A warning is a structured document, not a paragraph. Read the structure and you can see which parts came from radar, which came from a person, and which line your own report would land on.

In this lesson you will

  • Parse a tornado or severe thunderstorm warning: VTEC line, polygon, HAZARD/SOURCE/IMPACT, and storm motion.
  • Explain Impact-Based Warning tags, and how "radar indicated" becomes "observed."
  • Distinguish a tornado warning, a PDS tornado warning, and a tornado emergency.
  • Explain the severe thunderstorm damage-threat tags, including which one triggers WEA.
  • Explain SVS updates and why a report mid-warning matters.

The anatomy

Every warning carries the same skeleton. The placeholders below are where the variable content goes. The labels are fixed, and they are what you should be able to find at a glance.

VTEC /O.NEW.K[WFO].TO.W.[####].[start]Z-[end]Z/
LAT...LON the warning polygon, as a list of vertices

HAZARD... what is expected, e.g. tornado; quarter size hail
SOURCE... where the information came from
IMPACT... what it will do to people and property

TIME...MOT... storm position, direction and speed
TORNADO... tag: RADAR INDICATED or OBSERVED
TORNADO DAMAGE THREAT... tag: CONSIDERABLE or CATASTROPHIC

A skeleton, not a real product. Field order and exact wording vary by product and office; read a live one alongside this.

The VTEC line is the machine-readable identity of the warning: action, office, phenomenon, significance, event number and the time window. It is how every downstream system, including ours, knows an update is the same event rather than a new one.

The tags, and what upgrades them

TagWhat it means
TORNADO...RADAR INDICATEDRotation on radar. Nobody has seen anything at the ground.
TORNADO...OBSERVEDSomeone has seen it. This is the upgrade your report causes.
TORNADO DAMAGE THREAT...CONSIDERABLEThe particularly dangerous situation tier of a tornado warning.
TORNADO DAMAGE THREAT...CATASTROPHICTornado emergency. The most severe wording the product has.
THUNDERSTORM DAMAGE THREAT...DESTRUCTIVEWind of 80 mph or hail of 2.75 in. This tier triggers a Wireless Emergency Alert.

The tag ladder is why the distinction between a tornado warning, a PDS tornado warning and a tornado emergency is not a matter of tone. It is a field with a defined value.

Where your report lands in the text

It lands on the SOURCE line, in words close to SOURCE...Weather spotters confirmed tornado. That line is where the spotter appears in the product, and every person and system that reads the warning sees it.

A warning decision draws on three inputs: radar, the environment, and reports. Reports carry the most weight where the other two are weakest, which Lesson 1 showed you how to predict. When the beam over your town is 8,600 ft up, your observation is not corroborating evidence. It is the only evidence about the ground.

The warning gets updated

A warning is not issued once and left. Severe Weather Statements update it while it runs: narrowing the polygon as the storm moves, clearing part of it, upgrading a tag. That is why a report made in the middle of a warning still matters. The product it changes has not been issued yet.

Teaching point
The most useful report is often not the first one. It is the report that arrives while a warning is live and tells the office what the radar cannot: it is on the ground, it is not, or it has passed.

Check yourself

A severe thunderstorm warning is issued and it sets off phones as a Wireless Emergency Alert. Which tag value explains that?

  1. THUNDERSTORM DAMAGE THREAT...DESTRUCTIVE
  2. THUNDERSTORM DAMAGE THREAT...CONSIDERABLE
  3. TORNADO...RADAR INDICATED
  4. Every severe thunderstorm warning sends a WEA
Hint

One tier of the thunderstorm damage-threat tag is defined by a wind speed and a hail size.

Answer

THUNDERSTORM DAMAGE THREAT...DESTRUCTIVE

The destructive tier, 80 mph wind or 2.75 in hail, is the one that triggers WEA on a severe thunderstorm warning. Most severe thunderstorm warnings do not, which is where the complaint that "the alert never came through" comes from.

Sources

  1. NWS Directive 10-511, WFO Severe Weather Products
  2. NWS Impact-Based Warnings documentation
  3. NWS Weather Spotter's Field Guide (YPA-201154, 2011), section 1