Gedday,

 As you probably noticed we had a terrific East-Coast (or Eastern Bass
Strait) low over the weekend, which dumped over 150 mm at various Alpine
locations in eastern Victoria/Gippsland. 7-day rain to 9 am 27 April

Thursday morning (22 April) polar orbiter image

Friday morning (23 April)  polar orbiter image

Saturday morning (24 April)     polar orbiter image

Sunday morning (25 April)   polar orbiter image

 

Satellite loop of the event, from 00 UTC 23 to 03 UTC 25  (Orange colouring corresponds to GPATS lightning observations)
 

The upper charts were interesting with a good split jet situation.
The development was preceded by a southward advection of low level
tropical (high thetae) air in the northerlies ahead of Friday morning's
frontal passage, as can be seen on the loop of surface thetae at

http://comms.ho.BoM.GOV.AU:80/gasp/javaloop_gasp_thetaE_0991_sh_anals.htm
(Saved copies here:  thetae 00/22    12/22  00/23    12/23  00/24    12/24  00/25  )

Thetae_ Friday morning

We had a northerly jet core coming down over us on Thursday night before
the front and as the low began developing; but the main spin-up of the low
on Saturday occurred in the open area between/well away from either of the
two split jets.  Perhaps the mesoscale boys (Millsy/Jeff Feren) would like
to comment?

200 hPa (jet stream) analyses from GASP
00/22  12/22    00/23  12/23    00/24  12/24    00/25

Jet stream saturday morning

The cut off low at the surface has weakened and moved eastwards.  However
the split jet over our longitudes remains and we have a mass of moisture
and cloud up over the central Queensland coast associated with a left
entrance to a core of the low latitude branch of the split jet.

Interesting situation. I'll put a couple of charts on my web-page later
today.

John McB

Geoff Feren

Hi Jon,

Yes, I agree that the recent east coast low was a very significant event, particularly in light
of the apparent dearth of such events in recent years - at least about the southern N.S.W.
coast. I am away from the forecasting bench for the next few days, and did not save the upper
level diagnostics whilst on duty during the weekend. So, at this stage I can only offer some
general comments about the dynamics of the development (based on a less than perfect memory).

The upper level flow appeared to have classical characteristics for a significant surface
cyclogenesis event in our region. Thus, a relatively broadscale and mobile short wave trough
approached a downstream (Tasman Sea) block. A significant - i.e. >100 knot SSW'ly jetstreak on
the western flank of this system resulted in its amplification and cutting off (conversion of
shear vorticity into curvature vorticity). This was associated with a rapid shortening
half-wavelength between the trough (which developed a northeastwards tilt) and the developing
downstream, short wave ridge. The main phase of surface cyclogenesis subsequently occurred just
to the east  of the upper low/trough axis, beneath diffluent upper level flow, and in the exit region of the
jetstreak wrapping around the upper low. Satellite imagery provided supporting evidence of the
presence of strong upper level dynamics - i.e. some prominent, large wavelength banding
perpendicular to the upper flow was evident for a period in the baroclinic system as it moved
across Victoria (apparently signifying gravity wave activity due to geostrophic adjustment in
association with the upper level cyclogenesis). Also, from memory, water vapour imagery showed a
reasonably prominent dry slot immediately to the rear of this system (in association with the
upper level jetstreak).

Overall, the numerical models did a good job (at least 1 to 2 days prior to the event) in
forecasting the general development. However, I don't recall any of the models correctly
depicting the detail of the main surface cyclogenesis during Friday night to early Saturday
morning (23-24 April). This particular development had similarities to the "cyclogenesis in
reversed shear" 1998 Syney-Hobart Yacht Race Storm analysed by Graham Mills. Thus, the main
surface low developed on the western flank of the pre-existing, broad low pressure zone - i.e. a
"sting in the tail development", within a very similar thickness pattern - more of, later.

