Weather review for the vineyard
week ending 20 September 2026
Climate drivers, chill units, 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
Season BEDD to 19 Sep
97.6 °C days
Warmest September in six years
Chill units 1–18 Sep
-16.5 CU
Negative since 9 September
IOD index
+0.65 °C
Back in positive territory
Disease forecast
Easing
Risk drops after 19 Sep — watch ascospore
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 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.
BEDD
Biologically Effective Degree Days — heat units accumulated from budburst, used to track vine development and compare seasons.
Chill Units
A measure of cold accumulation. Warm weather subtracts units — a negative total signals an unusually warm run, as seen this September.
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 Disease Lab.
IOD back in positive territory —
worth watching this spring
- The Indian Ocean Dipole is positive again — latest weekly value to 13 Sep 2026 is +0.65 °C.
- You can see the high IOD from Oct 2023 to Apr 2024 on the chart — and the impact it had on us.
- Recent readings sit above the +0.4 °C threshold that marks a positive phase.
What it means
A sustained positive IOD (above +0.4 °C) typically brings drier winter–spring conditions to south-west WA — worth watching as the season builds.
Source: Australian Bureau of Meteorology, IOD index — weekly values to 13 September 2026.
The warm water has shifted
north towards India
- Weekly and monthly sea-surface temperature anomalies show the warm pool near Madagascar has moved north.
- That northward shift is the same signal the IOD chart is picking up — the two move together.
Takeaway
The warm pool near Madagascar has moved north — a pattern that could continue a less-than-normal rain period here in the South West.
Source: Bureau of Meteorology — weekly anomalies 7–13 September 2026; monthly anomalies August 2026.
SAM back to neutral —
after a big positive spike
- A big spike in the SAM recently contracted rain and wind towards the Antarctic.
- Now in the neutral zone (+0.04 on 16 Sep) — suggesting normal rainfall patterns for September.
- The positive May–June period supports the dry winter we have just experienced.
Source: Bureau of Meteorology (SAM, 16 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.
Warm since 9 September —
chill units running negative
- Just -16.5 chill units accumulated from 1 to 18 September (18 days of station data).
- Daily chill units have been negative since 9 September — a run of unseasonably warm days and nights.
- Consistent with the BEDD numbers below — heat is accumulating fast this season. BEDD calculator
Source: Datavit Metrics period calculator (Chill Units), on-site weather station data, 1–18 September 2026.
Warmest September to date
in six years
- Season-to-date BEDD (1–19 Sep) is 97.6 °C days — the highest of the six seasons recorded in Metrics.
- Same date in previous seasons: 88.6 (2024–25), 85.7 (2023–24), 69.7 (2022–23), 59.8 (2025–26) and 59.1 (2021–22).
- The monthly comparison shows September 2026 clearly ahead — matching the negative chill units of the past fortnight.
What it means
Vines are accumulating heat faster than in any of the last six seasons — development is likely running ahead of recent years.
Source: Datavit Metrics season comparison — BEDD (°C days), on-site weather station data to 19 September 2026.
High pressure mid-week —
humidity and leaf wetness to blame
- 16–19 September: high disease pressure — in particular a powdery mildew ascospore event and downy mildew.
- Driven by a combination of high humidity and leaf wetness, visible in the environmental drivers panel.
- Powdery, downy and botrytis risk ease after 19 September, but ascospore risk climbs again towards the end of the forecast window — keep monitoring. 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 20 September 2026.