AMOSSG/4-SN No. 17

10/9/03

AERODROME METEOROLOGICAL OBSERVING SYSTEMS STUDY GROUP (AMOSSG)

FOURTH MEETING

Montreal, 15 to 18 September 2003

Agenda Item	5:	Aerodrome observation requirements
5.1:	Reporting requirements and criteria
Agenda Item	7:	Aerodrome forecast requirements

HARMONIZATION OF SPECI CRITERIA WITH TAF CHANGE CRITERIA AND REMOVING THE TERM "PROB" IN THE TAF DURING THE FIRST 3 HOURS

(Presented by Akoa Benoit Okossi)

SUMMARY
This paper outlines the operational need for harmonizing the criteria for issuance of SPECI and those for including change groups in TAF, and a need of removing the term "PROB" in the TAF during the first 3 hours, and then suggests that the Group invites the Secretary to prepare a draft amendment to Annex 3 on this issue in time for the AMOSSG/5.

1.	INTRODUCTION

1.1	The group will recall that Recommendation 2/3 b) of the MET Divisional Meeting (2002) invited ICAO, in consultation with the WMO, to consider the need for harmonizing the criteria for issuance of SPECI and those for including change groups in TAF. Moreover, the Air Navigation Commission took action on a proposal to study the operational feasibility and desirability of requiring a greater accuracy in the first three hours of a TAF by inter alia removing the term "PROB" during that period.

1.2	Considering difficulties raised in most TAF verification schemes, this paper outlines the need for harmonizing the criteria for issuance of SPECI and those for including change groups in TAF. It also emphasizes a need of removing the term "PROB" in the TAF during the first 3 hours to improve the forecast during that period.

2.	DISCUSSIONS

2.1	Usually TAF verification schemes compare airdrome observation reports METAR/SPECI against different time blocks of a TAF. But the difference between the criteria for issuance of SPECI and those for including change groups in TAF makes this comparison sometimes very hard to handle.

2.2	NORTAF (1996), e.g. a TAF verification scheme from a group of European countries (Norway, Sweden, Finland and Denmark) shows that some of the most difficult problems in TAF verification concern the verification of PROB and TEMPO groups. This method sidesteps completely the PROB group problems by considering only the worst forecast conditions. The most obvious way to address the issue is to regard PROB forecast as probability forecast and then apply standard verification measures, but an inherent weakness of this approach is that the probabilities assigned to PROB forecasts are necessarily arbitrary.

2.3	So using the term "PROB" in the beginning of a TAF validity period (in the first 3 hours), not only will contribute to decrease the accuracy of that forecast but also can make TAF verification very hard to process.

3.	CONCLUSION

3.1	It can be concluded that there is an operational need to harmonize the criteria for issuance of SPECI and those for including change groups in TAF, and to remove the use of the term "PROB" from the forecast during the first 3 hours to improve the accuracy of the TAF during that period.

4.	ACTION BY THE GROUP

4.1	The group is invited to:

a)	agree that there is an operational need to harmonize the criteria for issuance of SPECI and those for including change groups in TAF, and that the Secretary should prepare a draft amendment to Annex 3 in time for the AMOSSG/5 Meeting; and

b)	agree that there is a need to remove the use of the term "PROB" from the forecast during the first 3 hours to improve the accuracy of the TAF during that period, and that the Secretary should prepare a draft amendment to Annex 3 in time for the AMOSSG/5 Meeting.

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