The last few model runs (both GFS and NOGAPS 00Z, 06Z and 12Z on Mon 19th) have all been relatively consistent. There is still some uncertainty (more of a 'wobble' than a 'flip-flop'), but there is no doubt that a severe frontal system is going to reach the SW-Cape on Sat 24th.
The charts I posted this morning (based on the 00Z GFS run) are still generally indicative of the likely effects of the cold upper trough, although the timing and intensity of the rain, surface winds and pressure will change.
This whole system will be driven by a vortex of very cold upper air (about 15C below the seasonal average). Snow is theoretically possible on Sat 24th, but would require an unusual combination of favourable conditions.
This custom GFS chart shows the disturbance in the 500hPa temperatures (roughly 5000m elevation) at 12Z on Sat 24th.
Compiled by Gordon Richardson - Stormchasing SA
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