Severe Weather Watcher Handbook

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A severe, worst-first storm can have large hail, high winds and an occasional brief, weak tornado. When persistent downbursts create a long-lived damaging wind complex affecting hundreds of kilo-metres it is known as a derecho storm. Another special variation on the worst-first type seen in hot, humid conditions with SE winds is a high-precipitation supercell, or "HP supercell ". This is a large, dangerous storm with very heavy precipitation both ahead of and behind the updraft region. The wall cloud, in this case, is shifted forward and is on its east side with rain behind it, but it may not be clearly visible due to nearby showers, low cloud bases or a very dark sky. HP supercells are most common in the Central and SE U.S. They are occasionally seen in Ontario and Quebec, but rarely farther west.

62  W/SE,  63  NW/SE   Two severe worst-first storms (HP supercells). This wide view (62) shows a massive forward flank along which warm air is rising ahead of an intense precipitation core. The strongest updraft is marked by a pronounced wall cloud, which is continually reforming southeastward (left and forward) as outflow pressures the inflow. Above the shelf cloud, the higher anvil material fans forward and blows NE. In 63, a very broad, dark forward flank can be difficult to decipher. Strong SE inflow winds ahead of the core (note inflow bands top right) meet the rainwall along a curved boundary (bottom). Here, inflow is turned to the right around the storm's wide mesocyclone and rises abruptly at lower right. The lighter sky (bottom right) is a hail curtain from this updraft, but there is even heavier precipitation at the back side of the mesocyclone, which is fed by a second inflow region lower left (dark base). That location had occasional dust plumes from microbursts and would be the site for any wall cloud formation.

When worst comes last

There is a certain logic to a worst-last experience . It feels so normal for a storm to begin gradually and wind up to a climax. That's because it happens regularly every summer with ordinary thunderstorms. But in the rare case when it is a severe storm, the worst-last scenario is much more dangerous. It may end with a tornado!! In worst-last storms there is a slow but steady increase in everything . The darkness of the clouds, the lightning and thunder, and the rain. As you watch these changes, keep tabs on how quickly the storm is approaching- some spring tornadic storms travel at 50-80kmh (30-50 mph)! When the rain begins it is light or moderate, but steady. The thunder is long and rumbly, but you may hear an occasional distant bolt too. Winds may be a bit gusty at first (during the rain),and they may turn briefly to the northwest or north, but they become lighter again soon after.

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