Southwest Pacific Tropical Cyclone Outlook Update
This update for the latter half of the 2014–15 Tropical Cyclone (TC) season (February to April 2015) suggests near normal activity can still be expected. The 30-year (1981-2010) average number of all (named) storms is 12.4 (10.4) in the Southwest Pacific1 each season for November to April.
Two named storms (TC Niko and Severe TC Ola) have occurred so far this season. The updated outlook indicates that, from an analysis of past analogue seasons, approximately seven additional named storms can be expected, which would bring the full season total for 2014–15 season to nine.
Changes from previous guidance (issued in October 2014) indicate late season TC activity may be slightly reduced to the north of Vanuatu, while elevated activity may occur in the North Coral Sea east of Queensland. There is also an outlook of normal or slighly above normal activity for countries flanking the International Date Line in places such as Fiji and Tonga.
This updated outlook for the late portion of the TC season reflects recent changes in the Equatorial Pacific Ocean with respect to El Niño conditions that developed during November through January. The TC outlook for islands like New Caledonia, Vanuatu, Fiji, and Tonga still indicates two or more cyclones could interact with each of those countries during the late season.
The past seasons that had conditions similar to those presently in place suggest a good possiblity for severe TCs (Category 3 or higher2) to occur. As a result, all communities should remain vigilant and follow forecast information provided by their national meteorological service. It should also be noted that all categories of TCs can present hazards, and vigilance when a TC warning is issued is always a prudent course of action. On average, New Zealand experiences at least one ex-tropical cyclone passing within 550km of the country every year. If an ex-tropical cyclone comes close to the country, the current background climate conditions suggest it may pass to the east rather than to the west of Auckland city.
Weak El Niño conditions are indicated by slightly above normal sea surface temperature anomalies across the central and eastern Equatorial Pacific Ocean; however the atmospheric circulation pattern to complement that climate state is not present in the southwest Pacific.
There is an expectation amongst a number of international forecast centres that a weak El Niño will continue in coming months. Taking this climate scenario into account, near normal TC activity can be expected for many islands in the Southwest Pacific during the remainderof the 2014–2015 season, with an additional 6-8 named storms expected to form.
Southwest Pacific TCs are grouped into classes ranging from 1 to 5, with 5 being the most dangerous. For the coming TC season, there is a good possibility that at least four such storms reaching at least Category 3 could still occur; these storms have maximum sustained wind (MSW) speeds of at least 64 knots or 118 km/h. Of those systems, three storms may reach at least Category 4 strength, with MSW speeds of at least 86 knots or 159 km/h.
Category 5 strength TCs (MSWs greater than 106 knots or 196 km/h) have occurred during seasons that had conditions similar to present, so this type of event is still possible. Therefore, all communities should remain alert and well-prepared for severe events. Tropical cyclones have a significant impact across the Southwest Pacific from year to year. Vanuatu and New Caledonia typically experience the greatest activity, with an average of 2 or 3 TCs passing close to land each year. The outlook for this season indicates close tonormal TC activity for the remainder of the 2014–15 season for many islands, with slightly reduced risk in the region to the north of Vanuatu.
On average, New Zealand usually experiences at least one interaction per season with an extropical cyclone during ENSO neutral conditions. Most of the analog seasons identified for this forecast update do not show an ex-tropical cyclone coming close (within 550 km) to thecountry, however there are a couple of analogs that suggest an ex-tropical cyclone interaction is still possible. Seasonal climate forecast information recently issued by the National Climate Centre about expected regional atmospheric pressure patterns for February - April suggest a decaying system could still be steered toward New Zealand. These types of systems can generate damaging wind, waves and severe rainfall even if they do not make landfall. Their effects can also be spread over a larger area when they meet a higher latitude ‘high’ pressure system.
Even though TC activity is expected to be near normal for the remainder of the season, historical cyclone tracks (see supporting information for this forecast, Figure 2) indicate that TCs can affect parts of French Polynesia (including the Society Islands and the AustralIslands). As with the majority of other years, the late TC season (February–April) is still expected to be the most active time in the Southwest Pacific.
All Pacific Islands should remain vigilant in case equatorial Pacific El Niño conditions change during the TC season. Past seasons with conditions similar to present have seen TC tracks with increased sinuosity (irregular or looping motions rather than a curvilineartrajectory), which means they have potential to linger and impact a large area.
New Zealand’s National Institute of Water & Atmospheric Research (NIWA) along with meteorological forecasting organizations from the Southwest Pacific, including the Australian Bureau of Meteorology, and the Pacific Island National Meteorological Services, have contributed to this tropical cyclone outlook.
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