General Information Notice Definitions

by Ron Gravelle VA3TVS

Updated March 23, 2001

Slight, Moderate and High Risk Days

SLIGHT risk implies well organized severe thunderstorms are expected but in small numbers and or low coverage. Expect a moderate probability of between 5-29 reports of 1 inch (25mm) or larger hail, and/or 3-5 tornadoes, and/or 5-29 wind events, OR low to moderate probability of this outlook being upgraded to a moderate or high risk issued later in the day/evening.

MODERATE risk implies a greater concentration of severe thunderstorms, and in most situations, a greater magnitude of severe weather. Many weather offices will include the phrase
"some thunderstorms may be severe". Environment Canada will include "remember some severe thunderstorms produce tornadoes". This category includes a high probability of at least 30 reports of 1 inch (25mm) or larger hail, or over 30 reports that might be associated with a squall line or 6-19 tornadoes.

HIGH risk is only issued a few times per year, because of rare circumstances, almost always means a major severe weather outbreak is expected. Greater coverage of severe weather with an enhanced likelihood of extreme severe (includes the possibility of violent F4+ tornadoes or extreme convective wind events over a large area). A high probability of at least 20 tornadoes with at least two of them rated F3+, or a derecho (powerful squall-line) causing 50+ widespread wind damage events with numerous high end straight line wind (130km/h) and or structural damage reports.

Information courtesy of Storm Prediction Center (SPC)  
1313 Halley Avenue, Norman, Oklahoma, 73069

Day 1 Convective Outlook
Day 2 Convective Outlook

Ron Gravelle VA3TVS
Waterloo Region

Kitchener-Waterloo Amateur Radio Club Inc.
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last updated    December 30, 2002 12:55