Hi Whisper,
A slight risk for severe weather means that the conditions in and near the risk area haven't quite come together to create a higher category. Usually an area of the country under a slight risk will range from 5 to 25% chance that it might happen and with the worst area being closet to the center of the risk area.
A moderate risk for severe weather means that the temperature, dew points, humidity, and winds are moderately favorable for the development of severe thunderstorms. The percentage of a moderate risk ranges from 25 to 75% area coverage in and near the moderate risk areas. We watch specifically for Moderate risk because with this type of pattern the severe weather ingredients are coming together faster than a slight risk area and this same area has the potential to move into a high risk area without much more change in the current atmospheric conditions.
A high risk area (75 - 100% chance of severe weather within a specified region) means all the ingredients to produce thunderstorm cells and super cells is already in place and can turn into tornadic storms at any time or place. Usually this area will sit inside the moderate risk area and looks like a bullseye on a weather map. On a high risk area we already have our gear loaded, our jump bags in the cars, trucks & suv's, and are waiting to take off once a meso scale thunderstorm pops up. Meso scales are super cell thunderstorms with tremendous power and are the cells that produce the tornado's.
There is one other classification that is used by the Storm Prediction Center in Norman OK. This is a PDS area. PDS stands for Potentially Dangerous Situation. If you see a PDS area on a weather station and it's on or near your location be prepared to take immediate action if your local weather office, tv or radio station says so. A PDS system may cover 100's of square miles. These are the bad boys of the severe storms. They can and do produce hail the size of softballs, multiple tornados and what we call Twin Sisters which is one tornado with a smaller tornado coming out of the same cloud as the larger one.
Here are 2 sites that we use non stop when we track severe weather. They are for public use so there is no copyright or trademark violations in posting them:
Storm Prediction Center - Norman OK
<A HREF="
http://www.spc.noaa.gov/products/" TARGET=_blank>
http://www.spc.noaa.gov/products/</A>
And literally my weather site which receives over 1,000,000 visitors and users per month. It's free and anyone here may bookmark it if you'd like. This is not an advertisement for my weather site either...this site is totally free of any advertising and I do not accept advertising to keep it running.
<A HREF="
http://www.fallsky.com" TARGET=_blank>http://www.fallsky.com</A>
Any other questions? I'm happy to help anyone with storm fears. One note, if you've seen the movie Twister? Believe about 10% of what that movie shows. It's a movie for the box office not much for the science or real live tornado chasers.
David
"May God grant us the wisdom to discover right, the will to choose it, and the strength to make it endure."