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7 Answers

Terminal Aerodrome Forecast’s (TAF’s)

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General Aviation

TAF's are issued every 6 hours at 0000z, 0600z, 1200z, 1800z. Why are they valid for 30 hrs if they are issued very 6 hrs.?

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7 Answers

  1. Sans on Dec 12, 2014

    Let’s assume that you have an 8 hours trip from London to New York. Your ETD is 2300Z and your ETA is 0700Z. During pref light you used the 0000z TAF, does 0000z TAF cover forecast for your destination at your ETA? No, because 0000Z + 30 hours is good until 0600Z (next day) only whereas your ETA is 07:00Z. TAF issued every 6 hrs to ensure availability of this important wx forecast to every flight, short or long.

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  2. Gary S. on Dec 13, 2014

    Sans, thanks for your answer. However, I’m not convinced. Let me propose another scenario to illustrate my original question.
    It’s Dec.13 in London and my ETD is 2300z to NYC. The flight will arrive in NYC at 0700z. For preflight I would have to had used the Dec.13th 1800z forecast for New York because the Dec.14th 0001z forecast had not been issued yet.
    When we depart London at 2300z, we could catch the Dec.14th 0001z TAF enroute, as well as the 0600z TAF 1 hr before landing.
    In my humble opinion TAF’s being valid for 30 hrs is a mistake because at no time, in my example, is any TAF more than 6 hrs old.
    I agree that the TAF is important. So who would use a 30 hr. old TAF when they could get one every 6 hrs.?

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  3. Kris Kortokrax on Dec 13, 2014

    Not all TAFs are valid for 30 hours.

    A quick check reveals that in the Chicago area, only O’Hare’s TAF is valid for 30 hours.
    Midway, Gary and DuPage are only valid for 24 hours.

    This is consistent with the AIM which states that aerodrome forecasts are valid for 24 or 30 hours.

    I do not recall when the 30 hour valid time came into being and am not aware of the criteria for determining who gets 30 hours.

    I thought it might be for Class B airports. Just did an abbreviated briefing for ORD STL JFK ATL PHX and MIA

    All had 30 hour valid times except for PHX and MIA. So, it’s not a Class B thing.

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  4. Kris Kortokrax on Dec 13, 2014


    If you are flying from London to New York, you won’t be calling Flight Service to get a briefing. If you are an airline flight, you will be using weather information provided by your airline. If you are a Part 135 flight or Part 91 flight, you will most likely be using weather provided by a handler such as Universal, Colt or ARINC.

    The big reason for you to look at the forecast (other than severe weather in the forecast) is to determine if you need an alternate airport. Prudence would seem to dictate filing an alternate in this situation regardless of the forecast weather.

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  5. Gary S. on Dec 13, 2014

    The question still remains…why would anyone use a 24 hr or 30 hr old TAF when one is available which is, at the most, only 6 hrs old?

    It doesn’t make sense…not to me, anyway. I’ll try to contact the guy’s that wrote Aviation Weather on Monday to see what they say. Perhaps this 24-30 hr thing is left over from yesteryear when TAF’s were only issued every 24-30 hrs. Hope we can get to the bottom of this – Gary

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  6. Matthew Waugh on Dec 14, 2014

    It’s a forecast – it’s used for many things beyond just what’s going to happen in the next 6 hours. You seem to be too focused on just one use case.

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  7. Mark Kolber on Dec 15, 2014

    I may be denser that usual this morning but I don’t see the issue. If I get a briefing now or look up TAFs I get the most recent one, not the one that was issued yesterday or even 6.5 hours earlier. If I’m solely interested in a single point in time “right now!” forecast (as I think Matthew correctly surmises you to be), then all I ever see is the “6-hour” one ((unless, of course, there is a system problem).

    So who would use a 30 hr. old TAF when they could get one every 6 hrs.?

    Two answers come to mind immediately and they both have to do with planning:

    Some of us like to plan trips in advance and want to see the longest range we can. If I’m planning a trip tomorrow, I might look at the TAF this morning to get a sense of whether, if cloudy conditions are forecast, I might be able to get out early or have to wait until temperatures warm up later in the day. 6 hours tells me nothing about tomorrow 24 or 30 gives me more of a clue.

    We can see trends. Forecasts are just that, forecasts. They change and the further out they are, the less exact they can be. If I see substantial changes between the TAFs issued between now and the time of a later flight, I can see the weather is changing more rapidly than the models predict and can lead me to question even the reliability of the 6 hour forecast. People regularly use METARs in that way – to get value from seeing a trend rather than a point-in-time.

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