Maps Of Nz

Hi-tech equipment improves forecasting

Weather forecasting has become so sophisticated that more than a week ago supercomputers started creating maps hinting at what is now blasting across the country.

Even 20 years ago it was rare for forecasters to risk warning of snow or such a severe polar blast more than a few days ahead.

So how do forecasters know whether it's going to be rain or sleet or snow that falls and how low down the snow will settle?

Basically, it's about the density of air. While warm air is light and likes to rise, cold air is much heavier and tends to sink, as we all know from feeling cold draughts blowing under doors.

Radiosondes attached to weather balloons measure temperature, air pressure and humidity at different levels of the atmosphere as they ascend.

Those observed values are then fed into the mind-bogglingly complicated computer models that in turn generate maps on which forecasters base their predictions.

Some of these maps show the "thickness" of the air, or in other words the thickness of a slice of the atmosphere between two standard heights. This is expressed in units of atmospheric pressure, hectopascals (hPa), rather than metres or feet.

Those two pressure levels are further apart in warm air, which expands because it is lighter, than in cold air, because it is denser and squashes them closer together.

Therefore, these "thickness" values show how cold or hot the lower layers of the atmosphere are.

Over New Zealand, the 1000hPa (near-surface) to 500hPa (about 6000m) "thickness" usually averages between 5350 and 5600 geopotential metres (gpm).

In hot northwesterly conditions it can be above 5700gpm, in cold southerlies below 5300gpm.

Forecasters know from experience that snow will fall to sea level when the thickness falls to 5250gpm or lower.

In the heavy Christchurch snowfall three weeks ago, it was down to 5200gpm; on Monday it was even lower, close to 5150gpm for a time, and yesterday it was hovering around 5230gpm.

Forecasters also use the temperature about 1500m above sea level to calculate how cold the lowest part of the atmosphere is.

Based on the average drop in temperature of 1 degree Celsius for every 200m you climb, a temperature of minus 8C at 1500m should see the freezing level come right down to sea level.

The weather map for this week's winter blast is similar to the big storm at the end of August 1992.

New Zealand has been trapped in a river of icy southerlies between a column of high pressure extending north-south from an intense anticyclone way to the south filling the Tasman Sea, and a mass of rotating low-pressure systems to the east.

Antarctic blast races closer (+ New Maps) | WeatherWatch.co.nz

Temperatures south of New Zealand are quickly plummeting well below zero this afternoon as an Antarctic blast races towards the South Island tonight and the rest of the country tomorrow night.

There are numerous snow warnings currently in force covering regions in both islands.

Polar air has surged over Macquarie Island, Australian territory about 1000kms south west of Invercargill.  The temperature there this hour is -3 degrees.

The air headed towards New Zealand is polar air - coming from directly over Antarctica and is currently racing racing northwards to reach southern New Zealand tonight. 

WeatherWatch.co.nz says the air is coming from near McMurdo Station, a New Zealand region, where it's currently -20 degrees.

Auckland and Campbell Islands (500 to 700kms south of Invercargill) remain several degrees warmer ahead of the cold change but Auckland Island is in the firing line for the snowy change shortly - then it's on to Southland in the next 6 to 9 hours (around midnight) with an even colder pool of air arriving a few hours after that.

Check out our two maps below - the top one shows where the Antarctic blast currently is, the other one below shows exactly where the air is coming from and why it's so super chilled.  Temperatures on the top map were valid as of 3:45pm Saturday.

That is a very good point - as no where in Southland or Otago was anywhere near 13 today!  It's either an incorrect reading or the icon is showing a West Coast reading (a few places reached 13 there today).

- WW

Edit:  Just checked, it's an out of date reading for Cromwell, all other stations appear to be correct though.  Oddly it now says 14 degrees there but in reality it's around 6 to 8 degrees.

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