For a Wet Day to occur we need considerable surface heating, a layer of reasonably moist surface air, alto (middle) level moisture and unstable air. This mix, under certain conditions, is what gives us in our arid climate the odd so-called Wet Day. The definition of a Wet Day is 60mm or more on one day.
Wet Days are often marked by severe hailstorms because of the massive convection inside the cumulonimbus clouds from which they originate.

In Namibia, over more or less 80 years of gathering rainfall data, the following Wet Day events stand out:
Ondangwa with 45 Wet Days leads the downpour brigade. Through the Kavango valley to Katima, most stations have 30 or more days on their record. The average highest measures are between 100mm and 160 mm in one day. This level of both occurrence and intensity extends westward to around Oshakati and southward to around Grootfontein. Further west and further south, the Wet Days over 80 years only number on average, around 20 days, although amongst this data, there are some spectacular one-day rainfall events.
The heaviest fall on record occurred at Ombalantu, Nakayale Hospital, on 2 February, 1986 with 249 mm. The previous day measured only 20 mm.
Nkuru Nkuru totals 42 Wet Days across 85 years, with 135 mm on 22 January 1986, the only 100 mm or more on a single day. Only eight of these Wet Days have occurred since 1980.
Okatana has 31 Wet Days across 70 years, 114.7mm fell on 24 March 1937, and four days have topped the 100 mm mark. Only 9 Wet Days were recorded since 1980.
Erundu shows 22 Wet Days across 75 years with 103.4 mm on 6 December 1951. This was the only 100 mm fall and since 1980, they have recorded only one more Wet Day.
Dunroamin (Marienhof in the Okahandja district) has eight Wet Days across 50 years. 100 mm fell on 9 November 1963. This was the only 100 mm fall with only one more Wet Day since 1980.
The Windhoek Weather Office shows 8 Wet Days across 80 years. A mere 84 mm is the highest which was recorded on 7 March 1954. Windhoek has never had a fall of 100mm in the recorded rainfall history. Only one Wet Days (60mm or more) occurred since 1980.
Kranzplatz has had 18 Wet Days across a 70-year record. 122 mm fell on 15 February 1939. Two of these were 100 mm or more. However since 1980, there have been 11 such days on record. February 2000 had five such days. This 2000 rainfall season set new records across the south and this was one of them.
Gellap Ost has 12 recorded Wet Days across their 85 years. 91.4mm fell on 20 February 1952 and 76mm in March 2006. Since 1980 they only had 4 Wet Days.
Finally, Warmbad has three Wet Days on record across 85 years. 103.6mm fell on 18 February 2000. This their only fall above 100mm and their only Wet Day since 1980.
Generally, the ability to experience falls of 200 mm in just one day seems very limited. Apart from Ombalantu, only Kamanjab with 208.5 on 18 February 1952 ascends that line.
The geographical limit extends from the interior plateau to the Namib. This includes, specifically the northern coast. 100 mm has been recorded at Terrace Bay in the very wet 2006.
Further south along the coast, the likelihood of Wet Days dissappears.
Although moist air can penetrate further south, even in very wet years the conditions are never capable of producing Wet Days in the south west.
Since rainfall in Namibia is characterised by thunderstorms, the rainfall that leads to a Wet Day does not occur as soft rain over a 24-hour period. Instead, it is marked by downpours of flooding proportions, usually less than two hours in duration.
Bearing in mind that our rainfall season ranges from the end of October through to April, it is not surprising to find that Wet Days are generally limited to these months, more specifically from January to March for the interior, from far north to far south.