Severe Weather Watcher Handbook

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5  E/NE  This core close-up shows very heavy rain and hail on the left, right beside strong updrafts in the smooth, black rain-free base. The abrupt transition from heavy precipitation to none suggests a well-organized storm.

The core is the active part of a storm where a sustained, strong updraft/downdraft combination produces heavy precipitation. Underneath it we see a rain curtain while above it, the tallest part of the cloud can be found. The downdrafts, or descending currents of air, originate in the falling precipitation, while the adjacent updrafts draw on nearby warm air and can be seen as a low, dark cloud base. This dark, flat cloud base usually extends away from the core to the S or W. It is called the rain-free base because no rain shafts are visible under it (all the air is rising). The rain-free base is right beside the precipitation region and is a part of the longer flanking line, a line of developing cumulus towers that lead up to the core.

Storms that survive more than an hour maintain a loose balance between the warm inflow air and the cool outflow. Downdrafts spread out at the ground to form a pool of cool outflow air on the NW-NE side of the storm. Meanwhile, the unaffected warm inflow air ahead of or on the SW-SE side of the storm feeds the new updrafts, which then grow into large towers that join the existing storm to keep it going.

Somewhere under the storm,warm and cool air meet. This inflow-outflow region can shift around over time and varies greatly from one storm ystem to the next. Outflow can spread forward far ahead of the core when surface winds are blowing roughly in the same direction as the storm motion. The outflow can also move with the storm, arriving abruptly as the core passes, or it can be left in the cloud's wake as a cool breeze flowing away from it.

6  NW/E  A sculpted gust front precedes a heavy rain core. The steady updrafts (dark baseline) suggest forward regeneration in small steps, since the inflow-outflow boundary under the storm is not separating from the rain.

Wherever this cool air meets the warmer air, you will notice low clouds, scud, bumps along the main base, etc. which indicate uneven mixing and irregular condensation. The cool air is also moist (having evaporated some of the rain) so when it is drawn back into the updrafts, it condenses below the main storm base to form a lowering. What you see here and how it changes can tell you exactly how the storm is doing and whether a risk of severe weather exists. The characteristics of severe storms are discussed in detail in the What to watch for section.

7 S/E A mixed sky of light and dark elements can be hard to assess.A thick,dark flanking line extends from a storm to the left.Chunks of scud in the cool outflow behind it are moving quickly to the right.They can look both light,when frontlit against a dark higher cloud,or dark,when backlit.

Cool outflow air, when held in check, helps a storm to survive by providing just enough "push" to the warm inflow air. When outflow becomes dominant, the cool air surges forward, undercutting the inflow. The leading edge of this advancing outflow boundary is the gust front, seen as a line or layer of low clouds moving quickly forward. This is the shelf cloud, sometimes called an arcus, which often has a smooth, sculpted forward surface but turbulent, choppy clouds behind the leading edge.

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