Tornado warning

A tornado in Douglas, Oklahoma.

A tornado warning is an alert issued by government weather services to warn an area that a tornado may be imminent. It can be issued after either a tornado or funnel cloud has already been spotted, or if there are radar indications that a tornado may be possible.

Criteria

A tornado warning is issued when:

• A tornado is reported on the ground or is indicated by algorithm analysis on a weather radar output
• A waterspout is headed toward landfall
• A funnel cloud is reported in the sky

It is also sometimes issued when, depending on the circumstances:

• A thunderstorm with a threshold strong, tight rotation signature is indicated by Doppler capable weather radar, or
• A rotating wall cloud is reported (in context of all other available information).

A tornado warning means there is immediate danger for the warned and immediately surrounding area -- if not from the relatively narrow tornado itself, from the severe thunderstorm producing (or likely to produce) it. All in the path of such a storm are urged to take cover immediately, as it is a life-threatening situation. A warning should not be confused with a tornado watch (issued by a national guidance center, the Storm Prediction Center) which only indicates that conditions are favorable for the formation of tornadoes.

Generally (but not always), a tornado warning also indicates that the potential is there for severe straight-line winds and large hail from the thunderstorm. A severe thunderstorm warning can be upgraded suddenly to a tornado warning should conditions warrant.

In the United States, local offices of the National Weather Service issue warnings for tornadoes and severe thunderstorms based on the path of a storm, although entire counties are sometimes included, especially if they are small. Warnings were issued on a per-county basis before October 2007.

In Canada, similar criteria are used and warnings are issued by regional offices of the Meteorological Service of Canada of Environment Canada in Vancouver, Edmonton, Toronto, Montreal and Halifax.

Tornado warnings are generated via computer then disseminated through various communication routes accessed by the media and various agencies, on the internet, to NOAA satellites, and on NOAA Weather Radio. Civil defense sirens are also activated for the affected areas if present.

The first tornado warning was issued by the meteorological staff of Tinker Air Force Base in 1948 and was also coincidentally the first successful tornado forecast.

Advances in technology, both in identifying conditions and in distributing warnings effectively, have been credited with reducing the death toll from tornadoes. The average warning times have increased substantially to about 15 minutes; and in some cases to more than a one hour's warning of impending tornadoes. The U.S. tornado death rate has declined from 1.8 deaths per million people per year in 1925 to only 0.11 per million in 2000. Much of this change is credited to improvements in the tornado warning system.

Recognition

The SKYWARN program, which teaches lay people how to spot tornadoes, funnel clouds, wall clouds, and other weather phenomena, is offered by the National Weather Service. In tandem with Doppler radar information, eye witness reports can be very helpful for warning the public of an impending tornado, especially when used for ground truthing.

Tornado Emergency

Main article: Tornado Emergency

When a large, extremely violent tornado is about to impact a densely populated area, the Weather Service has the option of issuing a severe weather statement with enhanced wording; this is called a tornado emergency. This category of weather statement is the highest and most urgent level relating to tornadoes; the ladder is as follows: relevant verbiage in the current day's Convective Outlook, Public Severe Weather Outlook mentioning tornado potential (not all outlooks do -- they are issued when derecho (q.v.) formation is forecast, which can reduce the likelihood of tornadoes after a certain severity level), Tornado Watch, Particularly Dangerous Situation Tornado Watch, Tornado Alert (formerly), Tornado Warning, and the Tornado Emergency variant. Tornado warnings can also be intensified by added wording mentioning that the storm is life-threatening, it is an extremely dangerous situation, a large, violent and/or destructive tornado is on the ground, &c.

Tornado Alert

For many years up until the early 1980s an intermediate type of tornado advisory known as a Tornado Alert was defined by the National Weather Service and issued by local offices thereof. A Tornado Alert indicated that tornado formation was imminent and in theory covered situations such as visible rotation in clouds and some other phenomena which are portents of funnel formation. The use of this advisory began to decline after 1974 but was still listed on public information materials issued by various media outlets, local NWS offices, &c. for another decade or so.

The kinds of situations which called for Tornado Alerts in the past now generally result in a Tornado Warning with clarifying verbiage specifying that the warning was issued because rotation was detected in one way or another, that a wall cloud has formed, and so on.

The Tornado Alert was eliminated both because it was made largely obsolete by the advent of Doppler weather radar which can detect tornado formation earlier -- often before it is visible to the naked eye of trained spotters and other members of the public -- and with fewer false-positives and also it was also eliminated to reduce confusion amongst the public, with the latter issue first discussed in earnest following the 03.-04. April 1974 jumbo outbreak. The preferred response to both the Tornado Alert and Warning is to take shelter immediately, so it could be seen as splitting hairs especially since prediction has improved.

The Weather Channel uses the same red crawl LDL when the warning is issued, but with the words "Tornado Warning" on the top of the lower display line.

Example of a Tornado Warning

TORNADO WARNING
OHC103-092115-
/O.NEW.KCLE.TO.W.0022.070809T2023Z-070809T2115Z/

BULLETIN - EAS ACTIVATION REQUESTED
TORNADO WARNING
NATIONAL WEATHER SERVICE CLEVELAND OH
423 PM EDT THU AUG 9 2007

THE NATIONAL WEATHER SERVICE IN CLEVELAND HAS ISSUED A

* TORNADO WARNING FOR...
  MEDINA COUNTY IN NORTHEAST OHIO...

* UNTIL 515 PM EDT

* AT 419 PM EDT...NATIONAL WEATHER SERVICE DOPPLER RADAR INDICATED A
  SEVERE THUNDERSTORM CAPABLE OF PRODUCING A TORNADO 7 MILES WEST OF
  LODI...OR ABOUT 15 MILES WEST OF MEDINA...MOVING EAST AT 40 MPH.

* OTHER LOCATIONS IN THE WARNING INCLUDE BUT ARE NOT LIMITED TO
  WADSWORTH

IF YOU ARE CAUGHT OUTSIDE...SEEK SHELTER IN A NEARBY REINFORCED
BUILDING. AS A LAST RESORT...SEEK SHELTER IN A CULVERT...DITCH OR LOW
SPOT AND COVER YOUR HEAD WITH YOUR HANDS.

THE SAFEST PLACE TO BE DURING A TORNADO IS IN A BASEMENT. GET UNDER A
WORKBENCH OR OTHER PIECE OF STURDY FURNITURE. IF NO BASEMENT IS
AVAILABLE...SEEK SHELTER ON THE LOWEST FLOOR OF THE BUILDING IN AN
INTERIOR HALLWAY OR ROOM SUCH AS A CLOSET. USE BLANKETS OR PILLOWS TO
COVER YOUR BODY AND ALWAYS STAY AWAY FROM WINDOWS.

IF IN MOBILE HOMES OR VEHICLES...EVACUATE THEM AND GET INSIDE A
SUBSTANTIAL SHELTER. IF NO SHELTER IS AVAILABLE...LIE FLAT IN THE
NEAREST DITCH OR OTHER LOW SPOT AND COVER YOUR HEAD WITH YOUR HANDS.

LAT...LON 4100 8217 4114 8216 4114 8207 4117 8206
      4116 8169 4099 8168

LAPLANTE

See also

• Tornado watch
• Tornado Emergency
• Severe weather terminology (United States)
• Emergency Broadcast System
• Emergency Alert System
• [http://www.earlywarningsys.com