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METWSG/1-SN No. 4
/ METWSG/1-SN No. 4
10/10/07

METEOROLOGICAL WARNINGS STUDY GROUP (METWSG)

FIRST MEETING

Montréal, 20 to 22 November 2007

Agenda Item / 6: / Amendments to the provisions related to the content and issuance of SIGMETs to meet the evolving needs of flight operations.
6.2 / Development of a set of quantitative criteria to be included in Annex 3 for the threshold intensity of the weather phenomena to prompt the issuance of SIGMET messages

COMPLETION OF THE DEVELOPMENT OF QUANTITATIVE

CRITERIA FOR WEATHER PHENOMENA INCLUDED IN SIGMET

(Presented by the Secretary)

SUMMARY
This study note presentsfor consideration by the group a proposed course of action how to complete the development of the quantitative criteria for the threshold intensity of the weather phenomena prompting the issuance of SIGMET messages.

1.INTRODUCTION

1.1The introduction in Annex 3 — Meteorological Service for International Air Navigationof a set of quantitative criteria for the threshold intensity of the weather phenomena prompting the issuance of SIGMET was called for by the Limited Middle East Regional Air Navigation (LIM/MID RAN) Meeting (Cairo, 7 to 17 January 1996) through Conclusion 4/7. At that meeting it was noted that there were increasing opportunities for MWO to obtain guidance from numerical weather prediction models when preparing SIGMET messages and that to facilitate the use of such guidance it was necessary to try to establish such numerical thresholds that described the phenomena prompting the issuance of SIGMET. It was also recalled that at that time there were already quantitative definitions for two of the weather phenomena prompting SIGMET, i. e. tropical cyclone (the 10-minute mean surface wind equal or greater than 63 km/h (34 kt)) and severe mountain waves (the accompanying downdraft of 3 m/s (600 ft/min) or more and/or with observed/forecast severe turbulence).

1.2It should be noted that as a result of actions taken as a follow-up of Conclusion 4/7 of the LIM/MID RAN Meeting, significant progress has been achieved. In this regard requirements for the criteria related to phenomena included in SIGMET messages have been included in Annex 3 over the past ten years. The group will be pleased to note that criteria for the characterization of the following weather phenomena prompting the issuance of a SIGMET are included in Annex 3:

a)areas of thunderstorms and cumulonimbus (OBSC, EMBD, ISOL, OCNL);

b)area of thunderstorm (FRQ);

c)squall line (SQL);

d)hail (GR);

e)severe and moderate turbulence (TURB);

f)mountain wave (MTW); and

g)tropical cyclone.

The group is invited to review the list in Annex 3, Appendix 6, 4.2. It may be noted that some of the criteria are qualitative (e.g. presence or absence of hail); however, the group may wish to agree that the criteria as stated is sufficient except for icing. Furthermore, there are two phenomena for which there are no criteria listed under 4.2:

a)sandstorm; and

b)duststorm.

1.3Regarding icing, the group may wish to note that the World Area Forecast System Operations Group (WAFSOPSG) has been tasked to define such criteria. In this regard the WAFSOPSG had invited the World Area Forecast Centre (WAFC) Provider States to work on the feasibility of developing an objective icing index intended to improve the harmonization of icing forecasts provided by the WAFCs and also to assist meteorological watch offices (MWO) in the issuance of SIGMETs. In this regard it should also be noted that researchers continued to believe that the temperature, relative humidity, cloud liquid water content (LWC) and drop-size distribution remained the key meteorological factors contributing to airframe icing accretion. It may be noted that the development of objective criteria for icing are expected to be completed with the operational introduction of GRIB2-coded WAFS forecasts for icing in 2010.

1.4Based on the foregoing, the group may wish to concur that to complete this sub-task, criteria for duststorm and sandstorm should be developed. In accordance with the Working Arrangements between the International Civil Aviation Organization and the World Meteorological Organization (Doc7475), it may be considered that WMO should take the lead in this effort since the terms “sandstorm” and “duststorm” are used in general meteorology. The group may wish to agree to the following action:

Action agreed 1/.. — Quantitative criteria for "sandstorm and "duststorm"
That the WMO member of the group be tasked to develop quantitative criteria for “sandstorm” and “duststorm”, to be used for the issuance of SIGMET in time for the METWSG/2 Meeting.

2.action by the metwsg

2.1The METWSG is invited to:

a)note the information in this paper; and

b)decide on the draft action proposed for the group’s consideration.

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