Primarily for general aviation discussion, but other aviation topics are also welcome.
By TopCat
FLYER Club Member  FLYER Club Member
#1872562
So first thing today, the relevant bit of the Heathrow TAF said this:

EGLL 260504Z 2606/2712 21003KT 6000 FEW040 PROB30
TEMPO 2606/2608 3000 BR BKN008
BECMG 2607/2610 9999

So hardly any cloud, and clearing up nicely by late morning.

A couple of hours later, and it's this:

TAF AMD EGLL 260755Z 2607/2712 21005KT 5000 BR BKN006
BECMG 2608/2611 9999 NSW SCT016


I've noticed this quite a few times in recent months - a TAF AMD is issued hot on the heels of the previous one, and it's invariably much worse.

What suddenly new information do they get in the intervening period, and, more to the point, why was the information the previous one was based on so bad?

A Heathrow TAF issued at 6pm used to be very reliable for the next day, but this has been much less the case in recent times.
By johnm
FLYER Club Member  FLYER Club Member
#1872569
Too much just parroting the model output rather than have @Simonweather interpret it alongside looking out of the window :-)
Spooky liked this
User avatar
By James Chan
FLYER Club Member  FLYER Club Member
#1872571
A lot of weather forecasting is based off weather models like GFS. Supercomputers and/or cloud computing is used to predict the future based on current and historical observations. These models are updated over time from new discoveries in science etc.

Certain conditions make accurate forecasting incredibly hard and even harder the further into the future.

The TAFs are a pilots translation of this output.
By TopCat
FLYER Club Member  FLYER Club Member
#1872576
James Chan wrote:Certain conditions make accurate forecasting incredibly hard and even harder the further into the future.

The TAFs are a pilots translation of this output.

Sure, I get that. I'm not talking about days in advance. Just a few hours.

Maybe I've just seen a few duds recently and imagined a trend. But I've been looking at the Heathrow TAFs on a Friday evening and again first thing Saturday and Sunday ever since they were available on the internet.

I've been getting an increasing strong feeling of "oh FFS not again" when they amend the TAF to something much worse, often just an hour or few after the last one.
By rdfb
#1872608
TopCat wrote:Sure, I get that. I'm not talking about days in advance. Just a few hours.


Yeah, but at a fixed point. The other day I cancelled a flight because the forecast showed a front arriving that I'd have had to fly along (the forecast synoptic chart at my planned flight time might as well have shown the front as my magenta line). The actual weather was perfect blue sky - because the front had slowed just a little sooner than expected, dissipated as expected, and therefore never actually arrived. Consider what a TAF might have said - it'd have been BKN004 or something and then AMD NSW 9999. A massive swing in the TAF resulting from a forecast that was only slightly out. But someone looking only at the TAF, and not at the big weather picture, would have been surprised.
#1872638
MattL wrote:There seems to be have been a lot of decaying fronts recently which are quite hard to model how they will manifest things on the day I think

This was certainly what caught me out on 19th with Project Propeller.
#1872647
Large blocking highs over Scandinavia seem to fox the modellers these days invaribly predicting a quicker decay and return to westerley (wet) airflow.

I've cancelled a couple of weekend trips recently based on Ensemble outputs on Tuesday/Wednesday only to sit under flyable skies 5 days later. In both cases the progression eastwards of rain etc just didn't happen.
User avatar
By Iceman
FLYER Club Member  FLYER Club Member
#1872665
This is why TAFs are often couched in terms of probabilities and ‘becoming’ time ranges. It’s not an exact science. Having said that, to amend a TAF only two hours after you issued the previous one seems to defeat the object of a probabilistic TAF, and looks more like a met man just ‘METAR chasing’ for the airports two hours ‘upwind’ of you.

Iceman 8)
TopCat liked this