How about this one
LFBZ 161339Z 1613/1712 28010KT 9999 BKN014 OVC030 TEMPO 1613/1616 BKN009 PROB30 TEMPO 1700/1706 -SHRA BKN014 BECMG 1706/1707 4000 RA BKN010 OVC020 BECMG 1710/1712 9999 NSW BKN015
Can you have NSW and BKN015 at the same time?
Doesn’t No significant weather simply mean no precipitation in this context?
Can you have NSW and BKN015 at the same time?
Yes, I think so. BKN015 is of no operational significance. “NSW” indicates that 4000 RA, i.e. precipitation, which is significant weather, will have ended latest at 1712.
I am also wondering that since the TAF itself ends 1712, isn’t the statement BECMG 1710/1712 a little suspect i.e. the improvement could take place after 1200?
Doesn’t the BECMGmeans that between 10 and 12 on the 17 the weather will change to NSW BKN015? It doesn’t mean that the NSW / BKN will end before 12.
Peter wrote:
Can you have NSW and BKN015 at the same time?
Yes. Clouds are not “weather”. Weather is the stuff that goes between the visibility and cloud groups. In this case it is the rain that will cease.
Noe wrote:
Doesn’t the BECMGmeans that between 10 and 12 on the 17 the weather will change to NSW BKN015? It doesn’t mean that the NSW / BKN will end before 12.
It means that the change from the previous weather to NSW BKN015 will happen somewhere in the time period between 1000Z and 1200Z, so by 1200Z at the latest you should have the NSW BKN015 situation.
In this TAF
EDNY 101100Z 1012/1112 VRB03KT 5000 -RA BR SCT005 OVC020 TEMPO 1012/1016 4000 RADZ BKN010 BECMG 1017/1020 03008KT BECMG 1020/1022 OVC012 BECMG 1107/1109 BKN015 TEMPO 1109/1112 04013KT
Is the -RA BR apply all the way through the TAF period (i.e. until 1112) or does it get cancelled by BECMG 1020/1022 OVC012?
I know for a fact it does not get cancelled by “BECMG 1017/1020 03008KT” since that is just a wind change.
My view is that it applies all the way through, strange as it would be to forecast 5km vis to be constant over such a long period.
Peter wrote:
Is the -RA BR apply all the way through the TAF period (i.e. until 1112) or does it get cancelled by BECMG 1020/1022 OVC012?I know for a fact it does not get cancelled by “BECMG 1017/1020 03008KT” since that is just a wind change.
My view is that it applies all the way through,
You’re right, it does apply all the way through. The “-RA BR” has to be cancelled explicitely by writing “NSW” (No Significant Weather). Similarly cloud info is cancelled by “NSC” (No Significant Cloud).
strange as it would be to forecast 5km vis to be constant over such a long period.
It could be that the visibility improvement is not large enough to warrant a change in the TAF. There can be surprisingly large expected changes without the TAF having to mention it.
In addition to NSW/NSC, CAVOK may also be used to cancel relevant weather phenomena and cloud for the period covered by the change group.
The technical reference for AA’s first comment is regulation 51.8.3 in WMO-No. 306 (Manual on Codes), Vol. I.1, Part A (link). A description of the significant changes in AA’s second comment are recommended in WMO-No. 49 (Technical Regulations), Vol. II – Meteorological Service for International Air Navigation, appendix 5, 1.3.2 (link). Note also point 1.3.3: “… in the case of significant changes in respect of cloud, all cloud groups, including layers or masses not expected to change, should be indicated.”