By 12Z on 23rd April, the broad surface low pressure zone had moved just to the east of Bass
Strait - which was the region where the models forecast the low would remain. Strong to locally
gale force southerly surface flow was affecting Bass Strait in its wake. Interestingly, by this
stage, the mesoscale surface analysis was showing an approximately east-west orientated trough
extending westwards from the Tasman Sea low pressure region through eastern Bass Strait. From
memory, south or southeasterly surface flow was present to the south of this trough, with
northwesterly surface flow to the north. This type of eastern Bass Strait surface flow is not
that uncommon (within a benign upper level environment) in a broad south to southeasterly
surface flow regime, particularly when ridging is increasing over Tasmanian latitudes or southwards. It
appears that such an east-west trough forms in the lee of the windward ridge generated by the
Tasmanian highlands. However, I've noticed on previous occasions that when a developmental upper
level flow becomes superimposed on such a surface trough, preferential development often takes
place within this region of pre-existing surface vorticity. This certainly happened in this case.

Cyclogenesis within eastern Bass Strait appeared to coincide with the development of a small
area of deep convection (and numerous GPATS strikes - see image below) on the southeastern flank
of the low.

Although surface pressure at the low centre only fell by a few hectopascals, there
was a marked increased in the south to southeasterly pressure gradient immediately on the
western flank of the system. Thus, storm force (50-55 knot) SSE'ly winds were recorded for a few
hours at Kingfish B oil platform (approx. 38.5S 148E), as well as significant (maximum) wave
heights of up to 7 metres (12 metres) - see graph below. I am not aware of how well this event
was forecast by the wave model - I was focussing on aviation issues at the time. I have included
below some comments passed on to RFC colleagues on the significance of this event, based on
passed climatology work on storm occurrence in eastern Bass Strait.

Finally, as indicated earlier, this event did have some similarities to the "reversed shear"
1998 Sydney-Hobart Yacht Race storm. In that case the assymetrical surface low pressure
circulation was associated with an intense band of W to SW'ly flow coincident with an ENE'ly
thermal jet. This resulted in a very low level wind max around 100 knots (from memory). In this
event, a similar thickness pattern was evident (see analysis below), and associated with an
assymmetrical surface low characterised by an intense band of SSE'ly surface winds.

RFC e-mail:

The major low pressure system which developed over eastern Bass Strait on Friday night generated
a peak significant wave height at Kingfish B platform (KFB) of just over 7 metres (max. wave of
12 metres) at approximately 3am on Saturday. This was associated with a 3-4 hr period of storm-force SSE'ly winds with peak average winds and gusts of 56 knots and 69 knots,
respectively. A climatology of storm occurrence in the oil rigs area of eastern Bass Strait for
the 40 years to 1990 (incorporating both observational data and the results of an ESSO-BHP
hindcast study of storm events back to 1950) indicated that 6-metre SE'ly storm events can be
expected on average once every two years.

Only two 7-metre SE'ly storms were recorded at KFB in the 12 year period of instrumented wave
data between 1978 and 1989. I am only aware of one more intense April SE'ly storm event back as
far as 1950 - i.e. the east coast low of April 21 1990, which was associated with major flooding
in Gippsland and other parts of eastern Australia. The most intense SE'ly events experienced in
the oil rigs area (since 1950) occurred in December 1970 and June 1978 and were associated with
significant wave heights peaking at 9 to 10 metres.

A previous analysis of pre 1990 6-metre SE'ly storm events in the oil rigs area found that most
events occurred either during a period of positive Southern Oscillation Indices, or during the
breakdown of an El Nino event - for example, the March 1983 storm.

Note that the restricted fetch of water present to the southwest of the platforms (i.e. due to
the presence of South Gippsland, Bass Strait islands and Tasmania) has limited the most severe
SW'ly wave-energy storms to a maximum significant wave height of approimately 7 metres. The most
severe SW'ly storms are typically associated with a small low passing just to the south of the
oil rigs area - e.g. the 1998 Sydney to Hobart Yacht Race storm.

