The Sydney to Hobart fleet are in for a nervous week after the long-range weather forecast revealed uncertainty over race conditions.
Typically, the forecast a week from Boxing Day gives crews an indication whether conditions on the 628-nautical mile race will suit their boat, and provides a chance to begin work on tactics.
But NSW's hot and humid weather, combined with the possibility of a low-pressure system forming this weekend, has made it difficult for the Bureau of Meteorology (BOM) to make conclusive predictions.
A low-pressure system forming over the Tasman Sea could set storms in motion from December 27, which would have significant impact on waves and wind direction.
As of Monday morning, the BOM's best guess was that the race will begin in southwesterly winds; but even that could change.
Navigator Adrienne Cahalan, sailing Alive in her 31st Sydney to Hobart race, said it was almost unheard of to have conditions so unclear a week out.
"If this low pressure (system) does develop, then it sort of throws everything out the window that we might've already had," she told AAP.
"While there's always the potential for that to develop, it creates uncertainty in the forecast.
"The models at each run are changing from having a low on the NSW coast to maybe having a high-pressure (system).
"This great diversity from model runs is something that's quite new."
The uncertainty means navigators are working overtime, with more insight to be gleaned from weekend forecasts.
But not all crews can wait that long to begin making tough calls.
"Some boats don't have to make decisions a week out," Cahalan said, outlining sail choice as just one crucial factor.
"We do. That's why this long-range forecast is really important for us.
"It just means it's double the work, because you've got to be prepared for everything, whereas in the past you might've been able to dismiss a couple of scenarios that you know won't happen."
URM Group, one of Alive's rivals for overall honours, is hoping to embrace the uncertainty.
The 72-footer has enjoyed a dominant lead-in to the annual event, recording wins at the Bird Island, Sydney Gold Coast and Flinders Islet races.
"I find it a little bit exciting when the forecast is this uncertain, because anything can happen," URM navigator Alice Parker said.
"There'll be opportunities for little boats and opportunities for big boats."
The variable forecast is perhaps even more alarming for the smallest boats in the two-hander division, which will require a handful of days to reach Hobart's Constitution Dock.
"We don't have one weather system to worry about, we probably have at least two to get through," David Henry, skipper and owner of Philosopher, said.
"We have double-trouble when it comes to predicting what the weather is going to be.
"The only thing that we definitely know is that we don't know (what the conditions will be)."