Weather review for the vineyard
week ending
A wet, cool October, a major Halloween disease event and a positive IOD at +0.69 °C — plus Datavit's independent reading of the local drivers and the first Chardonnay harvest estimate.
Prepared by Ben Johnston— Viticulturist & Vineyard Manager, DatavitDistributed by Cowaramup Agencies
IOD index
+0.69 °C
Sixth positive week — to 25 Oct
October rain
93 mm
143% of the October average
October BEDD
119
82% of the October average
Harvest estimate
12 Feb
Nine days early — Chardonnay
Measured on-site —
modelled in Datavit Metrics
This review is produced for Cowaramup Agencies by Ben Johnston. Every local weather figure comes from the property's own monitoring 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.”
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A wet, cool October —
and a major Halloween disease event
October finished cooler and wetter than average: BEDD 119 (82% of average), ETo 98 mm (104%) and 93 mm of rain (143%). Halloween's 30 mm made it the second-wettest October in eight years, while September–October rainfall now sits 5 mm above average.
Summer outlook — BOM's perspective: the Bureau's November–January forecast, issued 29 October, favours above-average rainfall for south-west WA and very likely warmer days and nights. It expects the positive IOD to return to neutral in early summer. This is BOM's outlook, not Datavit's.
What our local drivers say — Datavit's perspective: the positive IOD is still driving convection away from Western Australia, and the subtropical dipole is pointing the same way. Datavit's independent reading is that the drivers still favour a drier summer pattern — one wet month does not change the season's direction.
- BOM forecasts showers on Monday 2 November (1–6 mm, 90% chance), matching the minor disease event flagged by the Disease Lab for Monday night.
- After a possible morning shower on Tuesday, Wednesday to Saturday turns dry and sunny, with temperatures climbing to 22–24 °C — a useful protective-spraying window after the Halloween event. Disease Lab
“Sources: BOM long-range forecast Nov 2026–Jan 2027 (issued 29 October), IOD index to 25 October and Margaret River forecast (issued 1 November); Datavit Metrics and the on-site station.”
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Reading the numbers —
plain-language definitions
IOD
Indian Ocean Dipole — the temperature difference between the western and eastern tropical Indian Ocean. Positive usually means drier for south-west WA.
Subtropical IOD
A similar ocean-temperature see-saw further south, off WA's west coast. A positive phase reinforces the tropical IOD's drying signal.
SAM
Southern Annular Mode — the north–south shift of the westerly winds around Antarctica. Negative SAM pushes rain fronts towards us; positive contracts them away.
SST anomaly
How much warmer or cooler the sea surface is than average, in °C. Red is warmer, blue cooler — the raw fuel behind the IOD.
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 — accumulated warmth that drives vine development. A higher BEDD means a faster-moving season.
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
How favourable conditions are for powdery mildew, downy mildew and botrytis — driven by humidity and leaf wetness.
IOD remains high —
+0.69 °C, a sixth positive week
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- The Indian Ocean Dipole remains high — the latest weekly value to 25 October 2026 is +0.69 °C, well above the +0.4 °C threshold.
- It is driving convectional weather away from Western Australia.
- A sixth consecutive weekly reading above the threshold — and a positive subtropical IOD is now appearing in support.
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 25 October 2026, climatology period 1991–2020.
Positive subtropical IOD —
reinforcing the dry signal
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- A positive subtropical IOD is appearing in support of the positive tropical IOD.
- The two signals pointing the same way typically suggest a dry period ahead.
Takeaway
A positive subtropical IOD is reinforcing the positive IOD's dry signal for the South West.
Source: Bureau of Meteorology — weekly anomalies 19–25 October 2026; monthly average September 2026 (created 26/10/2026).
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.
Second-wettest October in eight years —
and 20% cooler
The current 2026–27 season is the 2027 column. October ETo reached 98 mm (104% of average), BEDD 119 (82%) and rain 93 mm (143%).
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Halloween's 30 mm made October the second-wettest in eight years. September–October rainfall sits 5 mm above average, while accumulated BEDD of 239 puts the season alongside 2022 and 2026 in the 230–240 range.
Source: Datavit Metrics, on-site Datavit weather station — ETo and rain in mm, BEDD in degree-days.
Major disease event on 31 October —
all models in play
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- Major disease event on 31 October — Halloween's 30 mm of rain and long leaf-wetness hours triggered it.
- Downy mildew, ascospore (powdery mildew) and botrytis were all in play — every model spiked together around the event.
- Watch: a minor event is flagged for Monday night, 2 November, with showers of 1–6 mm forecast. Disease Lab
How it's modelled
Past days come from the on-site station's measured history; the days ahead come from the Meteoblue forecast.
Source: Datavit Metrics Disease Lab — on-site Datavit weather station + Meteoblue forecast, 26 October–7 November window.
Harvest estimate: 12 February —
nine days early
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- Early budburst with the P58 and Gingin clone Chardonnays.
- The current prediction of harvest for these blocks is 12 February 2027 — nine days early, 174 days from budburst.
- The estimate uses actual station data from August to October 2026, with eight-year averaged algorithm weather data for the rest of the season. Explore Metrics
By the numbers
268.3 BEDD accumulated over 70 measured days, with 860.4 BEDD projected across the remaining 104 days towards the 1,120 target.
Source: Datavit Metrics Harvest Date Estimator — on-site Datavit weather station + eight-year averaged weather data.