Your Labour Weekend Forecast: Heavy Rain and Snow For the South Island
A large area of low pressure in the Tasman Sea propels a large front towards the country, making landfall during Friday afternoon for western areas. Large amounts of moisture shift down from the sub-tropics on a ‘atmospheric conveyor belt’ starting as north as Brisbane. This high level of moisture along with this large area of low pressure looks to create a rather largescale weather event for the central and upper South Island. Heavy snow could occur for the higher altitudes above 600m for Canterbury and 800m for Otago.. Unlike our usual westerly rain events, heavy rain is not restricted to western and northern areas of the South Island. The low pressure system also has a smaller low that is set to develop off the Canterbury coast. This low creates a ‘u-turn’ once the moisture moves southeast over the top of the south Island, sending it straight back up into canterbury, see Map 1. The North Island misses out on the bulk of the heavy rain except for Mt Taranaki and the eastern Bay of Plenty Ranges which may see moderate rainfall totals.
Map 1
Map 2 shows the Extreme Weather Index (EWI) for precipitation. The EWI is a tool that can highlight the potential for highly unusual weather, in this case, precipitation much more intense than would typically be expected. Precipitation EWI maps are given as values between 0 and 1. A value of 0 means that precipitation is either not expected, or is not expected to be unusually heavy for a given area. A value of 1 means that precipitation during a given timeframe is expected to be extremely unusual for a given area, and also that the forecast for these extreme values is very confident. Note that EWI values aren’t tied directly to how much precipitation is forecast to fall, but instead how unusual that precipitation is.
Map 2
Rainfall Accumulations for Saturday
Tasman and Nelson, 100-150mm could accumulate in the ranges west of Motueka, the Richmond Ranges and Nelson Lakes. 30-50mm could fall for the remainder of the region.
Marlborough, 100-150mm for the Southern slopes of the Richmond Ranges and surroundings ranges south of the Wairau River.
Westland and Buller, 150-200mm is forecasted to fall in the ranges for Buller and for the Westland Ranges. 100-150mm could fall along lowland and coastal areas.
Canterbury, 150-200mm could accumulate about the main divide with 100-130mm for the ranges further east until the foothills. 50-75mm could occur along the foothill and inland areas of the plains.
For all regions: Rivers may rise rapidly especially with high intensity rainfall in the headwaters of major rivers. Surface flooding may occur on roads and flood plains.
Heavy Snow
At this stage we have seen significant signs of heavy snow for Canterbury with 20-30cm above 700m, 10-20cm down to 600m and light falls around 5-10cm accumulating down to 500m. For inland Otago, 10-20cm could fall above 700m with 5-10cm around the 600m mark. Snow flurries are possible down to 400-500 metres.
Map 3 shows the distribution of snow and should only be used as a guide only as our forecasted have been hand crafted using multiple layers of analysis.
Map 3
Rainfall and snow accumulations may increase or decrease in the next 24 hours. Please check in regularly on our website. We will also post updates on our Facebook page.
Useful Links:
https://www.severeweather.co.nz/in-depth-analysis
