Sunday, February 12, 2006

Column 8: Severe thunderstorm forecasting (Originally posted October 6, 2005)

Severe thunderstorm forecasting. It's something I love to do, and something I hope to get better at.

I'm a devout follower of the ingredients-based forecast, as it seems to work out for me so well most of the time. And I'll continue following that tradition.

So what are we looking for when we're forecasting severe thunderstorms? There are 4 main ingredients: moisture, instability, lift, and shear. I'll talk about them all in turn.

First off, you need moisture in the low levels. Moisture is the fuel which feeds thunderstorms. We all remember that muggy summer day when a thunderstorm was welcome, as it came to cool us off. Moisture is measured more or less directly at the surface and aloft, in the form of the dewpoint. I prefer this measure, but some prefer the more nebulous "relative humidity".
How does moisture feed thunderstorms? When air rises, it cools and eventually reaches a point where condensation occurs. When this condensation occurs, heat is released--the heat that went into evaporating the moisture in the first place. This heat is known as latent heat. This latent heat becomes real heat, and warms up the air very near it. As we all know, warm air rises. So moisture in the air leads indirectly to upward motion.

Instability is the next ingredient necessary. Instability is essentially the tendency of the atmosphere to overturn. Usually measured as Convective Available Potential Energy (CAPE) or Lifted Index (LI), the higher the CAPE or the lower the LI, the more instability there is. Essentially this can be thought of as the temperature difference between a parcel of air rising from the surface, and the air already present around it.

Lift is an important ingredient, too. Some people like to use the word "trigger" instead, but I'm more partial to "lift". Lift can be present in many ways: at the surface--a cold front, a warm front, a dryline, an outflow boundary, a seabreeze, a surface trough, and there are others that I'm forgetting. Aloft, the upward motion helpers (not so much triggers, though) are left exit and right entrance of jet streams, vorticity maxima, cold pools aloft, and dry air intrusions aloft.

Finally, we get to wind shear. Wind shear is the ingredient that, without it, even if you have the others, you won't get a long-lived severe thunderstorm. Sure, you may get pulse-y severe storms, but not the beasts that I try to chase. Wind shear comes in 2 forms (that we count): directional and deep-layer. Directional shear is simply the changing of the wind direction with height. Near a warm front, the wind will often be from the east, and as you go higher, the wind will veer to southeast, south, southwest, and finally west. This is directional shear. Deep-layer shear is an arbitrary concept that was developed numerous years ago, but seems to work. It's the vector difference in wind velocity between the surface and 6 km. (Usually we use 500 mb as a proxy for that.) What both of these types of shear do is set things up so that the downdraft (air going down) doesn't fall into the updraft (air going up), essentially cutting off. In other words, shear separates the updraft from the downdraft.
Deep-layer shear is necessary for mid-level thunderstorm rotation, while directional shear (especially in the lowest 1 to 2 km) is ideal for tornado development.

Put these things together, and you can get severe thunderstorms.

But sometimes not. There are numerous things that can come up to destroy your perfectly "good" forecast of severe thunderstorms, leading to chasers getting sunburns and logging thousands of kilometres on their vehicles.

A storm-squasher is subsidence or, more correctly, lack of lift. I've seen a few occasions where things were primed to go, but there was no lifting mechanism in place, so it stayed sunny.
Lack of low-level moisture can sink a forecast, as well--especially if you're using a model forecast (more on model forecasts in a bit) for your storm determination.
Warmer-than-expected air aloft can stop updrafts from going. A parcel of air rises and keeps rising if it's warmer than what's surrounding it. If you have a lot of warm air aloft, having rising air warmer than its surroundings becomes less and less likely.
As well, the warm air aloft can create a "lid" on upward motion in the atmosphere, a temperature inversion. Moisture trapped under this lid, if lifted, can cause a deck of low cloud, not allowing the sun to shine and do its job. Too much cirrus overtop can have much the same effect, although to a lesser degree.
Finally, too **much** wind shear can cause your updrafts to get torn apart before they barely get started. This phenomenon, if you ever see it, is called a "turkey tower". Seriously. The storms trying to go up look like turkey necks.

So now, how does one forecast severe thunderstorm areas, based on model output? Of course, I prefer to forecast up to 12 hours in advance with real data, but after that, short term forecasting techniques don't really work.

Now, all of what I'm about to say is geographically dependent. The numbers I'll give are what a person would use in the plains states. The numbers aren't vastly different for most regions across Canada, but Alberta tends to have some different characteristics and thresholds.

First off, let's look at moisture. Moisture for severe thunderstorms needs to be deep enough that it still serves as fuel. If you have a shallow layer of moisture, it will tend to mix through the depth of the boundary layer once daytime heating and mixing occur, so you need the depth of the mixed layer to be less than or equal to the depth of the moisture. Traditionally we look at the dewpoints at 850 mb and at the surface. 925 mb is optional, too.
Dewpoint thresholds vary upon time of year, but it seems that once surface dewpoints go above 15 degrees C (10 in Alberta) there's likely ample moisture. At 850 mb, we tend to look for dewpoints above 10 degrees.

Instability is something that's related to the moisture in the atmosphere; the more moisture, the more unstable the airmass. So it wouldn't be surprising to find that the areas with the highest dewpoints, according to the model, are the areas most likely to have high instability. When assessing instability, I'll look at Surface Based CAPE (SBCAPE) and Mixed Layer CAPE (MLCAPE). SBCAPEs above 1500 j/kg (1000 in AB) and MLCAPEs above 1000 j/kg seem to be the minimum needed for severe storms, although I have seen less.

Lift is something that can be a bit more difficult to see on the models--especially the lifting mechanisms that are smaller in scope, like seabreezes and outflow boundaries. Fronts, though, can be identified by a tight gradient of the 1000-500 mb thickness pattern, accompanied by a tight temperature gradient at 850 mb. Drylines can be found simply by looking at the surface dewpoints. West of a dryline, the dewpoints tend to dry off by as much as 30 degrees C. (But the thickness pattern won't show any gradient--a sign of no front.)

Wind shear is perhaps most difficult to gauge. I like to simply look at the 500 mb wind and use its speed as a proxy for the 0-6 km shear vector. But in doing so you have to be careful; look at the surface wind direction, as if it's the same direction as the 500 mb wind, then using the 500 mb wind as a proxy for the shear isn't valid. Then you have to do vector differences.
As well, I look at the forecast winds at all given layers--surface, 925, 850, 700, 500, and 250. I then build a mental image of what the winds throughout the atmosphere are doing over any given point.

That's it--no fancy stuff, just plain old looking at information and synthesizing it. I've done this a lot, and I still need practise. In fact, I think I'll **never** stop needing practise. But it sure is fun!

A couple of tidbits to send you off with, and hopefully spark conversation and a few questions:

Cold fronts tend to produce lines of storms. Supercells can be embedded in the lines, but with a cold front it's more likely a squall line will occur.
Warm fronts can be prolific supercell producers. If a storm hugs the front, it can ingest moisture and copious directional shear, and these can go on to produce tornadoes for hours. If a storm crosses the warm front into the cooler air, though, it will still be severe--just producing large hail instead.
Dryline storms are fun to watch. Sometimes all you need to do is park yourself about 50 km east of a dryline and wait for things to go. Tornadic supercells frequently occur with these.
And finally:
Don't put too much credence into model output. What you have in front of you is **an** answer, not **the** answer. Model output is useful for strategic general placement, but for the day of?

I'll take real data any day.


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