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

[Harley Pearman]

Possible February conditions over Eastern Australia

La Nina has now peaked and should now start to wane although its impact will be felt into Autumn.

The POAMA (Predictive Oceanic Atmospheric Model for Australia) suggests a tendency toward drier conditions or more average conditions for western New South Wales and Victoria for February. In contrast the model suggests a wetter than normal month for eastern New South Wales and Victoria. Other models indicate a tendency for drier conditions over western New South Wales and Victoria with tendency for wetter conditions over the east of the state. Is La Nina starting to lose its impact over some areas? The northern Queensland coast looks set to be wet.

For temperatures, the POAMA model suggests a return to normal conditions for much of eastern Australia including Victoria and much of New South Wales. However, eastern New South Wales may be cooler than normal. The POAMA Model suggests 0.5C to 1C cooler than normal for February.

When looking at rainfall at least for Sydney for the period 1/1/2011 to 29/1/2011, unless any rain events occur between now and 31/1/2011, rainfall looks set to be below average for the month, more so in some locations than others. This would go against suggested POAMA rainfall forecasts made earlier.

So far at some selected sites being Camden, Campbelltown, Parramatta, Penrith, Richmond and Sydney rainfall varies between 27.6 mm to 55 mm for the month to date. When looking at rainfall for the closest weather station to where I live being Collins Street Seven Hills, rainfall for the month totals 38 mm. The median for the month of January is 82 mm and the average is 101.8 mm. Rainfall could fall well below average for January. It has been drier over the past two weeks and I am seeing some curing and drying of grass in open areas around Blacktown.  
  
The relevant plots as discussed are provided below produced on POAMA dated 26/1/2011.

Harley Pearman

[Harley Pearman]

January Weather for Blacktown for 2011

The January weather for my local area seems to have gone against the expected trend. It was a drier month with just 38 mm of rain falling on 9 days and a warmer than normal month where average daytime and night time temperatures were 1.4C above the long term average. There were 2 thunderstorms that passed over Blacktown and a third decaying storm passing to the north on the 5/1/11 and having no impact on my local area. In terms of monthly numbers since January 1998, a trend cannot be seen but for the season overall, a lesser number is becoming evident.

It reached 30C on 13 days and as at 31/1/2011, there had been 37 days this season where the maximum temperature had reached 30C or higher. It reached 35C or more on 6 days including 1 day where the maximum temperature reached 40C. The hottest day was 31/1/2011 when it reached 40.3C. When looking at my records, this is the first La Nina episode that I have experienced in Western Sydney where the maximum temperature has reached 40C. Of note it reached 40.3C on 31/1/11 and 41.3C on 1/2/11 (2 days in a row).

There has been a hot spell which has carried into the first week of February in which daily temperatures has exceeded 35C being 6 days in a row up to 4/2/11 and if 41C is reached today being 5/2/11 as forecast, then that would make it 7 successive days. This has made the nights uncomfortably warm making it hard to sleep.

The table for my home location is provided below. The rainfall derived is from my local weather station at Collins Street Seven Hills Station Number 67026 being less than 1 km from where I live.

Harley Pearman

[Harley Pearman]

January weather conditions for New South Wales

For the month of January, the following has occurred.

Rainfall

Rainfall has varied significantly to what was suggested by POAMA models. As mentioned in the previous post, it was a relatively dry month at Blacktown / Seven Hills with just 38 mm of rain falling. In particular:-

- The south west and north east of the state were wet as well as a small area around Lithgow / Oberon / Bathurst (Central Tablelands west of Sydney). Rainfall was as much as 100 to 200 mm above average in these areas.

- Below average rainfall occurred in a belt stretching from northern New South Wales to the coast including Sydney. Rainfall was as much as 50 to 100 mm below average for the month.

Maximum temperature anomaly:

For New South Wales, the north east and south west of the state experienced daytime temperatures that were 1C below the long term average. Elsewhere, the daytime temperatures were generally average or 1C above the long term average. It is noted that the January result has varied to some degree of what was suggested by the POAMA plot.

