Topic: La ~ Nina and her effect on the Australian storm season / General weather

[Antonio (stormboy)]

I was thinking for a while about starting this topic because i am very interested in some information on the Effect the La Nina system has on Australia's weather system.

i dont know if La Nina has anything to do with it but i have noticed over the past 4 - 8 weeks and much of winter Widespread cloud would develop over Northwestern WA and or Indian ocean and stream across Australia bringing Rain some heavy with it to the interior and the East Coasts.

and the last and i think most important Question to me is With the increase of Humidity and ocean temperature will la Nina produce more severe more longer lasting cyclones and also produce breathtaking super-cells in may areas?

Antonio.

[Harley Pearman]

The Bureau of Meteorology has an excellent discussion on El Nino and La Nina events impacting on Australia. The discussion includes numerous graphs, maps, rainfall plots and historical summaries of both El Nino and La Nina events.

The discussion can be found on the Bureau of Meteorology website at www.bom.gov.au

To find the discussion on the website, go to the heading "Latest ENSO Wrap Up" in the top right hand corner. Click on that. Then scroll down to "Impact on Rainfall" heading then "Average La Nina Patterns". There are other headings regarding El Nino and La Nina in this section for reading. This should assist.

Further, to this thread, I have produced on the POAMA (Predictive Oceanic Atmospheric Modelling for Australia) via the CSIRO site, the likely or possible rainfall prognosis for Australia for September to November 2010. You will see that the modelling suggests above average rainfall for Eastern Australia during this period. This is a big turnaround when compared to the same period for 2009.

To produce these on POAMA, I have set the month of September to Day 1 with a Lead Time of 0. The month of October is set to Day 1 with a Lead Time of 1 and the month of November is set to Day 1 with a Lead Time of 2. It is possible to model 6 months into the future but the accuracy is less certain. The plots are provided below.


Harley Pearman

[Antonio (stormboy)]

Harley thank you for the information will give that a read through im just so interested because i am only young and if i have lived through a La Nina i was too young or a non weather boy to see the difference with my new weather station i am looking forward to collecting the data i receive day to day and compare it to last years data etc. thanks allot

Antonio.

[Harley Pearman]

POAMA - Temperature anomaly prognosis for Eastern Australia (October to December 2010

Further to this thread, I have been looking at computer models (POAMA - Predictive Oceanic Atmospheric Model for Australia) for temperature anomalies, the possible impact of this La Nina and current La Nina trends. It is clear from the models and POAMA that a La Nina episode has set in and it is coinciding with a Negative Indian Ocean Dipole. Looking at records, it appears the last time this occurred was in 1974. This is looking like a moderate to strong La Nina and the POAMA models suggest the episode peaking in November / December and starting to weaken thereafter during February and March 2011.

Without repeating the technical information by the Bureau of Meteorology (Refer to their website as previously indicated), models suggest a cooler but wetter spring and summer ahead.

Possible Temperature anomalies October to December 2010

The October 2010 prognosis by POAMA suggests below average temperatures across eastern Australia being up to 1.5C below average. It is suggested that Sydney could experience temperatures that are slightly below average. It is suggested that western Victoria could experience temperatures that are near the average.

The November 2010 prognosis by POAMA suggests below average temperatures across eastern Australia from 0.5C to 1C. There is even an area where average temperatures may be lower. Victoria could experience temperatures that are close to the average.

The December 2010 prognosis by POAMA suggests below average temperatures across eastern Australia from 0.5C to 1C. For example, Sydney is within the range of below average temperatures by between 0.5C and 1C. Only Victoria is expecting average early summer temperatures.

POAMA 1.5 Experimental has recently been updated with new capabilities being added. I have looked at the new capabilities of this system but it is still in the experimental phase. The CSIRO is still working on the program.

To produce the above stated plots for October to December period, the variables are set to (Year - Latest being 2010), Month (Set to Latest being October), Day (Set to latest) but the Lead Times are set to (1) October, (2) November and (3) December. The plots are provided below.

Harley Pearman

[Harley Pearman]

September storm / rainy day observations (Relevant for Blacktown or Seven Hills only) - La Nina phase.

Since 1 January 1998, I made the decision to start observing the local weather and I have recorded the number of thunderstorm days since. I now have 12 to 13 years gathered. One will see at least for my local area (Blacktown) that there are no real trends emerging for the number of thunderstorms occurring during the month. Generally whether it is a La Nina phase or El Nino phase, I cannot see any specific trends emerging for the month. It is noted that at least one thunderstorm has occurred during the month for every La Nina phase that has occurred.

