TAF and 1-2-3 Rule

[DLconnection]

KTOL 292334Z 300024 05014G22KT P6SM SCT025 OVC035
TEMPO 0004 BKN025
FM0400 05014G22KT P6SM OVC025
TEMPO 0612 2SM -SHRA BR BKN004 OVC012
FM1200 04014G24KT P6SM OVC012
FM1800 02014G24KT P6SM OVC025=


1-2-3 Rule

The 123 rule states that 1hr before/1hr after ETA you need to have 2000ft ceiling and 3 miles vis.

I want to make sure that I am reading this wx report properly. Lets say that I was expecting to arrive at TOL at 0600Z. According to the report, temporarily between 0600Z and 1200Z we are expected to have poor wx conditions. Now, according to the WX services book, temporarily means that the wx described after the word TEMPO is forcasted to happen for only an hour somewhere in that time interval.

In this particular report we have a 6hr window. How exactly should I take this forcast into account when trying to use the 1-2-3 Rule? Im not sure since the weather is not Exactly forcasted to be there at 0600Z. The conditions simply might occur at that time.

 

[dispatchguy]

KTOL 292334Z 300024 05014G22KT P6SM SCT025 OVC035
TEMPO 0004 BKN025
FM0400 05014G22KT P6SM OVC025
TEMPO 0612 2SM -SHRA BR BKN004 OVC012
FM1200 04014G24KT P6SM OVC012
FM1800 02014G24KT P6SM OVC025=

If you're arriving anytime between 05Z-13Z, that tempo comes into play (dont forget the hour before/after), so according to that TAF for TOL, an alternate would be required for any ETA between 05Z-19Z, since you have 1200' OVC forecast til 18Z. TEMPO is the equivalent of the old CHC (chance). All it is is the forecaster hedging his bets that SOMETIME, even if for only a minute, during that time period is it going to be that TEMPO.

Also, even if those were FEW004 SCT012, I would probably add an alternate as well - the clouds could thicken, and voila, you've got a ceiling.

Keep in mind, also, that some airlines have their own rules about whether an alternate is required. One major has a 5000-5 rule, if its worse than 5000-5, you get an alternate.

 

[JetPilot500]

Also, even if those were FEW004 SCT012, I would probably add an alternate as well - the clouds could thicken, and voila, you've got a ceiling.

That may be good advice, but keep in mind, LEGALLY you don't need to file an alternate based on SCT or FEW.

You must file an alternate if during your one hour before and one hour after period you have ANY forcast of less than 2000/3, including PROBs or TEMPOs.

JetPilot500

 

[dispatchguy]

You are very correct, but I know some crews who prefer to have that added fuel and weather in their brief - into some airports (such as SFO), if I saw a FEW004, to me thats a mandatory alternate - "any other condition which may delay landing of the aircraft" (Factors for computing fuel required - who's FAR reference escapes me at the moment).

FEW clouds can mess up an operation - especially right along the final approach course.