Monthly Weather Review Northern Territory May 2013 Monthly Weather Review Northern Territory May 2013 The Monthly Weather Review - Northern Territory is produced twelve times each year by the Australian Bureau of Meteorology's Northern Territory Climate Services Centre. It is intended to provide a concise but informative overview of the temperatures, rainfall and significant weather events in Northern Territory for the month. To keep the Monthly Weather Review as timely as possible, much of the information is based on electronic reports. Although every effort is made to ensure the accuracy of these reports, the results can be considered only preliminary until complete quality control procedures have been carried out. Major discrepancies will be noted in later issues. We are keen to ensure that the Monthly Weather Review is appropriate to the needs of its readers. If you have any comments or suggestions, please do not hesitate to contact us: By mail Northern Territory Climate Services Centre Bureau of Meteorology PO Box 40050 Casuarina NT 0811 AUSTRALIA By telephone (08) 8920 3813 By email [email protected] You may also wish to visit the Bureau's home page, http://www.bom.gov.au. Units of measurement Except where noted, temperature is given in degrees Celsius (°C), rainfall in millimetres (mm), and wind speed in kilometres per hour (km/h). Observation times and periods Each station in Northern Territory makes its main observation for the day at 9 am local time. At this time, the precipitation over the past 24 hours is determined, and maximum and minimum thermometers are also read and reset. In this publication, the following conventions are used for assigning dates to the observations made: Maximum temperatures are for the 24 hours from 9 am on the date mentioned. They normally occur in the afternoon of that day. Minimum temperatures are for the 24 hours to 9 am on the date mentioned. They normally occur in the early morning of that day. Mean temperatures are the average of the daily maximum and daily minimum temperatures. Daily rainfall is for the 24 hours to 9 am on the date mentioned. This means a significant fraction of the rain may have occurred on the previous calendar day. Monthly rainfall is for the period from 9 am on the last day of the previous month to 9 am on the last day of this month. Maximum daily wind gusts are in the 24 hours from midnight to midnight. Climatological values The climatological averages shown in the text and tables are generally long-term means based on observations from all available years of record, which vary widely from site to site. They are not shown for sites with less than 10 years of record, as they cannot then be calculated reliably. Climatological extremes are generally taken from available observations from all available years of record. The number of years can vary substantially from site to site. The ACORN-SAT dataset is being used for temperature area averages from December 2012 onwards. The major change from earlier datasets is that the ACORN-SAT dataset commences in 1910, rather than 1950, and hence rankings are calculated using a larger set of years. Copyright © Commonwealth of Australia 2013 This work is copyright. You may download, display, print and reproduce this material in unaltered form only (retaining this notice) for your personal, non-commercial use or use within your organisation. Apart from any use as permitted under the Copyright Act 1968, all other rights are reserved. Requests and inquiries concerning reproduction and rights should be addressed to Commonwealth Copyright Administration, Attorney General’s Department, Robert Garran Offices, National Circuit, Barton ACT 2600 or posted at http://www.ag.gov.au/cca. Product code IDCKGC22R1. Prepared on 8 July 2013. ISSN 1836-392X Cover photograph Sunrise on a deck of altocumulus near Lilla Creek Station. Photograph by Gavin Heathertington-Tait, 2 May 2008. Used with permission. If you have a photo you think would make a good cover for the Monthly Weather Review, please contact us at the address above. Monthly Weather Review Northern Territory May 2013 Overview • Hot conditions linger into the dry season • Warm sea surface temperatures off NW Australia • Above average rainfall for most of NT • Northwest cloudbands influence Alice Springs weather Extremes in May 2013 Hottest day (Highest daily maximum temperature) 37.7 °C at Victoria River Downs on the 3rd Warmest days on average (Highest mean daily maximum temperature) 35.0 °C at Bradshaw Coolest days on average (Lowest mean daily maximum temperature) 23.9 °C at Kulgera Coldest day (Lowest daily maximum temperature) 13.5 °C at Yuendumu on the 15th Coldest night (Lowest daily minimum temperature) 2.6 °C at Alice Springs Airport on the 27th Coolest nights on average (Lowest mean daily minimum