Rio Tinto’s West Australian iron ore shipments have been hit by bad weather this quarter, putting its 2015 guidance under threat and helping spur a recent rally in iron ore prices.

But, in downbeat news for iron ore prices in the medium term, the company’s mines have not been affected. This means that once the longer than normal wet season at Rio’s Cape Lambert and Dampier ports passes, Rio will use expanded infrastructure capacity to put as much iron ore on to the market as it can to make up shipments over the rest of the year.

After hitting a 10-year low of $US46.70 a tonne in early April, iron ore prices surged as high as $US65.40 last week on lower imports into China, partly because of Rio’s outages, Chinese mine closures and renewed steel mill buying.

With reports Rio was back up shipping above capacity last week, iron ore prices were under pressure last night, threatening to fall back under $US60.

It is understood Rio’s ports (which are to the southwest of the unaffected Port Hedland harbour used by BHP Billiton and Fortescue Metals) have been shut down for various periods over April and May because of storm surges and tides.

This comes on top of lacklustre first-quarter Pilbara iron ore shipments of 69 million tonnes that missed analyst expectations by about 10 million tonnes.

So while Rio has been the most strident of the world’s big iron ore exporters when talking about its plans to continue expanding in the face of low prices and recent calls from Fortescue chairman Andrew “Twiggy” Forrest for production restraint, bad weather has made Rio the only big miner to actually crimp production.

The continued shortfall in Rio’s exports was picked up by analysts trying to monitor monthly shipments from Rio’s private Pilbara ports ahead of the miner’s quarterly report, released next month.

This is done by extrapolating Chinese imports, Port Hedland and Brazilian data and even by employing ship spotters in the Pilbara.

Rio had guided to 330 million tonnes of iron ore production from the Pilbara this year (including minority interests) and 350 million tonnes of shipments from its global mines (which include Canada).

But Macquarie analyst Hayden Bairstow says this might not be achievable, despite Rio this half targeting a boost in annual Pilbara export capacity to 360 million tonnes.

“In order to achieve its guidance, Rio will need to achieve a second half shipping rate of about 370 million tonnes per year, beyond the forecast capacity of 360mtpa,” Mr Bairstow said in a note to clients.

“Delivering a beyond-capacity result is not unachievable, however we believe a 350-360mtpa rate is more likely on a sustained basis.”

Macquarie cut its forecast for 83 million tonnes of second quarter Pilbara shipments to 76 million tonnes.

It cut its full-year (Pilbara) forecast by 5 million tonnes to 320 million.

Indicating the reduced production might have stopped and more ore is about to hit the market, Macquarie estimates Rio’s shipments last week hit a record annual rate of 370 million tonnes.

JPMorgan analyst Lyndon Fagan also lowered his full-year Rio shipment estimate, dropping it by 8 million tonnes to 320 million, saying the wet weather had probably delayed Rio’s increase in capacity.

“Recent weather impacts make the full-year target of 330 million tonnes unlikely,” Mr Fagan said.

But he noted the outage would not hit Rio’s bottom line.

“A spot scenario shows the positive pricing impact more than offsets weak volumes,“ Mr Fagan said.

The flood of Australian production that has driven prices down from $US135 at tonne at the start of 2014 will not let up any time soon.

As well as Rio’s extra tonnes, BHP is planning to put an extra 20 million tonnes of capacity on at some stage and Gina Rinehart’s Hancock Prospecting is set to free

55 million tonnes of new capacity this year at the Roy Hill mine in the Pilbara.

Roy Hill is expected to deliver first ore in September.

Growing output and a poor outlook for steel in China makes the future for iron ore prices far from rosy.

“While iron ore prices have strengthened in recent weeks, we do not believe the rally is sustainable without improved steel fundamentals,” RBC analysts said.

“We continue to anticipate weakness further into the third quarter as we move past China’s summer construction peak and supply continues to increase.”

Credit Suisse analyst Matthew Hope said China imported just 71 million tonnes of iron ore in May, down from 80 million tonnes in March and April.

“We know Brazil was down 1Mt sequentially, and for Australia, we suspect Rio Tinto’s output was flat, with no ramp-up through May,” Mr Hope said.

“We believe Rio Tinto failed to increase shipments as intended.”