NH Fall/SH Spring 2023 Discussion Thread

Northern Australia set to burst to life
( source: Weatherzone )


Accumulated weekly rainfall totals could reach 50-80mm, with isolated areas possibly picking up more than 100mm in a week.

This is enough rainfall for the northern rainfall onset to be officially declared in some areas across northern Australia. The northern rainfall onset is the first rainfall of the season that is sufficient to stimulate plant growth. This is defined as the date when 50 mm or more rainfall has accumulated after September 1.

The wet week ahead is a sign that the build up season is well and truly underway, which unofficially runs between September and December across northern Australia.

The build up can be an uncomfortable time for some northern Australians, with heat and humidity rising steadily during this time. The steamy atmosphere during the ‘build up’ also sparks regular thunderstorm activity across the tropics.

The build up is a precursor to the monsoon, which typically arrives late-December abd brings cooler temperatures, heavy rain, thunderstorms, and an increased chance of tropical cyclone activity.

However, this year's monsoon is expected to arrive late with El Niño active in the Pacific Ocean, most likely in January.

The potential late arrival of the monsoon means that rainfall should be well-below average during the first half of the wet season, but close to average in the latter half.

The map below shows that the northern rainfall during El Niño years tends to be less than average between October and December, particularly for parts of the Top End, NT and QLD.

During El Niño years, northern Australia typically observes below average tropical cyclone activity during the cyclone season. This can also reduce the heavy rainfall and flooding that typically occurs with these systems. Research has shown that Queensland is half as likely to see cyclones during El Niño compared to neutral years.

The map below shows one forecast model that is predicting drier than normal conditions across northern Australia in December 2023.

Wet season 2022/23

While the start of the 2023-24 wet season is shaping up to be drier than average, the 2022-23 wet season was the 6th wettest on record, with some pockets recording the wettest season on record.

Last season's above average rainfall was driven partly by La Niña, which was active in the Pacific Ocean between November 2022 and March 2023. October and November were particularly wet months across northern Australia, under the influence of a negative Indian Ocean Dipole (IOD) that eventually broke down in December 2022.

Last year's first monsoon onset occurred during December, but the wet season started 1-2 months earlier than usual thanks to La Niña. The green shading on the map below shows the areas where the wet season arrived earlier than usual last year.

The early arrival of the wet season meant that above average rainfall fell during spring and early summer. The map below shows that large areas of the tropics received very much above average rainfall last wet season.

This wet season is forecast to be dramatically different to the last, with more hot and dry weather forecast across the countries north, particularly early in the season.

Got up to 88F here today which tied the daily record set last year. All time high for November is 89F.

Looking like El Nino pattern may be starting to set in here. Front arrived last night and temps down to the mid 50's this morning. Picked up over half an inch of rain. Could be more showers the next couple of days and may get the heaviest rain we've seen in a while with a system early next week. High temps in 60's and low's in the 50's thru the middle of next week. 

Precip the next 7 days. South LA could see a good bit of drought relief. 

Nice rains down for LA, yeah. Will be lucky to get an inch this weekend here. Just 0.05" so far from today.

45/32F averages so far this month already. 

Interesting to see that the average air pressure stays more or less steady throughout the year but gets much more variable in winter, I guess that influenced by both the strengthening and shifting towards lower latitudes of the Icelandic low, and Siberian high pressure outbreaks. I wonder how cold it got in the 1989 winter.

Thunderstorms in and around Perth

The coldest temp was -4.6°C in November, so quite a mild winter.


Here's a example of the synpotics that prevailed during that high pressure period. European high, driving warm subtropical air towards western Europe, not unlike recent winters: