Monthly Weather Review Northern Territory April 2011 Monthly Weather Review Northern Territory April 2011 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 the years 1800 to 2009. They are not shown for sites with less than 10 years of record in that time, as they cannot then be calculated reliably. Climatological extremes are generally taken from available observations from the years 1800 to 2009. Copyright © Commonwealth of Australia 2011 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 IDCKGC12R1. Prepared on 23 May 2011. ISSN 1836-392X Cover photograph Sunset shines on altocumulus near Anningie. Photograph by Gavin Heathertington-Tait, 23 April 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 April 2011 Overview April began with heavy rains and gusty storms Significant weather across the Top End with several stations exceeding their previous record April rainfall Heavy rainfall at Katherine within the first week of the month. The following On the 2nd, Katherine Aviation museum weeks showed signs of the approaching dry measured 83.2 mm which was the wettest April season with cooler nights and fewer storms. This day on record, surpassing the previous record of was the coolest April on record in terms of both 64.0 mm set on the 25th in 1970. Katherine mean temperature and mean minimum Aviation Museum then broke this record again on temperature for the Northern Territory. the 5th when they received 88.4 mm. The average April total rainfall at Katherine is only Extremes in April 2011 38.4 mm. A taste of the dry season Hottest day The official start of the dry season is 1 May, but (Highest daily maximum temperature) the end of April gave a taste of the dry season to 35.8 °C at Victoria River Downs on the 26th Territorians in the Top End. On the 20th and 21st Warmest days on average the minimum temperature at Darwin Airport (Highest mean daily maximum temperature) dipped to below 20 °C for the first time this year. 33.1 °C at Jabiru Airport Another dry season indicator happened on the Coolest days on average 18th when, for the first time this year, the dew (Lowest mean daily maximum temperature) point in Darwin dropped below 15 °C. 26.1 °C at Arltunga Coldest day Eastern Arnhem thunderstorms (Lowest daily maximum temperature) On the 12th, a series of thunderstorms dumped 21.0 °C at Rabbit Flat on the 4th heavy rain over the eastern Arnhem District. Coldest night Rainfall totals from the event include 149.0 mm (Lowest daily minimum temperature) at Gove Airport and 148.0 mm at Yirrkala Tropical 2.0 °C at Arltunga on the 12th Gardens. Coolest nights on average (Lowest mean daily minimum temperature) 9.7 °C at Alice Springs Airport Synoptic summary Warmest nights on average On the 1st and 2nd, A low pressure system over (Highest mean daily minimum temperature) the Joseph Bonaparte Gulf pushed warm, moist, 25.4 °C at North East Island tropical air inland which produced heavy rainfall Warmest night and gusty storms across the western Top End. As (Highest daily minimum temperature) the low moved southwest the axis of the 27.2 °C at Point Stuart AWS on the 8th monsoon trough also moved south, bringing Warmest on average overall another burst of heavy rain to the area. This (Highest mean temperature) heavy rainfall during the first week of the month 28.2 °C at Jabiru Airport set new daily and monthly rainfall records for Coolest on average overall many stations across the western Top End. (Lowest mean temperature) 18.1 °C at Arltunga The rest of the month showed signs of transition from wet season to dry season. Occasional Wettest overall (Highest total rainfall) isolated thunderstorms brought heavy rains to 565.0 mm at Yirrkala Tropical Gardens the Top End including The Arnhem and coastal regions. As the month progressed these storms Wettest day (Highest daily rainfall) became less frequent. 162.6 mm at Tipperary on the 4th One of the largest contributors to dry season-like Highest wind gust weather patterns in the latter half of the month 65 km/h at Point Fawcett on the 2nd was a northward shift of the subtropical ridge. 65 km/h at Walungurru on the 2nd The ridge, which was located mostly over the Great Australian Bight for the first half of the month, slowly crept northward during the second and third weeks. This placed a broad high pressure area over South Australia, Victoria and New South Whales which reached into the southern NT and forced a cool and dry continental southeast flow over the Top End. page 1 Monthly Weather Review Northern Territory April 2011 Daily mean sea level pressure analyses 1 April 2011 2 April 2011 3 April 2011 4 April 2011 5 April 2011 6 April 2011 7 April 2011 8 April 2011 9 April 2011 10 April 2011 11 April 2011 12 April 2011 13 April 2011 14 April 2011 15 April 2011 16 April 2011 17 April 2011 18 April 2011 page 2 Monthly Weather Review Northern Territory April 2011 19 April 2011 20 April 2011 21 April 2011 22 April 2011 23 April 2011 24 April 2011 25 April 2011 26 April 2011 27 April 2011 28 April 2011 29 April 2011 30 April 2011 page 3 Monthly Weather Review Northern Territory April 2011 Temperatures The mean temperature for Northern Territory in April 2011 was 23.2 °C, which is 2.2 °C below the long-term average and ranked as the coolest April on record. Mean daily maximum temperatures were below average across the NT while the mean daily minimum temperatures were near average in the Top End and below average across the rest of the NT. Thick cloud cover in the beginning of the month kept daily maximum temperatures low while allowing minimum temperatures to stay relatively high across the Top End. The rest of the Territory, especially the Alice Springs District was strongly influenced by a series of high pressure systems streaming across the Bight that pushed cool, dry air into the Territory from the southeast. The warmest on average was Jabiru Airport with 28.2 °C, followed by Dum In Mirrie AWS with 28.0 °C. The coolest on average was Arltunga with 18.1 °C, followed by Alice Springs Airport with 18.4 °C and then Kulgera with 18.9 °C. New temperature records Record lowest April mean maximum temperature Years New record Old record of record Victoria River Downs 31.5 32.0 in 1983 43 Daly Waters AWS 30.7 30.9 in 1963 42 Centre Island 30.1 = 30.1 in 1999 37 Timber Creek 32.7 32.8 in 1985 31 Katherine Aviation Museum 31.4 31.6 in 2006 25 Record lowest April mean minimum temperature Years New record Old record of record Tennant Creek Airport 17.5 18.2 in 1971 41 Centre Island 24.3 24.5 in 2008 37 Record lowest April mean temperature Years New record Old record of record Tennant Creek Airport 23.1 23.2 in 1983 41 Wave Hill 23.9 24.1 in 1976 37 Centre Island 27.2 27.4 in 1999 37 Wollogorang 24.8 25.0 in 1997 25 page 4 Monthly Weather Review Northern Territory April 2011 Maximum temperatures Anomalies of mean daily maximum temperature in April 2011 There were several locations in the NT which recorded record lowest mean maximum temperatures, including Katherine Aviation Museum and Victoria River Downs.