The Cap: It's Boom or Bust!
by Tim Marshall
Storm Track, May 31, 1988
© Copyright 1988 Tim Marshall
Traditional methods of forecasting severe thunderstorms and tornadoes can be found in several publications including Col. Robert C. Millers, 1972, Notes and Analysis of Severe Thunderstorm Forecasting. Other publications are in the National Weather Service Technical Memorandums and Severe Storm Conference volumes. Most forecast methods rely on a combination of instability, moisture, and the position of large synoptic scale features such as troughs, cold pools, and jet maxima. But, all of these are secondary features in their importance to severe weather forecasting.
Take this true situation: The surface temperature is 95 degrees, the dewpoint- 70 degrees. There are strong southeast winds at 25 gusting to 35 mph in your area. The dryline is approaching with strong west winds behind it. All the jets are in place; the lifted index is -14 degrees C. Your at the triple point just as a tornado watch is issued for the area! Are you ready to sit back for a show?? In intense anticipation you wait patiently, eat lunch, then dinner. Just before the sun sets, you watch boiling cumulus along the dryline foam and froth then suddenly dissipate. At dusk, you find out there were no storms within three states. The tornado watch box is cancelled. It was a bust! What happened?
Blame it on the "cap". The cap is a thermal inversion filled with dry ai that suppresses deep convection. In the morning, it lies just above the ground and may be several thousand feet thick. Sometimes you can see it where air pollution stagnates in stable stratiform layers.
The strength of the cap can be determined by using sounding data taken from weather balloons. The sounding data includes temperature and dewpoint profiles. If you plot the data with height, it may look like the figure below.
Diagram from Snow, 1984, Scientific American article on "The Tornado".
The temperature profile appears as a nose as the temperature increases through the inversion. Note the moist layer below the capping inversion and the dry air just above it.
The strength of the capping inversion changes throughout the day as daytime heating erodes the cap from below. Also, the approach of cold air aloft with maybe a jet maximum can enhance vertical motion enough to lift and weaken the cap. In contrast, subsiding air and warming aloft can strengthen the cap throughout the day.
The behavior of the cap definitely makes or breaks a chase day. No matter how unstable the air is, no matter how much low level moisture there is, if the cap is too strong, storms are not going to develop. Consequently, most of my "bust" days can be blamed on the cap.
As you can see, outguessing the strength of the cap is tricky. Temperature and dewpoint soundings are taken only twice a day in locations about 500 miles apart. Subtle changes in the cap strength (ie. localized forcing) can and do go unnoticed. It's easy to be lulled into chasing on a day when the cap is too strong. But when all other parameters are there, you MUST take that risk. If a storm makes it through the cap, it could be a field day. For example, the cap was very strong on May 22, 1981. But an isolated storm developed and produced numerous tornadoes in west Oklahoma.
The bottom line is,when you're dealing with the "cap', there is little difference between thunderstorm BOOM or BUST. If the cap is too weak, the atmosphere can overturn early in the day and you are left with a squall line at 11 am. If the cap is too strong, you get a brilliant blue sky and maybe a sunburn. But if the cap burns off at just the right time of day (say 2 to 4 pm, the time of maximum heating) you may be left with isolated severe storms that can be possibly tornadic. Sounds like a goldilocks and the three bears story, doesn't it? If everything is just right, here is how I depict the evolution of storm development.
MORNING- The day begins with low clouds in east Texas which remain all day. Some light drizzle occurs there, even a few imbedded thunderstorms are possible. Farther west, the air is drier, the moist layer is not as deep. The sky remains clear throughout west Texas allowing for strong surface heating to begin mixing the moist layer with the dry air above. The capping inversion begins to erode from below.
AFTERNOON- By mid-afternoon, the dryline has pushed into the moist air increasing the low level wind and moisture convergence and enhancing vertical motion along the boundary. A line of boiling cumulus may be seen along the dryline. They appear ragged and torn, evidence of strong mixing. The mixing layer is deeper, extending higher, lifting and eroding the cap more quickly. With luck, the cap will break allowing one or more storms to develop just about the time of maximum surface heating. Slowly, the small cumulus will dissipate as mixing begins to shut down leaving the few surviving storms to feed on the buoyant warm, moist air from below. Of course, the right timing of upper short waves, jet maxima, instability, and moisture are important parameters needed for severe storm development. But, it i8 the cap stren8th which determines whether this instability can be released upward in the form of deep convection. To calculate the cap strength, you need to know how to read a sounding. The cap strength is computed from the moist adiabat along the tempersture profile. Use the following equation: CAP STRENGTH = SWB of cap - WB of surface, where SWB is the maximum saturation wet bulb potential temperature of the cap, and WB wet-bulb potential temperature of the surface.
When the cap strength is plus three or higher, the chances for severe storm development are minimized. Negative values suggest easy overturning of the atmosphere. The best values for severe storms are when the cap strength is plus one or two says Bill Read, NWS forecaster. The presence of the cap can enhance severe storm development by delaying over-turning until the lower layers of the atmosphere can heat up. Keep in mind the surface temperature changes throughout the day, and therefore, the cap strength values should be updated. Here are some boom/bust examples.
LITTLE/NO CAP - EARLY BOOM: On March 24, 1988, a squall line developed through the center of the U.S. The morning sounding (solid lines) at Monett, MO indicates a weak capping inversion of 3 degrees C at 770 mb. The moist layer was thick, about 180 mb. By mid-afternoon, the surface temperature rose considerably, the dewpoint remained nearly the same (dotted lines). At this time, the cap index was now -1, no cap existed. Air rising from the surface would not be restricted, and thus, early boom,
MODERATE CAP - LATE BOOM: On May 16, 1986, an storms developed in north central Texas producing several tornadoes. The morning sounding is similar to the first case, only the cap is stronger, about five degrees C at 800 mb. Deep convection was suppressed all day. By mid-afternoon, the cap index was +1. An upper short wave approached and one area of the cap broke sending a storm explosively into the sky.
STRONG CAP - BUST: On May 7, 1988, a very strong cap existed. Note the shallow layer of moisture, about 80 mb. Strong increases in surface temperature and dewpoint throughout the day were futile in breaking the cap. By mid-afternoon, the cap index was still +8. It was blue sky, no storms would develop.