Monday 25th July: 2011′s coldest day so far

In New Zealand, the coldest days of the year are usually in late July or early August.

The most significant cold snap since 1995 gripped the nation from late Friday 22 July until late on Monday 25 July. Significant amounts of snow fell in the south and east of both islands, with over 30 cm to near sea level reported in the Christchurch area. Many of the arterial routes through the North and South Islands were closed, including the Waioeka Gorge route from Gisborne to Opotiki.

On Wednesday 20 July forecasters advised, in a Severe Weather Outlook, of the potential for heavy snow during the upcoming weekend. As the weekend approached, various Severe Weather Warnings / Watches, Road Snowfall Warnings and Special Weather Advisories were issued, advising of significant snow from Southland to Hawkes Bay and describing accumulations down to near sea level likely in the South Island and down to 200m further north.

This cold episode was the result of a river of air flowing straight from the Antarctic ice shelf to New Zealand’s shores. For this to occur, a high pressure zone over Tasmania needs to occur at the same time as a low pressure system deepens over the Chathams area, so that between them a southerly flow extends from the Antarctic to New Zealand for a long enough time to allow polar air to reach the country.

MetService mean sea level analysis for midnight Saturday 23 July 2011.

As the weather map above shows, the “polar blast” was just arriving over the far south of the country at midnight Saturday 23 July (behind the old front); it spread across the country during the following two days. Because it is so cold, polar air is generally very dry. But as it passes over the (increasingly warmer) ocean between the Antarctic and New Zealand, it takes up heat and moisture from the sea surface: this is what drives the formation of showers.

Had the polar air arrived a day or so earlier it would have encountered warmer air associated with the low which brought heavy rain to parts of Northland, Bay of Plenty and Gisborne a few days earlier; the result would have been even more snow.

MTSAT-1R infra-red satellite image for 8:00am Monday 25 July as a good burst of snow was moving across Canterbury. The heaviest snow is likely to be falling from the coldest-topped (red) clouds. Image courtesy Japan Meteorological Agency.

MetService weather radar image for 8:00am Monday 25 July. Blue areas are heavier showers.

The coldest air made its way across the North Island early on Monday 25 July and a trough following on behind brought a further bout of heavy snow later that day. Heavy showers brought the snow level down as they passed: a centimetre or so of snow even fell in Greytown (about 100 metres above sea level) in the Wairarapa to the east of Wellington.

Location Maximum temperature on
Mon-25-Jul 2011
Lowest daily maximum temperature on record Month / year occurred in Record starts Monday’s max temperature is the lowest since …
Auckland 10.2 8.8 June 1976 1966 June 2002
New Plymouth 6.7 6.0 July 1951 1944 1944
Napier 7.4 3.6 September 1969 1940 May 1989
Paraparaumu 6.3 5.4 July 2004 1972 1972
Kelburn (Wellington) 5.6 4.5 August 1938 1931 July 1995
Christchurch 5.0 1.7 August 1992 1954 June 2007

 

Finally, this MODIS (Moderate Resolution Imaging Spectroradiometer) satellite image shows the snow coverage over New Zealand late on the morning of Tuesday 26 July. Snow areas are coloured red.

3 thoughts on “Monday 25th July: 2011′s coldest day so far

  1. Pingback: The Climate Show #17: the end of the peer show

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