Case Study Two: Intense Rainfall and Thunderstorm Activity during Autumn
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March rainfall amounts were average to above average in the far north areas and below average elsewhere. Low rainfall amounts were recorded in the areas surrounding the Kaipara Harbour (34% of normal at Pouto and 59% at Dargaville) and some areas south and west of Whangarei (46% at Marsden Point and 57% in the Brynderwyn Hills). Most rainfall fell over five days from 1-5 March, particularly in the far north areas where 60-100mm was recorded.
Moderate rises in water levels were recorded during this time. There was a relatively settled period during mid-March, with rain returning towards the end of the month. Typically, river flows were average to above average in the far north and below average in central, western and southern areas of the region. For example, river flows in the Kaihu, Mangakahia and Manganui Rivers were 60% of the normal March flows.
Northland experienced a series of active fronts during April, which produced severe thunderstorm activity and associated intense rainfall. Two severe rain events occurred during the periods 14-16 April and 29-30 April. Both events resulted in flash flooding.
On 15 April, thunderstorm cells developed over an area from the North Hokianga to Kerikeri. This band of mobile thunderstorms moved quickly southwards producing intense thunderstorm activity and severe lightning strikes. At the Omapere telemetered rain station, 67.5mm of rain was recorded in one hour and 97mm over 3 hours. Rainfall amounts of 20-27mm/hr were being recorded inland and along the western hills of Northland, from the Hokianga to the Kaipara Harbours. Flood warnings were issued for the west coastal areas, inland areas of the Mangakahia River catchment and the low-lying river flats at Tangiteroria.
On 29 April, the Meteorological Service warned of another series of active fronts, "bringing moist northerlies and thunderstorm activity that could produce some intense downpours, especially in the eastern hills. Rainfall rates were likely to be 20mm to 30mm/hr". As predicted, early on 29 April, heavy rain fell over the eastern hills of Glenbervie and the northern hills of Ngunguru. Rainfall amounts of 23-40.5mm/hr were recorded. The heavy rain and thunderstorm activity was more intense in the northern hills of Ngunguru where 86mm fell in three hours, resulting in slips and road closures.
During the evening of 29 April, thunderstorm activity returned to an area of the eastern hills from the Bay of Islands to Whangarei. Rainfall amounts of 31mm/hr were recorded at Puhipuhi, 55mm/hr at Oakura Bay and 65mm/hr in the northern hills of Ngunguru. Once again, flash flooding, slips and road closures resulted. Total daily rainfall amounts for 29 April were: Whangarei/Glenbervie 86-96.5mm, Oakura Bay 121.5mm, Puhipuhi 109mm and the northern hills of Ngunguru 178mm. Elsewhere, rainfall amounts varied from 10.5mm at Dargaville to 62.5 mm at Kaeo. Flood warnings were issued for Hikurangi Swamp, Wairua River catchment and the low lying river flats at Tangiteroria.
Northland continued to experience a series of active fronts during May, some of which produced severe thunderstorm activity and associated intense rainfall. On 25 May, 33mm/hr was recorded at the Paparoa rain-gauge station, compared to 9mm at Maungaturoto and 3mm at Ruawai.
On 27 May, a developing low to the north west of Northland was blocked by a high pressure system further to the south of the North Island. A band of heavy rain from south of the Bay of` Islands to Whangarei became relatively stationary and over a 12 hour period heavy rain fell over the hills north of Ngunguru and Whangarei. Rainfall amounts of 75mm and 78mm were recorded with intensities reaching 17mm/hr. Minor surface flooding resulted.
Thunderstorm activity through May resulted in varied rainfall amounts over the region, from 40% of the May average in the Pouto Peninsular to 160% of average in Whangarei. Lower than normal rainfall was recorded over the west coast areas and some parts of the far north. Above average rainfall was recorded, mainly along the east coast. River levels were normal or above normal for May.
Northland continued to experience a series of active fronts during June, some of which produced severe thunderstorm activity and associated intense rainfall. On average, there were 20 days of rain during June, with 60-70% of the monthly rainfall amounts recorded during mid to late June. Generally, below average rainfall was recorded in most areas of the far north and average to above average in the surrounding areas of Whangarei, inland south of Kaikohe and southern Kaipara areas south of Dargaville, Maungaturoto and Mangawhai. River flows were average to above average for this time of year.