Reporting of tornadoes

AMOFSG/7-SN No. 9 17/3/08

AERODROME METEOROLOGICAL OBSERVATION AND FORECAST STUDY GROUP (AMOFSG)
SEVENTH MEETING

Montréal, 9 to 12 September 2008

Agenda Item

5:

Aerodrome observations

REPORTING OF TORNADOES
(Presented by Bill Maynard)

SUMMARY

Both Annex 3 and WMO No. 306 are internally inconsistent regarding the use of the ‘+FC’ code to distinguish tornadoes and waterspouts from funnel clouds that do not reach that ground as indicated by “FC”. It is proposed that this situation be resolved in favour of enabling the use of “+FC”. .

1. 
   INTRODUCTION
   
   1. 
      The Aerodrome Meteorological Observing and Forecasting Study Group (AMOFSG) was established by the International Civil Aviation Organization (ICAO) Air Navigation Commission (ANC) to assist the meteorology secretary of the Air Navigation Bureau in reviewing the observation and forecasting standards and recommended practices in Annex 3. This study group is the successor to the Aerodrome Meteorological Observing Systems Study Group(AMOSSG) and has carried forward its work program with the addition of an explicitly wider scope to include forecasting topics.
      
   2. 
      Both Annex 3 and World Meteorological Organization Manual No. 306 - Manual on Codes – are internally inconsistent regarding the use of “+FC” to distinguish tornadoes and waterspouts from funnel clouds which are reported by “FC”. Both Canada have implemented the use of “+FC”, in METAR / SPECI and in TAF, when applicable. Excerpts from the applicable domestic standards are shown in Appendix A.
      
2. 
   Discussion
   
   1. 
      The use of the FC present weather code within METAR and SPECI is defined in Annex 3, Appendix 3, paragraph 4.4.2.3 as follows (in part):

      4.4.2.3 Recommendation.— In local routine and special reports and in METAR and SPECI, the following types of present weather phenomena should be reported, using their respective abbreviations and relevant criteria, as appropriate:…

      
      
      …d) Other phenomena…
      
      …Funnel cloud (tornado or waterspout) FC…
      
   2. 
      Similarly, TAF should include “funnel cloud (tornado or waterspout)” in accordance with recommendation 1.3.1 g) of Appendix 5 of the annex. Neither Tables A3-2 or Table A5-1 provide a basis for the inclusion of “+FC’ within METAR / SPECI or TAF.
      
   3. 
      Section 4.4.2.6 of Appendix 3 to Annex 3 and Table A3-2 reserve the use of “+” to indicate intensity. World Meteorological Organization Manual No. 306 – Manual on Codes – states specifically in paragraph 15.8.4 of Volume I.1 that “Intensity shall be indicated only with precipitation, precipitation associated with showers and/or thunderstorms, duststorm or sandstorm.”
      
   4. 
      By contrast with the references indicated in paragraphs 2.1 to 2.3; Appendix 1, Model A to Appendix 3, pertaining to OPMET, states that “+” is used for “heavy, or well-developed in the case of dust/sand whirls (dust devils) and funnel clouds”. Similarly, Code Table 4678, as referenced by paragraph 15.8.4 of WMO No. 306, states that “+” is to be used for “heavy (well developed in the case of dust/sand whirls (dust devils) and funnel clouds”, even though note (5) to the table restates that “intensity shall be indicated only with precipitation, precipitation associated with showers and/or thunderstorms, duststorm or sandstorm.” [Bold emphasis added].
      
   5. 
      In consideration of the above, either Code Table 4678 is in error or both note (5) to the Table and paragraph 15.8.4 are incomplete. Similarly there is an inconsistency between the description of intensity given in Appendix 1 of Annex 3 and the other references to intensity in the Annex.
      
   6. 
      It can be argued that the difference between tornadoes and/or waterspouts and funnel clouds that do not touch the ground may be significant to aviation for reasons which could include:
      

• 
  The potential for extensive foreign object damage associated with tornadoes;
  
• 
  The potential for damage to aerodrome facilities, including navigation aids and power sources associated with tornadoes;
  
• 
  The high probability of one or all of severe turbulence, intense lightning, erratic altimeter readings, wind shear, airborne debris, hail, heavy precipitation and very strong and rapidly shifting winds associated with tornadoes; and
  
• 
  The potential for damage to aircraft on the ground, and dangers to ground personnel associated with tornadoes.
  

3. 
   Conclusion
   
   1. 
      In the opinion of the author the hazards to aviation associated with tornadoes are such that the benefit of the doubt should be given to adopting the coding practice that differentiates between funnel clouds and tornadoes or waterspouts even if the merit for doing so is largely hypothetical. Therefore the annex should be revised to be consistent and to enable the use of “+FC” when warranted.
      
4. 
   ACTION BY THE GROUP
   
   1. 
      Review and comment upon the contents of this paper. Operators, in particular, are requested to validate or reject the assertion that explicit reporting of tornadoes is relevant to aviation.
      
   2. 
      Agreement with the conclusion stated in paragraph 2.7.
      

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AMOFSG/7-SN No. 9 Appendix

APPENDIX

Excerpts From Domestic Standards Referring To The Use Of ‘+FC’
A. From the Federal Meteorological Handbook No. 1 (September 2005):
8.3.3 Other Weather Phenomena
c. Funnel Cloud (Tornadic Activity)

(1) Tornado. A violent, rotating column of air touching the ground.

(2) Funnel Cloud. A violent, rotating column of air which does not touch the surface.

(3) Waterspout. A violent, rotating column of air that forms over a body of water, and touches the water surface.

[Note 4 to Table 8-5 states] “ Tornadoes and waterspouts shall be coded as +FC.”

B. From the Manual of Surface Observations as referenced by the Canadian Aviation Regulations
3.2 TORNADOES AND WATERSPOUTS (+FC). These phenomena are associated with a violently rotating column of air, pendant from a Cumulonimbus cloud. This violent whirlwind is nearly always observable as a cloud column or inverted cloud cone (funnel cloud), and a “bush” composed of water droplets raised from the surface of the sea or of dust, sand, or litter, raised from the ground (see photo on next page).
3.2.1 This phenomenon is called a TORNADO when it occurs over land and a WATERSPOUT when it occurs over water.
3.2.2 The observer shall note the direction of the storm from the station and the direction towards which it is moving. Intensity values are not ascribed to tornadoes or waterspouts. The plain language words TORNADO or WATERSPOUT shall be inserted in the coded weather reports whenever these phenomena are observed.
3.2.3 A Tornado or Waterspout shall not be reported when the vortex does not reach the ground, or when the observer is not sure that the vortex of the funnel reaches the ground (or water), that is, a “bush” is not observed. In this case, FUNNEL CLOUD and its direction from the station shall be reported.
10.2.10.1 Symbols for the conditions of weather and obstructions to vision which may be reported in Column 32 are listed below:
WEATHER PHENOMENA AND SYMBOLS

Tornado........................................................................+FC (TORNADO in remarks)

Waterspout...................................................................+FC(WATERSPOUT in remarks)

Funnel Cloud.............…………....................................FC (FUNNEL CLOUD in remarks)

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