temperature) 10.3 °C at Alice Springs Airport Warmest nights on average (Highest mean daily minimum temperature) 26.2 °C at Cape Don Warmest night (Highest daily minimum temperature) 28.2 °C at Cape Don on the 3rd Warmest on average overall (Highest mean temperature) 29.4 °C at Cape Don Coolest on average overall (Lowest mean temperature) 17.4 °C at Arltunga Wettest overall (Highest total rainfall) 167.0 mm at Litchfield Wettest day (Highest daily rainfall) 101.4 mm at Batchelor Aero on the 24th Highest wind gust 78 km/h at Cape Wessel on the 3rd page 1 Monthly Weather Review Northern Territory May 2013 Significant weather off the northwest coast was a primary driver of the wet conditions throughout the Alice Springs District. Northwest NT receives a dry season drenching May is a transition month, which means that the air mass over the Top End has to change from warm and humid to cool and dry. The mechanism for change is related to southern winter conditions by the presence of strong high pressure systems. These systems bring cool, dry southeasterly winds from higher latitudes and dry season conditions to the Top End. In May a southeasterly wind change reached the north coast on the 23rd. As the incumbent southeasterlies met the warm humid airmass over the Top End, a series of showers were triggered which broke several rainfall records for May (see below). Showers are always a possibility ahead of a southeasterly wind change as the cold dry air undercuts the warm humid air and lifts it to form cloud. What made this event distinct was the depth of moisture in the atmosphere. The warm SSTs off the northwest coast, coupled with prevailing easterly winds during the early half of May contributed to a deep layer of moisture and a good environment for showers on the 23rd and 24th. From this rainfall event several sites saw their highest May daily rainfall on record including Batchelor Aero with 101.4 mm on the 24th. There were no reports of Damage or flash flooding. Northern Territory Fires Throughout the month of May there were a number of wildfires started by unknown sources about the northwest Top End, but none that caused noteworthy damage. Western Top End residents would have observed a significant increase in smoke early in the month. This was associated with prescribed burning by Fire Authorities, as is normal practice for May. Warm SSTs drive AS rainfall The sea surface temperature (SST) remained very much above average off the northwest coast of Australia during May with some areas nearly two degrees above the long-term average and exceeding the previous May SST temperature record. The temperature of the sea surface is an important weather ‘driver’ for the NT. Warm SSTs off the northwest coast can provide tropical moisture to cloud bands that sweep across the central part of the continent. This Month the Alice Springs District saw 4 cloud band events providing ample rainfall to the region. Alice Springs Airport measured 42.6 mm this month, this is more than double their long-term average and their wettest May in 9 years. Across the whole Alice Springs District, every site measured above average rainfall. To the North of Alice Springs, Rabbit Flat (a WMO climate reference station) and The Granites both saw their wettest May on record with 82.0 mm and 93.0 mm respectively. Warm SSTs page 2 Monthly Weather Review Northern Territory May 2013 Synoptic summary The month began with TC Zane located off the Cape York Peninsula in the Coral Sea. The storm rapidly weakened to a tropical low on the eastern side of the Cape on the 2nd, and had dissipated by the 3rd. A shallow trough skipped across the Alice Springs District on the 5th and 6th and while the region experienced slightly cooler temperatures from the southerly wind change, the trough passage did not produce any rainfall. During the first fortnight, the sub tropical ridge was still located in a summertime position, with high pressure centres remaining mostly south of the continent over the ocean. The first northwest cloudband event to affect the Alice Springs District came mid-way through the second week of the month, around the 12th. A low pressure system over southern WA earlier in the week got pushed south as a high pressure system moved east over the region creating and another high over NSW and the Tasman Sea remained relatively stationary. This produced a mid-level trough over the central continent which created a river of moist air at mid-levels from the warm SSTs in the northwest. The Alice Springs region saw cloudy skies, cooler daytime temperatures and over 25 mm of rainfall at some places during this week. By the third week of May the warm SSTs off the northwest coast had induced a broad area of relatively low pressure over northern WA while a pressure ridge built across the continent.