Event 29 SEP - 2 OCT SE Aus Broad-based Low

Ken Kato

One of Us
Ski Pass
Jul 13, 2019
3,663
33,529
363
Brisbane
Today could be a big day for some people but it depends on a few important factors not ruining things.
Here's a copy and paste from part of a detailed post I made elsewhere (apologies for the somewhat dumbed-down nature of it for the social-media masses):

" Today (Friday) has the potential to be a big day for a number of locations (but obviously not everyone’s going to get a bad storm) however it depends on a few critically important factors not interfering too much as I’ll describe below. As an aside, the N to NW winds today will probably become strong at times in SEQ as well.

FACTORS - Most ingredients are favourable for showers and thunderstorms for many locations, some particularly nasty – strong shear, an unstable atmosphere, adequate moisture, and the approaching surface and upper level trough.

But this does depend on yesterday’s storms and any premature storms or rainfall this morning not stabilising the atmosphere too much (keep in mind that if this happens, any storms later on today would be weaker and more localised than expected).

However, I still don't think this will prevent showers, storms, and some rain areas from redeveloping in the lower east of Australia given how unstable the atmosphere is ahead of the troughing (there may be a temporary decrease in cloud cover in some areas during the day allowing more heating)…… and a number of locations in the striped areas on this map will probably get a particularly severe storm.
For the locations which do get a severe storm, the main threats are large to locally giant hail, damaging winds, and bursts of intense rainfall leading to local flash flooding.

Although there are also some ingredients present for the possibility of another tornado somewhere in that broader region, remember that they’re often very hard to predict accurately in Australia and if one were to occur, their damage would be restricted to a very narrow path rather than affecting a large area (but obviously destructive and dangerous along that narrow path).
There is some modest directional shear (winds changing direction with height) in the lower atmosphere near parts of the coast which is one of the ingredients, together with instability and modestly low cloud bases to storms in some areas.

TIMINGS - For the locations which do get a storm today, the more favourable times are later this morning and again this afternoon/early tonight for SEQ/NE NSW but with the approaching upper level trough destabilising the atmosphere, they’re possible at any time (before drier air starts moving in overnight tonight).

LOOKING AHEAD - Afternoons will gradually start getting warmer this weekend in SEQ and by around Monday, it should be much hotter than average by early October standards (afternoon temps getting close to the mid 30s by then away from the coastal fringe) but followed by a few cooler days after that. And much drier weather for awhile as well.
Some of the more sheltered parts of SEQ may also see fog overnight tonight/early Saturday morning but if the westerlies just above the surface become too strong, this will reduce fog potential.

FOOTNOTES - So to wrap up, obviously NOT all of you will get a bad storm but given the potential today, I’d definitely keep an eye on the weather radar and warnings today in case any come near you.
Here’s a thunderstorm potential map for between 7am today and 1am Saturday morning automatically generated by data from one of the models (this map is from a new experimental script currently undergoing testing against past cases). "


51535878067_6797878bb8_b.jpg
 

POW Hungry

Old n' Crusty
Moderator
Ski Pass
May 28, 2000
23,791
39,093
1,063
Eastern Burbs of Sydney
Today could be a big day for some people but it depends on a few important factors not ruining things.
Here's a copy and paste from part of a detailed post I made elsewhere (apologies for the somewhat dumbed-down nature of it for the social-media masses):

" Today (Friday) has the potential to be a big day for a number of locations (but obviously not everyone’s going to get a bad storm) however it depends on a few critically important factors not interfering too much as I’ll describe below. As an aside, the N to NW winds today will probably become strong at times in SEQ as well.

FACTORS - Most ingredients are favourable for showers and thunderstorms for many locations, some particularly nasty – strong shear, an unstable atmosphere, adequate moisture, and the approaching surface and upper level trough.

But this does depend on yesterday’s storms and any premature storms or rainfall this morning not stabilising the atmosphere too much (keep in mind that if this happens, any storms later on today would be weaker and more localised than expected).

However, I still don't think this will prevent showers, storms, and some rain areas from redeveloping in the lower east of Australia given how unstable the atmosphere is ahead of the troughing (there may be a temporary decrease in cloud cover in some areas during the day allowing more heating)…… and a number of locations in the striped areas on this map will probably get a particularly severe storm.
For the locations which do get a severe storm, the main threats are large to locally giant hail, damaging winds, and bursts of intense rainfall leading to local flash flooding.


Although there are also some ingredients present for the possibility of another tornado somewhere in that broader region, remember that they’re often very hard to predict accurately in Australia and if one were to occur, their damage would be restricted to a very narrow path rather than affecting a large area (but obviously destructive and dangerous along that narrow path).
There is some modest directional shear (winds changing direction with height) in the lower atmosphere near parts of the coast which is one of the ingredients, together with instability and modestly low cloud bases to storms in some areas.

TIMINGS - For the locations which do get a storm today, the more favourable times are later this morning and again this afternoon/early tonight for SEQ/NE NSW but with the approaching upper level trough destabilising the atmosphere, they’re possible at any time (before drier air starts moving in overnight tonight).

