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#1013729 - 22/09/2011 14:26 Sth WA: Deep lows and cold fronts 25th September - 2nd October 2011
Big_Pete Offline
Weatherzone Addict

Registered: 19/12/2004
Posts: 1955
Loc: Perth
Thought I'd just start this new thread for the coming systems, both deep lows coming on the 25th and 29th. The system on the 29th is looking the best so far, but it is seven days out. BoM models predicting it to be further north than the system on the 25th. Lets see how this one pans out.

GFS is predicting a decent cold pool behind Monday's system, they have backed off a bit on the lowers. Looks to be good SW'ly flow behind it, though the flow may switch more S'ly depending on how deep the second low is as it approaches behind the first one.

Highs are going to get pushed north again. The high on the 27th is predicted to be 1016hPa!

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#1013771 - 22/09/2011 17:00 Re: Sth WA: Deep lows and cold fronts 25th September - 2nd October 2011 [Re: Big_Pete]
Big_Pete Offline
Weatherzone Addict

Registered: 19/12/2004
Posts: 1955
Loc: Perth
Quote:
The hemispheric long wave pattern has remained stable in recent weeks. There are six main troughs. Currently the most significant troughs are near the longitudes of South Africa, the southwest Indian Ocean, the Indian Ocean, Western Australia, New Zealand, the south Pacific, and the southeast Pacific.
Summary:
Over southern and eastern Australia the cold front events with potential to bring widespread rain are now expected about 24 September to 28 September, 29 September to 3 October, and 14 October to 18 October. Rain events originating in the tropics and moving south are possible about 18 September to 22 September, 27 September to 1 October, and 15 October to 19 October.
Over Western Australia the strongest cold fronts should occur about 20 September to 24 September, 29 September to 3 October, and 3 October to 7 October.

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#1013838 - 22/09/2011 22:57 Re: Sth WA: Deep lows and cold fronts 25th September - 2nd October 2011 [Re: Big_Pete]
snowbaby Offline
Weather Freak

Registered: 05/08/2009
Posts: 295
Loc: Albany, W.A
I haven't looked at much of the detail yet of individual fronts but this upcoming general period I must say is looking fascinating...love the prospect of cold weather and possible southerlies..

Big_Pete would it be true to say the subtropical high belt looks like it's riding further north than we might expect at this time of year? If so it would vindicate an expectation of above average spring rain according to the authors of the Swaci.

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#1013849 - 22/09/2011 23:56 Re: Sth WA: Deep lows and cold fronts 25th September - 2nd October 2011 [Re: snowbaby]
Big_Pete Offline
Weatherzone Addict

Registered: 19/12/2004
Posts: 1955
Loc: Perth
Originally Posted By: snowbaby
Big_Pete would it be true to say the subtropical high belt looks like it's riding further north than we might expect at this time of year? If so it would vindicate an expectation of above average spring rain according to the authors of the Swaci.



Yeah, it looked like the highs were moving south there for a while, but now the maps are predicting the next high to be level with Geraldton. I'm sure we'd normally see highs that far north in July. I think it may have something to do with the negative SAM index and postive IOD. Not sure what exactly.

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#1013886 - 23/09/2011 09:32 Re: Sth WA: Deep lows and cold fronts 25th September - 2nd October 2011 [Re: Big_Pete]
snowbaby Offline
Weather Freak

Registered: 05/08/2009
Posts: 295
Loc: Albany, W.A
Its quite interesting to me how two of the conditions mentioned in the paper “A monsoon-like Southwest Australian Circulation System and its relation with rainfall in Southwest Western Australia” may be present at this moment - the subtropical high belt shifting north and somewhat of a cold sea surface anomally well to the west of western australia. The authors of the paper Juan Feng and Jian ping Li have postulated that these two conditions are possibly drivers of a positive SWAC index - that is above average rainfall. Anyone interested can find the paper on Dept of Climate Change Website...

The cold pool for Monday Tuesday is shaping up pretty good. Expect a fair bit of moisture about for day or two and GFS 12z run has 850 temps at -1 now - also Ecmwf looks to me to be slightly more positive on last nights run. Need just a few more runs to look at to lock it in I rekon.

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#1013896 - 23/09/2011 10:31 Re: Sth WA: Deep lows and cold fronts 25th September - 2nd October 2011 [Re: snowbaby]
Big_Pete Offline
Weatherzone Addict

Registered: 19/12/2004
Posts: 1955
Loc: Perth
I like the look of Sunday's sounding for Albany. cool


Yeah that cold pool is looking a bit better. Models are showing a bit of variation in the system on the 29th though.