Minimum temperature anomaly

The whole state of New South Wales experienced minimum night time temperatures that were above average. In particular, the south east of the state had minimum night time temperatures that were 2C to 3C above the long term average with the rest of the state being at least average to 1C above the long term average.  

The relevant plots are provided below produced on the Water and the Land site dated 6/2/11.

Further to this, during the period 30/1/11 to 6/2/11, a hot spell occurred right across Sydney which saw average daytime and night time temperatures being well above average. Maximum daytime temperatures for this period for Blacktown were 30/1 - 36C, 31/1 - 40.2C, 1/2 - 41.3C, 2/2 - 36.4C, 3/2 - 37C, 4/2 - 37C, 5/2 - 41.2C and 6/2 - 32.6C - Cooler change arrived after 2 pm Sunday bringing relief but no rain. This is 7 successive days where the maximum temperature for the day soared to more than 35C. Nights were also warm with a minimum of 26.5C on the morning of 6/2/11. This was a very uncomfortable period to be in Sydney.


Harley Pearman

[Harley Pearman]

February weather conditions for Blacktown 2011

February 2011 was unusually dry and warmer than normal. The weather conditions for February is in stark contrast to what the models were suggesting and is in stark contrast to that experienced in recent years. Blacktown has had a run of wet February's but February 2011 saw the second driest February I have seen since 1998 with just 14.8 mm of rain falling on 3 days. When looking at rainfall for January and February 2011, just 52.8 mm has fallen. The mean for the period is 215.2 mm and the median for the period is 174.7 mm at the nearby Collins Street weather station.

It seems that this La Nina episode has disturbed our January and February normal rainfall patterns in which it has been drier than normal. This is unexpected considering that many other regions are experiencing heavier than normal rainfall.

While I was out of Sydney for 7 days during the month, I am only aware of one thunderstorm occurring being the 28/2/11. This was not significant in nature and only 1 mm of rain fell where I live.

It reached 30C on 13 days taking the total number to 50 days so far for the season till 28/2/11. The average maximum temperature for the month at Blacktown was 30.2C which was above average. The average minimum temperature was 19.4C which was also above average. The start of March has been warm with the maximum temperature reaching 30C on 3 out of 4 days. It has reached 30C on 53 days at Blacktown to 4/3/11.

On a monthly basis, the number of thunderstorms show no trends but for the season, it is becoming apparent that the number of thunderstorms is much less. I have counted just 14 thunderstorms on 10 days with many of these being minor in nature and having little impact. 

It seems that when comparing the La Nina of 2007/2008 and 1998/1999 to that of 2010/2011, they seem to be all different with different weather conditions and different impacts. For example, I expected a wet February but instead a dry February occurred. The February of 1999 and 2008 (La Nina years) were wet in comparison.
 
The table for February is provided below.

Harley Pearman

[Jimmy Deguara]

Hi Harley,

This particular La Nina is now the strongest on a 5 month mean ever recorded from what I have heard. You would need to compare with 1974 and 1917 if I have the correct years which were particularly strong La Nina periods.

Regards,

Jimmy Deguara

[Harley Pearman]

La Nina rainfall my my local area

As suggested, I would need to compare other La Nina's for my local area to see what has happened. I have managed to locate old rainfall data going back to 1950 for the Collins Street Weather Station at Seven Hills. Data started here on 1 January 1950. There is nothing before that. I have used that to compile some rainfall for the period September to March but only for the known La Nina years since 1950 that I am aware of. Prior to that, I cannot find anything.

However the Prospect Weather Station has rainfall records going back to January 1877. I found some rainfall records for the 1917/18 La Nina period as well for that station.

I also found daily temperature records for Prospect going back to about 1976.

I cannot find records of the number of thunderstorms.

