Where rotation actually lives in a storm, what the word for it means, and why most of it never produces a tornado.

In this lesson you will

  • Label the updraft, forward-flank downdraft, rear-flank downdraft and gust fronts on a supercell plan view.
  • Define mesocyclone as a radar term, and state why it is not something a spotter reports seeing.
  • State the typical diameter of a mesocyclone.
  • Explain why most detected rotation is not associated with a tornado.

What makes a supercell a supercell

One thing: a persistent rotating updraft. Everything else about the storm follows from that. The Field Guide puts it as two characteristics rather than one, and both are worth having in your own words:

Field Guide, verbatim
Two important characteristics that distinguish supercells from ordinary thunderstorms are: Persistent rotation at the rain-free base, and a rear-flank downdraft (RFD): region of dry air wrapping along the back portion of the circulation within the storm.

Note that the Field Guide says rain-free base. Module 3 lesson 3 covered why the NWS now prefers updraft base for the same feature, and this module uses the newer term except when quoting.

Overhead schematic of a supercell moving north-east. A forward-flank precipitation core fills the north and east. The updraft sits on the rear south-west side, ringed by a dashed circle labeled mesocyclone with a curved arrow showing counterclockwise rotation. A rear-flank downdraft descends from the west and its dry air wraps around the south side of the circulation. A forward-flank gust front runs along the south-east edge of the precipitation and a rear-flank gust front curves around the south and west of the updraft. Warm inflow arrows feed the updraft from the south-east.
The plan view Lemon and Doswell established in 1979, and the map the rest of this module is drawn on. The mesocyclone is dashed for a reason, which is the next section. WxAlerts, after Lemon & Doswell 1979
Updraft
The rising column. In a supercell it rotates, it persists for hours, and it is the only part of the storm a spotter stands under and can see into.
FFD
Forward-flank downdraft. The main precipitation, falling out ahead of and beside the updraft. This is the part radar reads most cleanly, and the part that hides everything behind it.
RFD
Rear-flank downdraft. Dry air descending behind the updraft and wrapping around the circulation. Lesson 4 is about nothing else.
Gust front
The leading edge of outflow from either downdraft. Outflow, not inflow, which is the distinction lesson 5 turns on.
Occlusion
The stage where the RFD has wrapped far enough around the updraft to cut it off from its inflow. Tornadoes often occur near this point in the cycle.

Mesocyclone is a radar word

This is the correction the whole module is built on, so it gets the glossary in full rather than a paraphrase.

SR-145, verbatim
A storm-scale region of rotation, typically around 2-6 miles (3-10 km) in diameter. [It] should not be considered a visually-observable phenomenon (although visual evidence of rotation, such as curved inflow bands, may imply the presence of a mesocyclone).

Read the size again: two to six miles across. Standing underneath one, you are inside something wider than your whole field of view, and what you can actually see is a handful of features it produces. A wall cloud. Curved inflow bands. A clear slot. Those are observations. The mesocyclone is an inference from them, and a radar makes it better than you do.

Never report this
"I have a mesocyclone." A forecast office cannot act on that, because you have told them your conclusion instead of your observation, and their radar already has an opinion about the conclusion. Report the rotating wall cloud, the funnel, the debris. Those are things only you can see.

Most rotation is not a tornado

Trapp, Stumpf and Manross went through more than five thousand mesocyclone detections that met the radar algorithm's own criteria, and asked how many came with a tornado.

Detected mesocyclonesTornadic
All 5,322 detections26%
Low-altitude, base at or below 1 kmover 40%
Midaltitude15%

Trapp, Stumpf & Manross 2005. The pattern is the useful part: how close to the ground the rotation is detected matters more than whether rotation was detected at all.

The rate at which supercells produce tornadoes is quoted two different ways by two federal sources, and they are given here separately rather than averaged into one confident-sounding number. NSSL says as few as 20 percent of supercells produce tornadoes. The Field Guide's supercell page says 30 percent or less. Both are official; neither is a peer-reviewed statistic; quote whichever you are citing and say which.

Teaching point
Every number on this page points the same way. Rotation is common and tornadoes are not, so the presence of rotation is not the report. What is happening near the ground is the report, and near the ground is exactly where radar is weakest. That is the job this module is training you for.

Check yourself

A spotter radios in: "I have a mesocyclone on this storm." What is wrong with the report?

  1. A mesocyclone is a radar-scale feature, not a visual one, so they have reported a conclusion instead of an observation
  2. Nothing, provided the storm is a supercell
  3. Mesocyclones are too small to see from the ground
  4. Only the forecast office is permitted to use the term
Hint

Think about what the office can do with the sentence once it arrives.

Answer

A mesocyclone is a radar-scale feature, not a visual one, so they have reported a conclusion instead of an observation

SR-145 is explicit that a mesocyclone "should not be considered a visually-observable phenomenon". At two to six miles across it is wider than the spotter's field of view, and the office's radar already detects it better than a person can. What the office cannot get anywhere else is the rotating wall cloud, the funnel or the debris, so that is what goes in the report.

Sources

  1. NWS Weather Spotter's Field Guide (YPA-201154, June 2011)
  2. Branick, A Comprehensive Glossary of Weather Terms for Storm Spotters, NOAA Technical Memorandum NWS SR-145
  3. NWS online glossary
  4. Trapp, Stumpf & Manross 2005, Wea. Forecasting 20(4), 680–687: a reassessment of the percentage of tornadic mesocyclones
  5. Lemon & Doswell 1979, Mon. Wea. Rev. 107(9), 1184–1197: severe thunderstorm evolution and mesocyclone structure as related to tornadogenesis
  6. NSSL Severe Weather 101, tornado basics and tornado types