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Including significant weather and TCU in TAF

AMOFSG/7-SN No. 19

AMOFSG/7-SN No. 19

5/8/08

AERODROME METEOROLOGICAL OBSERVATION AND FORECAST STUDY GROUP (AMOFSG)

SEVENTH MEETING

Montréal, 9 to 12 September 2008

Agenda Item

Aerodrome forecasts

INCLUDING SIGNIFICANT WEATHER AND TCU IN TAF

(Presented by Kees Blom)

SUMMARY

Whilst investigating the new TAF code to be introduced according to AMD 74, on 5 November 2008, several inconsistencies were identified.

INTRODUCTION
1. According to AMD74 to Annex 3 a new TAF code will be introduced in November 2008, mainly concerning the extension of the validity period of the TAF and the fact that no longer “short” and “long” TAFs will be allowed for the same aerodrome.
2. These changes, however brought to the attention of users and service providers with AMD 74 in 2007, are putting a lot of pressure on users and providers to implement the required changes in a timely manner. Especially since the complete details became only available during 2^nd Quarter of 2008.
3. Furthermore several inconsistencies were identified which give rise to questions about the implementation and might lead to non uniform implementation, thus causing communication and interpretation problems. These problems will in turn lead to more costs for the users since in software either more probable coding should be allowed or TAFs might be rejected by communication or end-user systems.
DISCUSSION
ICAO Annex 3, APP 5, para 1.2.3 :

Weather phenomena

Recommendation.— One or more, up to a maximum of three, of the following weather phenomena or combinations thereof, together with their characteristics and, where appropriate, intensity, should be forecast if they are expected to occur at the aerodrome:

— freezing precipitation

— freezing fog

— moderate or heavy precipitation (including showers thereof)

— low drifting dust, sand or snow

— blowing dust, sand or snow

— duststorm

— sandstorm

— thunderstorm (with or without precipitation)

— squall

— funnel cloud (tornado or waterspout)

— other weather phenomena given in Appendix 3, 4.4.2.3, only if they are expected to cause a significant change in visibility.

The expected end of occurrence of those phenomena should be indicated by the abbreviation “NSW”.

ICAO Annex 3, APP 5, Table A5-1 Template for TAF:

Weather (C)4, 5

Intensity of

weatherphenomena (C)6

– or +

Characteristics and type of

weatherphenomena (C)7

DZ or RA or

SN or SG or

PL or DS or

SS or

FZDZ or

FZRA or

SHGR or SHGS or

SHRA or

SHSN or

TSGR or

TSGS or

TSRA or

TSSN

IC or FG or

BR or SA or

DU or HZ or

FU or VA or

SQ or PO or

FC or TS or

BCFG or BLDU or

BLSA or

BLSN or

DRDU or

DRSA or

DRSN or

FZFG or

MIFG or

PRFG

4. To be included whenever applicable.

5. One or more, up to a maximum of three, groups in accordance with 1.2.3.

6. To be included whenever applicable in accordance with 1.2.3. No qualifier for moderate intensity.

7. Weather phenomena to be included in accordance with 1.2.3.

This table seems to imply that e.g. –RA, -DZ, BR, HZ etc. are allowed in the TAF, but the notes are contradicting this by referring to 1.2.3. It might also be that these phenomena are only listed because of the last item in 1.2.3….. other weather phenomena given in Appendix 3, 4.4.2.3, only if they are expected to cause a significant change in visibility.

When this is the case the question arises whether these phenomena can only be used after change groups (this seems to be implied by significant change) or may also be used in the first group of the TAF?

WMO-No. 306, §51.5.1 does not indicate that “other weather phenomena” can be forecast. WMO only lists a limited number of “significant” weather phenomena. These, and only these, phenomena are allowed. §51.1.5 also indicates the w´w´ group shall be omitted if weather is absent or not significant. WMO’s list in §51.5.1 corresponds to the one given in ICAO Annex 3, para 1.3, f) and g) dealing with weather phenomena in change groups!
Another issue dealing with “significant” weather is the question of which phenomena are “allowed” to be indicated after a FM change indicator? An interpretation is that FM differs from the other change indicators since it is the start of a new, self-contained forecast, thus being subject to the requirements given in Annex 3, APP 5, para 1.2.3. This would mean that NSW is not to be used directly after FM. However several examples have been found of TAFs containing NSW after FM.
AMD 74 to Annex 3 also gives a new definition for “Cloud of operational significance”: A cloud with the height of cloud base below 1 500 m (5 000 ft) or below the highest minimum sector altitude, whichever is greater, or a cumulonimbus cloud or a towering cumulus cloud at any height. In here the addition of towering cumulus (TCu) causes inconsistency problems.
Annex 3, APP 5, para 1.2.4:

Cloud

Recommendation.— Cloud amount should be forecast using the abbreviations “FEW”, “SCT”, “BKN” or “OVC” as necessary. When it is expected that the sky will remain or become obscured and clouds cannot be forecast and information on vertical visibility is available at the aerodrome, the vertical visibility should be forecast in the form “VV” followed by the forecast value of the vertical visibility. When several layers or masses of cloud are forecast, their amount and height of base should be included in the following order:

a) the lowest layer or mass regardless of amount, to be forecast as FEW, SCT, BKN or OVC as appropriate;

b) the next layer or mass covering more than 2/8, to be forecast as SCT, BKN or OVC as appropriate;

c) the next higher layer or mass covering more than 4/8, to be forecast as BKN or OVC as appropriate; and

d) cumulonimbus clouds, whenever forecast and not already included under a) to c).

Cloud information should be limited to cloud of operational significance; when no cloud of operational significance is forecast, and “CAVOK” is not appropriate, the abbreviation “NSC” should be used.

As shown in 2.6 Cumulonimbus cloud is included, TCu is not, while in the last sentence reference is made to “clouds of operational significance”. Same is occurring in Table A5-1 Template for TAF: Under Cloud-type only CB is mentioned, not TCu.

CONCLUSION
1. Several inconsistencies exist in Annex 3 after AMD 74.
2. Questions have been posed on the use of “significant weather phenomena” in TAF, especially after change groups.
3. It is proposed to add some guidance to the relevant paragraph(s) and table(s) on the item of significant weather to avoid the misinterpreting (and not uniform usage) as it occurs at the moment.
4. It is not completely clear whether the inclusion of TCu in the definition of “clouds of operational significance” implies that TCu should always be included in TAF, in the same way as already done for CB.
5. It is proposed to use TCu exactly the same as CB in TAF, also to avoid misinterpretation and/or not uniform usage. This could be done by including TCu after CB in the relevant paragraph(s) and table(s).
ACTION BY THE GROUP
1. The group is requested to:

note the content of this paper; and
consider the proposal in para. 3.3 and, if supported by the group, formulate a draft amendment to Annex 3 for incorporation in Amendment 75.
consider the proposal in para. 3.5 and, if supported by the group, formulate a draft amendment to Annex 3 for incorporation in Amendment 75.

— END —

5 AMOFSG7SN NO 16 AMOFSG7SN NO 16 26508 AERODROME

Tags: amofsg7sn, aerodrome