I have presented 3 tables. One is for rainfall during La Nina periods September to March for my nearest weather station since 1950 being Collins Street, Seven Hills. This is for the La Nina's that I am aware of. One is for rainfall for the La Nina periods of 1910 and 1916/18 from the Prospect Weather Station. There appears to have been a La Nina around about 1917/18 and the third one is for the number of 30C days for the La Nina years 1983/84, 1989/90 as well as the most recent ones. I have used the Prospect station for this so there are minor variations.

I have discovered that a dry January and February has occurred before during a La Nina year for Blacktown (January and February 1965). I have discovered that La Nina's can bring very wet summers but also drier summers (The comparison of the wet summer of 1955/56 to the dry summer of 1964/65 is amazing). It appears on the whole that a La Nina does bring a wetter spring / summer period overall but occasionally, it can be drier. So far the rainfall for this La Nina 2010/11 is much lower than that of recent and previous La Nina's (September to March period).

When comparing the number of 30C days during La Nina periods (September to March) at least from 1983/84, there seems to be a reduction in number of very hot days although I have found that it reached 42C on 3/1/1990. It reached 40C three times this summer just gone at Blacktown.

It seems that this summer 2010/2011 for Blacktown has been quite reasonable and not too wet.

I found the data from the Bureau of Meteorology site "Water and the Land" in climate data. The tables referred to are provided below.

Harley Pearman

[Harley Pearman]

February conditions for New South Wales 2011

Weather conditions for the month varied considerably to what was suggested by POAMA. It appears that the POAMA plots for February did not accurately predict the conditions. Rainfall was expected to return closer to normal but instead heavy falls occurred in the southern Murray Darling Basin while much of NE NSW was dry. Further, a cooler than normal month occurred over the south west of the state while other parts of the state was warmer than normal.
 
Rainfall anomalies

An extremely wet month for parts of south west New South Wales contrasts with a dry February around Sydney and parts of the far north coast around the Tweed Valley. Significant rainfalls occurred over the southern Murray Darling Basin with the highest falls in order of 200 mm up to 400 mm above average for an area from Corowa east to Mt Kosciusko. West and south west New South Wales had above average rainfalls from 25 mm to more than 200 mm. The rest of the state was drier. Other than small pockets near Armidale and Barrington Tops, the rest of the state was rather dry. The driest areas occurred around Sydney, Gosford and the Tweed Valley where rainfalls were 100 mm to 200 mm below average.

Of note and not produced here, when looking at the rainfall plot 1 December 2010 to 28 February 2011, the area around Sydney / Newcastle / Wollongong and lower north coast has had the driest summer of any region in Australia.

Maximum temperatures for NSW

South west New South Wales experienced a cooler than normal month. Much rainfall occurred in that region and as a result of cloud cover and rain, maximum temperatures were 2C to 3C below average. Contrast to north east New South Wales, daytime temperatures were 1C to 3C above average for the month. The area around Sydney, Newcastle, Wollongong and the Hunter Valley had day time temperatures that were 2C to 3C above average. This co incides with a dry February.

Minimum temperatures for NSW

The plot shows that night time temperatures were for the whole of the state, average to above average. The warmest night time minimum temperatures occurred over the northern inland of New South wales where temperature anomalies were 2C to 3C above average.

Further to the above, I made a trip into southern New South Wales 19 to 22 February 2011 and the effects of the rain in that area was evident. Rivers such as the Murray was flowing swiftly. The Ovens River at Bright (Victoria) was at minor flood level following a heavy downpour. The Hume Reservoir was at 99.3% capacity and rarely seen at this level at this time of year. The Murray River at Albury was not at flood level but was unusually high. Further the region looked unusually green. I was at Albury when a new monthly rain record was set there (267 mm of rain in February being the highest ever). This is a big contrast to the recent drought years.

The plots referred to are provided below and have been generated from the Water and the Land site Bureau of Meteorology dated 12/3/2011.

