NIWA Outlook: May – July 2019
Outlook Summary
• A weak, central Pacific El
Niño continued during April, as patterns of enhanced
rainfall persisted in the vicinity of the International
Dateline. Although El Niño is forecast to continue during
the upcoming three-month period, it may weaken later in
2019.
• Air pressure is expected to be higher than
normal to the west and north of New Zealand and below normal
to the south of the country, resulting in more westerly
quarter winds than normal.
• Temperatures are forecast
to be above average in the north and east of the North
Island and east of the South Island and about equally likely
to be above average or near average in all remaining
regions. Warmer than average coastal and Tasman Sea surface
temperatures may influence several spells of unseasonable
warmth through the season, particularly in eastern areas,
contributed to by frequent westerly air
flows.
• Rainfall is about equally likely to be below
normal or near normal in the North Island and in the east of
the South Island, most likely to be near normal in the north
of the South Island, and about equally likely to be above
normal or near normal in the west of the South Island.
Influenced by El Niño and a progressively more active
Tasman Sea, frequent fronts are possible for western areas
through the season.
• During April 2019, marine
heatwave conditions continued in the Tasman Sea. Warmer than
average seas may lead to a reduction in the intensity and
duration of cold spells during the coming season, however
frosts are still to be expected.
May – July 2019 temperatures are forecast to be above average in the north and east of the North Island and east of the South Island (50-55% chance) and about equally likely to be above average (45% chance) or near average (40% chance) in all remaining regions of New Zealand. However, cold snaps and frosts are likely to occur, particularly in colder locations, as the season progresses.
May – July 2019 rainfall is about equally likely to be above normal (40% chance) or near normal (35% chance) in the west of the South Island, most likely to be near normal (45% chance) in the north of the South Island, and about equally likely to be below normal (35-40% chance) or near normal (35-40% chance) for all remaining regions of New Zealand.
May – July 2019 soil moisture levels and river flows are most likely to be below normal (50% chance) in the north of the North Island. For the west of the South Island, soil moisture levels are about equally likely to be above normal or near normal (35-40% chance) while river flows are most likely to be above normal (45% chance). For all other regions of New Zealand, soil moisture levels and river flows are about equally likely to be normal or below normal (40-45% chance).
Regional predictions for the May – July 2019 season
Northland, Auckland, Waikato, Bay of Plenty
The table below shows the probabilities (or percent chances) for each of three categories: above average, near average, and below average. In the absence of any forecast guidance there would be an equal likelihood (33% chance) of the outcome being in any one of the three categories. Forecast information from local and global guidance models is used to indicate the deviation from equal chance expected for the coming three-month period, with the following outcomes the most likely (but not certain) for this region:
• Temperatures are most likely to be above
average (55% chance).
• Rainfall totals are about
equally likely to be near normal (35% chance) or below
normal (40% chance).
• Soil moisture levels and river
flows are most likely to be below normal (50%
chance).
The full probability breakdown is:
Temperature | Rainfall | Soil moisture | River flows | |
Above average | 55 | 25 | 15 | 10 |
Near average | 35 | 35 | 35 | 40 |
Below average | 10 | 40 | 50 | 50 |
Central North Island, Taranaki, Whanganui, Manawatu, Wellington
Probabilities are assigned in three categories: above average, near average, and below average.
• Temperatures are about equally likely to be above
average (45% chance) or near average (40%
chance)
• Rainfall totals are about equally likely to
be near normal (40% chance) or below normal (35%
chance).
• Soil moisture levels and river flows are
equally likely to be below normal (40% chance) or near
normal (40% chance).
The full probability breakdown is:
Temperature | Rainfall | Soil moisture | River flows | |
Above average | 45 | 25 | 20 | 20 |
Near average | 40 | 40 | 40 | 40 |
Below average | 15 | 35 | 40 | 40 |
Gisborne, Hawke’s Bay, Wairarapa
Probabilities are assigned in three categories: above average, near average, and below average.
• Temperatures are most likely to be
above average (55% chance).
• Rainfall totals are about
equally likely to be below normal (40% chance) or near
normal (35% chance).
• Soil moisture levels and river
flows are about equally likely to be below normal (40-45%
chance) or near normal (40% chance).
