Rear Flank Downdraft

[Janette Bontempo]

Was wondering...I took the skywarn last year, and again this year. I noticed more emphasis on Rear Flank Downdraft and "Clearing" this year. Why is that?

[Andrew Stoller]

They're probably putting more emphasis on visually identifying the RFD as a clearing area that wraps around a mesocyclone, because it's an important aspect of tornado genesis. If you can relay to the NWS that you're seeing a wall cloud and then it starts wrapping up with an RFD clear slot, it could mean tornado genesis is immanent. It's another "ground truth", if you will.

[Paul Austin]

I would add that RFD often can and does cause quite a bit of damage over a relatively large area (when compared to the damage path of a tornado), though not typically as severe. Case in point: On May 22, 2008 the RFD of the tornadic cell near Wakeeney, KS blew the glass doors out of the service station, the cover off of one of the gas pumps, and the windows out of several parked cars where my brother and sister had positioned to observe the storm as it passed over, even though the tornado passed at least a mile away. RFD winds at the surface can often exceed 80 mph. So the RFD is important for visually identifying clues to tornado genesis, but also can be the cause of damage meeting severe criteria. Both aspects would be important for a spotter to be aware of.

[Bill Tabor]

RFD's...yeah you gotta love them. :D I remember an extremely large sup over LBB some years back with a coal black base over town. Toward the back there was a clear area or slot that I found so intruiging. I was just drawn in for a closer look. Then all hell broke loose - crap flying across the field and across the road with those big highway metal light polls breaking over. Someone further out with more perspective sighted a tornado in my area.

As a fairly green chaser the day before the Jarrell F5 tornado I was chasing in OK near Purcell and followed another chaser on the backside of a very large inverted bowl inflow wall / base only to be surprised with all hell / broken trees only to find a tornado newly developed to my east.

May 12th 2005 I thought I could live close to the updraft inside the notch as the tornado passed and dissipated. As it began redeveloping to my east I was in the path of the RFD blast wrapping around the hook and slinging very large hail nearly horizontal. Lucky I only got vehicle damage and a bruise on my arm.

Likely lots of other RFD encounters but these are off the top of my head. The RFD can be beautiful to watch like a cascading waterfall, but it can be an angry force that you don't want to be in front of either.

I'm not a brave or dare devil chaser - I just get caught sometimes. You will too if you chase. RFD can be the sign or cause of tornadogenesis that you should be aware of, but take care around it particularly with really large, mature, wound up supercells - likely IMO HP the worst. Sincerely..respect the wind.

[Brian Stertz]

It is those steady state RFD's that are truly fascinating....and are often seen with the more violent class of tornadoes. The May 3rd Bridge Creek OK and May 4th Franklin KS tornadoes both had these similar striking RFD cuts that allowed for great viewing on those chases...with seriously bad results for those that were in the path.

[Mikey Gribble]

If they are bring up "clearing" when discussing the RFD I would assume maybe they were mentioning it as a good indicator of a tornado forming or for locating where a tornado could form.
I don't know how many storms you've so I'll kind of explain this for somebody that doesn't know much about supercells, since this is the beginners portion of the forum.
The updraft base is a cloud base that is typically on the right rear flank of the storm. There usually isn't rain falling in the area and on a good storm that has tornado potential the cloud bases will usually be pretty low to the ground. All the updraft base is is where warm moist air from the surface is being sucked in and up into the storm. You can think of it like a giant shop vac drawing air in from the south side of the storm. The updraft "base" is simply the cloud base that forms when this warm air that is being drawn in hits its condensation level.
A rear flank downdraft is cold air on the back side of the storm that rushes down towards the ground and when that cold air hits the cloud base associated with the updraft it quickly erodes them leaving a clear slot. A lot of times it will create sort of a horse shoe shape in the updraft base.
Here is a horrible picture of an updraft base that doesn't have a well defined RFD.
http://www.loadedgunchasing.com/images/Albany1.bmp

Here is a good picture of a well defined RFD that has punched through the updraft base.
http://www.stormeffects.com/images/52099%20LakeMcTor4.jpg (http://www.stormeffects.com/images/52099%20LakeMcTor4.jpg)

