Weather review for the vineyard
week ending 4 October 2026
IOD battling to stay positive, the highest September ETo in eight years and a powdery mildew ascospore event — measured on-site and modelled in Datavit Metrics.
Prepared by Ben Johnston— Viticulturist & Vineyard Manager, DatavitDistributed by Cowaramup Agencies
IOD index
+0.76 °C
Battling to stay positive
September ETo
81 mm
Highest in eight years
September rain
72 mm
23 mm below the 7-year average
Winter chill
561.5 CU
Second coldest winter in 14 years
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.”
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.
Chill Units
A measure of winter cold accumulation (Utah–Richardson model). Hours at 2.5–9.1 °C add chill; hours above 18 °C take it away.
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 battling to stay positive —
+0.76 °C this week
- The Indian Ocean Dipole is battling to stay positive — the latest weekly value to 27 Sep 2026 is +0.76 °C, above the +0.4 °C threshold.
- You can see the high IOD from Oct 2023 to Apr 2024 on the chart — and remember the impact it had on us.
- Warm water building in the eastern Indian Ocean could be masking an actual IOD higher than currently measured.
What it means
A sustained positive IOD typically delivers a less-than-normal rainfall period for the South West — worth watching as the season builds.
Source: Australian Bureau of Meteorology, IOD index — weekly values to 27 September 2026.
Warm water building —
in the eastern Indian Ocean
- Weekly and monthly anomalies show a build-up of warm water in the eastern Indian Ocean.
- That build-up could be masking an actual IOD higher than measured — the same signal the IOD chart is picking up.
Takeaway
Warm water building in the eastern Indian Ocean could continue a less-than-normal rain period here in the South West.
Source: Bureau of Meteorology — weekly anomalies 21–27 September 2026; monthly anomalies August 2026.
SAM back to neutral —
less rain from the south
- SAM has returned to the neutral zone — fewer rain-bearing systems arriving from the south.
- The positive SAM through May and June supports the dry winter we have just experienced.
- A negative SAM typically brings wetter weather patterns with cooler temperatures; positive phases do the opposite.
What it means
With SAM neutral, the southern rain tap stays subdued for now — one less source of spring rainfall.
Source: Bureau of Meteorology — SAM, latest 12 months, climatology period 1980–2018.
Highest September ETo in 8 years —
rain 23 mm below average
Monthly ETo, BEDD and rainfall across seven recorded seasons. The current season is the right-hand column of each table.
September 2026: ETo 81 mm (highest in 8 years), BEDD 120 (warmest September in 6 years), rain 72 mm — 23 mm below the 7-year average of 95 mm.
Source: Datavit Metrics — on-site Datavit weather station. ETo and rain in mm; BEDD in degree-days.
Warm September — but still the
second coldest winter in 14 years
- Winter 2026 accumulated 561.5 chill units (ex October) — the second-highest total since 2013, behind only 2016 (851).
- September added just 26 chill units (ranked 9th of 14 years), reflecting the warm finish to the season.
- Chill accrues between 2.5 °C and 9.1 °C; hours above 18 °C subtract chill under the Utah–Richardson model. Chill calculator
Source: Datavit Metrics — Wilyabrup chill units (Utah–Richardson), on-site weather station data.
Ascospore event on 27 September —
another forecast for 8 October
- High disease pressure was experienced over 27 September — in particular a powdery mildew ascospore event.
- Driven by a combination of high humidity and extended leaf wetness, visible in the environmental drivers panel.
- Watch: another pressure event is forecast around 8 October, with rain and a leaf-wetness spike in the model — the ascospore curve is already climbing. 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 4 October 2026.