Couldn't find the papers on their site, snowbaby. Could you give me the link? smile

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#1013930 - 23/09/2011 14:32 Re: Sth WA: Deep lows and cold fronts 25th September - 2nd October 2011 [Re: Big_Pete]
Big_Pete Offline
Weatherzone Addict

Registered: 19/12/2004
Posts: 1955
Loc: Perth
EXTENDED PERIOD FORECAST FOR THE SOUTHWEST LAND DIVISION
Issued at 12:30pm WST on Friday the 23rd of September 2011

SYNOPTIC SUMMARY:
A ridge of high pressure is moving into the Bight, with isolated showers and
drizzle east of Esperance expected to clear later this afternoon as winds turn
more NE'ly.
A surface trough will develop inland from the west coast during Saturday, with
warmer and generally sunny conditions over most of the SW Land Division.

FUTURE DEVELOPMENTS:
The surface trough will move east on Sunday as a cold front and a low pressure
system develop to the southwest of the state. An unstable westerly flow will
produce isolated showers southwest of a line Perth to Walpole on Sunday morning,
extending to southwest of a line Geraldton to Hopetoun during the afternoon.
Showers will tending scattered over the Southwest district in the afternoon as
the low moves closer, with isolated thunderstorms possible. The surface trough
will also produce isolated thunderstorms and showers over the Southeast Coastal
district on Sunday morning, before contracting into the Eucla in the afternoon.
The low will deepen to the south of the state on Monday, with a cold front and
associated middle level disturbance moving through the SW Land Division.
Scattered showers will extend throughout, and isolated thunderstorms are likely
over southern and eastern parts.
The middle level disturbance will remain over the Division on Tuesday, with a
cool SW'ly flow pushing through as the low moves east. Scattered showers and
isolated thunderstorms are expected southwest of a line Lancelin to Merredin to
Hopetoun, with some small hail possible near the south coast. Isolated showers
will continue over the remainder.


FORECASTS FOR THE SOUTHWEST LAND DIVISION:
SATURDAY:
Warmer and generally sunny over most parts.

SUNDAY:
Isolated showers southwest of a line Perth to Walpole, extending to southwest of
a line Geraldton to Hopetoun during the afternoon. Showers tending scattered
over the Southwest district in the afternoon with isolated thunderstorms
possible. Isolated showers and thunderstorms over the Southeast coastal
district, clearing in the afternoon.

MONDAY:
Scattered showers extending throughout in the morning, with isolated
thunderstorms over southern and eastern parts.

TUESDAY:
Scattered showers and isolated thunderstorms southwest of a line Lancelin to
Merredin to Hopetoun, with some small hail possible near the south coast.
Isolated showers over the remainder.

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#1013936 - 23/09/2011 15:08 Re: Sth WA: Deep lows and cold fronts 25th September - 2nd October 2011 [Re: Big_Pete]
snowbaby Offline
Weather Freak

Registered: 05/08/2009
Posts: 295
Loc: Albany, W.A

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#1013982 - 23/09/2011 20:17 Re: Sth WA: Deep lows and cold fronts 25th September - 2nd October 2011 [Re: snowbaby]
Big_Pete Offline
Weatherzone Addict

Registered: 19/12/2004
Posts: 1955
Loc: Perth
Models still uncertain about what the cut-off low on the 28th/29th will do. Some models just predicting a cold front with an accompanying deep low near the south coast.
http://www.bom.gov.au/australia/charts/viewer/index.shtml

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#1014071 - 24/09/2011 12:14 Re: Sth WA: Deep lows and cold fronts 25th September - 2nd October 2011 [Re: Big_Pete]
Big_Pete Offline
Weatherzone Addict

Registered: 19/12/2004
Posts: 1955
Loc: Perth
Brilliant day today after a chilly start of 5.2 degrees and 2.5 degrees at the airport. smile

GFS predicting the 500's to peak at -31C and 850's at 0C. I doubt if they'll go subzero now, still a chance because some models are indicating that low will deepen below 984hPa, so there should be quite a strong southerly behind it as it moves east.

Looking forward to tomorrow, atmosphere looks unstable even without maximum heating, so should get another decent thunderstorm. Tops are looking to be around 25,000ft - 27,000ft tomorrow.

