NH Summer/SH Winter Thread 2024

Record-challenging high sitting over Tas this week
( source: Weatherzone )


A blocking-high pressure system will park itself directly over Tasmania this week, and is forecast to become so strong, it challenges an Australian record.

High pressure is crossing the Great Australian Bight after a cold front swept over southeastern Australia this weekend. While high pressure is common after a cold front, this particular high pressure system will stall, as it becomes a blocking-high over Tasmania from Tuesday morning.

You can see the high pressure system stall over the Apple Isle during the week below, with the centre (marked with a red 'H') remaining virtually stationary for nearly 7 days.

^^ Forecast MSLP from ECMWF.

Australia’s high pressure record

The record for the highest Mean Sea Level Pressure (MSLP) could be under threat this week. The all-time Australian record stands at 1044.3 hPa, recorded at Launceston on June 7, 1967.

The ECMWF model shows a forecast central pressure of 1045 hPa is expected on Saturday morning directly over Tasmania, meaning the record could be under threat. Other models show forecast central pressures of around 1042-1044 hPa in close proximity to Tasmania as well.


Due to its position and breadth of the highs’ ridges, it is possible that Melbourne (1041.2 hPa), Canberra (1042.9 hPa), and even Sydney (1040.0 hPa) could also see their pressure records under threat.

Why is this high locked in place?

This high will become a ‘blocking high’, locked in place by the jetstream. The jetstream is a fast-moving belt of westerly winds that circumnavigates the globe in both the northern and southern hemispheres. There are usually 2 jetstreams: one mid-latitude and one polar jet, although they can combine at times.

A ‘blocking-high’ occurs when the jetstream splits around a high pressure centre, locking it in place for days or even weeks at a time. A clear jetsream split can be seen over Australia this week.

^^Image: MSLP and the jetstream on Thursday morning. The colours show the positioning of the jets that have split around the high pressure system.

Adding onto this, is the presence of a cut-off low pressure system over South Australia this week. In a process known as 'anti-cyclonic intensification' rising air near that low will be transported to the high over Tasmania, where it will slowly start to descend. Since air is much slower sinking than it is at rising, air builds up over the high, increasing its central pressure.

Extended-range forecasts from ECMWF show that Hobart is expected to have a combined weekly pressure anomaly of +22hPa above average. This is well above the 99% percentile mark of +14hPa, indicating it is quite likely to be the highest weekly average pressure for the city.

What weather will this high bring across Australia?

High pressure is often associated with clear skies and light winds. So, for most of Tasmania, as well as large parts of Victoria and southern NSW, this high will bring a run of sunny days, but cold frosty nights. Launceston is forecast to fall below zero degrees for the next 7 or 8 mornings in a row.

In Queensland and northern NSW, the high pressure brings persistent southeastly winds to the coast, increasing the chance of showers each day, as well as established ocean swell.

For the remainder of northern Australia, the high to the south will increase winds, leading to warmer days. The increased winds also increase their fire danger, given it is now the start of the northern Australia bushfire season.

For parts of SA and WA, the high means the low pressure systems and cold fronts that are crossing those areas during the week are more likely to bring longer-lasting rain than normal. This is because those types of systems get 'backed-up' like being behind a slow car on the freeway, meaning they stick around for longer. SA may also see a rare boost of moisture coming in from Queensland, something that is very unusual to see during the winter months.

Such intense and focal high pressure systems are likely to impact weather for nearly every part of the country this week, so stay tuned to the Weatherzone News Feed for updates on all of those weather interactions.

Heavy rain in Riga on 1 July bringing flooded streets and the air temperature has dropped from 24C in the morning to 16C this evening.

Decent old frost on the Tasmanian Central Plateau at Liawenee this morning.


And even colder this morning !!

Germany had the wettest 12 month period since measurements began in 1881. 1070 mm of precipitation were recorded between July 2023 and June 2024. The nationwide average 1961-1990 is just 789 mm.

