Map from Google - Sudan

In Sudan the climate is desert in the north and on the coast of the Red Sea, while it becomes semi-desert or semi-arid in the south, which is affected by the summer monsoon.

On the coast of the Red Sea, winter is pleasantly warm, with highs around 25/27 °C (77/81 °F), while summer is stifling, with a combination of heat and humidity that is hard to bear.
Here are the average temperatures of Port Sudan.
Average temperatures - Port Sudan
Port SudanJanFebMarAprMayJunJulAugSepOctNovDec
Min (°C)191920222426292927252421
Max (°C)272729323538414038343128
Min (°F)666668727579848481777570
Max (°F)8181849095100106104100938882

The rains, poor, occur mostly between October and January, because in autumn and winter the prevailing north-east currents discharge a bit of humidity on the coast after passing over the sea. In Port Sudan, only 110 millimetres (4.3 inches) of rain per year fall, with a maximum of 50 mm (2 in) in November, while in Suakin, more to the south, rainfall reaches 120 mm (4.7 in) per year, so we are still at desert-like levels, although sometimes along the coast an intense downpour or thunderstorm may occur, which can unload an entire month's rainfall in a few hours.
Here is the average precipitation in Port Sudan.
Average precipitation - Port Sudan
Port SudanJanFebMarAprMayJunJulAugSepOctNovDecYear
Prec. (mm)411120930125025108

The sea is warm all year round, and in summer, when it exceeds 30 °C (86 °F), it becomes one of the warmest seas in the world.
Sea temperature - Port Sudan
Port SudanJanFebMarAprMayJunJulAugSepOctNovDec
Sea (°C)262626272930313131313028
Sea (°F)797979818486888888888682

While the coast can be considered a world apart, both for the higher humidity and for the rainfall pattern, the interior of Sudan is desert in the north, and progressively rainier as you move to the south, where the rains occur in the summer season.

Ancient pyramids in Meroe

In the north, occupied by a portion of the Sahara desert (whose part located to the east of the Nile is called Nubian Desert, while the western part is called Libyan Desert), the climate is desert, and in particular the northernmost area, near the border with Egypt, is one of the driest and sunniest in the world. The Nubian desert is particularly desolate, devoid of oasis and therefore of settlements, of course apart from the banks of the Nile, where the remains of the ancient Egyptian and Nubian civilizations are found, as well as more recent settlements. Winter temperatures are pleasantly warm during the day and cool at night, even cold at times, especially in the far north, where it can occasionally drop to around freezing (0 °C or 32 °F). The rest of the year is definitely hot, with highs around 40 °C (104 °F) from May to October, but with records of 50/52 °C (122/126 °F). Wind can raise sandstorms at any time of the year.
Here are the average temperatures of Wadi Halfa, located in the far north, on the shores of Lake Nasser.
Average temperatures - Wadi Halfa
Wadi HalfaJanFebMarAprMayJunJulAugSepOctNovDec
Min (°C)781216212323232219149
Max (°C)232631364041414038363025
Min (°F)454654617073737372665748
Max (°F)73798897104106106104100978677

Already at the latitude of the Bayuda Desert, in the "S" described by the course of the Nile, during summer there is a slight increase in humidity, a decrease in temperature, and a few rare downpours in July and August, brought by the last offshoots of the summer monsoon.
A few hundred kilometres (or miles) to the south, we find the capital Khartoum (Al Khurtum), located at 400 metres (1,300 feet) above sea level, where the White Nile and the Blue Nile meet. With an average annual temperature of 30 °C (86 °F), Khartoum is one of the hottest capitals in the world. Even winter is hot, with highs around 31 °C (88 °F) in January, but sometimes it can get cold at night, in fact the cold record is only 1 °C (34 °F); in spring the temperatures increase rapidly, so that they reach 40 °C (104 °F) already in April; in April and May sometimes the temperature have reached as high as 47 °C (117 °F).
Here are the average temperatures.
Average temperatures - Khartoum
Min (°C)161720242728262626262117
Max (°C)313337404241393839393532
Min (°F)616368758182797979797063
Max (°F)8891991041081061021001021029590

In July and August, under the influence of the monsoon, which brings a bit of rain, the temperature decreases slightly, but it remains very high, around 38/39 °C (100/102 °F), and then between October and November, at the end of the monsoon, it increases slightly again, rising to 39/40 °C (102/104 °F). Annual precipitation in the capital is very low, and amounts to 150 mm (6 in), including 70 mm (2.8 in) in August. Here is the average precipitation.
Average precipitation - Khartoum
Prec. (mm)000157507025400162

In May or June, when the front of moist air from the south invades the central part of Sudan, it can give rise to a phenomenon called Haboob, a sand storm that advances like a wall, and then greatly reduces visibility.

Haboob in Khartoum

In the Western Darfur region, lies Jebel Marra, a mountain range with volcanic craters occupied by lakes, such as Deriba Caldera, about 3,000 metres (9,800 ft) high. Here the climate is temperate because of altitude, and the landscape is greener than the surrounding areas, because the rains are more abundant, i.e. at the level of the southernmost areas.

Continuing south, annual rainfall increases gradually, reaching 700 mm (27.5 in) in the southernmost part. Almost all of Sudan is practically arid or semi-arid, because the greener area, geographically and culturally different, has been separated in 2011, giving birth to South Sudan. Anyway, in the southernmost part, eg in the Nuba Mountains, the rainy season is quite long (from May to October, but especially from June to September) and intense, so that it gives rise to a savannah-type environment. The climate in the Nuba Mountains, but also in the south-east, in the Dinder National Park, despite the altitude of around 500 metres (1,600 feet), is hot even in winter, with highs above 30 °C (86 °F), while in spring the temperature rises to around 40 °C (104 °F) and more, then summer is muggy and rainy, and finally autumn becomes torrid again.
Here are the average temperatures of Nyala, in South Darfur, at 650 metres (2,100 ft) above sea level.
Average temperatures - Nyala
Min (°C)151720222323222121211916
Max (°C)313336383836323133353331
Min (°F)596368727373727070706661
Max (°F)88919710010097908891959188

In Nyala, 430 mm (17 in) of rain per year fall, including more than 100 mm (4 in) in July and August. Here is the average precipitation.
Average precipitation - Nyala
Prec. (mm)00031750125140752000430

When to go

The best time to travel to Sudan is winter, and in particular the months of December and January. The weather is nice everywhere, except on the coast of the Red Sea, where there can be a bit of cloudiness and some showers; in the south and the east it's hot during the day, but generally cool at night, while in the north it can get cold at night. In February, the temperature begins to rise across the country, and in the south, where it was already intense, it becomes quite torrid.

What to pack

In winter: light-coloured clothing, light and long shirt and pants of natural fibres (cotton, linen), especially in the south where it is hot even in winter; turban for the desert and the wind from the desert, sunglasses (even graduated instead of contact lenses), a jacket and a sweater for the evening, desert boots or sandals. For the Red Sea coast, light clothing, a sweatshirt for the evening, raincoat or umbrella; equipment for snorkeling. For the reef, equipment for snorkeling, water shoes or rubber soled shoes.

In summer: for the inland areas: lightweight clothes, long and made of natural fabric (cotton or linen), desert turban; possibly sweater and sleeping bag for overnight stays outdoors. For the south, light clothes, light raincoat or umbrella for the rains, light sweatshirt. For the Red Sea coast, lightweight clothes; for the reef, equipment for snorkeling, water shoes or rubber soled shoes. For women, it is best to avoid shorts and miniskirts.