What is this?

[Carla Bisby]

Earlier this year I was chasing in Illinois and saw this area of rotation but I'm not sure what to call it. I don't think it would be classified as a funnel but maybe as a rotating wall cloud? It had about 8 fingers hanging down and it was rotating (although weakly). Any insight? I'm new to chasing and am still trying to learn so I appreciate any comments anyone may have. (I have a box drawn on the photo to highlight the rotating area I'm talking about). Thanks .

http://community.webshots.com/photo/151004...151004976sAhKWy

 

[Laura Duchesne]

It is very difficult to tell what it is... you would have to give us some more info. It's definately not a funnel and it looks too narrow to be a wall cloud... wall clouds are more round... that almost looks like a developing shelf cloud because there are some outflow characteristics in the clouds behind it.

Do you remember what type of thunderstorm it was? Linear, multicell or supercell?

Did you feel any outflow or inflow?

What direction are you facing in and where is the precip relative to that vantage point? What direction was the storm moving?

 

[Carla Bisby]

The area was round although it does not show well in the pic. I was facing south. The storm was moving west to east. The precip was to the east of this area. Here's a pic of the cell it was in. And the area was rotating. Am I correct that scud does not rotate?

http://community.webshots.com/photo/151004...151004991QWpFRk

 

[Mike Peregrine]

Carla - was the storm tor-warned? I pulled up a radar mosaic for that day (by the way - does anyone know where a person can get individual site radar archives now? - NIDS no longer seems to do the trick - I could only pull up a mosaic from the NCDC ... thanks).

The radar image taken at 5:00 p.m. on May 23rd shows widespread convection across Illinois - from the same system that produced a highly tornadic day further west in Nebraska the day before.

Here are the storm reports from Illinois that day:

http://www.spc.noaa.gov/climo/reports/040523_rpts.html

As you can see, there were several tornadoes across the state that day.

Your image appears at a glance to possibly be a shelf feeding into an area of precipitation over the horizon. I would be most interested in taking a look at the area where the shelf actually intersects the downdraft ... but from all of the information provided, it is quite possible that the storm was indeed rotating and that you were in the right vicinity to intercept the rotation. It's possible, depending on all of the information you have ... the shelf itself appears to be a little on the outflowish side. Typically as far as positioning is concerned, a chaser would want to be south and east of the location in your photo (on the other side of the shelf here), which would increase your ability to make a good judgment. If you could verify rotation that was sustained for at least 15 minutes, then chances are you have a portion of a wall cloud in this photo (possibly occluding as the storm core progresses eastward). Like Laura mentioned, there are a lot of factors that would allow you to finally make that conclusion if you were there witnessing the event.

Hope this helps -

 

[Laura Duchesne]

Okay, had a feeling you were facing S, but wasn't sure. Well given the position of that lowering, if it was rotating and round, then it could very well be a developing wall cloud.

How long did this hold together?

Scud are actually cloud tags that rise up a majority of the time. It can be found under developing wall clouds, near precip cores or in outflow areas. Scud is actually short for Scattered Cumulus Under Deck :wink:

Edited... I noticed Mike posted before I did... but is slight rotation possible along shelf clouds? I agree that it does look outflow dominent... not somehting chasers usually want to see.

 

[Mike Peregrine]

Just speaking from personal experience - I've been on several storms that have a broad inflow area feeding into the core (sometimes with similar tags as the one in this shot), which then seem to get pushed away from the storm base as a type of gust front ... this can actually be occurring at the same time as the rotation closer to the core organizes prior to tornadogenesis. Anyone can feel free to correct this of course, but you can get storms that appear to be gusting out from one angle that are actually not 'gusting out' at all ... but instead are quite well balanced and mature storms, capable of producing a tornado. The Hallam, Nebraska storm just the day before this one, started with a broad (and high!) rain-free base leading into the downdraft - that very suddenly achieved tornadogenesis. The wall cloud/mesocyclone became suddenly apparent as it sort of just cascaded out of the RFB. In the plains a person seems to be able to get a feel for when a storm is about to really get with it and suddenly produce. There are other storms - like the one in this shot - that are much tougher. You can end up following for hours and things don't shift much beyond what you see here. In the end it has to just be a judgment call from the person on the scene ... video helps a lot in this situation too because you can review it at higher speeds to better judge rotation.

 

[Laura Duchesne]

I am just curious, is direct linking from chaser sites allowed? That's taken by Bobby Eddins... nice shot, but maybe you should provide the link to that page instead?

 

[Glen Romine]

Carla - was the storm tor-warned? I pulled up a radar mosaic for that day (by the way - does anyone know where a person can get individual site radar archives now? - NIDS no longer seems to do the trick - I could only pull up a mosaic from the NCDC ... thanks).

There is http://locust.mmm.ucar.edu/case-selection/ , which at least provides access to regional composites among other obs. As for individual sites beyond the last few days, you probably have to request and download it from HDSS. For recent events, there is of course RAP's radar page, which goes back about 5 days.

Glen

 

[Angie Norris]

I'd say this was a shelf cloud on the leading egde of the storm. Lisa, was the rotation you saw horizontal or vertical? If it was horozontal, then it was probably a roll cloud. Doesn't really look like a wall cloud...looked like it was detached from the cloud base, so scud is also a good possibility.
Angie

 

[Laura Duchesne]

Actually, the more I look at it, the more I think it's a shelf cloud... it looks like it's on a leading edge, like Angie said... and there are outflow characteristics behind it.

Lisa, did this feature pass you? Did you feel any change in wind, ie. from inflow to outflow? It is really difficult to tell on this one I think... gotta be there to actually see what was going on. I am curious about the rotation though.