Lake Pukaki Twizel highway speed reduction in black ice and freezing fog conditions

NZ Transport Agency Waka Kotahi (NZTA)’s current measures to request people to slow down south of Tekapo through to Lake Pukaki/Twizel will remain in place while the cold hoar frost and black ice conditions continue.

Yesterday signs advertising a new temporary speed limit of 50km/hour were put in place between Simons Pass and Lake Ruataniwha south of Twizel, a distance of 28 kms.

“We have started with a blanket speed limit approach, but we will refine this to high risk areas on SH8 over coming days, based on weather forecast and conditions on the ground,” says Mark Pinner, System Manager Central South Island for NZTA. “This will mean that some areas of highway less prone to persistent frost or fog may go up to 70km/ hour or even 100km/hour in coming days.”

“We acknowledge that this is an unusual situation which has required us to take these measures and it will be actively managed until the end of winter for everyone’s safety.”

Today is a fine and sunny day through the Mackenzie Basin with no fog, says Mr Pinner, but the potential for frozen/black ice road conditions in many places has not immediately changed. People need to slow down significantly hence the need for daily monitoring and adjustments which will reflect the road and weather conditions.

While other parts of the South Island road network are also experiencing more ice and fog-related weather this season, in general they are much less affected than this area of SH8 through the Mackenzie Basin.

Part of SH8 Pukaki highway last week, around 11 am, 18 July in fog with black ice.

Places like the Lindis Pass, west of Omarama, benefit from slightly more wind but the Pukaki area has had windless conditions in recent weeks allowing the hoar frost and fog to last for hours and a persistent/long-lasting frost effect on the highway keeping temperatures extremely low.

“We are working closely with our Police partners and MetService to ensure people get to their destinations safely and without endangering themselves or other road users. If it becomes necessary, we will close the road, so people should check our highway map before setting out around the central and southern South Island, particularly overnight.”

Highway map(external link)

Police will be patrolling the area over the winter period while the freezing conditions continue. “It is essential people ‘Drive to the Conditions,’” says Mr Pinner.

“We encourage all drivers around the South Island to slow down where there is the potential for black ice on bridge decks and in shaded places and in fog, drivers need to slow significantly.”

NZTA has also added a fourth gritting crew into the early morning/daytime highway maintenance in the Mackenzie basin while the freezing temperatures continue.

MetService overview of current 2024 winter weather conditions

So why is this winter proving challenging on some southern South Island/Te Waipounamu highways and in the Mackenzie Basin in particular? MetService supplied us with this overview:

• The difference largely comes down to preceding years’ winter conditions being unusually warm.
• The winter of 2024 has an absence of oceanic heat (marine heatwave), which is noticeable to the average kiwi. This follows several record warm winters from 2020 to 2023.
• By comparison, 2024 is not a warm winter, but more what we would call a ‘normal winter’. This means conditions that would be more typical for winter like black ice, are more common than they have been.
• Most winters see a period of “blocking”, when a large, slow-moving high pressure system sits somewhere near New Zealand. This leads to settled weather and light winds, allowing temperatures to plummet in inland areas. We have seen this through much of July so far.
• If there is enough moisture in the air lying over the country, fog will form and fill the Mackenzie basin. This freezes and forms the hoar frosts we saw over recent weeks. Unfortunately, this also leads to black ice conditions on roads.
• Further periods of black ice are possible through the rest of this winter, especially with another cold outbreak looking increasingly likely next week starting 29 July, with the potential for snow to affect many South Island roads. However, the length of the ice risk period will depend on how long high pressure persists after a trough moves away.
• In the long term, most weather modelling indicates systems becoming more mobile as we head towards spring and a return to the more usual westerlies.