Harald Richter

Date: Tue, 27 Apr 2004 16:42:27 +1000 (EST)
From: Harald RichterGeoff and list,

I think this article would make a nice and worthy addition to the Storms Conference in Brisbane in July (if they still accept submissions). It seems you can consider your 2-page extended abstract done. :)

One of the nuggets in your comments below is the role of Tasmanian topography in promoting lee cyclogenesis N of Tasmania in the 'wake' of departing lows.  Is this a commonly observed feature?

Cheers, Harald

John Nairn
John Nairn just sent me an email saying that HE has put in a paper to
the Storms Conference on the development of an intense low in the Great
Aussie Bight, that occurred last month (the low... not the Bight).

The  Nairn low was on 8 March 2004.  A catamaran out in it experienced 18
hours  of gale force winds.  Some supporting data and a link to John's
extended  abstract for the Storms Conference are at

http://www.sa.bom.gov.au/~johnn/2004march8storm/

Geoff Feren
Hi Harald/Jon et al,

Just some further comments on the event:

(1) I can't answer Harald's question re the frequency of lee cyclogenesis N of Tasmania. Certainly it is far more common to see the approximately east-west orientated trough over Bass Strait (with south to southeasterly flow to the south, and northwest to southwesterly flow to the north) without any clear indication of an embedded cyclone centre. I became familiar with such lee wave cyclogenesis developments after examining Type 3 east coast low developments many years ago.
Holland et al (MWR, 1987) identified these particular systems as the small-scale, potentially very intense lows which tend to propagate northwards along the NSW coast on the western flank of a synoptic scale Tasman low. In some of these cases, it appeared that the initial cyclogenesis occurred over eastern Bass Strait within these east-west orientated lee
troughs. It's possible that such developments were more likely in sea surface temp patterns characterised by large SST gradients between a warm eddy off the east coast and the cooler water to the west.

(2) Richard Russell has created a link which displays the wave height
output for 3 MESOLAPS runs during/immediately prior to the event -

http://serva.vic.bom.gov.au/rgn/rfc/archivemainseachart1.htm

Again, Kingfish B (38.6 S 148.3 E) recorded a maximum sig wave height of  just over 7.0 metre at approximately 3am 24/4 (17Z 23/4). Thus, the preceding 00Z 23/4 model run showed sig waves of approx 4.5 metres there at that time, peaking at 5.0 to 5.5 metres at KFB at 7am. Progged sig wave heights peaked at 6.0 metres further to the northeast within eastern Bass Strait.The subsequent 12Z run generally picked up the sig waves by about 1 metre, but the peak of about 6.5 metres in the KFB vicinity was not progged until late Saturday morning/early afternoon.

(3) As indicated in the previous e-mail, one of the most interesting aspects of this event was the presence of some similarities with the 1998 Boxing Day storm.Thus, in both events the main cyclogenesis over eastern Bass Strait occurred after the initial trough/low pressure zone had already move out to the Tasman Sea. The following displays the thickness anals (extracted from the archives -

unfortunately, formatted differently) for both events immediately prior to the severe eastern Bass Strait developments. Note that the thickness contours (red lines) for the recent event are analysed at 4 gpdm intervals, compared with 8 gpdm intervals for the Boxing Day Storm. Thus, stronger baroclinicity was present for the Boxing Day storm, but otherwise the patterns were similar.

Geoff.

John McBride

On Fri, 30 Apr 2004, Geoff Feren wrote:

>
> (3) As indicated in the previous e-mail, one of the most interesting
> aspects of this event was the presence of some similarities with the
> 1998 Boxing Day storm.Thus, in both events the main cyclogenesis
over.........
 

Another interesting similarity between this event and the 1998 Boxing day storm is that there was a lot of rain in the cold front as it passed over Melbourne, immediately prior to the formation of the meso-low.

I know this because the current event was just last weekend, and in the case of the 1998 storm, like many people I sat for most of the day in the rain in the outer of the MCG before I finally borrowed a mobile phone and called the VRO who advised me to go home.
[P.S  Strangely, whenever I call in at the VRO, they still tell me to "go
home"]

Have a good weekend

John McB
 

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