Today marks the anniversary of the biggest snowfall in Blue Mountains recorded history. 90cm fell in Blackheath on this day in 1900.
( source: Blackheath Weather )
The 4th of July.
Today marks the biggest snowfall in Blue Mountains recorded history. 90cm fell in Blackheath on this day. 90cm! 🫨
We all know that snowfalls have declined a lot since the old days but snow will return, eventually. There are a LOT of non-science based theories out there about our declining snowfalls but IMHO, stick to the science. It's not perfect but it's a LOT better than BBQ or pub theories - I promise. 🙏
We won't see 90cm ever again but we may see 9cm. A good fall will come. When will it snow again?
Not yet. We have incredibly stubborn high pressure systems much further south than usual (courtesy of a huge split in the jetstream) which is blocking cold fronts and giving us onshore SE drizzle and showers.
Hold onto hope. Always hold onto hope. Late July and through August and September give us a chance of an outlier event or two.

Why Has the Winter of 2024 Felt So Cold Over Southeastern Australia If There's Been No Cold Fronts?
Thursday July 4, 2024
( source: WeatherWatch )
As yet more cold weather records tumble across Southeastern Australia (Liawenee rebroke the Tasmanian July record this morning falling to -13.5C, after yesterday falling to -12.9C) the question is, why is it so cold if there's been a distinct lack of cold fronts this winter?
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Why Has the Winter of 2024 Felt So Cold Over Southeastern Australia If There's Been No Cold Fronts?
Thursday July 4, 2024
As yet more cold weather records tumble across Southeastern Australia (Liawenee rebroke the Tasmanian July record this morning falling to -13.5C, after yesterday falling to -12.9C) the question is, why is it so cold if there's been a distinct lack of cold fronts this winter?
FrostyCreek.jpg
It seems counter-intuitive, but cold fronts across southeastern Australia can actually bring warmer temperatures! This is because in winter, the ocean is often warmer than the land because the ocean retains more heat. Currently the sea surface temperatures off Tasmania and Victoria are ranging from 12 to 15C, and to record colder sea surface temperatures (below 10C), you need to travel many hundreds of kilometres further south again. When cold fronts sweep through, they bring Southern Ocean air into southeastern Australia - this airmass (while still cold), is limited by how cold it can get due to the ocean temperatures being comparatively milder.

In the Northern Hemisphere, there are significant landmasses (such as Canada and Russia) that rapidly lose their heat in winter which provides a significant source of cold air. This is why locations of equal latitude in America and Eurasia are frequently colder in winter compared to their Australian equivalents. As an example, Atlanta in the US which is at a similar latitude to Sydney has had a record low of -22C! Even if we take into account that Atlanta is well inland, the Australian all-time record which is in the Snowy Mountains at nearly 2000m is -23C.

If we look at the months of May and June so far for 2024, they've been characterised by a lack of cold fronts. This means no injection of "mild" ocean air, and the air has just been sitting over the same spot. Since land loses its heat more rapidly than the ocean, there's no change in airmass and the only heat is the sun during the day to briefly warm temperatures up.

A lack of cold fronts often means lighter winds - and light winds contribute to cold nights and drop the overall average temperature (that's the average minimum + maximum temperature). Not to mention cold fronts often bring cloud and rain - further factors that normally keep minimum temperatures warmer at night (though can bring colder temperatures in the daytime).

What happened to all the cold fronts?

The frequency of cold fronts is linked to the Southern Annular Mode (SAM). When the SAM is negative, then cold fronts extend further northwards into Australia. When the SAM is positive, they retreat southwards closer to Antarctica. Positive SAM events occur due to unusually high pressures over Antarctica. What causes the higher pressure anomalies to develop is still unknown - but it's likely linked to an overall warmer climate compared to previous decades. There has been a trend towards a higher number of positive SAM events - which may mean less winter cold fronts on average in the future.

