Change in TAF's coming down the pike


30 hours TAFS coming in November.

Excerpt from the above link

On November 5, 2008, with the 1800 UTC issuance, the NWS will change the TAF format to conform to international standards outlined in ICAO Annex 3. The change includes the initiating of 30-hour TAFs for high-impact U.S. airports. To accommodate the extended TAF period, the format of all TAFs will change.

Does anybody know what the criteria for “FRM” is when they describe significant and rapid change? I don’t see that much significant change in their examples provided in the page, (unless winds from 110 to 220 is considered significant?)

I would have thought it would have preceded the 0400 time frame from their example?

I wonder how many aviation related computer programs this will break in the English translations :astonished:


When I first moved overseas I saw a list of what changes dictate a FRM change, but don’t remember where. Basically anything that generates a TEMPO, PROB30 or PROB40 gets a FRM change.
A 110 degree wind change I think would qualify. Also, as a general rule, a change in forecast that moves the wx from one category to another, VFR to MVFR to IFR to LIFR, would qualify.
I also saw, maybe in the same document, a comparison of what the FAA used in those days to the ICAO format. TEMPO and PROB30 are essentially informational while PROB40 must be used in your weather calculations, i.e. do I need an alternate or does this weather disqualify the airport as an alternate…etc.
Obviously the above is my wording, not from the official publications, but it’s how I remember it!!



Very interesting thoughts, as all I knew was the 123 rule for alternates.

But even with what you say above, I generally use the NWS public forecast for probabilities of thunderstorms and for the kind of flying I do, anything 30 percent or greater, I just do not fly.

I talked to a meteorologist, and the percentage represents coverage over the area of forecast and I find for me that 30 percent just isn’t comfortably enough airspace between me and the building CU’s.

I have only noticed the PROB come up with T’storms.

Is that your experience too or is there other criteria for that to pop up in the TAFs? (I.E. I never seen it for TEMP conditions or rain forecasts, I don’t think anyway!)


Good questions;
I wrote up this long winded answer but then found this, which you can be sure is more accurate and informative than my memory: … t/icao.htm
Down almost at the bottom they talk about froms and probs.
Here are the current TAFs for Stansted and Riyadh for you reading enjoyment;
TAF EGSS 071608Z 080024 10010KT CAVOK PROB40 0008 7000
TEMPO 1317 11015G25KT
TAF OERK 071535Z 071818 12006KT 8000 FEW040
BECMG 0204 26006G16KT
TEMPO 0212 4000 DU/BLDU SCT040
(visibilities are in meters in the above TAFs)
Looks like there is no point in having the airplane washed before I go to Riyadh tomorrow.
The Riyadh TAF is pretty typical for here, they could have said CAVOK TEMPO MAYOCTOBER 4000 DU/BLDU and been about right. :confused:


Very informative. Suffice it to say, if you see prob30 in a TAF of an airport I am going to, it will be by a four wheel version of a vehicle :smiley:

Let me guess instead of having to pay to get paint stripped off your birds, you just fly ‘em in mother nature’s natural “sand blaster”? Bet all that dust doesnt’ bode well for windows and glareshields?


Just got back from the exciting (not) RON, 6000 meters in blowing dust and 39 degrees when we landed over there. You can imagine what the dew point was. It’s not really true summer yet, the top of the dust layer was only about 10,000 feet. By July it will be 18,000. The dust had cleared up today. You are right, a lot of sandblasting takes place here.

Now I remember where I read the PROB 40 thing. It was in the company manual, they had decided that chickening out was the wise thing to do with CBs. ANY thunderstorm activity, regardless of ceiling, visibility, severity etc. warranted an alternate being calculated.