The number of days when it rains appear to be steady.

Perhaps there is a lesser likelihood of experiencing the first 30C days of the season during a La Nina episode. Similar to 2007, September 2010 did not see any 30C days occurring across Blacktown.

I will work on this exercise for the duration of this current La Nina phase but please note, many of the figures presented are from my observations since 1 January 1998 around Blacktown (Western Sydney). Beyond the region of Blacktown, Parramatta, Mt Druitt, Prospect and Baulkham Hills, the figures presented here will not be valid. The rainfall is provided from the closest official weather station being Collins Street Seven Hills being less than 1 km from where I live.

The table is provided below.

Harley Pearman

[Antonio (stormboy)]

That table was handy to see keep up the good work.

Antonio

[Harley Pearman]

La Nina influences - Temperature and rainfall for Eastern Australia - September 2010

La Nina was dominating eastern Australia during the month of September and the relevant plots for temperature and rainfall are provided below:

Average maximum temperature anomaly

Cooler than normal days appear to be the trend for September. Other than for coastal New South Wales and Queensland, daily temperatures were 1C to 3C below average. Closer to the coast, temperatures were closer to the average.

At Blacktown where I live, maximum temperatures failed to reach 20C on 9 days. There were no 30C days when compared to 2009 and the maximum temperatures of 26 to 27.2C occurred between 24 and 28 of the month. When a comparison is made to the El Nino year of 2009, it is found that there have been no 30C days so far this season from 1 September to 8 October 2010. For the same period of 2009, there had been 5 such days. Further it has reached 27C at Blacktown on 3 days for the same period of 2010 but during the same period of 2009, there had been 9 such days.  

Average minimum temperature anomaly

Minimum night time temperatures were warmer than normal except for south west New South Wales where minimum night time temperatures were 0C to 2C below average. Cloud cover would have kept night time temperatures warmer than normal across a large region of eastern Australia.

Cooler than normal days and warmer than normal nights appear to be the trend for September 2010.

Rainfall:

Across eastern Australia, average to above average rainfall occurred except for coastal areas north and south of Sydney. These areas experienced rainfall deficits of 25 to 50 mm below average for the month while the rest of the state of New South Wales was satisfactory. It is noted that cloud bands moving across Queensland during the month resulted in significant rainfalls. Rainfall anomalies reach up to 400 mm above average. There were rain events in Victoria including Southern New South Wales and Queensland. The regions affected are identified in the Rainfall Anomaly Plot.

Given that La Nina is now dominating the weather pattern, here in Blacktown, the first 8 days of October have been characterised by cooler, cloudy weather with regular showers. So far, it has rained on 5 out of 8 days. Friday 8/10/2010 turned out sunny but only after a cloudy morning. There were heavy showers Wednesday afternoon that delivered 8 to 10 mm but no storm so far for the month. Saturday morning has turned out cloudy with isolated showers. Hence the start of October here in Western Sydney is one of clouds, showers and cooler temperatures.

The relevant plots produced from the Bureau of Meteorology Land and Water site for temperature and rainfall are provided below.

Harley Pearman

[Harley Pearman]

December 2010 rainfall prognosis on POAMA 1.5 Experimental

So far, the POAMA Experimental 1.5 Model has been accurate in forcasting above average rainfall for September and now October across eastern Australia. Current October rainfall for many areas of the Murray Darling Basin are significantly above the long term average following two recent rain events and it has been wet in South East Queensland as well.

A review of the November models suggests little change. Above average rainfalls are still expected across eastern Australia for the month of November and now December. The December model indicates wetter than normal conditions for much of New South Wales, especially for areas closer to the coast. Hence for Sydney, the months of November and December according to POAMA 1.5 Experimental may be wetter than normal.

Latest models indicate a moderate to strong La Nina influence (ENSO Currently Plus 26) with a peak during the months of November to January then declining thereafter. Hence it is likely that the current La Nina should prevail into the first quarter of 2011 but decaying as the summer progresses toward Autumn 2011.

The POAMA plot for December 2010 is attached below.

Harley Pearman

[Harley Pearman]

Trends for October around Blacktown as La Nina takes hold

Please find attached, the table for October 2010 specific to my locality around Blacktown. The table produced is based on my observations and weather records (BOM data etc) specific for Blacktown and or where I live but it could not apply elsewhere.