^^
Why Has the Winter of 2024 Felt So Cold Over Southeastern Australia If There's Been No Cold Fronts?
Thursday July 4, 2024
As yet more cold weather records tumble across Southeastern Australia (Liawenee rebroke the Tasmanian July record this morning falling to -13.5C, after yesterday falling to -12.9C) the question is, why is it so cold if there's been a distinct lack of cold fronts this winter?
FrostyCreek.jpg
It seems counter-intuitive, but cold fronts across southeastern Australia can actually bring warmer temperatures! This is because in winter, the ocean is often warmer than the land because the ocean retains more heat. Currently the sea surface temperatures off Tasmania and Victoria are ranging from 12 to 15C, and to record colder sea surface temperatures (below 10C), you need to travel many hundreds of kilometres further south again. When cold fronts sweep through, they bring Southern Ocean air into southeastern Australia - this airmass (while still cold), is limited by how cold it can get due to the ocean temperatures being comparatively milder.
SSTs for July around Australia - compared to land, ocean temperatures even off Southern Australia tend to be warmer. Source: Bureau of Meteorology.
In the Northern Hemisphere, there are significant landmasses (such as Canada and Russia) that rapidly lose their heat in winter which provides a significant source of cold air. This is why locations of equal latitude in America and Eurasia are frequently colder in winter compared to their Australian equivalents. As an example, Atlanta in the US which is at a similar latitude to Sydney has had a record low of -22C! Even if we take into account that Atlanta is well inland, the Australian all-time record which is in the Snowy Mountains at nearly 2000m is -23C.

If we look at the months of May and June so far for 2024, they've been characterised by a lack of cold fronts. This means no injection of "mild" ocean air, and the air has just been sitting over the same spot. Since land loses its heat more rapidly than the ocean, there's no change in airmass and the only heat is the sun during the day to briefly warm temperatures up.

A lack of cold fronts often means lighter winds - and light winds contribute to cold nights and drop the overall average temperature (that's the average minimum + maximum temperature). Not to mention cold fronts often bring cloud and rain - further factors that normally keep minimum temperatures warmer at night (though can bring colder temperatures in the daytime).
The "Wind Drought" across Southeastern Australia has resulted in lower outputs of wind energy for 2024 compared to previous years.
What happened to all the cold fronts?

The frequency of cold fronts is linked to the Southern Annular Mode (SAM). When the SAM is negative, then cold fronts extend further northwards into Australia. When the SAM is positive, they retreat southwards closer to Antarctica. Positive SAM events occur due to unusually high pressures over Antarctica. What causes the higher pressure anomalies to develop is still unknown - but it's likely linked to an overall warmer climate compared to previous decades. There has been a trend towards a higher number of positive SAM events - which may mean less winter cold fronts on average in the future.

^^Pressures across large parts of Antarctica have been well above average for June so far. This has contributed to the persistent positive SAM phase. Source: NCEP NCAR Reanalysis.

^^Pressures through the Southern Ocean have also been above average for June due to a lack of large winter lows. However, the Tasman Sea has experienced below average pressure anomalies. This is likely due to the warm water across eastern Australia resulting in successive low pressure systems occurring from upper trough activity. Source: NCEP NCAR Reanalysis.
Does this mean all of winter will be cold?

Not necessarily - and this is where the patterns get complex. The Sun has begun its march back towards the Southern Hemisphere as the earth begins to slowly tilt the other way - by August there'll be a lot more heat radiation hitting northern Australia and if there's no introduction of cold air pushing into these areas, then these areas will become warm quickly. In other words, while the lack of cold fronts can allow for cold air to sit over and recirculate across southeastern Australia, in northwestern Australia, the lack of cold fronts can allow for warm air to sit over and recirculate. This could mean a shorter winter and an earlier start to warmer temperatures. Though this isn't always clear cut because a positive SAM (especially in a potentially cool neutral ENSO phase that we may see this year) can help bring onshore easterly winds over eastern Australia.

As the climate continues to change it will continue to make climate forecasting more challenging in the coming years as patterns that were true in the 90s and 00s will potentially bring different expected patterns.

Dry air coming in from the NW

Beryl reintensifying in the Gulf is a real possibility, and for this one model ensembles have been biased too far south/west 

It is official: After many months of above average temperatures, the UK is having the coldest start to July in 22 years. The rich get richer and the poor get poorer!