Harley Pearman

[Harley Pearman]

Final effect of La Nina - Summer rainfall anomaly

I have produced on the Water and the Land site, the rainfall anomaly plot for the whole of Australia. Of note worthy significance is the area around Sydney / Newcastle / Wollongong / Lower Hunter and Lower North Coast. Well below average rainfall has occurred in this area for the period 1 December 2010 to 28 February 2011. The anomalies or below average rainfall is the worst in Australia. While the rest of Australia has had rain from this La Nina event, this area is suffering well below average rainfalls during the period. The plot is shown below.

This area really needs some rain and it is dry. The grass has dried right out and the region is rather brown from what I have seen from the lack of rain of recent weeks / months.

Harley Pearman

[Harley Pearman]

March weather records for Blacktown

The month of March will conclude this topic.

During the month there were 9 days when the maximum temperature reached 30C taking the final tally to 59 for the season 1 October to 31 March. A near normal season occurred in terms of the number of 30C days. There were 2 thunderstorms with one occurring on 21 March which was weak and another on 30 March. It rained on 10 days but the final rainfall tally will not be known. The weather station at Collins Street Seven Hills quoted here so often appears to have malfunctioned on 19 March and was not operational until 1 April. However there were good rains during the period 19 March to 22 March following a dry start to the month.

The second table provides the number of thunderstorms experienced in my local area since 1998/1999 for the period 1 October to 31 March each year. When looking at the table it is hard to identify a trend. It seems that each season is different. The 1998/99 La Nina produced few thunderstorms for my local area but the 2007/08 La Nina produced many thunderstorms. During the 2010/11 La Nina period, there were 15 thunderstorms on 11 days but I noted that many storms were weak and had little adverse impact.

The tables referred to are provided below. Table one provides the monthly records for Blacktown and Table 2 provides the number of thunderstorms experienced in my local area commencing with the 1998/1999 season. Each season is from 1 October to 31 March. Some additional comments are provided below each table.

Harley Pearman

[Harley Pearman]

I have had some difficulty loading the tables. The tables are provided below.

Harley Pearman

[Harley Pearman]

Final comments on the 2010 / 2011 La Nina for NSW including March 2011

This is the final statement for this topic from me and provides some final conclusions to the La Nina summer of 2010/2011.

For the month of March, the wettest areas were the far west, southern Central West of the state, Riverina, South Coast, Illawarra and north east Sydney. Rainfalls were up to 200 mm above average on parts of the South Coast. The driest areas of the state were the North Coast and Northern Inland. A couple of pockets on the North Coast and the Hunter Valley had rainfalls that were 50 to 100 mm below average. In Western Sydney, rainfall returned but rainfall was average or just below average for the month.  

For the 6 month period 1/10/10 to 31/3/11, South West New South Wales has done very well with rainfall right across the summer. However La Nina has left Western Sydney, Newcastle and Hunter Valley with rainfalls that are below average. Rainfalls of 100 to 200 mm below average has occurred. In most areas, La Nina delivered strong rainfalls or at least average rainfalls except in the areas mentioned. There was rain in March across Western Sydney or Sydney in general but deficits remain. South West Western Australia was also a little dry but the deficits are greatest around Sydney and Newcastle region.

Looking at the March to October Rainfall Anomaly, there is no drought anywhere in New South Wales following summer rainfall. However La Nina has not been as good as expected for Western Sydney, Newcastle area and parts of the Hunter Valley. It was drier than expected in the areas mentioned. Amazingly and unexpected, an area of the Lower Hunter around or near Singleton started to experience a serious rainfall deficiency. This is more pronounced in the January to March period. An area near Tamworth and a small area near the New South Wales / Queensland state border also started to experience a 3 month rainfall deficiency till 31/3/2011.

Further to the above, there was was 75 mm of rain last month at the Collins Street Weather Station site at Seven Hills. The records have been recovered to provide a better outcome for Blacktown and to complete the records.

The plots described above have been produced on the Land and Water site Bureau of Meteorology on 10/4/2011. However I will upload these as soon as I fix the problem with my computer with respect to uploading plots.

Harley Pearman