La Nina has taken hold but it is noted that the number of thunderstorms for the month of October appears to be steady with no specific trend taking place. There were at least 4 occurring on at least 3 days. There was a storm cell that passed well to the north of Blacktown late Sunday 31 October but it had no impact whatsoever across my locality and I did not even hear a thunderclap. The figure includes the storm of 23/10/10.

It seems that this La Nina is proving to be very efficient in terms of the number of days when it has rained in Blacktown (15 for the month of October). Further, the number of days when the temperature reaches 30C is less when compared to recent years (3 in total for the month of October and nil for September). Also the highest maximum temperature recorded for the month was only 31.6C on the 31/10/10. So far, there have been no days when the temperature has reached 35C. The total rainfall for the month was slightly above average. It appears so far that this La Nina episode is reducing the number of hot days that may occur but at the same time increasing the number of days when rainfall may occur.

The table for the month of October for my locality is provided below.


Harley Pearman

[Harley Pearman]

La Nina impacts on New South Wales for October 2010

La Nina appears to be having an impact on New South Wales in terms of rainfall (Some of it significant) and cooler than normal maximum daytime temperatures.

Maximum temperature anomaly

As suggested by the October POAMA ensemble (Predictive Oceanic Atmospheric Model for Australia), the majority of the state of New South Wales was cool or cooler than normal. In particular:-

- Maximum daytime temperatures in the south east quadrant were at best average to 1C above average.
- Everywhere else saw below average temperatures for the month.
- The cooler than normal temperature anomaly is more pronounced across the north of the state from 2C to 3C below average.
- Sydney is within the range of average to 1C below average for the month.

Minimum temperature anomaly

Minimum night time temperatures across eastern New South Wales ranged from 0C to 2C above average with the warmest areas occurring over the north east of the state and inland south east of the state. Minimum night time temperatures were generally average or slightly above average. Night time temperatures were 0C to 1C below average across the western half of the state.

Rainfall

As suggested by the October POAMA ensemble (Predictive Oceanic Atmospheric Model for Australia), much of New South Wales experienced average to above average rainfalls. A small area south west and south of Sydney around Goulburn and Batemans Bay experienced below average rainfall in the order of 25 to 50 mm below average but generally, October was a wet month across the state. In particular:-

- New South Wales North coast experienced rainfalls of 10 mm to 400 mm above average for the month.
- Upper Murray (Southern New South Wales) experienced rainfalls of 50 to 200 mm above average for the month.
- Parts of North West New South Wales experienced rainfalls of 50 to 200 mm above average for the month.


The relevant plots prepared on the Bureau of Meteorology "Water and the Land" site dated 6/11/2010 are provided below.

Cool wet start to November for Blacktown

Here in Blacktown, the first week of November 2010 has seen rainfall occur on 5 of 7 days. It has been cool with much cloud and rainfall has varied from 35 to 43 mm. Additionally, early Sunday morning 7/10/2010, a winter appearance took hold soon after dawn because a thick fog developed across the region. Once the fog lifted a sunny day ensured. Generally La Nina is resulting in much cloud, shower activity and cooler daytime temperatures. Further to this point, large areas of Sydney have had much larger rainfalls during the same period.

Harley Pearman

[Harley Pearman]

Trends for November 2010 as La Nina continues

Please find attached the table for November 2010 for my local area being Blacktown. Of particular interest, the number of thunderstorm days for the month was lower than that in recent years being 2 days but the number of thunderstorms overall being 5 is generally consistent with that of recent years. A general trend in the number is not apparent.

There were 9 days when it reached 30C but so far this season there have been no days when the maximum temperature has exceeded 35C. There have been 12 days overall for October and November where the maximum temperature has reached 30C with the hottest day so far being on the 14/11/2010 where it reached 33.7C.

Rainfall occurred on at least 14 days. The total rainfall figure of 110.8 mm is at the Collins Street Weather Station Seven Hills being the closest weather station to where I live.

So far this season it is becoming apparent that:-

1 - The number of very hot days is less when compared to recent years.
2 - The number of days when it rains is more when compared to recent years except 2007 which was also a La Nina year.

The data is compiled from the local weather stations via Weatherzone and from personal observations made over the years from 1/1/1998 but can only be applied to my local area being Blacktown and nowhere else.

