Significant weather advisory
A significant weather advisory (alternately termed as a "significant weather alert" or a "special weather statement", the terminology varies depending on the local National Weather Service Weather Forecast Office) is issued when Doppler radar indicates a strong thunderstorm is producing small hail or high winds whose strength does not reach the criteria to be designated a severe thunderstorm. It does not necessarily account for lightning or flooding.
This alert product was created in the early 2000s, and is sometimes a precursor to a severe thunderstorm warning. Many versions of the First Warning weather alert system used by broadcast television stations have begun including significant weather advisories into their systems, however, these systems usually classify such an advisory terms such as "Heavy T-Storms", "Heavy Storms" or "Strong T-Storms" (terms used by the system prior to the inclusion of the product).
The National Weather Service in the United States issues a significant weather advisory for strong thunderstorms (and occasionally, other weather situations such as downbursts) that may pose limited to no threat to life and property; it is typically issued if the hail size in a thunderstorm is less than one inch (2.5 cm) in diameter, and/or winds less than 58 miles per hour (90 km/h). The product is mainly used by forecast offices located in the National Weather Service's Southern Region Headquarters.
A significant weather advisory or warning is not an official National Weather Service product, instead it is issued within a special weather statement. Special weather statements are also used for any type of weather approaching watch/warning/advisory levels. Many NWS offices do not use significant weather advisories, instead issue normal special weather statements mentioning much of the same text used in such an advisory.
If severe weather is detected, alerts (mainly a severe thunderstorm or tornado warning) are issued by the National Weather Service (effectively replacing a significant weather advisory), and the Emergency Alert System may activate and broadcast the alert. EAS alerts begin with a SAME header, which sounds as three digital bursts and automatically activates the EAS, a 1050 Hz attention signal, the alert as processed by text-to-speech software, then the SAME End of Message tone to end the alert broadcast. Also, sirens may be activated to warn the public who are not listening to media to take cover.
Example of a significant weather advisory
SPECIAL WEATHER STATEMENT NATIONAL WEATHER SERVICE NORMAN OK 1251 AM CDT THU JUL 14 2011 OKZ015-140600- DEWEY OK- 1251 AM CDT THU JUL 14 2011 ...SIGNIFICANT WEATHER ADVISORY... THIS SIGNIFICANT WEATHER ADVISORY IS FOR DEWEY COUNTY. AT 1251 AM CDT...A STRONG THUNDERSTORM WAS LOCATED OVER CAMARGO... MOVING EAST AT 15 MPH. HAZARDS INCLUDE... HAIL UP TO ONE-HALF INCH... WIND GUSTS TO 50 MPH... MINOR FLOODING IN AREAS OF POOR DRAINAGE... SEVERE WEATHER IS NOT EXPECTED AND NO WARNINGS ARE ANTICIPATED AT THIS TIME. LAT...LON 3613 9899 3593 9900 3595 9934 3613 9933 $$ WR