The Forward Flanking Downdraft and Rear Flanking Downdraft can both fan out with shelf cloud gust fronts. A rear flanking downdraft will often make a wall cloud fan out into more of a shelf cloud. The rear flanking downdraft clear slot often punches a nice hole through the updraft base and then fans out into the region where the wall cloud might be.
Your moderskeppet shot shows a rainy downdraft and surrounding gust front clouds. Whether this is from the forward flank of any type of storm, or the rear flank of a supercell isn't very obvious in the photograph. It could be a mediocre storm, complex, or an HP supercell.
I did a talk for spotters recently and was trying to stress situational awareness on supercells. I often use the RFD clear slot and the horseshoe shaped RFD gust front as a guide to get my bearings on where I'm located relative to the storm and from where the tornado might form.
Here's a classic hook echo:
The ball at the end of the hook, while commonly referred to as a debris ball, is usually or mostly just precipitation within the rear flanking downdraft and not debris (unless you've got dual pol data that says otherwise). This ball of rain, punches a donut hole in the updraft base of the storm, and then fans out in the forward direction of the storm making a backwards C or horseshoe shape. If you draw a line around the forward part of this ball, you're drawing where the rear flanking gust front is. This can be seen visually in the storm structure.
I've drawn a blue line around the rear flanking downdraft using the end of the hook echo as a guide. This blue line translates to a feature we can see visually on the storm as a rear flanking gust front. The tornado most often, but not always, forms at the top of this horseshoe shaped gust front. I put an orange triangle where the tornado most often occurs on a textbook supercell.
It took me about a dozen times seeing these shapes visually in the field before they really sunk in, so I thought I would show a bunch of examples here to get kind of a general theme going. In each, I've translated the blue line from the above radar scan to the structure we see visually in photographs. These are different storms than from the above radar scan, but they all presented on radar with recognizable hook echoes (unless otherwise states).
Looking north at the most textbook example I've seen in the field. We are south of the hook, looking due north at it. A violent EF4 tornado is located right at the top of the horseshoe shaped rear flanking gust front, right where it should be.
Another textbook example. Again, looking north at the southern end of the hook at a horseshoe shaped rear flanking downdraft gust front (blue). The RFD clear slot is the hole in the clouds behind or left of this blue line.
If there is no tornado, but we have the same horseshoe shaped gust front present, we can guess where a tornado is most likely to form:
Indeed a several tornadoes formed in that location a few minutes later.
Even if there is no wall cloud present, we can still identify the likely location of a tornado if the RFD clear slot and horseshoe shaped RFD gust front are present:
Looking WSW from ENE of the hook.
The RFD clear slot as it's first presented as a donut hole in the updraft base, followed by the rear flanking downdraft expanding a few minutes later. Looking north:
This storm did not produce a tornado, mainly because it occurred in Iowa. But we'd have a good idea of where one might have formed thanks to the prominent RFD clear slot and horseshoe shaped base.
How about from other angles? Looking south from inside the forward flanking precipitation core where the hail is coming down:
How about from behind the hook echo looking east? This view is usually blocked by rain, but sometimes on a classic or LP supercell you can look right at the back side of an RFD clear slot and horseshoe shaped RFD gust front with a view of a tornado:
Looking east, toward the backend of a hook echo. This is a difficult spotting position, since the horseshoe isn't immediately obvious and just looks like a bank of convection. It's there though. it's generally a safer position too since the storm is most likely moving directly away from you, so it's less of a concern trying to identify it here.
How about on a high precipitation supercell? From east of the hook looking west, the horseshoe shape isn't very obvious since we're looking at it head on and can't see the curvature very well. The clear slot is also not clear, and is filled with rain.
I think most folks not well acquainted with supercell structure would assume the tornado would be in in the darkest part of the storm, on the right side of the image. This is actually the forward flanking precipitation core, the area north of the hook echo. You can, however, make out the curvature of the RFD gust front in this photo, and use it to trace a line where that horseshoe shape should be. You'd also know this if you had a radar scan and knew you were looking west toward at a hook echo.
The tornado is most likely to be at the top of the horseshoe, in the lighter shades of rain in this photo. This shot is an example of why I think chasers who panic may pick escape routes that aren't the safest. They cut south across the path of a potential tornado since the area to the south looks less severe than the area to the north, even though the lighter colored bands of rain within the RFD represent the most hazardous part of the storm.
Same storm looking north as the storm passes to our north:
Looks just like a typical shelf cloud doesn't it? It pretty much is. You can't easily make out the horseshoe shape because the clear slot is filled with rain. We can assume it's there though because we know we're looking north at a hook echo thanks to our radar scan. Follow the curvature of the rear flanking gust front to figure out where this horseshoe shape is. From this we'd guess this tornado would be buried within the rain of the RFD, and indeed they usually are on HPs.
A tricky HP, looking west:
Try to find the curving rear flanking downdraft gust front since we don't have an obvious horseshoe shape here. Use the barrel shaped mesocyclone above the gust front as a guide for where it might be. Use the wall cloud and tail cloud as additional clues. The wall cloud and tail cloud point away from where the tornado and top of the horseshoe shaped gust front should be. Follow them in to the base of the storm:
Don't assume your tornado will always be there. They can form under the thick part of that wall cloud to the right. They can also form anywhere along that blue line, and also under adjacent updraft bases north and south of this blue line:
Anticyclonic funnels and tornadoes form on the southern/bottom end of the horseshoe shaped RFD gust front:
The El Reno supercell, looking northeast toward a rain obscure horseshoe shaped RFD gust front as an anticyclonic tornado is forming (right), the area where I'd expect the cyclonic tornado is marked by the orange arrow (left):
Textbook looking northwest:
Textbook looking north:
Of course not every storm is textbook, and not every storm presents a horseshoe shaped gust front. Most do not. Here's a weird storm, looking west:
Is that a wall cloud? Where is the horseshoe shape? Does it wrap around that lowered area, or extend more to the right?
The answer is that I don't know! This storm was not warned and did not produce a tornado, but I don't underestimate it either. I don't have my bearings on this storm so I give it a wide berth and don't drive underneath it.
A more linear looking gust front is usually associated with a linear bow echo. Weak tornadoes are possible, but damaging straight line winds and gustnadoes are much more likely:
If your squall line or bow echo does have a curved or horseshoe shaped base or gust front, treat this as a supercellular RFD gust front. You might have an embedded supercell, and tornadoes do form within kinks in these lines.
How about this messy HP looking west?
It's junky and outflowy so a tornado might not be imminent but we have plenty of clues to guess where it might be. Look for that curving RFD gust front and follow the pointy tail clouds into the base:
Most storms will present with mediocre gust fronts, and most of these do not produce tornadoes. The good news is that large, violent tornadoes usually present textbook hook echoes and supercell structure with RFD clear slots and horseshoe shaped RFD gust fronts. El Reno for example:
Initially difficult to discern from a distance due to the HP state of the storm, but the faint RFD gust front could be made out. Look at the barrel meso above it and follow that tail cloud in for clues though:
Closer to the storm, the structure was exquisitely textbook:
Don't rely on the wall cloud. It's not always there. Don't rely on your supercell schematics either. Every storm is different. Try and find some landmarks to help you identify FFD from RFD: horseshoe shaped gust fronts, barrel shaped updraft towers and mesocyclones, clear slots, wall clouds and tail clouds, shelf clouds, and hook echoes. On many storms these features are not obvious, and these are the ones to be careful around, not underestimate, and admit you don't have situational awareness.
