Changes to severe weather products coming, including Impact-Based Warnings

There are several recent and upcoming changes to severe weather products issued by both SPC and local National Weather Service (NWS) offices, including Memphis, that you need to be aware of.

SPC Convective Outlooks (effective date: October 2014)

We've talked about the changes made last October to SPC's Day 1-3 convective outlooks in a previous blog. To recap, the three-tier system of Slight, Moderate, and High risk categories has become a five-tier system, adding Marginal and Enhanced categories on either side of a Slight risk. We invite you to read the linked blog above for details on these changes. We'll pair the 5-point numerical value of the forecast category when discussing the risk for the Memphis metro to help eliminate confusion associated with the new names.

In addition to the Day 1-3 outlooks, the Day 4-8 experimental outlooks issued by SPC previously only displayed areas with a 30% probability of severe weather during this period. In December 2014, these outlooks were also modified to display areas under either a 15% or 30% risk of severe storms, which provides meteorologists and the public with a heads up to lower risk severe weather areas in the day 4-8 period that weren't previously highlighted.

Impact-Based Warnings (IBW) (effective date for Memphis: April 1, 2015)

While SPC is responsible for nationwide issuance of Tornado and Severe Thunderstorm Watches, local NWS offices issue the warnings for their area of responsibility. Starting April 1, the Memphis office of the NWS will join many other offices across the nation in an experimental change to the format of Tornado and Severe Thunderstorm Warnings that began a couple of years ago with a handful of offices in the central U.S.

Areas where impact-based warnings will be used in 2015

Called "impact-based warnings," the format change was made in response to the findings of a service assessment done after the Joplin, MO tornado in May 2011. That assessment found that "credible, extraordinary risk signals prompt people to take protective actions." One of the intended outcomes of IBW is to motivate proper public response to warnings by distinguishing between low and high impact events. In addition, it allows the warning system to be optimized within the existing infrastructure, realigning the message in terms of societal impacts, thereby communicating recommended actions more concisely.

The changes will have no effect on the mechanics of warning polygons, but will provide more detailed hazard and action information within Tornado and Severe Thunderstorm Warnings. That detail will include the impacts the severe storm could cause, which will be reflected in the updated format of the warning bulletins. The changes include the addition of "event tags" at the bottom of each Tornado and Severe Thunderstorm Warning. These tags include "Tornado" and "Tornado Damage Threat," which are described below. In addition to those tags, Severe Thunderstorm Warnings will carry maximum hail size and wind speed tags. Tornado Warnings will also carry a maximum hail size tag.

The Tornado tag will have three possible options:

• "Possible" - Reserved for Severe Thunderstorm Warnings, indicating that the storm has some potential to produce a tornado, but forecaster confidence is not high enough to issue a Tornado Warning.
• ""Radar Indicated" - Evidence on radar and near storm environment is supportive of a tornado, but there is no confirmation.
• "Observed" - A tornado is confirmed by spotters, law enforcement, etc.

The Tornado Damage Threat tag will also have three options:

• No tag - Used most of the time when tornado damage is possible, but the tornado is expected to be short-lived.
• Considerable - Used rarely, when there is credible evidence of an ongoing or imminent tornado capable of producing considerable damage. Tornado is expected to be long-lived.
• Catastrophic - Used exceedingly rarely, when a severe threat to human life and catastrophic damage is occurring. Only used when reliable sources confirm a violent long-lived tornado. This tag will trigger the "Tornado Emergency" warning.

The example below summarizes the possible tag options and also shows the Impact bullet in the warning that describes the threat to life and property.

Some additional examples of warnings using the new tags are shown below (changes are highlighted in red).

Severe Thunderstorm Warning with new tags.

Tornado Warning for radar indicated tornado.

Tornado Warning for an observed tornado producing considerable damage

Tornado Emergency for an observed tornado producing catastrophic damage.

More information from the NWS on the 2015 IBW Demonstration is contained within the YouTube video below created by NWS-Memphis and on this website.

Severe weather watches add "Summary" section (effective date: April 14, 2015)

Finally, beginning April 14, all Tornado and Severe Thunderstorm Watches issued by SPC will contain a "Summary" section. According to SPC, the summary will provide a "general 1-2 sentence statement of the severe weather expected in and close to the watch area. This new section facilitates consistent, forecaster-driven, concise communication for public consumption." For more information on this change, please see this page from the SPC.

Erik Proseus
MWN Meteorologist

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