Snow Watch 2017

How's it looking today, Jon?

I have annotated the plot I loaded up in an earlier post which shows the forecast for Saturday 13 May made from 10 May 2017. The red arrows show where the wind is going and where it has come from. Most of the cold air has to be brought in (in climate terminology - advected) and the source of that cold air is from the south. The green line shows the boundary between relatively warmer air and relatively colder air. The plume of cold air and the wind lines should make it clear how the air has been advected. The high pressure system which spins anticlockwise is also marked. That is the system responsible for driving in the cold, southerly flow over the Berg.


The plot below shows the flow (lines) and temperature (colour) for an altitude of about 3000 m (pressure level 700 hPa) as forecast for Saturday at 1300 from 12 May. The plot can be directly compared with the forecast that was shown in an earlier post of mine/the annotation shown in the plot above. The change in the forecast issued today from the forecast issued at longer lead time is that the air is slightly warmer (-2.7 deg C compared with -3.4 deg C) near Giant's Castle. The flow is no longer simply being fed from the Southern Ocean by the big ridging high. Instead the flow is more mixed up. This has happened because of something going on higher up in the atmosphere.



Here is a plot of the flow higher up in the atmosphere nearer 10 000 m and this is where the forecast for 13 May has changed quite a lot over the last few days. A deep plume of cold air has been moved over southern Africa. A cold core of air has been cut off from the parent supply of cold air to the south so that there is a closed flow around a cold cored low. This is a system called a cut-off low. Note how strong the winds are NE of this cut-off low: 160 km/hr. The cold cored cut off low has had its own influence on the feed of cold air lower down in the atmosphere.

1300 on Saturday 13th: white-out on Sani escarpment.
1 deg C near Vultures Retreat.

Hats off to JonWells' vigilance and the science of numerical modelling.

Thanks for the great info Tiska and Jon.

On one of the hikes I did at Lotheni earlier this year (either the 21 March long weekend or Easter weekend, I can't remember which one), we saw quite a significant area of snow at Vergelegen, although it appeared quite a thin dusting and probably didn't last long.