This is a overhead view of what a typical supercell looks like. The updraft part of the storm is where air is flowing in from the surface and up into the storm
http://upload.wikimedia.org/wikipedia/commons/5/5e/Supercella.png (http://upload.wikimedia.org/wikipedia/commons/5/5e/Supercella.png)

Below is a link to a picture from a tornadic storm in Kent County Texas. It shows the RFD notch pretty well. You can see the flat updraft base, the clear notch on its back side from the RFD and the wall cloud that has developed along the edge of the RFD/updraft intersection.
http://www.loadedgunchasing.com/images/DSC_0041.jpg

[John Hudson]

To take it a step further, I wanted to clarify how the RFD comes into being. Is it true the RFD forms when a robust primary updraft tower acts as an obstacle to the upper level flow, forcing its winds to flow downward?


John
VE4 JTH

[Dean Baron]

HERE (http://s21.photobucket.com/albums/b251/dean681/?action=view&current=IMG_1598.jpg) is an example of a wall cloud and and RFD that I took last year while chasing a TOR warned storm northwest of Minneapolis. I also want to add that the RFD should be avoided whenever possible, it can pack a serious punch. I got slammed by the RFD from the storm in the pic I posted and the gusts were easily in excess of 50 mph, and they can be a lot stronger than that.

[Bill Tabor]

To take it a step further, I wanted to clarify how the RFD comes into being. Is it true the RFD forms when a robust primary updraft tower acts as an obstacle to the upper level flow, forcing its winds to flow downward?
John
VE4 JTH

I don't believe they are entirely certain. My personal theory is it is a byproduct of vortex breakdown of the updraft as it matures and cycles. Likely related to collapse of an overshooting top too IMO.

[Brian Stertz]

To take it a step further, I wanted to clarify how the RFD comes into being. Is it true the RFD forms when a robust primary updraft tower acts as an obstacle to the upper level flow, forcing its winds to flow downward?
John

Yes from what I have seen in my chases, I find this to be true with the stronger tornadoes. I think this shows up often with the cascading rain blob that shows up when the meso begins to intensify and tornadoes develop. Every tornadic supercell has kind of its own individuality so grouping all things will do this and all things will do that is definitely challenging. Then comes the HP vs Classic vs LP issue....or the freakish hybrids that occur.

[Dustin Wilcox]

Was wondering...I took the skywarn last year, and again this year. I noticed more emphasis on Rear Flank Downdraft and "Clearing" this year. Why is that?


This is the best visual reasoning I can provide. Early in the video the first 10 sec. or so you can see the first formation of a "wall cloud" and just to the left of it, the tiniest "cut". Then the video skips ahead (no more than 5 min real time) and that tiny cut is now a large RFD/Clear Slot, you can see the sun shining through. Just a classic example of how an RFD impacts tornadogenisis...
http://www.youtube.com/watch?v=Byym59xE13k

[Matt Sellers]

My thought is that the RFD occurs as an effect of the rapid deepening of the meso- or micro-scale surface-h8 low underneath a rapidly-strengthening mesocyclone. Consider the mesocyclone as the center of a rapidly-deepening surface low (in the mesoscale): strong northerly (to NW/W) surface winds can be expected on the back side of the mesocyclone, as is normally the case with rapidly-deepening synoptic-scale lows.
The surface pressure differential can become strong enough to induce RFD winds of 100+ mph, especially if a tornado is nearby and <a href="http://www.crh.noaa.gov/fsd/storms/tor062403/samaras/samaras_fig4.gif">inducing pressure drops of up to 100mb</a> in its immediate vicinity, in extreme cases. <br>
Link is to NWS Sioux Falls, SD website, and references data recorded by Tim Samaras and crew via remote device in a F4 tornado near Manchester, South Dakota on June 24, 2003.

[Angel Dimitrov]

Few links for advanced readers:
http://ams.allenpress.com/perlserv/?request=get-abstract&doi=10.1175%2F2007MWR2285.1
http://ams.confex.com/ams/pdfpapers/142136.pdf
http://members.cox.net/jdavies1/waf796/waf796.htm

[Paul Knightley]

I think there is still some debate about the exact origins of the RFD although from some stuff I've read, some researchers are suggesting that it's tied into a descending precip core at the rear of the storm, which then acts to 'focus' low-level vorticity, in order to bring it to a high enough level for vortex stretching to occur.