The full probability breakdown is:
Temperature | Rainfall | Soil moisture | River flows | |
Above average | 55 | 25 | 20 | 15 |
Near average | 35 | 35 | 40 | 40 |
Below average | 10 | 40 | 40 | 45 |
Tasman, Nelson, Marlborough, Buller
Probabilities are assigned in three categories: above average, near average, and below average.
• Temperatures are about equally
likely to be above average (45% chance) or near average (40%
chance).
• Rainfall totals are most likely to be near
normal (45% chance).
• Soil moisture levels and river
flows are about equally likely to be below normal (40%
chance) or near normal (40-45% chance).
The full probability breakdown is:
Temperature | Rainfall | Soil moisture | River flows | |
Above average | 45 | 25 | 20 | 15 |
Near average | 40 | 45 | 40 | 45 |
Below average | 15 | 30 | 40 | 40 |
West Coast, Alps and foothills, inland Otago, Southland
Probabilities are assigned in three categories: above average, near average, and below average.
• Temperatures are about equally likely to be above
average (45% chance) or near average (40%
chance).
• Rainfall totals are about equally likely to
be above normal (40% chance) or near normal (35%
chance).
• Soil moisture levels are about equally
likely to be above normal (40% chance) or near normal (35%
chance) and river flows are most likely to be above normal
(45% chance).
The full probability breakdown is:
Temperature | Rainfall | Soil moisture | River flows | |
Above average | 45 | 40 | 40 | 45 |
Near average | 40 | 35 | 35 | 35 |
Below average | 15 | 25 | 25 | 20 |
Coastal Canterbury, east Otago
Probabilities are assigned in three categories: above average, near average, and below average.
• Temperatures are most likely to be above
average (50% chance).
• Rainfall totals are about
equally likely to be near normal (40% chance) or below
normal (35% chance).
• Soil moisture levels and river
flows are about equally to be below normal (45% chance) or
near normal (40-45% chance).
The full probability breakdown is:
Temperature | Rainfall | Soil moisture | River flows | |
Above average | 50 | 25 | 15 | 10 |
Near average | 35 | 40 | 40 | 45 |
Below average | 15 | 35 | 45 | 45 |
Background
During April 2019, the atmosphere once again responded to a warm pool of water in the central and western Pacific, with anomalous atmospheric lift, rainfall, and convection centred along and just west of the International Dateline. The NINO3.4 Index anomaly for April (to the 28th) was +0.76C, consistent with what occurred during March. The NINO1+2 Index (in the far eastern Pacific) had an anomaly of +0.57C.
Rainfall and SST patterns remained consistent with a weak, central Pacific El Niño.
At the end of April, the Madden-Julian Oscillation (MJO) became active and was expected to constructively interfere with the El Niño base-state during the month of May. As this occurs, the atmospheric response may become more aligned with an east-based El Niño (e.g. westerly quarter winds becoming more common for New Zealand).
The Southern Oscillation Index (SOI) was near neutral during April (preliminary value -0.1), an increase compared to March (-0.6). The rising branch of the Walker Circulation focused along and just west of the Dateline and a sinking branch over the central and eastern Pacific. This supports the idea that the ongoing El Niño event is of the central-based type, nudging the synoptic pressure patterns near Darwin and Tahiti away from what might be considered their typical positions.
The probability for oceanic El Niño conditions, according to the consensus from international models, is 83% for the May – July period. For August – October, the probability is 64%. For November 2019 – January 2020, El Niño remains the most likely outcome at 58%. This continues to suggest the potential for a ‘protracted’ event (multi-year duration).
New Zealand’s coastal water temperatures for April 2019 continued above or well above average in all areas. Marine heatwave conditions continued in the Tasman Sea.
New Zealand Coastal Sea Surface Temperatures
during April 2019 North NI West NI East
NI North SI West SI East SI
+0.9C +0.8C +0.9C
+1.2C +2.1C +1.0C
Warmer than average sea surface temperatures are forecast to persist during the coming three-month period. These warm seas may contribute to spells of unseasonable warmth and invigorate low pressure systems as they approach New Zealand from the west.
The
Southern Annular Mode (SAM) was variable during the month of
April. Continued variability is forecast during May. As the
upcoming season continues, the SAM may have a tendency to
become more negative (e.g. 2nd half of May and June) as the
Tasman Sea begins to turn more
active.