The snow that closed the Desert Road and Napier-Taupo Road on Sun-Mon-Tue 4-5-6th  October 2009 was unseasonable.  It was caused by a low pressure system deepening over the area at the same time as a cold southerly flow arrived, resulting in moist air being cooled from below in a cauldron of lowering pressure.  This produced an unusually heavy amount of snow over a wide area.

The weather map for noon on Sunday shows the low pressure system forming over the Central North Island

Wetaher Map 1pm Sunday 4 October  2009 (0000UTC)

Wetaher Map 1pm Sunday 4 October 2009 (0000UTC)

And the satellite image for the same time shows the extent of the cloud ---skies had already mostly cleared over the South Island

Satellite image (visible) 1pm Sunday 4 October 2009 (0000UTC)

Satellite image (visible) 1pm Sunday 4 October 2009 (0000UTC)

MetService issued a Severe Weather Watch for heavy snow on Saturday night and upgraded this to a Warning soon after 9am on Sunday, advising of 20-30 centimetres of snow over the higher ground, including the Hawke's Bay hills.

MetService maintains a number of NZTA (New Zealand Transport Agency) weather stations on the Napier-Taupo and Desert Roads with their locations shown in this Google Map image:

 

roadsites
The following stations line up along the Napier-Taupo road from west to east:

APA Taupo Airport

HLX: High Level Road

ERX:  Eastern Rangitaiki

TOX: Te Haroto

TEX: Te Pohue

NRA: Napier Airport

For these sites, the temperature (degrees Celsius) and rainfall (mm/hr) graphs from 1am NZDT Saturday 3 Oct (02 1200 UTC) until 1pm NZDT Tuesday (06 0000 UTC) are shown below.

NapierTaupo

Note the warm afternoon on Saturday (20 C in Napier), the rapid cooling to zero degrees air over the road around noon on Sunday – at around a third of the way through the rainfall event-- and the steady warming on Tuesday morning.

The following stations line up from north to south along the Desert Road

TTX Tongariro

MRX Moturoa

DRT: Desert road

RUX Waiouru

For these sites, the temperature (degrees Celsius) and rainfall (mm/hr) from 1am NZDT Saturday 3 Oct (02 1200UTC) until 1pm NZDT Tuesday (06 0000 UTC) are shown below.

DesertRoad

Comparing these traces with the Napier-Taupo trace we can see that the temperature drop and rise were more gradual on the Desert Road but the rainfall events were similar.  That peak of rain at Waiouru around noon on Tuesday is due to snowmelt – the snow that had accumulated in the rain gauge melting as conditions warmed..

It seems that around 30 centimetres of snow fell on the upper parts of these roads,  and the wind blew this around producing some drifts of around half to one metre. It took several days for the Napier-Taupo Road to reopen.

In most of the surrounding cities, Monday’s temperatures remained in single digits.  Only Gisborne managed to get above 10.  It was reported that snow lay on the roads around Rotorua:

cities

Decode: AAA for Auckland Airport, GSA for Gisborne Airport, HNA for Hamilton airport, ROA for Rotorua Airport, and TGA for Tauranga Airport. Temperatre in degrees Celcius, and timestep from  1am NZDT Saturday 3 Oct (02 1200UTC) until 1pm NZDT Tuesday (06  0000UTC) are shown above.

This was indeed a freakish early October snow event, so late in the year, so low, so heavy and so widespread -- these details deserve being kept as a bench mark for possible future comparisons.