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Last Cold Front Of The Season?

Just as many people in Gauteng (and I’m sure elsewhere in the country) were getting used to the “early Spring” warmth, we (in Gauteng at least) woke up to some rather chilly temperatures yesterday morning. This was thanks to a bit of a cold front, associated with a Cut-Off Low (COL) centred SE of East London this morning.

One of the nice things though is that some of the haziness/pollution that is typical of winter in the interior of the country got cleared out a bit as it was pulled southward into the COL. This effect could actually be seen on satellite imagery (see below), with pollution from biomass burning being pulled down from N of Zimbabwe.

As this COL moves to the SE a high pressure will ridge in behind it, advecting (transporting) low level moisture into the N’ern interior from the N/NE. This, combined with the instability forecast by the GFS model means that N’ern Gauteng (among other places) could see some relatively early spring-time thunderstorms later today and possibly even Friday and Saturday (see rainfall maps).

The development of storms today will be dependent on when exactly the ridging takes place. If it starts late the rain/storms may only come on Friday (thanks to a friend noting that 🙂 ). Hail from thunderstorms at this time of year would not surprise me, but conditions do not appear to be very favourable for the formation of large hail at this stage.

Unfortunately, the ridging mentioned above will bring with it cooler temperatures as well.

If you were hoping for nice hot weather this weekend in the NE’ern interior, unfortunately it seems the GFS model is showing maximum temperatures in the low- to mid-teens range. These stretch from the KZN and Mpumalanga escarpment through NE’ern Free State, Gauteng and into the E’ern half of the North West and central and SW’ern Limpopo (see maps below).

Personally, I don’t look at the actual temperature values forecast by GFS, but I do use them to gauge how much the temperature will change in the coming days.

The influx of cold air at the surface will also result in the formation of yet another upper air low pressure system, however it is forecast to be relatively weak.

The upper low had already started forming late yesterday over the Eastern Cape and by today should start to become more prominent in the upper air over the N’ern parts of the Northern Cape. It should then start moving NE and be situated over W’ern Limpopo by Saturday and then move SE off the N’ern KZN coast, provided the forecast stays the same.

Will it be the last Cold Front of the season?

Well, if the models are to be believed, unfortunately not. The GFS model is showing another cold front system moving into the SW’ern Cape by early Monday next week. And it seems as though the next one could be a lot worse in terms of cold weather (see below). Another model is also showing the formation of another (much more intense) COL towards the middle of next week.

Bear in mind though, that deterministic forecasts (ones which predict rainfall amounts, temperatures, etc.) become inaccurate beyond 3/4 days, so we’ll see how things play out over the coming days.

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