24 January - climate report

Current situation

The weekend rain event of 21 and 22 January provided a small interlude to the dry weather experienced through December and January so far. Much of this rain fell on the east coast of the Far North and around the Hokianga Harbour, with stations in the Whangārei and Kaipara districts recording far less rain. Rainfall at many of these stations remain below the long term median for January.

Flows in many of the waterways around the southern half of the region remain low, reflecting the low overall rainfall experienced for the months of December and January to date. Some of these remain below, or close to design drought flows.

Following such dry conditions experienced in December, soil moisture to 22 January remains below average for much of Northland, excluding Kaitaia.

Rainfall

In some parts of the Far North, recorded rainfall totals for January are at or above expected January rainfall totals based on recorded long term medians. The highest rainfall over this event was recorded at Touwai (Whangaroa) with 90.5mm. Rainfall throughout most of Northland, particularly in the Whangārei and Kaipara districts remains significantly below what would be expected in January based on long term monthly medians, with around 20-30mm recorded at stations in these areas (Figure 1 and Table 1 below).

River flows

Flows at most monitoring stations in the region experienced a small to moderate increase from this rain event. However, flows in many waterways south of Whangārei and west towards Dargaville remain low and without further rain will continue to display a downward trend. Most rivers in the region will be back to the level recorded before this rain event within the next two weeks with no further rain.

Soil moisture deficits

Soil moisture at all NIWA recording stations except Kaitaia remain below average for this time of year, with 100-125mm deficits. These deficits are expected to continue to decline through the remainder of January. Typically soil moisture levels bottom out through December and January, beginning to rise around February.

Figure 1

Map – Recorded Rainfall for 19 – 22 January 2017

Table 1

Rainfall to 23 January in comparison to long term January median values
(note that gaps in median values represent insufficient data for the calculation)

Outlook

The MetService has advised that the current dry conditions may prevail through January, February and possibly March. The high pressure systems over New Zealand have been sitting at higher latitude than normally seen at this time of the year. This has resulted in an increase in winds from the west and south west. Fronts of rain coming from these directions have dissipated just south of Northland. Low pressure systems from the north have been pushed to the east by the large stationary high pressure systems. To date the low pressure systems that normally develop off the Australian coast have been non-existent.

The current weather pattern does increase the chance of a deep low forming north of New Zealand; the timing will determine if it can penetrate the high pressure system to reach Northland.