Reading the Mountain: MetService Forecasts + Personal Observation

A weather forecast tells you what is likely. A personal observation tells you what is actually happening. The trampers who consistently make good decisions in the New Zealand backcountry use both, and weight them differently as the day unfolds. Here is the system we use, and the MetService products that drive it.

The forecast products that matter#

MetService publishes more than the consumer-app shows. The ones that count for trampers:

1. Mountain forecasts (metservice.com/rural)#

Aspect-specific forecasts for every major NZ range — Tararuas, Ruahine, Kaweka, Nelson Lakes, Mt Aspiring, Fiordland, Aoraki/Mt Cook, Kaikōura. They cover wind speed at the tops, freezing level, cloud base, and precipitation, broken down by 3-hour blocks for 48 hours.

Read the wind speed at the tops, not the valley speed. A "moderate" 25 km/h in the valley often means 70+ km/h on a ridgeline. Anything over 60 km/h on tops is hard going. Over 90 km/h, plan to stay below the bushline.

2. Marine area forecasts (counter-intuitively useful)#

For the West Coast and Fiordland, the Tasman Sea marine forecast often gives you the next 24 hr of weather sooner than the rural forecast, because that is where it is coming from. If a 1000 km front is offshore, you have 12-18 hr.

3. Severe weather warnings and watches#

MetService issues warnings (highly likely, immediate) and watches (possible, plan ahead). Both pages worth bookmarking. Cross-reference with the regional council's emergency alerts — they are usually first to post road closures.

4. Rain radar#

The animated radar at metservice.com/maps/rain-radar gives you the previous 6 hr of rain across the country. Useful at hut breakfast time: you can see whether the front is east or west of the divide right now.

5. NZ Avalanche Advisory (avalanche.net.nz)#

Winter and shoulder season only. Bulletins by region — Aoraki, Two Thumb, Craigieburn, Tongariro, Fiordland. Read the text, not just the colour. Covered in detail in our winter post.

The personal-observation layer#

Forecasts are a starting point. The mountain itself tells you what is happening.

Cloud signals#

• Lenticular clouds (smooth, lens-shaped, hovering over a peak) = strong upper winds. Plan a low day.
• Lowering cloud base through the morning = front arriving. The cloud usually drops 100-200 m/hr ahead of weather.
• Cap cloud sitting on a single summit = wind shear, often the early sign of a southerly.
• Wave clouds parallel to the ranges = strong, persistent westerly. Do not commit to crossing tops.

Wind signals#

• Tussock moving in waves at 30+ km/h. Below this, walking is fine. Above, the gusts will push you.
• Sudden silence in valleys is often the trough between two fronts. The next round can be the worst.
• Wind backing from NW to W to SW = classic frontal passage. Plan for 6-12 hr of bad weather, then improvement.

Pressure signals (if you carry an altimeter watch)#

• A 5 hPa drop in 6 hr = significant front incoming.
• Pressure rising steadily through the morning = clearing weather, push for tops.
• Pressure rising fast = strong post-frontal wind, expect gusts.

Key tip: If your forecast says "fine, light wind" and you are watching cloud lower, lenticulars form, and pressure drop — believe what you are seeing, not the forecast. Forecasts are 80-90% reliable in NZ. The 10-20% they miss is when local conditions diverge, and the mountain will tell you that before MetService does.

A morning routine that scales#

Every morning of a trip, before leaving the hut:

1. Check phone forecast if signal exists (rare in the Tararuas, possible in Fiordland tops, often in Mt Cook valleys).
2. Walk outside, look at cloud base, wind on the trees, feel the temperature change since dawn.
3. Listen — birds going quiet ahead of weather is real and worth trusting.
4. Tap the altimeter if you have one. Pressure change since last night.
5. Decide: full plan, alternate plan, or rest day.

That whole routine takes 5 minutes. It has saved us from at least three trips that would have ended badly.

The decision tree we use for committing to a tops day#

Before we cross a saddle or commit to a high traverse, we ask:

1. Forecast says winds under 60 km/h on tops? If no, drop.
2. Cloud base above the saddle, with no signs of lowering? If no, drop.
3. Pressure stable or rising in the last 6 hr? If no, drop.
4. Daylight hours sufficient with a 90-minute weather margin? If no, drop.
5. Whole party fit and well? If no, drop.

Five yeses, we go. Any no, we wait, retreat, or take the low alternate. There is always another day.

Tools we carry#

• Phone with MetService app, plus a backup like Yr.no for cross-reference.
• Altimeter watch (Suunto, Garmin Fenix, Coros). Pressure trend is gold.
• Print of the relevant forecast at the trailhead, before signal disappears.
• PLB for when judgement was wrong.

The mountain is not punishing. It is indifferent. It will let you be late, slow, or wrong without flinching, and it will not warn you before it changes its mind. Reading it well is just paying attention. The forecast is half the story, the sky is the other half.