Strange METAR or TAF / METAR or TAF interpretation

The TEMPO group changes the previous state within a given timeframe. What is not modified, remains and what is modified returns to its previous state after the group unless there is a transition to a new state.

Achim’s explanation covers it

TEMPO is a temporary weather change for a given timeframe, while BECMG indicates a permanent weather change.

Here’s a breakdown:

EDDH 150500Z 1506/1612 07005KT 9999
This is the general weather forecast for the period 1506/1612

TEMPO 1506/1508 3000 MIFG BKN007
Between 1506 and 1508 there is shallow fog expected

TEMPO 1508/1517 08010KT
Between 1508 and 1517 the wind will be 08010KT

PROB30 TEMPO 1602/1607 4000 BR BKN007
A 30% probability that there will be mist between 1602 and 1607

If no cloud was expected during the TEMPO period, the TAF would have read:

EDDH 150500Z 1506/1612 07005KT 9999 SCT020 TEMPO 1506/1508 3000 MIFG BKN007 TEMPO 1508/1517 08010KT SKC PROB30 TEMPO 1602/1607 4000 BR BKN007

or, maybe more likely,

EDDH 150500Z 1506/1612 07005KT 9999 SCT020 TEMPO 1506/1508 3000 MIFG BKN007 TEMPO 1508/1517 08010KT NSC PROB30 TEMPO 1602/1607 4000 BR BKN007

I’d read this as that generally for the period between 1506/1512 it will be scattered at 2000 ft.Between 0600 – 0800 dont expect to see your scattered cloud as there will be low viz and cloud, mist, fog and broken at 7000. I dont know the elevation there but I assume that not very high AGL. Temporarily the wind will pick up a little between 0800 and 1700 and will hopefully clear that MIFG away. Then the wind will die down again later and not unexpectedly you will see ground fog / mist again.

I have 2 basic things I say in my head. 1: If the TAF (using a standard monitor landcape view) is about one line long, then the weather will change so I need to a) look more closely at other TAF’s in front and behind a planned route b) consult other sources for a 2nd opinion (national weather site or whatever you deem accurate enough. 2: If the TAF is more than than 1 line long, (meaning there is loads of PROB30 / PROB 40 / TEMPO’s etc, often with TSRA) – then just stay at home.

I will be honest, if I didnt have the IMCr in the UK, with the ability to plan and execute flights in IMC, I’d have given up a long time ago. Certainly in the UK, the chance of a good despatch rate with “good VFR” over a wide area is either a) pretty low b) a bit of a gamble based on loads of different TAF’s and the reality that they might well be different. I always combine TAF analysis with the last 4 METAR reports and make my judgement.

I can sometimes be wrong though, like when I flew to Deauville recently from the UK and was in IMC with some rain for 40 odd minutes. This time I didnt bother to look at a rainfall radar as all of the TAF’s I used all had cloud about 2000 ft and some even CAVOK.

The big gotcha with TAFs is that any forecast event whose end point coincides with the end of the TAF itself is going to potentially (almost certainly in fact) extend beyond the end of the TAF.

NSC

Ah yes, the good old NSW or NSC. I treat these with more caution than anything except TSRA ! NSW in IMHO just means no rain or significant cloud, and does not always mean good VMC flying conditions, and NSC as a prediction tool I also found to be pretty useless.

I’d read this as that generally for the period between 1506/1512 it will be scattered at 2000 ft.Between 0600 – 0800 dont expect to see your scattered cloud as there will be low viz and cloud, mist, fog and broken at 7000. I dont know the elevation there but I assume that not very high AGL. Temporarily the wind will pick up a little between 0800 and 1700 and will hopefully clear that MIFG away. Then the wind will die down again later and not unexpectedly you will see ground fog / mist again.

Agree with achimha, lenthamen, Airborne Again. Just two quick points in response to this:

• TAF altitudes, like METAR altitudes, are AGL, so BKN007 is 700’ AGL (not that it matters for EDDH )
• This is a fairly typical high-pressure situation. Large-scale subsidence sets up a temperature inversion at low levels with fog/low stratus. Heating at the surface during the day then potentially burns off the cloud. There is a feedback in the system — once the clouds break up, a lot more radiation reaches the ground, reinforcing the breakup — and so the timing is very hard to predict.

b) consult other sources for a 2nd opinion (national weather site or whatever you deem accurate enough.

Since the TAF was also issued by the national weather service, that “2nd opinion” won’t contain extra information. What is useful is comparing a few models, say ECMWF, GFS, COSMO, UKMO. That’s what makes the forecast discussion interesting reading.

b) consult other sources for a 2nd opinion (national weather site or whatever you deem accurate enough.

Since the TAF was also issued by the national weather service, that “2nd opinion” won’t contain extra information. What is useful is comparing a few models, say ECMWF, GFS, COSMO, UKMO. That’s what makes the forecast discussion interesting reading.

Although that is true, maybe I havent analysed it enough, but their representations of the weather (even if it the same data) may be more correct / incorrect than just a textual interpretation of some lines of text. Also, looking at rainfall radars, whether they comes from (in the UK), the MetOffice, the BBC, or on the Aviation Portal website, is again useful as a comparison just against TAF’s which are localised.

Now that we are moving to general tips the biggest one by far is that any forecast of a dissipation of a persistent condition (fog) is optimistic at least 90% of the time.

I will not depart on a flight if the destination is fogged in, until the fog has actually cleared at least to approach minima, regardless of what the TAF says about when it should clear.