Thursday, April 25, 2024

Equinox fires the starting gun on autumn

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Traditionally, more chaos comes into our weather patterns once we’re past the equinox.
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By Philip Duncan, WeatherWatch

This week is the autumn equinox. At precisely 4:06pm on Wednesday, March 20, is when it occurs. 

To many, this marks the official start of autumn and certainly from a physical point of view many plants start to change now. Right now we have basically equal-length days and nights, but in the weeks ahead the nights become longer and the days shorter. 

The cooler mornings are certainly being welcomed by many who find summer nights too hot for sleeping in some regions. Parts of the country are basically in drought. 

The way droughts are announced in New Zealand can be slow and clunky. Last year when forecasters were looking for the likely peak of the dry weather from El Niño it was expected by us (and others overseas) that late summer and early autumn might be the driest point.
Droughts – or big dries, or whatever you want to name these dry periods – end in NZ in two ways. Either one big rain event comes in and reverses everything in a day or two, or – as is often the case –  the ending is death by paper cuts. By that I mean over the next few months we have so many showers that eventually they cumulate to bring normality back to most places.
Around NZ it’s hit and miss as to who is dry. In recent weeks we’ve seen some very stubborn dry weather, but we have also had downpours and a few showers. 

The regions around NZ have variety of conditions – some parts are about right for this time of year, others wetter, many others drier, all within the same one region. But it’s safe to say that much of the eastern side of NZ is drier than normal. 

Soil moisture maps lately look like classic El Niño with the dry weather to the north and east of both main islands but wetter in the west.
Our forecast for March was one of a lot of dry weather and cold fronts trying to move in. But as we head towards April we’re seeing the speed wobbles come back into the high-pressure belt. Instead of it looking like a straight line from west to east, they are moving around more and we’re seeing breaks, which will allow some low pressure and wet weather to move through. 

Will this line up to bring relief to your property? That’s harder to say. But certainly the highs look to be “wobbling” around a bit and that gives wiggle room to low pressure, showers and rain bands. 

The tropics is also very active – and while we need these stubborn highs to break up further, the right timing of that can allow a tropical low to drop south. In short, we have some wet weather slowly returning going into April, scattered across both islands, but still plenty of dry this week.

Upcoming highlights:
• Southerly flow kicks off this week
• High pressure moves in, centred over the South Island on Tuesday
• High pressure dominates much of NZ this week
• Low pressure may form over NZ this weekend bringing rain relief to some – not locked in

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