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#1501170 - Today at 11:44 Re: SEQLD / NENSW Day to Day Weather - 2019 [Re: gleno71]
Ken Kato Offline
Meteorological Motor Mouth

Registered: 05/03/2012
Posts: 6065
Originally Posted By: gleno71
One thing I would love to know if Ken or someone can answer this please.

I have in the past spoke about how we tend to get showers and rain from High pressure systems and so forth. It seems to mainly effect eastern Australia.

Now if you take out the cold pool and trough factor, would this high still produce rain from the SE flow?

Highs create sinking air, which means no rain or clouds can form. So what part of the high pressure creates sinking air? I am presuming because we are at the outer boundries of the high here in SE Qld, is the rule of thumb the further away from the high we are, the less chance the air sinks? Apologies if it's a confusing question.


To answer your first question - yes highs can still produce coastal showers in the SE flow to their north without a cold pool aloft or trough due to the uplift of air as the winds come onto the coast due to orographic lifting, roughness of the terrain causing greater surface friction resulting in wind deceleration, and diurnal heating of the land. Of course this is provided these influences outweigh any counteracting inhibiting ones (insufficient moisture, insufficient onshore component to the flow due to the coastline's orientation, etc, etc). Just that a cold pool aloft or a trough often significantly enhances this shower activity.

As for your second question, you basically answered part of it yourself. Generally speaking, the closer you get to the centre of a high, the stronger and deeper the subsidence (sinking air) which tends to suppress cloud and rainfall. Conversely, the further you get away from the centre, the weaker the subsidence. Also, the height of the trade wind inversion (below which there's often moisture and instability) tends to get higher the further north (in the SH) you move away from a high's centre - this allows for convective clouds to bubble up underneath it to greater heights the further north you go along the coast.
These reasons are why the southern and central coasts of NSW often get fine weather when a high moves across that region even though those regions may be in a weak onshore flow a bit to the north of the high's centre.... and as you move further north along the NSW coast and up the QLD coast, showers tend to increase as do convective cloud depths in the more well-defined onshore flow, especially along the sections of the coastline which are oriented more perpendicularly to a SE flow.
The fact that SE winds are also often relatively cool compared to winds from more northerly directions, aids in the instability in the lower levels as they travel over relatively warm surfaces.

The phenomenon of coastal showers in onshore flows on the equatorward side of highs isn't just restricted to the east coast of Australia either. They occur in other parts of the world along coastlines which face onshore flows.
If we were on the west coast of the continent and a high passed by to our south, we'd typically get fine weather instead, unless of course there was another mechanism causing cloud/rainfall.

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#1501171 - Today at 11:45 Re: SEQLD / NENSW Day to Day Weather - 2019 [Re: Seabreeze]
NotsohopefulPete Offline
Weatherzone Addict

Registered: 24/12/2008
Posts: 1395
Loc: Toowoomba
Hi again glenno71. I just realized CJI has explained some aspects, but I assumed you were after a more local type explanation. And now Ken as well.


Edited by NotsohopefulPete (Today at 11:51)

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#1501186 - Today at 16:28 Re: SEQLD / NENSW Day to Day Weather - 2019 [Re: CJI]
gleno71 Offline
Weatherzone Addict

Registered: 09/11/2007
Posts: 1853
Loc: Australia
Originally Posted By: CJI
High pressure results from sinking air Gleno. Think of dry air from the Hadley and Ferrel cells piling up a bit as it falls toward the surface, compressing downward before escaping.

Farther away from the centre of the high, moist air from the ocean is mixed with the air moving from high to low pressure, but subject to the Coriolis force. On the east coast this occurs to the north of a high as moist SE maritime air moves onshore.



Thanks appreciate the feedback

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#1501188 - Today at 16:32 Re: SEQLD / NENSW Day to Day Weather - 2019 [Re: Seabreeze]
gleno71 Offline
Weatherzone Addict

Registered: 09/11/2007
Posts: 1853
Loc: Australia
Thanks Ken and everyone else for there replies. This could explain as why the Barometer cannot be relied on in this neck of the woods as Ken has explained in the past.

The high pressure fine and low pressure rain scenario seems to work well in latitudes down near Melbourne, Adelaide and perth.

Where as Darwin Cairns etc, are in constant low pressure but have mainly fine weather during the dry season.

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#1501189 - 36 minutes 37 seconds ago Re: SEQLD / NENSW Day to Day Weather - 2019 [Re: gleno71]
Ken Kato Offline
Meteorological Motor Mouth

Registered: 05/03/2012
Posts: 6065
Originally Posted By: gleno71
Thanks Ken and everyone else for there replies. This could explain as why the Barometer cannot be relied on in this neck of the woods as Ken has explained in the past.

The high pressure fine and low pressure rain scenario seems to work well in latitudes down near Melbourne, Adelaide and perth.

Where as Darwin Cairns etc, are in constant low pressure but have mainly fine weather during the dry season.


Re the use of barometers in our region, it depends on the setup. For example, sustained falling pressure over a period of more than a day or two indicated on barometers can often indicate a nearby developing or approaching surface trough, low, or front which can in turn indicate potential rainfall.
But great caution needs to be used because daytime heating often also causes diurnal falls in pressure (which can often even be greater in this region on hot days than those caused by synoptic systems), and highs moving further away to the east which aren't always followed by any rainfall before the next high moves across. There can also be times when it's just an upper level feature that's producing rainfall with no surface system as well as the coastal showers associated with east coast ridges.

Places like Darwin actually experience higher pressures (for their location) during their dry season and lower pressures during their wet season but the difference is that pressures in the tropics don't tend to change anywhere near as much on short timescales as say Melbourne where pressure can be 1030hpa one day and down to 995hpa several days later due to the midlatitude systems sweeping across.
In other words, it's not so much whether the pressure is low or high in a given location that's an indicator of approaching surface synoptic systems - it's more the sustained changes in those pressures which matter.

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