Severe Weather Watcher Handbook

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The next important change comes when the rain becomes heavy, possibly termittently, and mixes with or changes to hail. This is a warning that the updraft region is getting nearer and you may now see a more concentrated darker area of cloud coming toward you. In very hot, humid conditions, large quantities of rain and hail may fall; but if you are in line for the storm's core, the rain eventually decreases and changes over to mainly hail. If the hail becomes larger and larger it will also end soon - but this is no time to become complacent! You are right at the strongest part of the storm, where most of the air is rising above you. Although the cloud base is black, you can probably see brighter sky beyond it. If a tornado risk does exist, a wall cloud will show you where the rotation is. The air will be dead calm for this last few minutes. After the darkest clouds pass, the sky brightens rapidly - often becoming completely clear - and winds freshen from the SW-W. The severe version of a worst-last storm is also a supercell type, and is either called a "classic supercell" (the standard supercell type) or an "LP (low-precipitation) supercell" - a dry-weather, high wind shear version. An LP supercell is characterized by a very long anvil, a small core region, a wall cloud at the back, and a thin precipitation curtain that usually lets some light through. Tornadoes can occur, but the main severe threat is usually large hail. The classic supercell has a larger, darker appearance and accounts for most of the major tornado events everywhere. Many spotters have loosely adopted the LP and HP terms as a quick way to differentiate all severe storms.

64  N/E,  65  W/NE   Two severe worst-last storms (Classic and LP supercells)
Most of this storm (64) has passed by to the NE, but at the very back (SW end) is a compact updraft which has created everything else before it. A disorganized lowering is seen under this base, while above it the anvil flares out. Outflow from rain is helping to guide warm air toward the updraft along an inflow bank (bottom).
An extreme example of a worst-last storm can be seen in an LP supercell whose small, severe core survives desptie strong winds aloft due to a very buoyant updraft. In this wide view (65), warm air converges on a small updraft base with almost no flanking line. The air then rises in a stream of closely-packed towers which are tilted sharply forward aloft. Precipitation (mostly hail) is sparse, letting lots of light through. The rotating updraft column has a small wall cloud below it (bump on base, bottom left).

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