What exactly does it mean? TEMPO in TAF is well defined, but sometimes they also come in a METAR, as it did today: TEMPO 18015G25
I mean, TAF is a forecast, METAR is the current weather, what is TEMPO supposed to mean for current weather?
This is the body of the TAF. The basic format is:
Wind – Visibility – Weather – Sky Condition – Optional Data (Wind Shear)
The wind, visibility, and sky condition elements are always included in the initial time group of the forecast. Weather is included in the initial time group only if significant to aviation. If a significant, lasting change in any of the elements is expected during the valid period, a new time period with changes is included. It should be noted that, with the exception of a FM group, the new time period will include only those elements which are expected to change; i.e., if a lowering of the visibility is expected but the wind is expected to remain the same, the new time period reflecting the lower visibility would not include a forecast wind. The forecast wind would remain the same as in the previous time period.
Any temporary conditions expected during a specific time period are included with that time period.
Yes, TAF is OK. But TEMPO also comes in the METAR. What is a TEMPO actually suppose to mean in a METAR ?
The METARs from some airports include a “trend forecast” (sometimes called “landing forecast”) after the current weather. It applies to the next two hours after the time of observation. If no major change is expected the trend forecast is given as NOSIG.
As they are issued more frequently than a TAF and have a shorter time of validity they can be expected to be more accurate than the TAF.
https://en.wikipedia.org/wiki/Trend_type_forecast
It’s a Trend type forecast. In Italy (not sure if it’s the same in the other countries), it’s valid for the next 2 hours after the METAR emission. The Trend forecast (in Italy) is provided by the Meteorological Forecasting Unit to the Aeronautical Meteological Observer (which only manually inserts it into the METAR message). It’s provided for the major airports only.
(Aeronautical Met Observer)
I still don’t get the exact meaning. METARs come every 20 minutes. How can a TEMPO in an old METAR be valid when a new one comes?
I still don’t get the exact meaning. METARs come every 20 minutes. How can a TEMPO in an old METAR be valid when a new one comes?
It’s not. New METAR overrides the old one, so TEMPO in old one is valid only until new METAR is released. It’s just a trend forecast valid from the moment of METAR validity time for next 2 hours if not overridden by next METAR.
https://met.nps.edu/~bcreasey/mr3222/files/helpful/DecodeMETAR-TAF.html
LeSving wrote:
I still don’t get the exact meaning. METARs come every 20 minutes. How can a TEMPO in an old METAR be valid when a new one comes?
It means that at the time of issue of the METAR, the weather during the next two hours is forecast to be generally the same as at the time of observation, but temporarily changing according to the TEMPO group. When the next METAR comes, there will be a new forecast.
It’s no different than with TAFs. E.g. a 9-hour TAF is generally issued with 3-hour intervals. A new TAF replaces the old one.
(Nitpick: METARs come every 30 minutes. At 20 and 50 minutes past the hour.)
OK, got it (I think 
)
LeSving wrote:
How can a TEMPO in an old METAR be valid when a new one comes?
It is not. The moment a new METAR with a new trend comes out, it superseeds the old one. Like with a TAF as well.
METARS are issued every 30 minutes normally, some airports do it hourly. Every time a trend is considered which is valid like the METAR for 2 hours maximum. If for any reason no new METAR comes (e.g. technical) then the last issued METAR can be used for a maximum of two hours.
What we do with the Trends is to predict the weather in the validity of max 2 hours if it changes beyond predefined trigger values. I can’t list them all obviously as a) they are different at each airport and b) there are way too many. But some examples:
- Visibility: 200,300,500, 1000, 3000, 5000 m. In other words, CATIII, II, I and then some steps to VMC (5km).
- Cloud/VV: 100,200, 300, 500, 1000, 1500 ft. Same thing. Also if a FEW or SCT layer becomes BKN or OVC.
- SigWX. RA, FG, SN, SHRA, e.t.c. always only when moderate or heavy. No trend for light rain or snow.
- Wind directions and speeds beyond trigger values
Most of the time, the time span for something to happen is not 2 hours but more within the next 30 minutes or to the next METAR, but we have to consider everything.
So if say a front arrives, we will start issuing Trends the moment we expect the first trigger values go below the maximum, mostly cloud and vis and or if we see SigWX approaching. For cold fronts we will give TEMPO TSRA starting approximately 2 hours before if we see it coming here, but sometimes CB’s pop up short notice too.
For Fog we will usually start with giving a BECMG below 1500m vis or 1500 ft ceiling and then gradually give the next steps as they go. It can and does happen that fog pops up within 10 minutes… expected and unexpected. You can be CAVOK and get a trend BECMG 0500 FG. As with any trend, always expect the worst. Improvement at the end of validity, decreasing values immediately.
It is quite interesting work and very satisfying if you get it right, frustrating if not.
