Why does the Weather Service issue "tornado warnings" for spinning thunderstorms?
Jun 28, 2010
in
Tornado Shelters
I understand that the Weather Service uses two types of alerts: Watches and Warnings.
Watch – conditions are favorable, and a tornado might happen today.
Warning – Seek shelter NOW!
15-20 years ago, a tornado warning meant "there’s a tornado on the ground, seek shelter NOW"
Today, it’s "there’s a thunderstorm spinning overhead. It might produce a tornado, but it often doesn’t".
To me, that shows a need for a third level of alert.
Watch – Don’t plan on spending the day outside.
(whatever the new term is) – Spinning thunderstorm in the area.
Warning – Seek shelter NOW! – Tornado on the ground.
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8 comments
UALog on June 28, 2010 at 2:15 pm
The NWS does not have a policy where if rotation is detected with a thunderstorms, that a tornado warning should be issued for that cell. When the first Doppler radars were being installed across the nation back in the early to mid 1990s, the radar operators were trained that if a bounded weak echo region (BWER) was detected (which would indicated a very strong updraft existed) and rotational movement of the air was detected in at least three elevation slices (this would indicate deep low level rotation), then you would likely have a mesocyclone. In this case, a severe thunderstorm warning should be considered.
What would usually trigger a tornado warning would be if you have above and also have one of the following:
- the detection of an appendage or a hook echo in the base reflectivity radar product,
- the storm cell having a history of producing tornadoes
- the detection of strong low level gate-to-gate wind shear near or below the base of the convective cloud deck. This would would be seen with the storm relative motion velocity radar product and would likely indicate a lowering of the rotating wall cloud at the base of this mesocyclone.
-current wind profile data showing very strong speed and directional low level wind shear are still present.
- any ground truth reports of a funnel cloud/tornado forming from a reliable source.
The main reason for this is because many thunderstorms will show rotation. And only a small percentage of these rotating cells would produce a tornado. So if a tornado warning is based mainly on the detection of a rotating storm cell, the false alarm ratio would be very high.
The main problem with to warn or not to warn is simple. We really don’t know exactly what triggers the formation of a funnel cloud from the base of the a convective cloud. We have plenty of well educated guesses, but nothing that will tell us for sure until it really forms. A warning will not be very helpful if it can not be issued until a funnel has started to form because it will give little or no time for people to react and take action. Therefore, meteorologist can’t wait until we have ground truth of a funnel forming in order to issue a warning.
So a radar based warning would be issued early when the right ingredients seems to be converging rapidly towards the pot of boiling water, but whether all the ingredients will fall into the pot to get the right mix will have to wait and see.
And I agree with one other earlier answer that it is best to keep it simple because there are still a lot of people who still don’t understand the difference between a watch and a warning. Just keep an eye on Yahoo Answers and see how often that question pops up.
One other comment…even though technology has greatly reduced the time when the meteorologist decides to go with a warning to the time the warning actually reaches the public, it still takes at least about 2 minutes to complete this process. And in a tornadic warning situation, two minutes is still a very long time to get a warning out to the public. So lets keep the warning process simple.
Shalimar on June 28, 2010 at 2:15 pm
It’s a good idea but is often hard expesially if you live in a large area, meterologist maps can see tha there may be spining thunderstorms in the area that look like it’s going to be there all day, they can’t easily predict when a tornado may arrive from them but it’s a general warning, sometimes they do give these warning if it is in a major area
TJ on June 28, 2010 at 2:15 pm
A tornado warning is issued when there is strong level of storm rotation, rotating wall cloud, funnel cloud, or tornado is present. When a mesocyclone is present, it is one of the ingredients to form a tornado.
× ÏƒÐ½Ð¸ ∂єєÑÑ” fαÑÐ¼Ñ”Ñ on June 28, 2010 at 2:15 pm
A rotating thunderstorm is a good indicator that a tornado is forming or will form soon. So a rotating thunderstorm is a good time to seek shelter in case a tornado does drop from it. The weather service is just trying to make things safer for people around tornadoes.
Skywarn storm spotter MAN-477A on June 28, 2010 at 2:15 pm
When the Doppler Radar is spinning in a thunderstorm, that means the Doppler Radar is indicating a tornado is NOW occurring. The watches and warnings are still the same definition.
cicocg62 on June 28, 2010 at 2:15 pm
I understand your point but you have to understand that the general public barely cares about weather. Simplifying the alerts is best. The more complicated you make it, the less likely people will understand and take the right precautions. It’s worth a shot pitching to the NWS and gain their thoughts, however.
. on June 28, 2010 at 2:15 pm
The rotation in a thunderstorm has the potential to produce a tornado. Yes sometimes the rotation does not produce a tornado but sometimes it does which is also pretty much the only way a tornado forms. The National Weather Service defines a tornado warning as strong rotation being detected in a thunderstorm on the doppler radar but a tornado might already be on the ground or if a funnel cloud or tornado was detected or reported. The rotation could produce a tornado at any given moment and they want people to protect themselves just in case it does form. There are also times where the radar doesn’t detect tornadoes and the radar could be showing only rotation meanwhile there’s a tornado on the ground. If they created a third level alert, people wouldn’t take that as serious as a tornado warning and if a tornado does form then it would be too late to keep themselves out of harms way. Also, it depends on where you live. If you live in an area where tornadoes aren’t too common then it’s true that most rotation doesn’t create a tornado but a lot of them do in areas where tornadoes are common.
Carly on June 28, 2010 at 2:15 pm
Watches mean conditions are favorable for Tornadoes to form & Warnings mean that there has beena Tornado spotted and you should immediately take cover. If i were to add a 3rd term (and i know what you mean) i would put that term first before a Watch and perhaps call it a Chance or a Chance of tornado development later in the day.