Schematron rule SIGMET.SIGMET-5 is too restrictive

[blchoy]

There were reports (cbs-tt-avxml:1086 and cbs-tt-avxml:1142) saying that the schematron rule SIGMET.SIGMET-5 is too restrictive.

Checking history, the rules comparing phenonemonTime against validPeriod first appeared in IWXXM v2.1 as below:

• SIGMET.SECC1: When SIGMETEvolvingConditionCollection timeIndicator is an observation, the phenomenonTime must be earlier than or equal to the beginning of the validPeriod of the report.
• SIGMET.SECC2: When SIGMETEvolvingConditionCollection timeIndicator is a forecast, the phenomenonTime must be later than or equal to the beginning of the validPeriod of the report.
• AIRMET.AECC1: When AIRMETEvolvingConditionCollection timeIndicator is an observation, the phenomenonTime must be earlier than or equal to the beginning of the validPeriod of the report.
• AIRMET.AECC2: When AIRMETEvolvingConditionCollection timeIndicator is a forecast, the phenomenonTime must be later than or equal to the beginning of the validPeriod of the report.

In IWXXM v3.0 they have changed to:

• SIGMET.SIGMET-5: iwxxm:analysis//iwxxm:phenomenonTime must be equal to iwxxm:validPeriod//gml:beginPosition.
• SIGMET.SIGMET-6: iwxxm:forecastPositionAnalysis//iwxxm:phenomenonTime must be less than or equal to iwxxm:validPeriod//endPosition.
• AIRMET.AIRMET-5: iwxxm:analysis//iwxxm:phenomenonTime must be equal to iwxxm:validPeriod//gml:beginPosition.

While there are no explicit descriptions on how the elements should be related in Annex 3, Paras 3.5.2.3 and 3.5.2.4 of the Regional SIGMET Guide Template (there was a working paper METPWGMOG/7/SN/30-rev discussing an update to it in 2018) does mention:

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3.5.2.3. The following regulations apply when determining the validity period:

• The period of validity of a WS SIGMET should not be more than 4 hours;
• The period of validity of a WC or WV SIGMET should not be more than 6 hours;
• In case of a SIGMET for an observed phenomenon, the filing time (date/time group in the WMO header) should be the same or very close to the time in the date/time group indicating the start of the SIGMET validity period;
• When the SIGMET is issued for a forecast phenomenon:
  • the beginning of validity period should be the time of the expected commencement (occurrence) of the phenomenon in the MWO area of responsibility;
  • the time of issuance of a WS SIGMET should not be more than 4 hours before the start of validity period (i.e., expected time of occurrence of the phenomenon); and for WC (tropical cyclone) and WV (volcanic ash) SIGMET the lead time should not be more than 12 hours.

3.5.2.4. The period of validity is that period during which the SIGMET information is valid for transmission to aircraft in flight. Examples:

1. First two lines of a SIGMET for an observed phenomenon: WSTH31 VTBS 241120 VTBB SIGMET 3 VALID 241120/241500 VTBS–
2. First two lines of a SIGMET for a forecast phenomenon (expected time of occurrence 1530) WSSR20 WSSS 311130 WSJC SIGMET 1 VALID 311530/311930 WSSS-

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   Therefore SIGMET.SIGMET-5 (and also AIRMET.AIRMET-5) should only be valid when @timeIndicator="FORECAST". This should considered to be a bug and should be fixed.

Taking also into account of the good things we have in v.2.1, the following revision to SIGMET.SIGMET-5, SIGMET.SIGMET-6 and AIRMET.AIRMET-5 are proposed:

1. For iwxxm:analysis (applies to both SIGMET and AIRMET):

   • if @timeIndicator="OBSERVATION", the phenomenonTime should be less than or equal to the start of validTime
   • if @timeIndicator="FORECAST", the phenomenonTime should be greater than or equal to the start of validTime

2. For iwxxm:forecastPositionAnalysis (applies to SIGMET only):

   • the phenomenonTime should be within the start and end of validTime

Views are most welcomed.

[blchoy]

Fixed in Version 3.0-dev.

For record purpose SIGMET.SIGMET-5 and AIRMET.AIRMET-5 was split into 5a and 5b respectively to cater for the two cases when @timeIndicator = 'OBSERVATION' and @timeIndicator = 'FORECAST'.