[Greg McLaughlin]

I believe one reason the NWS is adding emphasis to identifying the RFD is to help spotters to be more accurate when identifying wall clouds and tornadoes. Over the years I have seen many false reports where someone thought a scud tag or shelf cloud was a wall cloud or funnel. If a chaser or spotter understand what an RFD slot looks like they are more likely to accurately identify tornadic features.

Someone earlier stated that RFD is cold air that wraps around the meso, and while that can be true, often RFDs that promote tornadogenesis contain relatively warm, boyant air that literally wants to rise back into the meso.

[Andrew Stoller]

This is the best visual reasoning I can provide. Early in the video the first 10 sec. or so you can see the first formation of a "wall cloud" and just to the left of it, the tiniest "cut". Then the video skips ahead (no more than 5 min real time) and that tiny cut is now a large RFD/Clear Slot, you can see the sun shining through. Just a classic example of how an RFD impacts tornadogenisis...
http://www.youtube.com/watch?v=Byym59xE13k

In this video, you can see the downword motion of the RFD interacting with the upward motion of the updraft. That is cool!

[Andrew Stoller]

I believe one reason the NWS is adding emphasis to identifying the RFD is to help spotters to be more accurate when identifying wall clouds and tornadoes. Over the years I have seen many false reports where someone thought a scud tag or shelf cloud was a wall cloud or funnel. If a chaser or spotter understand what an RFD slot looks like they are more likely to accurately identify tornadic features.


Many times in my young career as a storm chaser, I end up focusing on the RFD gust front too much, thinking it's part of the updraft base. Identifying the RFD clear slow in relation to the updraft has helped me focus more on the action spot of supercells.

May 23rd, 2008 Wakeeny, KS evening supercell was a classic example. That storm cycled and produced a new mesocyclone with a big clearslot. It was beautiful because the sunset was allowing all kinds of bluish/orange light to come through the RFD, but I kept focusing on that and the RFD gust front, only to look a bit further north to see the actual meso!

[Janette Bontempo]

Thank you so much everyone for all of your posts and information!! Sorry I didn't get back sooner....I was mia for surgery and in the hospital....its looking like it might be busy for the reg's the later part of this upcoming weekend and early part of next week. Stay Safe!!!

[Stephen Locke]

Someone earlier stated that RFD is cold air that wraps around the meso, and while that can be true, often RFDs that promote tornadogenesis contain relatively warm, boyant air that literally wants to rise back into the meso.

If it is generally agreed that RFD contains (cold) air from aloft why are so many RFD's reported to be warm. Does the air mix out with warm air at the surface or does the air actually heat up due to compression when the cascading vertical
column strikes the surface. And, would this compression be a viable energy source to help kick start tornadogenisis.

-Stephen

[Paul Knightley]

The fact that RFDs around tornadic storms often seem to be 'warm' is a major area of research at the moment. Some theories centre around Fujita's original 're-cycling' theory, whereby air which has already entered the storm from being lifted along the forward-flank downdraught then descends in the RFD and re-cycles into the tornado, perhaps bringing increased vorticity with it. How/why this air is warmer is not really known, although some theories suggest that low LCLS/high RH in the boundary layer prevents too much evaporative cooling of in the RFD (as precip evaporates) and thus the RFD reaches the ground with some CAPE.

[Stephen Locke]

The fact that RFDs around tornadic storms often seem to be 'warm' is a major area of research at the moment. Some theories centre around Fujita's original 're-cycling' theory, whereby air which has already entered the storm from being lifted along the forward-flank downdraught then descends in the RFD and re-cycles into the tornado, perhaps bringing increased vorticity with it. How/why this air is warmer is not really known, although some theories suggest that low LCLS/high RH in the boundary layer prevents too much evaporative cooling of in the RFD (as precip evaporates) and thus the RFD reaches the ground with some CAPE.

Are the cold RFDs typically dry? Does a warm RFD feel moist?

-stephen

[Paul Knightley]

Are the cold RFDs typically dry? Does a warm RFD feel moist?

-stephen

Do you mean dewpoint-wise? A cold RFD can have quite high relative humidity. It's the amount of CAPE contained within the RFD which seems to be key, thus a warm, moist one is more conducive than a cold one, whether it's wet or dry.