Watching for a Negative SAM: A Possible Twist in the Snow Season

Early signs of a negative SAM could shake up winter patterns, bringing a mix of cold snaps and warmer bursts — though the outlook remains uncertain.

Thanks to strong Tasman ridging, the next 4–5 days are set to deliver classic mid-season conditions. While onshore easterlies will feed moisture and showers into the east coast, the mountains will see favourable in-and-outbound windows that make the most of a near-peak snowbase this week.

By Tuesday, the ridge settles well south of the Alps, funnelling in cold south-easterlies and drawing low-level Tasman moisture. This will bring occasional fog to the Eastern Slopes, though only minor freeze–thaw conditions below 1900 m. In contrast, the Western Slopes look sunny from tomorrow through Friday. Typically, these ridges lean warm, but with this system holding a meridional structure (a Rex block), the mid-latitude atmosphere stays unusually cool.

A rex block is a ridge of high pressure that typically sits south of surrounding low-pressure systems (and the polar jet stream). This big anomaly dominates the weather over the east coast of Australia and the Tasman tomorrow through to Friday.

Kicking off this week, New Zealand sees a much-needed moderate top-up, as outlined in last Monday’s update. Canterbury & Otago resort and alpine areas are expecting 20-30cm over the next 48 hours.

By late Friday, the Australian Alps pick up a weak front sliding through the southeast. With the system’s peak energy landing on the west coast mid-week, its northerly flow drags warm, moist air across the continent. As the ridge reasserts itself, a light top-up of 2–3 cm is possible above 1800 m by Friday night - some element of rain will be in the mix, regardless.

Long Range Outlook

There’s some prospect in the long-range outlook as we try to see past this rex block that persists into next weekend. The fortunate news is that there has been a wobble in the stratospheric polar vortex.

In recent weeks, the polar vortex has eased from its sustained winter strength back toward a more typical phase. This transition has triggered warming in the stratosphere and, in turn, some striking shifts in the long-range outlook. Expect a few erratic model runs as they adjust to this mid-winter anomaly in the polar “engine room.”

Around 30 km above 60°S, the upper polar vortex has eased from near-record wind speeds back to more typical levels over the past week. This deceleration has sparked localised warming around the vortex, a shift that often triggers a wobble in its structure. Coupled with the potential of a negative SAM event (late Aug, early September), this could lead to some unusual variability in the winter pattern. This could also point to the first major indication that the polar vortex has reached its winter peak.

10-hPa temperature change in a week in the Southern Hemisphere. The contour interval is 5 °C. The pink and light blue shadings indicate warming and cooling, respectively. The red shading denotes warming for values higher than 25 °C.

This disruption of the polar vortex is particularly significant, aligning with strong ridging over Antarctica — a classic marker of a potential negative SAM event. If realised, it would drive greater variability in our winter, alternating between sharp cold outbreaks and warmer phases. The loop above highlights the long-range outlook in the lower stratosphere, showing the extension of various ‘limbs’ (polar nodes, or cold outbreaks) as the circulation wobbles.

The GEFS/NOAA’s AAO outlook (or SAM as we know it) suggests a slow transition to negative in the coming weeks. It should be noted that long range accucary of SAM beyond 10 days is low. Source: NOAA

Australian & New Zealand Long Range Outlook

With a hard ridge in place, it’s difficult to predict how frontal systems will move across eastern Australia and the Tasman, though global models are currently showing a strong system. Beyond 7–10 days, confidence remains low in the absence of a dominant climate driver.

Around 26–30 August, a strong polar node is expected to push through southeast Australia, as highlighted in recent Snowstack posts.

EC’s Ensembles product highlights the strong system building over South-East Aus and pushing through New Zealand. Source: ECMWF

Eyes are locked on this system as it hinges on the development of that potential negative SAM phase. Right now, it’s got some good support between various models and looks strikingly cold for now… But alas, it needs time to consolidate further support.

ECMWF’s long-range deterministic outlook favours a strong polar push ~27th August. Source: ECMWF

Unfortunately, for NZ this could lead to a draw down of warm, moist air ahead of the node, resulting in marginal conditions, followed by some snowfall. But it’s a long way out for NZ. The 30th of August to the 1st of September looks to be the next reasonable snow prospect for New Zealand. Stay tuned for further updates.

South American Outlook

South America’s having a shocker - no two ways about it. But that’s all about to change for some resorts.

Snow extent from remote sensing suggests the continent is collectively having the worst snow extent of the last 20 years.

South of Santiago, a strong polar front is set to sweep across southern South America from Tuesday through next weekend. This could be one of the most powerful systems to affect southern SA so far this winter.

Kicking off early Tuesday morning in the deep south (Nevados de Chillán and surrounding areas), abundant instability and cold air will produce snow down to sea level across large swathes of Patagonia and southern Chile.

Expect Chilean resorts to be gloating on social by late-week as their meagre snow base is about to get jacked.

40-60cm is likely across some Chilean resorts (south of Santiago) this week. Source: Meteoblue.