McMahon 2pm forecast

I’ve had this before…

Wind speed 4mph, gusting to 38mph.

Where is this 38mph coming from?

The whole forecast is pretty lame. It has it cloudy, yet it’s blue sky.

First thing I’d say is that it’s a forecast and as such there’s never any guarantee that it will be perfect, or even good. Here’s what the people with multi-million pound forecasting computers say about your weather at 2pm…

UKV - Sunny, 24C Wind 9km/h Gusts 26km/h
ECMWF - Sunny, 21C Wind 9km/h Gusts 21km/h
GFS - Sunny spells, 22C Wind 10km/h Gusts 9km/h (figure that one out!)
Meteo-Blue - Sunny, 25C Wind 5km/h Gusts 8km/h
Icon-EU - Sunny spells, 22C Wind 8km/h Gusts 15km/h
Arome - Sunny, 22C WInd 6km/h Gusts 18km/h

Even with millions of pounds worth of hardware and software and with a lot more weather data input they still can’t get closer than the temperature being 22 - 25C, or whether it’s sunny or there’s some cloud and whether the wind is 5-10km/h and gusts are 8-26km/h.

If you’re mixing GFS and ECMWF then at 2pm there’s a chance that you’re mixing GFS 06z data with ECMWF data from 00z and on top of that your local data from 2pm. They may be giving you variations that are difficult to reconcile.

I agree that 4mph wind speed and 38mph gusts seems quite a wide spread but it’s very difficult to know why. If you want to investigate then I’d suggest looking at your own local data and see what that says for 2pm and then do some extra runs using the same set of GFS/ECMWF/local data where you run with 100% GFS, then 100% ECMWF and see what answers you get from those. That might start to give you an idea about where the odd looking values came from.

I run my forecast at 3pm where I have both gfs and ecmwf the same at 06z and got 9mph gusting to 18mph, not unreasonable.


6pm forecast says wind speed 3mph, gusting to 29mph. Bizarre.

What do you get with 100% GFS and 100% ECMWF?

By that, do you mean uncheck GFS Model data and GFS Advection?

Something seriously wrong going on now. RAOB data over 15 hours old.

GFS data not found in wdata.txt, even though I have ECM selected

ECM run took like 1 minute and come back with a sort of default forecast, which is wrong

GFS comes back with 4mph, gusting to 23mph. It’s so far off, it’s worthless.

It’s always the same for my forecasts, the wind gust values are always too low when strong winds are forecast, or too high when light winds are forecast.

We’re not talking an expected error of margin here, they are categorically wrong and don’t match the models either.

No. In Euromix set GFS to 0% and ECMWF to 100% then mix and run WxSim. Save the output. Then in Euromix set GFS to 100% and ECMWF to 0%, then mix and run WxSim. Compare the outputs to see if one or the other gives unexpected gust speeds.

I also see at the bottom of your plaintext.txt file…

  • No initial surface data found - METAR and/or local station data may be missing *

Missing local data will affect your forecast.

Yes, you’re right. I’m not sure why that was unchecked. Synop was checked, but not metar :tired_face:

I tried both separately, and they both give high gusts for the today/this afternoon/tonight/rest of the night.

The latest forecasts show wind speeds of 9mph, gusting to 29mph. It’s so far fetched.

Tomorrow’s forecast is 16mph / 29mph which is still too high, but more reasonable.

I just don’t know what it is with wind forecasts on my WXSIM set up, they have always been extremely poor and not worth looking at.

The wind forecasts it produces for this location never tallies with ECM and GFS’ raw data, or any other model.

WxSim produces a hyperlocal forecast for the few miles around you. The GFS and ECMWF models at best get to 0.25 degrees of longitude and latitude so they work in approximately 30km squares. Anything below that is interpolated which is just a linear estimation of conditions at your location based on conditions in the corners of that square.

For locations near the sea interpolation can give weird effects if one or more of the corners are in the sea, so you can’t necessarily trust the results it gives you.

It would be worth you talking to Tom about the customisation he did for you. That should take notice of the local topography, etc, so it’s possible that he’s made a mistake with one of your local parameters that’s affecting your gust speeds.

I just checked my forecast and it shows 9mph gusting to 17mph and UKMO shows about 7mph gusting to 14mph. So for me it is quite acceptable. Maybe you should talk to Tom about all this.


Have you checked the Wind gust controls in WxSim (Preferences | Settings | button 1 - bottom right hand corner). Tom may set this if it needs to be different for a specific location and doesn’t recommend changing it, but it might be something you could consider experimenting with. In my settings the fields are set to (in order top left by line to bottom right) -

1 / 10 / 2 / 1.5 / 1.89 / 2.27 / 2.73 / 10 min / 24.2

How do the WxSim gust values compare to your measured gusts? WxSim would usually be higher because it assumes you’re measuring at 10m in an open environment. Most amateur station anemometers are lower than 10m and also usually have buildings and trees which significantly affect wind and gusts speeds. Neverthless, it’s still an interesting comparison.

Still getting this problem. Earlier today, wind forecast was 9mph, gusting to 34mph. This evening, 8mph, gusting to 31mph.

The forecast winds for Thursday look to be higher. WXSIM showing winds at 17mph, gusting to 30. So how can it be showing such high gust values during lighter winds?

Another big problem with the gust forecasts from WXSIM, is that is greatly under reads values when high winds are forecast - 40 mph=.

There’s been numerous times where 50mph+ gusts have been forecast by the Met Office (and I usually hit those values on my anemometer) and WXSIM has only shown 30 to 35mph.

On the flipside, WXSIM will greatly exaggerate average wind speeds when 40 to 50mph is forecast. I remember on many occasions that WXSIM was showing 35 to 40mph average wind speeds. Those sorts of speeds are rare as hens teeth here, and if they did occur, gusts would be in excess of 60mph. I know this due to my data, and what the Met Office will forecast. Even if the Met Office are showing gusts to be in the mid-50’s, wind speeds are circa 30mph, and it’s usually spot on.

For the TLDR;
WXSIM forecasts excessive wind speeds and gusts when low winds are forecast.
WXSIM forecasts excessive wind speeds and low gusts when high winds are forecast.

I have reported this on a few occasions, but nobody seems to be able to help.

Just to add; My anemometer is at 10m, free of any obstructions for several hundred metres. My wind readings are pretty much in-line with what the Met Office shows for my location. (UKV model)

I did exactly that, but whether GFS or ECM is moved to 100%, it always shows this in lastret:

Percent use of ECMWF = 0

What has Tom said on the subject. Forum users will try to give advice but if we’re not helping then your only route is to go direct to Tom.

It was quite some time ago. I do remember he asked me to send some specific files over, which I did, but I can’t remember what happened after that.

My latest (auto) forecast:

WXSIM forecast for Wareham, initialized at 22:00 27 Aug, 2023
Percent use of ECMWF = 0

ECM is set to 100% in Euromix

I’ve also noticed that when I manually run a forecast, the time goes backward:

WXSIM forecast for Wareham, initialized at 20:50 27 Aug, 2023

I ran that a few moments ago.