LOOKING AHEAD - Afternoons will gradually start getting warmer this weekend in SEQ and by around Monday, it should be much hotter than average by early October standards (afternoon temps getting close to the mid 30s by then away from the coastal fringe) but followed by a few cooler days after that. And much drier weather for awhile as well.
Some of the more sheltered parts of SEQ may also see fog overnight tonight/early Saturday morning but if the westerlies just above the surface become too strong, this will reduce fog potential.

FOOTNOTES - So to wrap up, obviously NOT all of you will get a bad storm but given the potential today, I’d definitely keep an eye on the weather radar and warnings today in case any come near you.
Here’s a thunderstorm potential map for between 7am today and 1am Saturday morning automatically generated by data from one of the models (this map is from a new experimental script currently undergoing testing against past cases). "


51535878067_6797878bb8_b.jpg
Well said. Top notch synoposis.
 

Billy Bob

One of Us
Nov 23, 2020
171
411
163
Bungendore
That ACCESS-C nails the 'Oatley split' very well!

There was a comment yesterday about urban heat islands and the effects on storms. Topography is the main factor affecting storm intensity, but the UHI effect in Sydney is significant. Penrith has both factors working in its favour: a natural heat sink at the foot of the Blue Mountains (which attract storms) and an urban core which enhances convection.

If storms make it that far (mainly from the SW), they can intensify once they pass the Cooks River, where the urban core is at its densest. Time and time again storms would pass Oatley with barely a whimper, then reintensify over the hotter urban areas to the north and east. Hail is extremely rare around the Georges River but more common in places like Ashfield and Annandale.
 
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Edge72

Hard Yards
Apr 3, 2020
96
191
83
Sydney
That ACCESS-C nails the 'Oatley split' very well!

There was a comment yesterday about urban heat islands and the effects on storms. Topography is the main factor affecting storm intensity, but the UHI effect in Sydney is significant. Penrith has both factors working in its favour: a natural heat sink at the foot of the Blue Mountains (which attract storms) and an urban core which enhances convection.

If storms make it that far (mainly from the SW), they can intensify once they pass the Cooks River, where the urban core is at its densest. Time and time again storms would pass Oatley with barely a whimper, then reintensify over the hotter urban areas to the north and east. Hail is extremely rare around the Georges River but more common in places like Ashfield and Annandale.
Thankyou for the explanation, so is ur summary saying the coastal areas of sydney are more effected by the UHI thus limiting convection or ? My observation on this was debunked and brushed aside yesterday.
 
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Perfect Storm

One of Us
Jul 5, 2019
161
314
163
Carramar, South West Sydney.
Stunner of a day today.

Temperature judging 24.3 degrees
on the Bankstown AWS with a northerly. Olympic Park 24.7 degrees.

If the wind can hold to remain from more of the northern quadrant and keep the NE/easterly at bay (Obs Hill currently showing an easterly) we may be in with a bit more of a chance of seeing something.

Has been a relatively dry September for Bankstown and surrounds at 20.8 mm for the month of September so would be nice to get a bit of of a top up of H2O..
 
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warrie

One of Us
Ski Pass
Jun 15, 2008
1,044
1,101
363
Sydney

Severe Thunderstorm Warning​

for DAMAGING WINDS and LARGE HAILSTONES​

For people in Central Tablelands, Southern Tablelands and parts of Hunter, Metropolitan, Illawarra, South Coast, South West Slopes and Australian Capital Territory Forecast Districts.

Issued at 12:20 pm Friday, 1 October 2021.

Severe storms beginning to develop throughout the state
 

warrie

One of Us
Ski Pass
Jun 15, 2008
1,044
1,101
363
Sydney
Radar image 12:49 pm. The storm line from Jenolan to Braidwood seems to be not drifting eastwards. So is Sydney safe from any spillage over the GDR? The earlier Nullo Mtn cell fizzled but another has developed. Maybe it will decay too before reaching metro.
1633057225641.png
 
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warrie

One of Us
Ski Pass
Jun 15, 2008
1,044
1,101
363
Sydney
BoM warning update now at 1:10 pm has a cell heading for the Gong. Hope the Donz is not out walking LOL
 
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warrie

One of Us
Ski Pass
Jun 15, 2008
1,044
1,101
363
Sydney
I and about 4 million others live in the so far no threat nor warning area of the Metro. There goes the 2022 house/car insurance premiums if a big hailstorm or tornado hits later this afternoon. And the chances are....

1633063382945.png
 

warrie

One of Us
Ski Pass
Jun 15, 2008
1,044
1,101
363
Sydney
Rain at a rate of 0.001 mm/h and now increased to 0.01 at B1.
Bankstown01/02:50pm22.317.310.8486.4ENE283315181001.70.013.8
06:01am
26.5
01:24pm
ENE39
02:45pm
21
02:45pm
Bellambi01/02:50pm17.918.315.2841.6NNW7945-0.013.5
05:40am
19.8
01:34pm
NW39
03:52am
21
 

warrie

One of Us
Ski Pass
Jun 15, 2008
1,044
1,101
363
Sydney
Premiums safe .... for now. Impressive view from B1 of NE side of C-town cell below
1633064365532.png
 
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