Harley Pearman

[Harley Pearman]

La Nina Impacts on New South Wales for November 2010

As suggested by the POAMA (Predictive Oceanic Atmospheric Model for Australia), the month of November around New South Wales was regarded as:-

- Cooler than normal especially during the daytime (With some variations).
- Wetter than normal especially across the inland of the state.

Generally conditions were a close match to that suggested by the POAMA models.

Maximum temperature anomaly

The following analysis is provided:-

- The south east of the state and areas near the Victorian state border experienced average daytime temperatures of 0C to 1C above average. Although this is close to the long term average for the month.
- Coastal areas and areas in a line from Batemans Bay to Coffs Harbour then south west to the Victoria State border experienced temperatures of 0C to 1C below average. Although this too could be regarded as being close to the long term average.
- Coastal areas in north east New South Wales through to central New South Wales towards south west New South Wales experienced daytime maximum temperatures of 1C to 2C below average for the month.
- Northern New South Wales experienced daytime maximum temperatures of 2C to 3C below average for the month.
- Northern border areas experienced average daytime temperatures of 3C to 4C below average for the month.

Minimum temperature anomaly

The following analysis is provided:-

- The South West slopes and Albury / Wodonga to Wagga Wagga then east to the foothills of the Great Dividing Range experienced minimum nightly temperatures of 2C to 3C above average.
- South east to southern New South Wales experienced minimum nightly temperatures of 1C to 2C above average for the month.
- Northern New South Wales stretching towards the south west of the state experienced minimum nightly temperatures of 0C to 1C above average.
- North west New South Wales experienced minimum nightly temperatures of 0C to 1C below average.
- An area around Tibooburra experienced minimum nightly temperatures of 1C to 2C below average.

Rainfall

Inland New South Wales experienced a wet month and above average rainfall was spread throughout the Murray Darling Basin. Interestingly, an area in south east and north east New South Wales experienced below average rainfall. An area across south east Queensland experienced falls of 25 to 100 mm below average for the month which stands in stark contrast to what happened across the remainder of eastern Australia.

When analysing the various plots for temperature and rainfall it is becoming evident that rainfall patterns stands in great contrast when compared to 2009. La Nina is impacting on eastern Australia and it is noted that the Murray Darling Basin is benefiting heavily from the event with many localities enjoying average yearly rainfall which is now well above average.

I have provided the relevant plots for temperature and rainfall anomalies for the whole of Australia so other states can be compared. The plots are produced at the Bureau of Meteorology Land and Water site dated 8/12/2010.

All models indicate La Nina to continue into early 2011 but it will weaken over time as late summer progresses.


Harley Pearman

[Harley Pearman]

Possible Rainfall and temperature prognosis for January 2011

I have been looking at POAMA Models and rainfall and temperature scenarios for January 2011. The POAMA (Predictive Ocean Atmospheric Model for Australia) has been accurate at predicting the outcome for this current La Nina phase and rainfall and temperature outcomes are generally close to predictions made.

It is clear that La Nina will continue into early 2011 but the model is indicating a weakening phase as late summer progresses being February 2011. The IOD (Indian Ocean Dipole) has returned to neutral as expected but the ENSO is still well above La Nina Thresholds which should continue into early 2011 but waning.

The January Rainfall scenario suggests higher than normal rainfall for eastern Australia. When focusing on New South Wales, south east New South Wales may experience a wet month. It is noted that rainfall in western and south western Victoria may start returning closer to normal as La Nina starts to lose its influence in that part of the state. The model suggests a wet January for much of Queensland.

The January temperature scenario suggests a continuation of below average temperatures especially for eastern Australia and New South Wales (Including Sydney) by as much as 1C below average. However western Victoria may see a warmer than normal January with average temperatures by as much as 0.5C above average. Cooler than normal temperatures of up to 0.5C below average may prevail across inland Queensland with near average temperatures suggested along the coast.

The models produced on POAMA are provided below. Please note that the model program is produced by the CSIRO, is still in the experimental stage but is being updated with new products.

Harley Pearman

[Harley Pearman]

December 2010 storm and rainfall records for Blacktown and Sydney

I have compiled the daily weather records for the month of December for my local area being Blacktown. Of interest, the following is noted:-

a) There were three thunderstorms on 2 days. In terms of trends since 1998, the number is lower but not the lowest. There were fewer storms in December 1999, none in December 2002, and 2 in December 2003 and 2009. During the La Nina year of 2007, there were 6 storms on 5 days. It is hard to identify any specific trends occurring for my local area. December 2004 saw more storms than normal for my local area.

