap Index plots say there is too much of a cap. How can that be?" With a strong cap, thunderstorms still can go up. However, they fire above the capping inversion, meaning they pull their inflow not off the surface, but much higher up in the atmosphere. These are called elevated storms, and can still pose a risk for severe weather, but you need surface based supercells if you want a shot at a tornado. Elevated storms often have rather junky looking structure as well. If your cap erodes, and storms fire, but then the cap quickly fills back in, your storms may also become elevated unless there is a good chunk of low level instability left.
So in conclusion, once you've got an overlapping area of all the necessary ingredients, look for the best sources of left, and when and where there is going to be a hole in the cap. That's your best bet for storm initiation. If you've got your other ingredients, but no source of lift, you've got no storms. So don't pick a target with 6000 CAPE that's 200 miles away from the nearest boundary or trough, as there's no lift there. Then its a matter of waiting for the cap to erode. If your Cap Index plot never drops below 3 for an extended period of time, you'll probably find yourself on a sun tanning trip despite having all the best ingredients you could ask for. All the CAPE in the world will not help to break a cap. You need strong lift and a weakening cap.
