Common Severe Weather Watches and Warnings Questions:

What's the difference between a watch and a warning?
A watch means severe weather is possible during the next few hours, while a warning means that severe weather has been observed, or is expected soon.

What is a Severe Thunderstorm or Tornado Watch?
A Severe Thunderstorm Watch outlines an area where an organized episode of hail 1 inch diameter or larger and/or damaging thunderstorm winds are expected during a three to eight hour period. A Tornado Watch includes the large hail and damaging wind threats, as well as the possibility of multiple tornadoes. Typical watches cover about 25,000 square miles, or about half the size of Iowa.

Why are watches not issued for all severe storms?
Many severe thunderstorms affect only a small area for a short period of time, making watches impractical. Watches are issued primarily for areas where well organized or significant severe weather is possible, or the severe weather threat is expected to persist for many hours.

What is the watch status line (line with an arrow inside a watch) and how is it interpreted?
When a watch is issued, the original threat is contained within the entire watch area. When the SPC determines where the severe weather threat continues within a severe thunderstorm or tornado watch, usually on an hourly basis, we issue a watch status message. Unless the whole watch area remains valid, the status report includes points on a line that indicates where the severe weather threat remains from our perspective. The severe weather threat is to the right of that line, as in our outlooks. As adjustments are made to the watch, you will see that yellow "status" line shift across the original watch area. This gives emergency managers, weather forecast offices, media and the general public guidance as to where the threat remains for severe weather.

How does the National Weather Service define severe weather?
There are many forms of hazardous, or severe weather. The primary mission of the National Weather Service is to provide forecasts and warnings for the protection of life and property. Forecasts can range from the climate and seasonal outlooks for agricultural interests provided by the Climate Prediction Center (www.cpc.ncep.noaa.gov), to the daily county forecasts for the public issued by local NWS offices. The NWS issues warnings for heavy snow, freezing rain, high winds, flash flooding, river flooding, thunderstorms, tornadoes, tropical storms, and hurricanes. In a broad sense, these are all defined as severe weather as any of them can and do pose a threat to life and property.

How does the National Weather Service (NWS) define a severe thunderstorm?
The term severe thunderstorm refers to a thunderstorm producing hail that is at least quarter size, 1 inch in diameter or larger, and/or wind gusts to 58 mph or greater, and/or a tornado. Although lightning can be deadly, the NWS doesn't use it to define a severe thunderstorm. If it did, every thnderstorm would be severe, by definition. Also, excessive rainfall may lead to deadly flash flooding, but heavy rain is not a severe criterion either. The flood threat is handled through a separate set of watches and warnings from your lcoal NWS forecast office.

Courtesy of the National Weather Service