NWS Testing Tiered Warning Levels for Tornadoes, Severe Weather

The National Weather Service has started testing a new tiered warning level system that will be included in all of their tornado and severe weather warnings, The testing and feedback period began April 2 and will run through November 30. The change will hopefully make the public better aware of the significance of a warning and to better convey the urgency of the situation during severe weather periods. Below is an excerpt from the press release.

"The Tornado Warning (TOR), Severe Thunderstorm Warning (SVR), and any Severe Weather Statement (SVS) issued as a follow-up to a TOR or SVR, are alphanumeric products issued by NWS offices to provide short-fused warning information on hazardous conditions associated with intense thunderstorms which pose a threat to life and/or real property. In the case of the TOR, the warning forecaster also believes there is compelling evidence the subject storm will spawn one or more tornadoes. These products are prepared by each National Weather Service (NWS) Weather Forecast Office (WFO) for their County Warning Area (CWA) of responsibility.

The TOR is based on scientific evaluation of atmospheric conditions, or an actual observation, indicating tornado development is imminent or occurring. Hazards associated with severe thunderstorms are nearly always attendant to the tornado threat as well – those hazards being winds gusting to 58 mph or greater, and/or hail of one inch (1”) diameter or greater. The SVS is a “follow-up” statement which provides updated information as to the status of storms within a TOR (or SVR) warning area.

This product enhancement attempts to take a positive evolutionary step toward enhancing risk communication and focusing on impact-based information in NWS warnings. This represents the next logical step to address recent service assessment findings (see Purpose section below) and builds on the successful use of coded tag lines which have been appended to Central Region warnings since 2010. For this experiment, the warnings themselves will be stratified into categories which distinguish extreme cases from the base convective warnings. Additional enhanced wording will be included to convey information about associated impacts, specific hazards expected, and recommended actions - both within the bullet statements and as part of the tag line codes. Use of tags to explicitly include severity to prompt faster risk assessment by key users and partners also represents a partial migration to CAP compliant formats (minus a certainty expression).
For severe thunderstorms (SVR), forecasters will be presented two options within the WarnGen GUI:

1. a storm (or line of storms) may be identified as a traditional, baseline SVR threat, or
2. cells of interest may be identified as extreme storms, i.e., those for which forecast hail is baseball-size or larger (>2.75") and/or wind gusts are predicted to exceed 80 mph (approaching low end EF1).

* Invoking the latter choice will trigger inclusion of enhanced verbiage concerning impacts and calls to action. It will also insert a highlight statement such as "This is a very dangerous storm". Finally, the option to include a tag line which explicitly states "TORNADO POSSIBLE" will be available. This second-tier category with messaging which highlights elevated danger may be selected during the issuance of an original warning (SVR) or in a follow-up statement (SVS).For tornado warnings (TOR), forecasters will be provided three options. The standard or “base” tornado warning represents the first option. The chief improvements for these base warnings – likely the most common type issued through the season – will be bullets which plainly and clearly communicate hazard and impact information, calls to action rephrased by social science partners, and tags which identify whether the tornado is observed or radar-indicated (implied statement of confidence in evidence), predicted hail size, and the option to add strength of non-tornadic thunderstorm-related wind. This warning type will be selected for cases in which there is credible evidence of a tornado.

The second level of tornado warning – one for which there is substantial evidence of a significant tornado coincident with a high impact event – will include the phrase "This is a Particularly Dangerous Situation" and incorporate enhanced wording within the second warning bullet to identify a high level of risk, dramatic description of expected damage and impacts, and promote serious urgency in taking action to seek shelter immediately.The "PDS" warning will also append an explicit damage threat reference in the form of a tag line code "TORNADO DAMAGE THREAT...SIGNIFICANT", rather than simply discriminating between observed or probable.

The third and highest level of tornado warning will be reserved for those rare cases in which a known, violent tornado is approaching which is likely to experience devastating damage - events such as Joplin. For these situations, the enhanced wording will include a "TORNADO EMERGENCY" announcement, the recommended action will be brief, clear and extremely urgent (e.g., IF YOU ARE IN OR NEAR LIBERTY...SEEK SHELTER IMMEDIATELY!), and the tag line will read: TORNADO DAMAGE THREAT...CATASTROPHIC.
* - These are the default thresholds but they may be configured locally.
 

The allowable values for hail and wind tag lines are coded and defined as follows:
Tornado Values for TOR and TOR related SVS:
TORNADO…RADAR INDICATEDTORNADO…OBSERVEDTORNADO DAMAGE THREAT…SIGNIFICANTTORNADO DAMAGE THREAT…CATASTROPHIC

For SVR/SVS the tornado tag line typically will not be appended. However when conditions warrant the option to identify possible tornado formation will be available in the form of the following tag:TORNADO…POSSIBLE

Hail Values for TOR/SVR/SVS
(Flexibility is given to local offices to add other events with 0.75 being the lowest allowable specified value, other than “no hail” and “smaller than three-quarter inch” values. The lowest value of hail size considered to meet “severe thunderstorm” criterion is 1-inch in diameter.):
0.00 Equates to no hail<.75 Small hail expected0.75 0.75 inch hail (penny-sized)0.88 0.88 inch hail (nickel-sized)1.00 1.00 inch hail (minimal SVR criterion for hail size)1.50 1.50 inch hail (ping pong ball-sized)1.75 1.75 inch hail (golf ball-sized)2.50 2.50 inch (tennis ball-sized2.75 2.75 inch hail (baseball-sized)4.25 4.25 inch hail (softball-sized) or larger and is the highest allowable value

Wind Values for TOR/SVR/SVS
Dimensions are in MPH. Flexibility is granted to add other events in 5 mph increments, but default template value choices are:<50 Wind gusts below severe criteria and lowest allowable value (for SVR hail only)60 60 mph peak wind gust and is the first allowable value above <50 (severe criteria)70 Used for warnings where wind is expected to be GTE 70 mph but LT 80 mph80 Used for warnings where wind is expected to be GTE 80 mph but LT 90 mph90 Used for warnings where wind is expected to be GTE 90 mph but LT 100 mph100 100 mph or higher peak wind gust and is the highest allowable value (significant structural damage)"

So the next time you see a warning, please note the severity level within the message. I "think" that I've completed my tornado page updates (for now). So for your viewing pleasure...

• Tornado Graph Slideshow
• Georgia Tornado Graphs and Maps
• US Annual Tornado Maps

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