Meteorologists estimate that 25 litres of water per square metre fell in this small time period, transforming parts of the city into a raging river.
Considering northern areas have up till now avoided the kind of flooding horrors experienced in urban areas of Lisbon just before Christmas, Saturday's deluge has seriously worried city authorities.
Deputy mayor Filipe Araújo admitted that "for the first time" water rose 'up' to the surface and did not drain into the underground river.
"We have a river that passes below that normally transports (rain) water. If we don't detect ways of correcting (the problem), it could happen again..." he said.
Mr Araújo stressed there are works ongoing to extend the city's Metro, and that these "could have contributed to the (extraordinary) scenario experienced this Saturday".
All efforts now are on understanding what exactly went wrong, and how to sort it out.
Yesterday's scenes were unprecedented, particularly at São Bento Metro station, which saw escalators and stairwells turned into raging waterfalls.
Shops were flooded, basements filled, pavements and road surfaces destroyed, manhole covers lifted and carried with the force of raging waters.
The 'damages' caused are as yet unclear; individual shopkeepers are already describing losses in the tens of thousands of euros.
"It all happened so quickly", explained one. "We suddenly saw the terrace being washed away by the water. We had time to rush inside, if not we would have been swept up as well..."
In less than two hours, emergency services registered over 250 calls for help due to flooding of homes and businesses.
"It was a phenomenon the city has never seen", admitted Filipe Araújo.
Social media was filled with videos cataloguing the chaos and showing just how strong the waters were at the peak of the incident
Mop-up efforts are still going ahead as the north and centre remain on orange alert throughout today.
Some roads remain closed in Porto's downtown - for instance Avenida Gustavo Eiffel, which was already closed to traffic due to a building collapse, and Rua Mouzinho da Silveira, where the floodwaters "opened up a crater".
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