Here is the scenario- it is early May in Dallas and it is hot and humid. The temperature is approaching 90 degrees and the dew point is 70 degrees. Surface winds are out of the SE at 25-30 knots, pumping up rich Gulf moisture, veering and strengthening with height, and pressure is falling. There is a surface low in SW Oklahoma, an attached warm front running along the Red River, a dryline extending south into west central Texas, and a strong, protruding cold front extending from the low southwesterly, back behind the dryline.

The National Weather Service has issued a Tornado Watch for entire North Texas area. The latest 12Z sounding out of KDFW indicates CAPE values are at 2700 and the Lifted Index is at -12. All the parameters are here for a major severe weather outbreak. As an avid storm chaser, I look forward to these days. My adrenaline starts pumping in, as I race out to my car in search of the perfect storm. Storms are initiating out west ahead of the dryline, and moving in my direction. I drive out to intercept them. However, by the time I reach them, they have weakened or totally dissipated. What happened?

As I head back home, I take a more thorough glance at my Skew-T sounding; I noticed something strange on the graph. From the surface to the 850 mb level, temperatures decrease with altitude, which is very normal. However, once at the 850mb level, temperatures start to increase. What is the phenomenon that creates this abnormal behavior? It is called a thermal inversion, or "Cap".

The cap is basically a lid that is put on the atmosphere. We can think of the planetary boundary layer as a pot of water. As this water heats up, it begins to boil. If there is no lid, then energy (latent instability) can escape without obstruction. If a lid is put on the pot, then all this energy will be trapped within the pot. This potential energy will keep increasing with time, as long as the lid is kept on. If the lid is removed, then all that energy will escape upward. However, if the pressure within the pot increases to a point to where the lid can no longer be held down, then that energy can punch through and explode violently.

Every spring and summer, the Dallas/ Fort Worth area forecasters are challenged with the prospect of severe storms and how the cap will influence the outcome of their forecasts. Information on forecasting revolves around processes that occur both vertically and horizontally in space and time. These criteria are important when looking at overall weather patterns. One pattern revolves around weather to our south and west. This is the best place to start describing the atmospheric processes that produce this cap, with its birthplace, in Mexico.

The geography of Mexico, our neighbor to the south, is made up of coastal areas, mountain ranges, plateaus, and deserts. Two mountain ranges, the Sierra Madre Occidental in the west and the Sierra Madre Oriental in the east are located in the northern part of Mexico. In between these two mountain ranges lies a higher plateau that include deserts. We can think of the mountain ranges as barriers as the desert air can only move north or south. When this air gets advected to the north from the higher terrain in response to mid-latitude cyclones, this creates the inversion that overlies the Dallas/ Fort Worth area.

Differential advection of a deep mixing layer from the planetary boundary layer in this hot, arid, elevated air mass over a shallow, more cooler, and moist mixing layer produces a mid-level front known as the "Elevated Mixed Layer Front". This front is located in the mid levels of the troposphere and overrides the cooler, moister air mass in the low levels. This front establishes the boundary will produce the inversion, as this forms a barrier for lifting mechanisms to produce thunderstorms.

The location of this front and its movement revolves mainly around two criteria. The first has to deal with wind vectors. Wind vectors describe the direction and magnitude of wind in relation to troughs of low pressure that either move in or develop west of the area. A favorite location for this development that concerns the Dallas/Fort Worth area is on the lee side of the Colorado Rocky Mountain ranges in SE Colorado. The counter-clockwise rotation around the low allows for southerly and southwesterly winds ahead of it. This will pull the elevated mixed layer northward. How far north depends on the magnitude of the wind vector associated with the low.

The second has to with seasonality. As we go from May into June, the main belt of the Westerlies shifts north. This allows the southern plains states to exhibit a more barotropic profile, allowing for further poleward advancement of the elevated mixed layer. This enables the cap over the Dallas/Forth area to be stronger and more solidified. As mid-latitude cyclones move across the northern tier states during the summer, the Dallas area rain chances become scarce, as there are virtually no synoptic scale storm systems to come along and erode the cap. Climatologically, the area experiences five days of capping, from April through June, and then decreasing to two days by August.

The strength of the cap is once again determined by seasonality. Late May is once again the most difficult time of year to break the cap. The reason for this is that the air within the elevated mixed layer and the air in the moist low levels possess characteristics within their source region of potential temperature. A formula to determine the strength is:

CAP STRENGTH= Saturation Wet Bulb Potential Temperature of the cap - Wet Bulb Potential Temperature of the surface.

If the calculation is three or higher, then the cap is very strong and severe storms are unlikely. Negative values will indicate a weak cap.

There are several ways, however, to erode the cap. This can best be illustrated by using a Skew-T chart. The first is with daytime heating. An air parcel will rise adiabatically from the surface if the air parcel temperature on a sounding is warmer than the surrounding environmental air. In the case of the elevated mixed layer, the environmental temperature will be warmer than the temperature of the air parcel. As the heating of the day occurs, the air parcel temperature will begin to rise. If it raises enough to surpass the environmental temperature, then the air parcel will be free to rise on its own.

Another way is synoptic scale forcing or lifting. This situation does not have to rely on surface thermal heating. If a mechanism, such as a front, should come through, lifting of the air can be forced up to where it will help erode the capping inversion. Temperature changes in the vertical and mesoscale features can also aid in the erosion of the cap. All of these factors have to be considered in forecasting weather in the Dallas/Fort Worth area.

The consequences or the ability to predict such situations is vital in forecasting for the area. The cap can make or break a day for severe weather. It doesn't matter how much low level moisture is being pulled in, if the cap is too strong, no storms will develop. If the cap weakens before the primary heating of the day, then we might be looking at a linear squall line developing. If the cap breaks at the right time during the afternoon, then severe storms with potential tornadic evolution is possible.

It is also difficult to evaluate the condition of the cap during various parts of the day. Soundings profiles are the best tool to determine the strength of the cap. However, in the Dallas/ Fort Worth area, soundings are taken only twice a day. The first sounding is taken locally at 1200 UTC (7:00 AM CDT/ 6:00 AM CST) and at 0000 UTC (7:00 PM CDT/ 6:00 PM CST). As a forecaster, it is very difficult to know when and what the strength level of the cap will be in the future, using information from a pre-existing sounding, hence, a real challenge.

One expert opinion comes from meteorologist Steve McCauley of WFAA-TV in Dallas. He calls this phenomenon the "Mexican Plume", obviously due to source region. He uses this term frequently in his forecasts and graphically outputs this data so that his audience can visually comprehend this event. He also explains that two ways for the cap to go away is for the upper level winds to "blow it away", or for an upper level disturbance to lift the cap vertically, causing it to cool, weaken, and then eventually go away.

It is difficult to come up with ideas to better forecast this problem. There are numerous tools that can be used to predict if and when the cap might break. The use of models, satellite imagery, and data from other observation stations upstream from the forecast area can be used to determine timing and strength of synoptic scale features that could influence the strength of the cap. Also, more frequent launches of weather balloons can help forecast, using real time information in determining the progression of processes that are influencing the cap.


The Cap: It's Boom or Bust!
Tim Marshall
Storm Track-1988

Mid-Latitude Weather Systems
T.N. Carlson
American Meteorological Society 1998
Chapter 16-pages 448-480

Wild Weather
Steve McCauley
November 2005