Weather 101 '" Other Localized Thunderstorm Patterns

In summer, when weather conditions are favorable, thunderstorms form over mountain areas earlier and more often than they do over nearby valleys or flat lands. This is linked to the fact that sun heats the slanted terrain facing it faster (more direct rays) and that an elevated heating source fuels convection (rising air currents) more easily. Such a pattern is common across most of the interior western United States in summer (Figure 1). It also happens along the Appalachian ridges in the eastern U.S. to a lesser degree (the mountains are older and smaller and have gentler slopes).

Urban centers are also know for their "heat island effect." This should give cities a slight advantage when it comes to supporting thunderstorm formation. However, daily wind patterns may transport some of that heat downwind from the urban center, fueling storms nearby, especially when linked to localized convergence zones (e.g., lake or sea breezes).

Since heating at lower levels can fuel thunderstorms, then forest fires and even erupting volcanoes can spawn thunderstorms and many times have done so. Believe it or not, sometimes fire-induced storms can spark lightning that ignites new fires.

And almost anywhere, there are differences in land cover due to vegetation variations, land use options and even whether or not it rained the day before.

All of these and more act to focus where thunderstorms may develop.

Outflow Boundaries

Thunderstorms can also interact with one another to enhance or destroy new thunderstorm development. This is linked to thunderstorm outflow boundaries.

Think back for a moment to one or more memorable thunderstorm events. These don't have to be severe storm events. In one or more of these, you probably experienced a rush of chilly winds blowing outward from the thunderstorm. These winds probably preceded the storm's rainfall (maybe even its thunder) by several minutes. The phenomenon is more common in drier locales in the western U.S.

This outrush of winds is linked to what meteorologists call a downdraft or in more severe cases a downburst or microburst. While updrafts and rising air are linked to the formative stages of a thunderstorm, sinking air starts to dominate during its mature stage and overpowers the storm towards its demise. As air sinks, it eventually reaches the ground and has no choice but to spread out in all directions. If the winds from higher altitudes are blowing strongly, that air motion is carried downward to become part of the outward flowing air. Similarly if a storm is moving in a particular direction at a certain speed, the outflowing air is more likely to move in that direction. These outflow features are often referred to as outflow boundaries, but can also be likened to miniature cold fronts (places where colder air displaces warmer air).

If you want to see this effect, just take a few cups of water outside. Pour one cup quickly onto a more or less level sidewalk or driveway. You should see a star pattern. Follow this nearby with water poured quickly at an angle or with a push in a particular direction. The star pattern becomes skewed in the direction of the push. If you want to view your experimental activities longer-term, use cornstarch, not water, to represent the downward moving air.

Such outflow boundaries, especially from individual or lines of thunderstorms, can often be seen on satellite and radar imagery. They usually take the form of curved arcs (resembling a backwards letter C) with the heaviest or most intense thunderstorms along the arc (Figure 2). Sometimes the outflow can race out ahead of the thunderstorms and be marked by a thin curved line on a radar display. If two outflows meet (or an outflow intersects a sea breeze, convergence is enhanced and new storms can develop or existing storms can be re-energized. The bowing backwards C-shape shows the push of the winds radially outward from the thunderstorm.

If the storm just produces its own outflow, then the storm rains itself out, leaving just some mid- and high-level clouds that quickly dissipate. What remains is a more or less circular, cloud-free ring. The dying thunderstorm cools the lower atmospheric levels, stabilizing the air and preventing new thunderstorms from forming for several hours or more (Figures 3 and 4).

Larger scale storm outflow boundaries can be linked to derechoes (large scale, long-lasting outflow boundaries linked to clusters of intense and/or severe thunderstorms). This radar image (Figure 5) shows a derecho with a leading outflow boundary that had lasted most of the day.

And, of course, any weather front or left over outflow or other boundary can become a focusing mechanism for thunderstorms at almost any time (even at night).

What all this means is that thunderstorms don't just form randomly. They form because of meteorological, geographical and other factors. And while meteorologists can't always forecast their onset, we are getting better at it. Meteorologists now have some skill in forecasting the nocturnal development of mesoscale convective systems (MCSs) that can lead to derechoes. They are also better able to key on how thunderstorm outflow boundaries can interact with sea breezes.

So, the time you hear a forecast that talks about "pop up afternoon thunderstorms" or "hit and miss showers," you'll know that the eventual development and movement has some type of basis other just randomness.