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Case Study Two: A stormy spring

September 2005

Apart from some areas along the eastern hills, rainfall amounts were below normal for September 2005, varying from 63% of the normal September rainfall at Dargaville to 132% at Puhipuhi.

Three moderate rainfall events produced most of September's rainfall:

· Rainfall from a northeasterly front on 9 to 11 September resulted in localised, intense rainfall along the eastern hills, where rainfall amounts varied from 76 mm at Kaeo to 107 mm at Puhipuhi. While, only modest amounts of 10 to 40 mm were recorded elsewhere over the region.

· On the evening of 14 and 15 September thunderstorm activity produced localised, intense rainfall as the front moved southward in a period of twelve hours.

· On 17 September, MetService issued a severe weather warning. A deepening low had developed in the Tasman Sea, west of the North Island and was expected to bring heavy rain and gale force winds from Northland to the East Cape. As the system approached Northland, wind speeds of 100km/hr, gusting to 140 km/hr were recorded at Cape Reinga. From midday 17 September to midday 18 September the barometric pressure at Dargaville dropped from 1012 hPa to 983 hPa (normally is 1014 hPa). Both coasts of Northland were ‘battered' by gale force winds, stormy seas and elevated spring tides. Minor flooding was experienced in some low areas of Dargaville as spring tides reached 0.3 metres above the predicted tide heights.

River flows improved from the low flows experienced during August, however flows were still 20% to 40% below their September averages. Groundwater level monitoring indicated that levels were near their lowest recorded for this time of year.

October 2005

October was generally a wet month for Northland with rain falling, on average, over 21 days. Heavy rainfall was recorded in early and mid October. Total rainfall amounts varied from 93 mm at Kaitaia to 276 mm at Tutamoe. The MetService issued a severe weather watch on 19 October indicating that Northland could receive 50 to 80 mm in localised areas over a 24-hour period. Rainfall amounts from this event were recorded between 30 to 50mm, mainly in eastern and southern areas of the region. This event was not upgraded to a warning, although a warning was issued to farmers on the lower Wairoa River flats at Tangiteroria as a result of elevated water levels in the Mangakahia River. Apart from some coastal areas to the east of Kaitaia, Northland received 146% of the expected October rainfall. Similarly, river flows were generally normal or above their normal flows for this time of year.

Analysis of the September groundwater level data for Northland indicated a trend in water level recovery at most sites. As a result of some heavy rainfall periods during October, particularly on 3 and 20 October, many of the groundwater stations showed a continuation of the water level recovery during October. Results from the automatic recorders showed that water levels at Ruawai, Taipa, Poroti West and Puriri Park (Maunu) were higher than normal for this time of year. The Tara and Mangawhai aquifers showed slightly lower levels.