Oh and thanks for the link, snowbaby. smile

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#1014105 - 24/09/2011 14:58 Re: Sth WA: Deep lows and cold fronts 25th September - 2nd October 2011 [Re: Big_Pete]
Big_Pete Offline
Weatherzone Addict

Registered: 19/12/2004
Posts: 1955
Loc: Perth
SYNOPTIC SUMMARY:
A high pressure system lies in the Bight with a surface trough developing inland
from the west coast and a cold front approaching from the west. Fine and
generally sunny conditions are expected about the SW Land Division today, apart
form some isolated showers developing about the SW Capes towards midnight.
The surface trough will move east on Sunday as a cold front and a low pressure
system develop to the southwest of the state. An unstable westerly flow will
produce isolated showers southwest of a line Lancelin to Walpole on Sunday
morning, extending to southwest of a line Geraldton to Hopetoun during the
afternoon. Showers will tending scattered over the Southwest district in the
afternoon, with possible isolated thunderstorms as the low moves closer. The
surface trough will also produce isolated thunderstorms and showers over the
Southeast Coastal district, far southeast Wheat Belt and far east Great Southern
on Sunday morning, contracting southeast during the day and clearing late
evening.

FUTURE DEVELOPMENTS:
The low will deepen to the south of the state on Monday, with a cold front and
associated middle level disturbance moving through the SW Land Division.
Scattered showers will extend throughout, although tend more isolated in far
eastern inland parts, and isolated thunderstorms with possible hail are likely
southwest of a line Mandurah to Hopetoun. Isolated afternoon and evening
thunderstorms are also possible about the far east of the Southeast Coastal
district.
The middle level disturbance and associated cold air mass will remain over the
SW Land Division on Tuesday, with a cool SW'ly flow pushing through as the low
moves east. Scattered showers, isolated thunderstorms and possible hail are
expected southwest of a line Lancelin to Southern Cross to Israelite Bay, with
scattered showers over the remainder of the SW Land Division.
On Wednesday a weak high pressure system will develop as the low pressure system
continues to contract eastwards. Scattered showers with isolated thunderstorms
and possible hail are expected about the Southeast district at first, but will
ease to isolated showers from the west during the day. Isolated showers are also
forecast southwest of a line Lancelin to Narrogin to Ravensthorpe, but will
clear during the day.


FORECASTS FOR THE SOUTHWEST LAND DIVISION:
SUNDAY:
Isolated showers southwest of a line Lancelin to Walpole, extending to southwest
of a line Geraldton to Hopetoun during the afternoon. Showers tending scattered
over the Southwest district in the afternoon with isolated thunderstorms
possible. Isolated showers and thunderstorms over the Southeast coastal
district, clearing late evening.

MONDAY:
Scattered showers extending throughout in the morning, with isolated
thunderstorms and possible hail southwest of a line Mandurah to Hopetoun.
Isolated afternoon and evening thunderstorms about the far east of the Southeast
Coastal district.

TUESDAY:
Cold, scattered showers and isolated thunderstorms southwest of a line Lancelin
to Southern Cross to Israelite Bay, with hail possible. Scattered showers over
the remainder.

WEDNESDAY:
Scattered showers with isolated thunderstorms and possible hail about the
Southeast district at first, easing to isolated showers from the west during the
day. Isolated showers southwest of a line Lancelin to Narrogin to Ravensthorpe,
clearing during the day.

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#1014183 - 24/09/2011 20:52 Re: Sth WA: Deep lows and cold fronts 25th September - 2nd October 2011 [Re: Big_Pete]
Big_Pete Offline
Weatherzone Addict

Registered: 19/12/2004
Posts: 1955
Loc: Perth
Well latest run of the models looks good, but I think snow on the Stirlings isn't that likely. That cold pool will hang around for over a full day, good SW'ly push behind it as BoM mentioned. Hoping for 20mm-40mm over the next 7 days. smile

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#1014209 - 24/09/2011 23:37 Re: Sth WA: Deep lows and cold fronts 25th September - 2nd October 2011 [Re: Big_Pete]
Vinnie Offline
Meteorological Motor Mouth

Registered: 17/05/2006
Posts: 6470
Loc: Mulambin , Yeppoon Central Qld
Hmm that is weird, EC is only going for about 12 mm of rain for Perth before the month is out, what happened to the 40mm + ?

http://www.yr.no/place/Australia/Western_Australia/Perth/long.html
_________________________

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#1014317 - 25/09/2011 09:39 Re: Sth WA: Deep lows and cold fronts 25th September - 2nd October 2011 [Re: Vinnie]
Big_Pete Offline
Weatherzone Addict

Registered: 19/12/2004
Posts: 1955
Loc: Perth
Bah, that can't be correct. With conditions as unstable as they are predicted on Monday and Tuesday, there's a good cold pool there, so convective showers will be quite heavy, you could see 5mm from one shower alone! I reckon Perth will get 15mm-30mm over the next week.

