Radar Indications

[Alex Carlson]

I am fairly new to weather forecasting, so this may be a dumb question.

When the NWS puts out a tornado warning that says "doppler radar indicated a severe thunderstorm capable of producing a tornado," what exactly are they seeing on radar?

I understand the obvious things like a hook, but what other good indications are there. If i remember correctly, looking at the base velocity will sometimes reveal a couplet of red and green indicating rotation.

 

[Jared Thompson]

This is the couplet from Yazoo City Tornado from yesterday just before it hit the city. This is SRV1 level 2 those are the inbound and outbound wind speeds from the radar. The radar was located SE of the storm. This of course was screaming tornado..

 

[Tim Supinie]

I wouldn't call a hook a reliable sign of rotation because lots of things can look like hook echoes that have nothing to do with rotation (it's just an oddly shaped storm), and no hook doesn't necessarily mean no rotation. The more reliable method for determining rotating storms is the Storm-Relative Velocity radar product. This is similar to the Base Velocity product, but it has the storm motion subtracted out of it. The reason is that the storm motions can sometimes mask areas of rotation on the radar, so subtracting the storm motion out of the velocity field makes it easier to see where there's rotation in the thunderstorm. Then, yes, you can look at where you have green inbound velocities close to red outbound velocities to see where you might have rotation. Another criterion for issuing a tornado warning is that the rotation has to be close to the ground (the lowest radar tilt, usually).

And no question is a dumb question.

 

[jladue]

I agree with Tom that viewing a hook echo by radar reflectivity alone is not always sufficient to say that a storm has got a mesocyclone unless you've got access to the 1 min interval data like with the TDWR (Terminal Doppler Weather Radar) where you can see the reflectivity patterns in motion. The hook could be a coincidential structure caused by a surge in the gust front causing new convection, or a merging cell. But a forecaster's confidence of a good mesocyclone goes up when the reflectivity pattern deforms because of a mesocyclone.

BTW, Doppler radar indicated really means a forecaster with significant experience is interpreting the data and making an assessment of a storm's potential to produce a tornado, all in the context of the near storm environment. One day, the environment may look quite bad and so it may take a stronger mesocyclone before a forecaster decides to issue a TOR warning. On another day the environment may look so good that a forecaster doesn't want to wait for a mesocyclone to become as strong. Anyway because the forecaster is really making the decision, and not the radar, many offices use the terminology 'NWS meteorologists detect a storm capable of producing a tornado'.

 

[MatthewCarman]

I have seen hooks on radar with no rotation associated with them that I could tell. I was using GRLevel3 when the tornado hit Yazoo and I just used SVR1 and could see clear rotation by the bright red and green colors coming togther.

 

[Alex Carlson]

Thanks for the information everyone, im just trying to learn as much as i can right now.

 

[Shane Lee]

Somewhat along the same lines....if there are any of you that use the Radarscope App....Which of the "tilts" are the lowest, and which are the highest? On Base Reflectivity it goes from 1-4 etc....I would assume that "1" is the lowest, but I'm not sure.

Also, what is the difference between the "enhanced" products and the regular products?

 

[Rob H]

Somewhat along the same lines....if there are any of you that use the Radarscope App....Which of the "tilts" are the lowest, and which are the highest? On Base Reflectivity it goes from 1-4 etc....I would assume that "1" is the lowest, but I'm not sure.

Also, what is the difference between the "enhanced" products and the regular products?

Tilt 1 will be a .5deg elevation angle, Tilt 4 will be 3.5deg - so your assumption is correct - 1 is lowest. The enhanced products are the higher resolution level 3 products. You can not only see more detail, but further away from the station as well.

 

[rdale]

1 is the lowest.

A few weeks ago the NWS started upgrading the resolution of the lowest tilts. That's probably where the "enhanced" term comes from.

 

[Matt Hunt]

Rain wrapped tornadoes won't have a hook echo, or you may only be able to detect a hook in the red portions. In the last several days with all of the action down in Arkansas, Mississippi, etc, I've watched several tornado warned cells on grlevel3 where I see no sign of a hook, and sometimes didn't really see any rotation on SRV or BV, so I'm not sure what they were looking at in those cases. Any ideas?

 

[Chris Nuttall]

For starters, the NWS is looking at level 2 data, which is at a much higher resolution than level 3. More subtle features can often be "smoothed" out of the level 3 data, but might stand out fairly noticeably in level 2.

As for as the storms in the southeast go, post a radar image of a storm you have a question about. Otherwise, it's going to be hard to answer any questions.

 

[Mike Smith]

I'm going to post a minority view with regard to hook echoes.

A hook, based on 39 years of experience, is a reliable indicator of a tornado. The caveat is to make sure you are able to see the development of the hook (i.e., it gets better defined with time). We are seeing in the very high resolution DoW data that almost every tornado has a hook.

Yes, there can be "hook imposters" -- random echoes that look like hooks. But, if you see the classic development stages of a hook it is a reliable tornado indicator.

 

[James Aydelott]

Mike has a great point. If you have been "on the storm" and seen the evolution of storm structure and the pendant "hook" you have a good idea whether or not its mesocyclonic.

This also includes looking above and beyond the 0.5 base reflectivity product-if you're seeing a WER or BWER above the hook, there's a strong likelihood that's its a mesocyclonically generated hook.

 

[Bob Hartig]

One point that hasn't been mentioned: color tables for velocity products can vary. While green and red are the traditional scheme, you'll find a whole array of alternatives. Most of them do a good to great job of highlighting couplets, but the colors of some can throw you. Consult the table to make sure you know what you're looking at.