Severe Weather Watcher Handbook

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57-59 N, NW & N/SE  Series showing the approach of a supercell. From a distance (57), this storm has a long, streaming anvil. Although it may look irregular (suggesting weakness) it has almost no pulse spacing and the ruffled edge is part of the extreme growth being blasted by strong upper winds. When the storm is nearer (58) you can see the organized structure: a large inflow band lower left and lines along the flank. The dry airmass prevents a full lowering, but scud from the rain is converging at the main updraft. In 59, the inflow-outflow axis is almost overhead. An older core (distant) will soon be replaced by a strong new phase (now underway, centre) which later dropped baseball hail. It is tempting to see the bright area (lower left) as a clearing edge - but there, a curtain of golfball hail was falling into the sunshine! The storm was a cyclic supercell (supercell phases interrupted by briefly weaker ones) with periods of rotation and high wall cloud structure, but always with total control over outflow.

When the storm has arrived

Once it is raining and clouds have engulfed the area, you must rely on the storm's behaviour to determine its next move. Three things to pay attention to are the precipitation,wind,and lightning/thunder. The expected precipitation pattern for most supercells is a steady transition from lighter anvil rain to heavy rain, and finally, rain mixed with or changing to hail. If this occurs, hailstones will become larger and less frequent,then end just as the lowering arrives.

Always check for a lowering to the W-SW during very large hail. If rain (or small hail)begins as brief, separate showers or a sudden cloudburst when the rain curtain arrives, the system is more likely propagating forward and this particular storm will wind down without any other severe effects. Watch the wind direction. If the wind shifts to the W or N (after being S) and persists, outflow predominates and there's no immediate concern. A calm wind, after SE-SW warm winds ahead of the storm, always suggests updrafts.Unless the gust front is just about to pass (very brief calm), the main updraft is overhead and you should check the sky for a lowering. Once winds shift to the W-N and remain so, most of the danger will have shifted to your east.

60  W/E,  61  W/E   Comparison of a disorganized stormy sky with a potentially severe one. In 60, thickening anvils and a scattering of lower clouds herald distant showers. There is no single focal point here. The dark cloud bank (centre) has some new convection but no inflow structure or lowerings. This scene is typical of many weaker cold fronts. But in 61, a severe storm has imposed control over the flow. A smooth, high anvil (seen top left) spreads forward from the black, distant core where a sharp boundary from rain to inflow exists. Note the inflow bands, especially the thick one at top, heading into the storm. New updrafts are concentrated along the forward side and at the SW end (bottom, centre). When the sky is drawn together like this, it is usually a severe situation.

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