Types of Thunderstorms

Thunderstorms are usually classified as frontal or air-mass thunderstorms.

Frontal Thunderstorms

Frontal thunderstorms are caused by warm, moist air being forced over a wedge of cold air. This lifting may occur with warm fronts, cold fronts, or occluded fronts.

• Warm-front thunderstorms are usually embedded in large stratiform cloud masses. They are likely to be the least severe of frontal thunderstorms because of the shallow slope of the warm-front surface. Surface wind conditions, in the cold air wedge beneath the warm front, may be unaffected by the thunderstorms above.
• Cold-front thunderstorms are generally more severe and occur in a more-or-less continuous line. Their bases are normally lower than those of other frontal thunderstorms.
• Thunderstorms occurring along a squall line are similar to those along a cold front, but may be even more severe. Heavy hail, destructive winds, and tornadoes are usually associated with squall-line thunderstorms.
• Thunderstorms are often associated with a warm-front type occlusion. In this case, they occur along the upper cold front and are set off by the lifting of the warm, moist air. They are usually more severe than warm-front thunderstorms and less severe than the cold-front type.

Air-mass Thunderstorms

Air-mass thunderstorms are unaffected by frontal activity. They are usually scattered or isolated. Air-mass thunderstorms may be further classified as convective or orographic, although these lifting processes often act together.

• Convective thunderstorms formed by convergence may occur day or night, but they tend to be most active in the afternoon. Those produced by instability resulting from advection of low-level warm air or high-level cold air may also occur day or night. The nocturnal, or nighttime, thunderstorm, which is common in the Midwest during spring and summer, is usually due to low-level warm-air advection and convergence. These storms are among the most severe found anywhere.
• Orographic thunderstorms develop when moist, unstable air is forced up mountain slopes. They tend to be more frequent during the afternoon and early evening because heating from below aids in the lifting process. Storm activity is usually scattered along the individual peaks of mountain ranges, but occasionally there will be a long unbroken line of thunderstorms.

High-level or Dry Thunderstorms

One type of air-mass thunderstorm, the high-level or dry thunderstorm, deserves special consideration because of its importance in starting wildfires. The lifting process may be orographic, convergence, cold-air advection aloft, or a combination of these, often aided by surface heating over mountain ranges. High-level thunderstorms occur most frequently in the mountainous West during the summer months.

Their distinctive feature is that their cloud bases are so high, often above 15,000 feet, that precipitation is totally or mostly evaporated before it reaches the ground. As a result, lightning strikes reaching the ground frequently start fires in the dry fuels. The downdraft and outflow usually reach the ground even though the precipitation does not. The cold, heavy air is generally guided by the topography into downslope and downcanyon patterns, but crossslope flow may also occur.

There are two principal weather patterns which produce high-level storms. One is the inflow of moist air, usually from over the Gulf of Mexico but occasionally from over the eastern subtropical Pacific, at levels of 10,000 to 18,000 feet. Thunderstorms are set off by lifting over mountains, and by heating and upslope thermal winds at higher levels in the mountains, as the moist air spreads northward from New Mexico, Arizona, and southern California. These storms usually develop in the afternoon and may extend into the evening hours.

The second important weather pattern in high-level storms is the cold Low aloft. With this pattern a closed low-pressure system aloft becomes cut off from the main belt of westerlies. The cold air within this closed Low produces instability and causes convective currents to develop. If sufficient moisture is present, thunderstorms will form. They can develop at any time of the day or night, but are most active in the afternoon when they are assisted by daytime heating. The movement of a closed upper Low is erratic and very difficult to predict. The Low may move in virtually any direction, may deepen or fill, or may be picked up by a trough moving eastward at a higher latitude.

The Far West is a favorite place for closed Lows to develop. They may meander around for several days or a week before finally dissipating or moving on.