The dry downdraft that wraps around the circulation, the bright slot that gives it away, and why its arrival is the opposite of good news.

In this lesson you will

  • State the NWS definitions of the rear-flank downdraft and the clear slot.
  • Describe the wrap-around or horseshoe appearance of an occluding mesocyclone.
  • Relate the appearance of a clear slot to the timing of tornadogenesis.
  • Summarize the warm-versus-cold RFD research at a spotter level, and state what part of it is not visually knowable.

Two definitions that describe one thing

RFD, NWS, verbatim
A region of dry air subsiding on the back side of, and wrapping around, a mesocyclone. It often is visible as a clear slot wrapping around the wall cloud.
Clear slot, NWS, verbatim
A local region of clearing skies or reduced cloud cover, indicating an intrusion of drier air; often seen as a bright area with higher cloud bases on the west or southwest side of a wall cloud. A clear slot is believed to be a visual indication of a rear flank downdraft.

The RFD is the process. The clear slot is what you see of it. Dry air descending and wrapping round the back of the circulation evaporates cloud as it goes, which is why the signature is brightness in the wrong place: a lighter wedge working into a dark base, with visibly higher cloud bases inside it.

The horseshoe

Watch the slot over several minutes and it does not sit still. It advances around the wall cloud, usually from the west or south-west, and as it wraps it can leave the lowering standing in a horseshoe of clear air open on one side. That is the visual of an occlusion: the RFD closing around the updraft and cutting it off from the inflow feeding it.

Field Guide, verbatim
Clearing skies working into the rain-free base, which suggests a part of the rear-flank downdraft is wrapping around the mesocyclone. This often precedes or accompanies tornado formation in supercell thunderstorms.

Precedes or accompanies. Not follows. The single most common misreading of this feature is that the storm is clearing out and weakening, and the truth is close to the reverse: you are watching the part of the cycle in which tornadoes most often appear.

A dark, rounded lowering hangs beneath a storm base over open farmland with a narrow cone-shaped tornado beneath it. To the left of the lowering the cloud base is visibly brighter and higher, with a wedge of lighter sky cutting in toward the tornado
The same photograph as lesson 3, looked at differently. The bright wedge cutting into the base beside the lowering is the clear slot, and there is a tornado underneath. This is what "precedes or accompanies" looks like. Photo courtesy of NSSL, public domain, via SPC
Safety, forward to module 8
A clear slot advancing around a wall cloud is telling you where the circulation is about to be. Do not be in its path, and do not drive into the bright area to get a better view of the dark one.

Warm RFD, cold RFD, and the limits of your eyes

Markowski, Straka and Rasmussen drove instruments into rear-flank downdrafts and found a real difference: the RFDs of tornadic supercells were warmer and more buoyant at the surface than those of nontornadic ones, by a few kelvin. One dataset put the mean difference at about 1.9 K, with roughly 3 to 5 K across the wider literature.

What you can do with that
Almost nothing, in the moment, and that is the honest answer. A few kelvin is not something you can judge by eye or by how the wind feels on your face, and this course is not going to pretend otherwise. Know it because it explains why the RFD matters rather than being an incidental downdraft, and because you will meet the terms. Do not turn it into a field test you cannot run.

Check yourself

A bright, higher-based slot of clearing sky is working into the storm base on the south-west side of a rotating wall cloud. What does it most likely mean?

  1. The rear-flank downdraft is wrapping around the circulation, which often precedes or accompanies tornado formation
  2. The storm is weakening as dry air erodes the updraft
  3. The wall cloud is dissipating and the threat has passed
  4. A new updraft is forming to the south-west
Hint

The Field Guide has an explicit sentence about clearing skies working into the base.

Answer

The rear-flank downdraft is wrapping around the circulation, which often precedes or accompanies tornado formation

This is the clear slot, and the Field Guide says clearing skies working into the base "often precedes or accompanies tornado formation". Reading it as weakening is the classic error and a dangerous one, because the slot marks where the circulation is heading.

Sources

  1. NWS online glossary
  2. NWS Weather Spotter's Field Guide (YPA-201154, June 2011)
  3. Branick, A Comprehensive Glossary of Weather Terms for Storm Spotters, NOAA Technical Memorandum NWS SR-145
  4. Markowski, Straka & Rasmussen 2002, Mon. Wea. Rev. 130(7), 1692–1721: direct surface thermodynamic observations within the rear-flank downdrafts of nontornadic and tornadic supercells
  5. Markowski 2002, Mon. Wea. Rev. 130(4), 852–876: hook echoes and rear-flank downdrafts, a review
  6. SPC public domain tornado images, scanned from the NSSL storm intercept archive