Weather review for the vineyard
week ending 6 September 2026
Climate drivers, winter rainfall, ETo, BEDD and disease pressure for the Margaret River region — measured on-site and modelled in Datavit Metrics.
Prepared by Ben Johnston— Viticulturist & Vineyard Manager, DatavitDistributed by Cowaramup Agencies
Winter rainfall 2026
490 mm
Driest of the last three winters
Vs last winter
-432 mm
2025 recorded 922 mm
IOD index
+0.22 °C
Neutral — but the ocean is warming
Disease forecast
Clear
No pressure events until 11 September
Measured on-site —
modelled in Datavit Metrics
This weekly 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 and BEDD 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
Reference evapotranspiration — the atmosphere's water demand, in mm. High ETo means vines need more irrigation; low ETo means rain goes further.
BEDD
Biologically Effective Degree Days — heat units accumulated from budburst, used to track vine development and compare this season against previous years.
Leaf wetness
Minutes the leaf surface stays wet from rain or dew. Sustained wetness is what allows fungal diseases to infect.
Disease pressure
Modelled infection risk (0–100%) for powdery mildew, downy mildew, botrytis and ascospore — read daily in the Datavit Metrics Disease Lab.
IOD currently neutral —
but the ocean is warming
- Currently sitting Neutral — latest weekly value to 30 Aug 2026 is +0.22 °C.
- You can see the high IOD from Oct 2023 to Apr 2024 and the impact it had on us.
- Temperatures appear to be heating up east of Madagascar — a positive IOD is probable later this season.
What it means
A sustained positive IOD (index above +0.4 °C) typically means drier winter–spring conditions for south-west WA — worth watching as the season builds.
Source: Australian Bureau of Meteorology, IOD index — weekly values to 30 August 2026.
Ocean temperatures are climbing
across the Indian Ocean
- Weekly and monthly sea-surface temperature anomalies show the tropical Indian Ocean warming from west to east.
- The warm pool east of Madagascar is the region to watch — it is the engine of a positive IOD.
Takeaway
Quite a noticeable increase in ocean temperature over the last month — consistent with the positive-IOD signal on the previous chart.
Source: Australian Bureau of Meteorology, GAMSSA SST — weekly average 30 August 2026, monthly average July 2026.
A positive winter SAM
matches our dry season
- A negative SAM typically provides wetter weather patterns with cooler temperatures.
- The positive May–June period supports the dry winter we have just experienced.
Source: Bureau of Meteorology (SAM, issued 1 September 2026) · Rainfall: Datavit Metrics station records.
The driest winter
of the last three
Winter 2026 finished 432 mm below last year — the driest of the last three winters, recorded by the on-site Datavit weather station.
Seven years of ETo, BEDD and rainfall,
side by side
Keep an eye on the 2027 column — monthly and accumulated comparisons over the last seven years, generated in Datavit Metrics from the on-site station's records.
ETo and rainfall in mm · BEDD in degree-days · Days over 34 °C / 37 °C and heat events tracked below the tables.
High pressure behind us —
a clean forecast ahead
- 30–31 August: high disease pressure across powdery mildew, downy mildew and botrytis, driven by high leaf wetness — 948 minutes.
- No pressure events forecast until 11 September — a good window for canopy work and spray coverage to hold. Spray windows
- Ascospore risk sits in the moderate band — keep monitoring as spring conditions build.
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 6 September 2026.