Note:- I did not hear any thunder or see any lightning from the rain event of 26/12/2010.

b) It reached 30C on 11 days but the hottest day was on the 31/12/10 when the maximum temperature for the day reached 33.3C (Prospect Weather Station). There have been 23 days since 1/10/2010 when the maximum temperature for the day has reached 30C and the hottest day this season to 31/12/2010 was on 14 November when the maximum temperature reached 33.7C.

One noticeable conclusion for the 1 October to 31 December period at least for my local area is that the maximum temperature has not reached 35C and there is a total absence of very hot days. This is a big contrast to 2009.

c) Rainfall was above normal. Weather Station Number 67026 at Collins Street Seven Hills (Closest to me) recorded 97 mm for the month. The median is 58.5 mm and the mean is 70.4 mm. Rainfall was above average and it rained on 9 days.

Sydney rainfall for 2010

When tallying rainfall for Sydney, I have discovered that while much of New South Wales enjoyed a wet year being wetter than normal, parts of Sydney had rainfall below average for the year. With a La Nina effect in place, above average rains fell in some parts of Sydney such as Campbelltown and Penrith but not Parramatta, Sydney and Terrey Hills (Refer to the second table attached). At Prospect, rainfall for 2010 was 1.8 mm below average (effectively an average year for rainfall).

Looking at Models and information statements, La Nina has now peaked and should now start to weaken. It should hold through to the end of summer and continue to weaken thereafter.

I have prepared 2 tables in the attachment for viewing compiling the data from the local weather stations (Bureau of Meteorology Station sites) and through my own observations. The tables are produced 1/1/2011.


Harley Pearman

[Harley Pearman]

December 2010 rainfall and temperature for New South Wales

Weather conditions in much of the state of New South Wales for December reflected that suggested by POAMA (Predictive Oceanic Atmospheric Model for Australia) although some variations have occurred.

Rainfall anomalies

Rainfall ranged from heavy to light depending on region. In particular, the Tablelands (North, Central and South) and the Central West experienced rainfall of 100 to 200 mm above average for the month. There was flooding in many inland areas some of which I saw at Dunedoo, Dubbo, Parkes and Canowindra in the first few days of the month. Above average rains fell in the south west slopes and north coast. The majority of the state experienced above average rains but some areas were drier including parts of the far west, lower north coast, lower Hunter and an area just north of Sydney.

The rainfall plot for New South Wales is attached.

Maximum daytime temperatures

All parts of New South Wales experienced cooler than normal daytime maximum temperatures and it is clear that La Nina is impacting on daytime maximum temperatures. Further there was an absence of extended hot spells across the state.

The greatest level of cooling occurred across the north central region where maximum temperatures were 4C to 5C below the long term average. The southern Half of the state had maximum daytime temperatures of 1C to 2C below the long term average while coastal New South Wales had temperatures of 0C to 1C below average.

The plot for Queensland and Victoria is provided to show that cooler than normal temperatures prevailed in the two states.

Minimum overnight temperatures

The minimum temperature trends for New South Wales are as follows:-

- Across the eastern part of the state, the nights were warmer than normal being 0C to 1C above the long term average. However there were two regions on the North Coast that had overnight minimums of 1C to 2C above the long term average.

- The north west inland of New South Wales saw minimum temperatures of up to 2C below average.

A similar situation occurred in Queensland as well. Victoria as a whole experienced minimum night time temperatures of 0C to 1C above the long term average.

The plots referred to are provided below and are produced on the Bureau of Meteorology "Water and the Land" site dated 14/1/2011.

Here in Blacktown, the first two weeks of January 2011 has seen rainfall on 8 out of 14 days. However heavy downpours have not occurred. Rainfall has been light in the form of showers / drizzle or light rain. The maximum temperature reached 38.5C on the 1/1/11 but since that day, the maximum temperature has not approached this level. There have been 2 thunderstorms so far this month although thunder was heard from a weakening storm to the north on the 5/1/11.

Models show that the current La Nina phase has now peaked and will start to wane. The SOI (Southern Oscillation Index) has dropped from Plus 27 to Plus 24 but La Nina will not breakdown fully until Autumn set in. It is suggested that the ENSO should return to a more neutral phase for next season.

Harley Pearman