Cumulus Report: 23 April 2020

Rainfall will mostly be in the form of scattered showers or thundershowers, moving west-east over the southern half of the country on most days. Due to the nature of the upper air, some thundershowers could again become severe, however, only in very isolated cases. In general, it seems possible that there will be much more active weather (rain / wind / cloud) from the weekend into early next week compared to conditions during the last few days.

The following is a summary of weather conditions during the next few days:

• General:

o The northern half of the maize-production region should be dry.
o The Free State, most of the Cape provinces, KZN and southern Mpumalanga should receive near-normal to above-normal rainfall.
o It will on average be somewhat warmer than normal over the interior.
o No widespread frost is expected during the next few days according to current forecasts.
o There are no indications of a strong cold front to influence the country (including the winter rainfall region) during the next few days.
o There is a possibility according to current forecasts that an upper-air system may result n rainfall as far west as the winter rainfall region later this week.
o Southeasterlies will dominate over the southwestern Cape, becoming strong on Sunday (26th) and Monday (27th).

• Rainfall:

o Isolated to scattered showers and thundershowers will occur over the Cape provinces until Friday (24th).
o It will become cloudy and cooler with scattered showers or thundershowers over the southern to eastern escarpment (as far north as the southeastern Limpopo) during the weekend. Isolated thundershowers should remain in place, especially over the southern and central parts, but spreading to the eastern interior by Sunday (26th).
o From Sunday (26th) into next week, another upper-air system will deepen in the southwest. It should result in more widespread thundershower especially over the western, southern and central parts.
o Widespread thundershowers could also move into the Free State, Eastern Cape and surrounding areas of adjacent provinces according to current forecasts by Monday (27th) and Tuesday (28th).

• Temperatures:

o Temperatures will on average decrease during the period, especially maximum temperatures, associated with progressively more inclement conditions towards the weekend and early next week when cloud cover will be more abundant.
o It could be cool, cloudy and rainy over the central parts by Monday and Tuesday (27th / 28th).
o It will be hot over the Lowveld on Saturday (25th).
o Maximum temperatures over the western maize production areas will be in the order of 21 - 26°C - decreasing throughout the period. Minimum temperatures will be in the order of 12 - 15°C.
o Maximum temperatures over the eastern maize-production region will range between 20 and 24°C, on a decreasing trend through most of the period. Minimums will be in the order of 8 - 10°C.

Overview of expected conditions over South Africa during the next few days

Two upper-air low pressure systems will result in scattered showers and thundershowers mostly over the southern half of the country. An associated high-pressure system ridging around the country will result in an influx of moisture, enhancing the conditions for rainfall from the weekend onwards.
Conditions in main agricultural production regions (22 - 28 April)

Maize production region:

The area will be partly cloudy and mild at the start. By Friday (24th), isolated thundershowers are possible over the southwestern parts. During the weekend, thundershowers and showers will occur mostly over the eastern parts where is will also become a few degrees cooler during the day - and cloudy. Thundershowers are expected to become somewhat more widespread by early next week, when it will be cooler over the entire area with abundant cloud cover. Maximum temperatures over the western maize production areas will be in the order of 21 - 26°C - decreasing throughout the period. Minimum temperatures will be in the order of 12 - 15°C. Maximum temperatures over the eastern maize-production region will range between 20 and 24°C, on a decreasing trend through most of the period. Minimums will be in the order of 8 - 10°C.

Cape Wine Lands and Ruens:

It will be mild to warm over the region, trending somewhat warmer towards next week. Associated with a lack in frontal activity, winds should be southeasterly for the most part, especially in the southwest. It will be partly cloudy on Thursday and Friday, with a possibility of thundershowers over the interior - especially on Thursday. Depending on the movement of the rain-producing low over the interior, significant falls may occur over the mountainous areas. Winds in the southwest will become strong southeasterly by Sunday into early next week when the onslaught of moisture from the south may produce light showers along the Garden Route while it will be warming along the West Coast.

Possible extreme conditions - relevant to agriculture

According to current model projections (GFS model) of weather conditions during the coming week, the following may be deduced:
• Thundershowers over the Western Cape interior on Thursday may become severe. Depending on the exact position of the low-pressure system over the interior, significant falls may occur especially over mountainous areas.
• Thundershowers over the Eastern Cape interior on Friday (25th) may become severe.
• Thundershowers over the southern parts of the Free State and Eastern Cape interior may become severe on Monday (27th).
• Strong southeasterlies are expected over the southwestern parts of the winter rainfall region by Sunday (26th) into early next week. Where vegetation is dry, this may enhance the possibility for the development and spread of wild fires.