A thunderstorm on 16 November released small marble sized (10+mm) hail onto Grasshopper Rock damaging young shoots and florets. Fortunately, the outcome is only light damage and long term effect negligible.
Hail of this severity is not something we have experienced before. With the potential to create significant damage in the vineyard, especially if hailstones were bigger in size and fell over an extended period, we have taken a closer look at the event.
Spring thunderstorms are not uncommon. The Alexandra and Roxburgh basins (100-300 m above sea level) with the high Old Man/Kopuwai Range (1600 m asl) to the SW is ideal geography for thunderstorms. The warm moist air rises out of the basin and if it is met by cooler SW winds/front coming from the Southern Ocean over the high hills, the rising moist air is likely to freeze. (See diagram below). When the frozen droplets recirculate in the updraft and downdraft, they can grow into large hail stones.
On 16 November warm WNW air created a vigorous warm/moist updraft above the Alexandra basin and especially on the southern side of the basin where Grasshopper Rock is and south towards Fruitlands. Temperatures peaked in the vineyard at 21 degrees by 2 pm. Around this time, cool air moved in from a SW direction and met the warm air rising from the basin. Temperatures dropped quickly to 16 degrees by 3 pm and by 6 pm when the rain and hail hit temperatures had dropped to 10 degrees. (See our vineyard weather station record for 16 Nov below).
Possibly the hail had been building in size for up to 3-4 hours. This seems to be quite an unusual event for Alexandra. The local geography does not seem to allow enough time to develop large hail stones capable of serious damage.
The southern part of Grasshopper Rock, along the hill, was worst affected and the northern extremities of the vineyard were barely affected.
The damage is not significant in terms of yield. We expect to harvest the same quantity as usual. The damage will mean some blocks do not need as much, if any, work to keep the bunches evenly spread along the vine.
The following images record the worst of the damage.
- Florets were laid over by the force of the wind and hail and the exposed florets were knocked off in some instances.
- Stems were left bruised and some stems had the tips knocked out or in the worst cases, quite large stems were smashed. Leaves were shredded by the hail, especially the older leaves.
Video of damage
In conclusion, we escaped the largest hail stones in the last 15 years with only light damage. Production is not likely to be impacted. From this experience, it is evident the effect of any hail will depend largely on the timing in terms of growth stage. The vine growth stage is identified as 15 - approx 8 leaves per shoot, single flowers in compact groups. Historically, November, which is midway between winter and summer, does seem to be the higher risk time when warm northern air and cool southern air meet.
When viewing this diagram, imagine you are looking to the west. Alexandra and Grasshopper Rock would be on the right hand side and south is on the left hand side.
Diagram credit: http://www.physicalgeography.net/fundamentals/8f.html