The probability of severe hail, which is hail over three-quarter inch in diameter, is known as POSH, and is displayed as a percentage within the POSH column on the storm attribute table.

The probability of hail, which could be any size, is known is POH, and is also displayed as a percentage on the storm attribute table.

Both POSH and POH are determined for each thunderstorm on radar as a whole, given they have a cell identification code.

Cells a great distance from the radar site, as well as those which have just developed, may have an unknown, or undetectable, probability. Check the next scan as to if the unknown probability has been resolved, as it may. If not, try using a different radar site that the storm cell may be closer to. Thunderstorm cells that have an unknown probability also have an unknown maximum hail size. Either all hailstone-related information is present, or none of it is.

Hail algorithms are basic given the radar is supplied with the height of the freezing level (0° C) and the placement of the -20° C layer in the atmosphere. Therefore, the hail probabilities and estimates can be misinterpreted by a user who does not have a great deal of experience.

Different atmospheric conditions can alter the size of hail. As a result, radar operators sometimes have their own techniques to determine hailstone size and probabilities.

Look for the largest hailstones and best chances of it under areas of high reflectivity on the composite radar scan.

A radar cannot detect the probability of general hail and severe hail well, or possibly at all, if a thunderstorm is close to the radar site. Beams are unable to extend into the updraft at such a degree from the radar's transmitter.


Jordan Gerth, May 2007