November 2024 will see big changes to SIGWX Charts

What’s a SigWx Chart?

Really? OK, well it is one of these things:

SigWx stands for, wait for it, ‘significant weather’.

So this ‘map’ of SigWx is used by flight planners, pilots, sometimes even ATC to see what’s out there in the airspace. Or what might be out there later when folk are flying through it – because while these are based on satellite and air data info, they are also forecasts.

What’s going on here?

A quick guide to this rather elaborate puzzle.

The big black arrows are jet streams, the puffy things are cumulonimbus clouds, the numbers in rectangular boxes are the tropopause heights and the bit that says CAT areas are clear air turbulence areas. These are numbered and you can find the corresponding region pictured by dashed line sections.

That isn’t all that’s on there, but its enough for what is relevant with this post (and if you need more info on how to read a SigWx chart then this probably ain’t a post for you anyway).

OK, ready for the news about SigWx Charts?

Wait, I need to mention something else quickly as well. OK, so, CAT – clear air turbulence – is forecast by using satellite imagery and spotting what they call ‘tropopause folds’.

At the upper-tropospheric boundary between air masses, vertical shearing at the jet stream combined with the ageostrophic convergence of polar, subtropical and stratospheric air produces a region known for its potential for clear air turbulence called a tropopause fold.

Basically, they measure temperature gradients, winds, all that sort of stuff and can determine, with a pretty decent level of accuracy, how likely CAT is in certain areas.

So how come with still get caught out by turbulence?

Well, exactly, that’s the question. The answer is, partly, that this is all a little bit of (very sciencey) guesswork with a chunk of subjective forecasting thrown in. In other words, while they can fairly accurately predict it, those predictions are based off a whole load of dynamic, rapidly changing and often ever so slightly unpredictable factors.

Which finally leads us to the Big Change…

The big change(s)

Coming in November 2024 are brand new (well, significantly updated) SigWx charts.

These are the charts produced by WAFS. The modelling system they use (IWXXM) is getting a big update (thanks to help from the UK Met Office and NOAA), and will use a broader range of data, gathered and assessed through advanced technology (they call them ‘Schematrons’) for more accuracy and usefulness.

The big changes:

• They will be ‘multi timestep’ forecasts which space T+6 and T+48, issued at 3 hourly intervals
• FL100-FL600 will be covered on one chart, so no more medium and high levels (for ease of planning and use)
• Areas of icing will cover the globe, instead of just being regional
• Tropopause info will be shown in 5000′ contours, instead of spot heights
• Areas of turbulence in cloud and icing will become only icing areas
• ISOL EMBD CB will no longer be shown (these are not produced bt a met model, but by subjective assumptions so are not super accurate). CBs of 50% or greater coverage (OCNL and FRQ) will remain
• CAT will become ‘turbulence’ because the new model can detect orographic type and clear air.

What will pilots see?

Well, it depends on your briefing pack provider. These charts are produced by WAFCs (World Area Forecast Centres) and not generally not considered ‘briefing quality’. Instead, flight planning providers generally whack them into their system and pull out the data they want. So pilots may not see a change on their SigWx charts in briefing packs, (aside from things like ISOL EMBD CB no longer showing), but should see more up to date, relevant and accurate weather forecast information.

Hopefully, this will lead to better flight planning, in particular better turbulence avoidance route planning, as well as better in-flight awareness for turbulence and other significant weather. This is definitely needed – in 2018, weather was the second most common reason for flight delays (randomly old statistic, very likely it has been in the following years as well), and turbulence-related accidents were the most frequent accident type for the past two years running.

The ‘Schematron’ (or what we hope it all looks like)

And then?

Not enough for you? Jeez. OK, well in 2027 they are planning on bringing in a “probablisting WAFS hazard data set” which will be able to predict stuff like icing severity, CB maximum tops height, turbulence day dissipation rates etc more accurately. It will also include 6 turbulence levels, 5 icing levels.

You can get all the info here on the Met Office website.