The phrase 'unexpected rain' is correct and usable in written English.
You can use it when you want to talk about rain that has come unexpectedly or suddenly. For example, "We were about to start playing tennis when it started to rain unexpectedly."
Caught in an unexpected rain, Gabilan catches a cold and, despite Billy Buck's ministrations, dies.
As he spoke, an unexpected rain began to fall, but no one in the group abandoned the sandy court.
He spent a morning shooting gophers out of his pickup-truck window and waiting for his fields to dry after an unexpected rain.
When the BMX flat stage final had to be cancelled in Montpellier due to unexpected rain, several competitors decided to stick around until the showers stopped and just entertain the crowd with their skills.
When she is 16, Anna falls suddenly in love with Cooper - Ondaatje describes the moment, with typical precision, as her hair darkens quickly in unexpected rain - and they subsequently meet every afternoon at his shack in the woods, the first secret the family has ever had.
The lyrically complicating hand of Mr. Maysles, who shot the film with several collaborators, is everywhere apparent in the way the camera picks a path around coherence, refusing narrative in favor of the thick detail of life unfolding: a small sea of umbrellas drips on a patio; several old women hobble from the clubhouse with plastic bags protecting their heads from an unexpected rain shower.
In recent days, unexpected rains have shut off the camp to UN flights, although plenty of supplies are on the spot.
Otherwise I would spend a lot of money and labour on my fields and unexpected rains would just wash everything away.
On Tuesday, Google's booth flooded, due to unexpected rains in the Vegas desert, according to Business Insider.
Rainfall was favourable however the country wide unexpected rains of October/November 2008 resulted in flooding of areas lying along Baro, Golo and Akobo rivers.
On the other hand these unexpected rains caused flooding in Gambela and low lying areas of Somali, Oromiya and SNNPR regions.