Clear skies here and the doesn't look to change anytime soon. Awesome, the more heating before the front arrives the better. If we can above 20 degrees here, CAPE levels will reach nearly 1000j/kgs!

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#1014509 - 25/09/2011 19:21 Re: Sth WA: Deep lows and cold fronts 25th September - 2nd October 2011 [Re: Big_Pete]
Big_Pete Offline
Weatherzone Addict

Registered: 19/12/2004
Posts: 1955
Loc: Perth
The front stalled, so we didn't have any trigger, storms kinda failed for us. frown Esperance is really copping some thunderstorms, it had quite a cap over that region, so storms must of exploded up fairly rapidly. smile
Tomorrow arvo looks good for coldies, should see some thunder action in the evening.

Quote:
ALBANY:
Remainder of Sunday: A few showers. Moderate to fresh W/NW winds.
Monday: A few showers. Possible thunder and small hail in the evening. Fresh
W/NW winds tending W/SW during the morning.
Temperatures Min 10 Max 15
UV Alert: 9:20 am to 2:40 pm, UV Index predicted to reach 6 [High]

Outlook for:
Tuesday : Few showers. Thunderstorm risk. Min 6 Max 14
Wednesday : Few showers clearing. Storm risk. Min 8 Max 16
Thursday : Shower or two. Min 6 Max 17

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#1014597 - 26/09/2011 01:48 Re: Sth WA: Deep lows and cold fronts 25th September - 2nd October 2011 [Re: Big_Pete]
Vinnie Offline
Meteorological Motor Mouth

Registered: 17/05/2006
Posts: 6470
Loc: Mulambin , Yeppoon Central Qld
One thing is for sure with 7 mm already and the current forecast 100mm + is certain now for September which means 4 months of 100mm + , compared with just 1 month last year !!
_________________________

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#1014970 - 27/09/2011 19:43 Re: Sth WA: Deep lows and cold fronts 25th September - 2nd October 2011 [Re: Vinnie]
theweatherman Offline
Cloud Gazer

Registered: 10/07/2011
Posts: 33
We got some brief lightning in Bby this (tues) morning, and very light small hail around 7am this morning too. Third hailing in as many weeks.

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#1015168 - 28/09/2011 10:44 Re: Sth WA: Deep lows and cold fronts 25th September - 2nd October 2011 [Re: theweatherman]
snowbaby Offline
Weather Freak

Registered: 05/08/2009
Posts: 295
Loc: Albany, W.A
Last night(Tuesday 28th)The abc tv news reported "light snow" on the Stirlings. I'm not sure where they got this from. In the afternoon of that day there were fairly infrequent sleet&snow showers and possibly more after I left (about 4.30pm)....from what I experienced I think it is most unlikely that there was any snow settling...

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#1015594 - 29/09/2011 10:29 Re: Sth WA: Deep lows and cold fronts 25th September - 2nd October 2011 [Re: snowbaby]
snowbaby Offline
Weather Freak

Registered: 05/08/2009
Posts: 295
Loc: Albany, W.A
There is a post on WAWG forum about snow on Bluff Knoll on Tuesday 27th. Not sure if it's from actual observation tho. I'm sure it was snowing on the summit again as I left because there was a fairly heavy sleet shower as I was walking down at about 4.30pm. It is possible that any snow showers after that time might have started to stick as the temps dropped down in the night - but there didn't seem to be a lot of moisture left at that stage.

BTW no sign of Big_Pete lately..?

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#1016612 - 02/10/2011 16:30 Re: Sth WA: Deep lows and cold fronts 25th September - 2nd October 2011 [Re: snowbaby]
Ruckle Offline
Meteorological Motor Mouth

Registered: 03/08/2005
Posts: 6322
Loc: Woodend VIC 579m ASL - where w...
Dont know about "deep lows" but pretty good upper trough just offshore (or right on it) and surface trough showing some lovely rotation of echoes on the Perth radar atm. Eastern cells (inland) nearer surface trough might even get a bit interesting as the afternoon goes on.
_________________________
Check out my Youtube channel http://www.youtube.com/user/Ruckle18?feature=mhum for more storms and even some snow

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