Rain and storm for Cpt pt 2

OK, everyone can calm down now! Yr.no is no longer showing crazy numbers for this morning (Wed 23rd). The forecasts are still higher than I expect, but within reasonable bounds for this type of trough/instability event. I'm not going to list all the specific forecasts, but most are about one quarter of last night's numbers, with a definite increasimg trend further south. We will know within the next 6 hours whether it will be very wet, or mostly noise and not so much rain.

Edit: Models are supposed to do 'sanity' checks on data, but sometimes garbage gets through the filters, and upsets the calculations. One strategy to deal with this is to run the models backwards in time (the laws of physics are symmetrical), and identify faulty weather stations. See:

Edit: This may be a complete coincidence, but two amateur weather stations (Atlantis and Sunset Beach) are both reporting relative humidity of 1% right now, which is implausible and clearly wrong. It is unlikely that a single data point would upset the models so much, but a reminder that all computers depend on the accuracy of the input data.

Edit: There are very few stations west and south of CT (mostly ships), which places excessive reliance on remote satellite data, with limited actual spot measurements. Anyway, just rambling now...
 
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akescpt said:
Weather says rain this morning…not convinced
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No rain from a cold-front like we normally experience in winter, but definitely some convective potential today. The aviation forecast is the most fickle I have seen, with light thundershowers from a high cloud base, but frequent updates as to the timing.

The yr.no forecasts keep changing as well, currently predicting 29mm for Simonstown this morning, 12mm for Kommetjie, and 41mm for Noordhoek, but zero for Hout Bay (less than 15km apart). I don't know how that is physically possible...
 
joshuatree said:
Yr's prediction for Milnerton is 15mm from 8h - 12h.
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Well, it is now 10am, and I have seen zero precipitation on the Milnerton webcam. The latest yr.no forecast shows 5mm possible between 10am and 12noon. I would call that forecast a complete bust...

Being generous, I would suggest that a model grid update has not been incorporated into the forecasts, and the smoothing function that would normally apply to an 'area' forecast for this kind of event is not working. Yr.no has previously been quite good at showing location specific rainfall, particularly in areas with orographic enhancement, but in this scenario I would say that it has failed completely...
 
Gordon_R said:
Well, it is now 10am, and I have seen zero precipitation on the Milnerton webcam. The latest yr.no forecast shows 5mm possible between 10am and 12noon. I would call that forecast a complete bust...

Being generous, I would suggest that a model grid update has not been incorporated into the forecasts, and the smoothing function that would normally apply to an 'area' forecast for this kind of event is not working. Yr.no has previously been quite good at showing location specific rainfall, particularly in areas with orographic enhancement, but in this scenario I would say that it has failed completely...
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Yip, not a drop so far. Sun breaking through now Durbanville Hills side, not as cloudy as previously.
 
Not a sign of rain and unlikely to materiaize SP side,, thunderstorm or otherwise...temps and humidy also down considerable from yesterday with some cooling effect from mild Northerly although that's going to swing Southerly by midday in all likelihood....
 
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