Weather review for the vineyard
week ending
IOD getting more positive at +0.88 °C, a cool first half of October with soils still at field capacity, and powdery mildew pressure easing — measured on-site and modelled in Datavit Metrics.
Prepared by Ben Johnston— Viticulturist & Vineyard Manager, DatavitDistributed by Cowaramup Agencies
IOD index
+0.88 °C
Getting more positive — week to 11 Oct
October BEDD
115
20% below the October average
Rain 1–17 Oct
35 mm
Soil still at field capacity
Irrigation need
−16.5 L/m
Negative — no irrigation needed
Measured on-site —
modelled in Datavit Metrics
This review is produced for Cowaramup Agencies by Ben Johnston. Every weather figure on this page comes from one system:
- Datavit weather station — research-grade, 100% solar, recording the site's real microclimate every 15 minutes, day and night. See the station
- Datavit Metrics platform — Disease Lab risk models, ETo, BEDD and chill-unit calculators, rainfall records and seven-year comparisons in a single login. Explore Metrics
- Bureau of Meteorology — IOD, SAM and sea-surface temperature charts for the broader climate picture.
“Weather, disease risk, ETo, BEDD and rainfall — what would take multiple tools, in one login.”
Scroll horizontally or use the arrow keys to view the full figure.
Reading the numbers —
plain-language definitions
IOD
Indian Ocean Dipole — the sea-temperature difference between the western and eastern Indian Ocean. Sustained values above +0.4 °C mean a positive phase, which usually reduces rainfall in south-west WA.
SAM
Southern Annular Mode — the north–south shift of the rain-bearing westerly winds across the Southern Ocean. A negative SAM often brings wetter, cooler weather to the south-west of WA.
SST anomaly
How much warmer or cooler the ocean surface is than the long-term average — the raw signal behind the Indian Ocean Dipole and the seasonal outlook.
ETo
Evapotranspiration — how much water the atmosphere pulls from soil and vine, in mm. Higher ETo means greater irrigation demand.
BEDD
Biologically Effective Degree Days — heat units accumulated from budburst, used to track vine development and compare seasons.
Leaf wetness
How long foliage stays wet after rain or dew, in minutes. Fungi need sustained wet hours to infect — long wetness means higher disease risk.
Disease pressure
Modelled infection risk (0–100%) for powdery mildew, downy mildew, botrytis and ascospore — read daily in the Disease Lab, driven by humidity and leaf wetness.
IOD getting more positive —
+0.88 °C this week
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- The Indian Ocean Dipole is getting more and more positive — the latest weekly value to 11 Oct 2026 is +0.88 °C, well above the +0.4 °C threshold.
- The index is starting to reach the levels we saw in the summer of 2024.
- Weekly values have now held above the positive threshold for four consecutive weeks — sustained values at this level are typical of a positive IOD event.
What it means
A sustained positive IOD typically delivers a less-than-normal rainfall period for the South West.
Source: Australian Bureau of Meteorology, IOD index — weekly values to 11 October 2026, climatology period 1991–2020.
Week vs month —
warming over the western Indian Ocean
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- A distinct increase in water temperature over the western Indian Ocean compared with the September average.
- Significant momentum towards a potential high (dry) IOD for the summer.
Takeaway
Warming over the western Indian Ocean is exactly what drives the IOD index higher — the same signal the IOD chart is picking up.
Source: Bureau of Meteorology — weekly anomalies 5–11 October 2026; monthly average September 2026 (created 12/10/2026).
SAM back to neutral —
average rain from the south
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- SAM is back to neutral — less rain from the south than during negative phases.
- Neutral represents average rainfall from southern systems — neither a boost nor a shut-off.
- A negative SAM typically brings wetter weather patterns with cooler temperatures; positive phases do the opposite.
What it means
With SAM neutral, expect near-average rainfall from the south — the IOD is the driver to watch this month.
Source: Bureau of Meteorology — SAM, latest 12 months.
A cool first half of October —
soil still at field capacity
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- A cool first half of October: heat accumulation (BEDD) is tracking 20% below the October average, with ETo near normal (98%) and mild wind.
- Projected October rainfall of 64 mm sits at 98% of average — yet the fortnight itself stayed dry enough to earn a “Dry” comment.
- Water requirement comes out negative — −16.49 litres per metre (≈ −54,950 L for a 1 ha block): rainfall has more than covered crop water use.
Bottom line
Soil still at field capacity — no irrigation needed. Cool conditions are slowing heat accumulation: growth is steady, not rapid.
Source: Datavit Metrics — season status (projected) and irrigation calculator, on-site Datavit weather station data, 1–17 October 2026.
Powdery mildew pressure since 14 October —
easing this week
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Scroll horizontally or use the arrow keys to view the full figure.
- Significant powdery mildew pressure since 14 October, riding on the week's humidity and leaf wetness.
- The next seven days provide a low-pressure period for spraying.
- No ascospore or primary infection events are predicted — but keep an eye on Wednesday 21 October, when showers return. Disease Lab
How it's modelled
The Datavit Metrics Disease Lab combines the last 7 days measured by our station with the next 7 days from the Meteoblue forecast.
Source: Datavit Metrics Disease Lab — risk timeline and environmental drivers, week ending 18 October 2026.