[Rasp] RASP Beta Inaccuracy vs old RASP 1.3 and 4km Res models

Sun Jul 6 23:19:26 SAST 2025

Hi Ria

Thanks for the feedback. A model is only useful if there is a "closed 
loop" to verify that the predictions match actual conditions experienced 
on the day.

First thing I need to check/fix is the timestamps on the Beta forecasts. 
I have not had a chance to look at this yet. These are labelled "1100 
SAST (1100Z). There is a two hour discrepancy between those two times as 
1100 local time is 0900 GMT. I suspect the 1100Z is correct and that the 
beta forecast is actually for 1300 local.

The archiving of the beta models is still not properly setup but I 
managed to find ZS2+1 (ie one day earlier) forecasts for 0900, yesterday 
5th July, which, if the above is correct, match the times of your actual 
readings. I have attached 2m and 10m surface wind forecasts together 
with the head and side graphics which have the header and scale 
information. I think these are closer to the legacy RASP and the actuals.

When you look at a forecast like RASP, the forecaster needs to:

  * Interpret from the forecast data into actually flying decisions. (Ie
    can I fly? etc)
  * Determine a confidence level in the forecast. (Do I believe this
    forecast?)

Disclaimer - I am not familiar with wind conditions required for foot 
launching at Signal Hill. so the following is a general comment.

If the conditions are "unusual" then confidence level drops. You can see 
from the charts a significant wind speed difference between Cape Point, 
Scarborough and Table Bay. Somewhere between those points the wind 
becomes "too strong" to foot launch. The only difference between the 
models is where that line falls. A few km either way becomes is 
significant to your flying decisions. You have to watch the forecasts 
and compare them to actual conditions and develop interpretation skills 
and confidence levels to help make that call. In other weather 
conditions, the decisions might be much easier.

Note there are technical differences between the RASP models. Both are 
based off the "GFS" weather model:

  * The legacy forecasts extrapolate off standard resolution GFS data,
    that was available when the code was written. It uses version 2 of
    the WRF forecast program to extrapolate the high resolution
    forecasts. The extrapolation requires a two iteration "window"
    forecast to reach the 1.3km resolution.
  * The beta forecasts use new high resolution GFS source data. It
    extrapolates with version 3 of the WRF program and it does it in a
    single run to a 2km resolution.

I would like to try a window run of the Beta model to get a higher 
resolution than 2km. But there is more basic housekeeping that has to be 
fixed first.

Please keep us updated with your observations.

Thanks!

Ian

I am not sure if that would explain your discrepancies?

Regards

Ian

On 7/5/25 11:31, Ria Moothilal wrote:
> Hi
>
> I'm giving feedback as requested. Today, the RASP Beta was wrong 
> again, and interestingly, the old RASP 4km model was more accurate.  
> Below are the forecasts for today, the 5th July at 11 am, as well as 
> the actuals for Signal Hill taken by 3 live weather stations. Both the 
> 1.3km and 4km old RASP models show that the north-west wind slows down 
> before it reaches Signal Hill / Table Mountain. The RASP Beta 2km does 
> not. This suggests that it's not just a resolution difference on the 
> RASP Beta, but also either a source data or model parameter change 
> that has taken place. The old RASP was accurate when compared to 
> actuals, but the RASP Beta was completely wrong
>
> The old RASP was the most accurate weather forecast for the Western 
> Cape. Nothing came close. I think it would be sensible to merely 
> recreate these same models on the new user interface
>
> 1. RASP Beta 2km
> image.png
>
> 2. Old RASP 4km
> image.png
> 3. Old RASP 1.3km
> image.png
> 4. Actuals from weather stations at Signal Hill and Sea Point:
> image.png
>
> Ria Moothilal
> Chief Flight Instructor
> +27 78 095 8136
> www.airschool.co.za <http://www.airschool.co.za>
>
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