This morning the MetService updated the Orange Heavy Rain Warning for Tasman northwest of Motueka to extend the duration of the event by 24 hours. It now covers the 54 hours between 6pm Thursday and midnight Saturday, and associated total has increased. The low pressure weather system will continue to draw warm moist air from the tropics through to Sunday/Monday and there remains uncertainty in the location, timing, intensity and duration of rain. Peak rainfall rates have also been increased slightly to 15 to 25mm/hr. MetService advise that there is a possibility that the duration of this Warning may be extended yet further into Sunday/Monday.
Based on the current weather forecast we are expecting Moderate flood flows and impacts over the long weekend in Tasman northwest of Motueka, and Minor flood flows in the eastern area including Nelson and Richmond. However, because the weather system is an ex tropical cyclone, small changes in course, timing and speed can result in quite different impacts so a conservative approach to planning would be prudent, especially if flooding consequences are severe for you.
For Tasman northwest of Motueka, from 6pm Thursday through to midnight Saturday, including the Golden Bay catchments and Takaka Hill current forecast rainfall is 250 to 350mm about the ranges, and lesser amounts near the coast, with peak rainfall rates of 15 to 25mm/hr. The wind direction will be North Easterly to Northerly and will change as Cyclone Tam moves position across the weekend. It’s possible that MetService will extend this Heavy Rain Warning into Sunday. MetService also advise that there is a low to moderate risk of thunderstorm activity during Friday, Saturday and Sunday. On Saturday and Sunday as the weather system moves closer to the top of the South Island thunderstorm activity could create localised bands of heavy rain that are slow moving in the area of Tasman northwest of Motueka.
- In main river catchments of Golden Bay, expect Moderate flooding with Annual Flood flows or less in the Aorere and Takaka Rivers.
- In the Riuwaka River expect Moderate flooding of an Annual Return Period and allow for a 5 year Return Period with this catchment being more impacted by rain from a North Easterly wind direction and as thunderstorm activity is possible.
- Rivers may peak multiple times during this multi day event. Current rainfall forecasts see one peak late Saturday morning.
- There is the potential for heavy falls in amongst this event, and short intense bursts of rain could result in more significant flooding in small streams and cause surface flooding.
- Expect some slipping to occur in prone areas.
For the eastern side of our region, the MetService have indicated that a Heavy Rain Watch may be issued tomorrow morning, with 60 to 80mm possible during the 24 hours of Saturday. Rainfall totals are not currently expected to reach Warning criteria. There is some uncertainty in the placement and intensity of rain bands. There is a low to moderate risk of thunderstorm activity and localised heavy rain during Saturday.
We are not anticipating storm surge as the low pressure system tracks to the west of the region. The wind direction is initially Easterly – North Easterly and larger waves are likely within normal beach zone. Tides over the weekend are not particularly high with Nelson high tide on Friday being 3.6m around 1pm.
Keep up to date with the latest MetService warnings here(external link).