Blair Trewin: Records associated with the event:
To give credit where credit is due, it's actually VRO who have done this - I'm only the
messenger!

I am currently working on a report on the event from a climate perspective,
concentrating on rainfall/temperature records (and non-records) across the high-quality
station network, and combining stations where necessary (in addition to the lists that
various regional people have posted, there are quite a few stations where it is
possible to create composites to get meaningful records from a short-period AWS site).
I should have this ready in draft form, at least, on Monday.

Two interesting things which have already come out of the process of compiling this:

- Mt Hotham (-0.2) recorded the first-ever sub-zero February maximum temperature in
Australia (discounting a few clearly bodgy obs from the Mt Wellington AWS in 1994).
While this is not quite as impressive as it looks - there were few reliable summer obs
from the alpine stations prior to AWS installation in the early 1990s - Cabramurra
(with 44 years of reliable data from two stations) set a February record, so it was
clearly an extreme event in the alpine region

- in addition to the low maximum temperatures already discussed, there were abnormally
low minimum temperatures across a big swathe of eastern Australia north of the main
cloudband - not that many records, but a lot of places which had their coldest February
min for 10-50 years - examples were Alice Springs (lowest since 1954), Longreach
(lowest at airport site, open since 1966), as well as places like Charleville, Bourke,
and Woomera.

Blair

John McBride/Harry Hendon: Influence of Southern Hemisphere Annular Mode (SAM)
Gedday,

I received this from Harry Hendon way back on the day after the event; but with the big pile of emails I received that day, I hadn't noticed it.  For some time now, Harry has been researching the influence of the Southern Hemisphere Annular Mode (the SAM) on Australian rainfall.  Another name for the SAM is the Antarctic Oscillation (AAO).  Indices of the SAM are put up in realtime on one of the NCEP sites:
    http://www.cpc.ncep.noaa.gov/products/precip/CWlink/ENSO/verf/new.aao.shtml

References/backround on the SAM are Mo, J. Climate 2000, and Thomson and Wallace, J. Climate 2000: both are linked to on my web-page.

With that background, the email from Harry is self-explanatory.

Date: Thu, 3 Feb 2005 13:49:58 +1100 (AEDT)
From: harry hendon

you might be interested in this.
sam has just  been at it highest value in at least 6 months (aao spread plot).

high sam means low pressure on the pole, enhanced westerlies at 60S and easterly anomalies
at 35-40S (250uwnd plot).

 

The current cutoff over us formed on the equatorward side of the associated, quasi-zonally symmetric,
 high heights  centered  along about 50S (200z plot). locally, this looks like a classic Rex block
 

It is interesting that the verifying sam value (sprd plot) was outside the ncep ensemble spread even at 7 days, which again emphasizes that this
was a pretty extreme event.

if you believe Thompson and me, high sam in summer equates to wet in the SE.

Harry

Dave Williams : Sheep Loss
   I wasn't going to contribute to this because I happened to have days off during the
event. I've asked some of my colleagues who were on during the event to contribute to
John's question about the convection on Tuesday night. In relation to Blair's comment
about cold temperatures, I think one could also find that the 24hr temperature falls
will be extreme as well. Hay went from 40.3 on the 1st to just 13.0 on the 2nd and 13.5
on the 3rd. The previous record lowest Feb. temp was apparently 17.7, based on nearly
50 years worth of data. Couple this with nearly 50 mm's of rain over 48 hours and it
was a massive contrast. Deniliquin went from 39.9 to 11.7 then 14.0. There was over 80
mm's in the 48hrs from 9am on Wednesday.

    I was most concerned about the sheep loss potential with conditions like this and
mentioned so on the ABC @ Wagga. I got one phone call back and spoke to a chap who was
clearly shattered. He'd lost 400-500 sheep by the sounds of it, though we'd had a sheep
graziers alert out. For whatever reason he'd failed to react, so may be there is
something there for us to consider. I'm not aware of any other circumstances in
relation to sheep losses etc, but did some approximate calculations for Deniliquin that
suggested that the sheep stress was over 300 {extreme] for at least 24 hours. Given
that it was 40 deg. the day before it would surely have to rank as one of the most
significant 24-48 hour changes in weather conditions  ever recorded.
 

    Dave.

James Taylor: New South Wales Convection:
Date: Sun, 06 Feb 2005 16:47:30 +1100
From: jamest

Hi John,

The event for NSW kicked off in the afternoon of the 1st of Feb with widespread convection
along the ranges and southeastern NSW. The triggering mechanism for this convection was the
initial stages of the low as it passed through southern NSW supported by the upper cold air
that was beginning its path through southeastern Australia. The result being an extremely busy
night shift for those concerned as up to 70mm in Sydney's west and around 35mm in Sydney's
east fell as the thunderstorm activity developed into a rain band. Media attention was
unwavering for the SPOC in the hot seat that night (Sean Carson).  This caught us under
staffed as the expectation was for the convection to ease into the evening (mesolaps had no
significant rainfall over Sydney on the previous ooz run and NTFGS eased the surface
convection and midlevel convection into the evening, US had rainfall over Sydney but not to
the extent experienced). Haven't had a chance to look at the reasons behind this in any
detail). Unfortunately a school girl perished when a tree fell on a tent in the Southern
Highlands on the afternoon of the 1st. On the better side of the coin, there was a decent
amount of rain in Sydney's water catchment.

Wednesday morning was the "calm before the storm" (perfect cliche at the time), It had been
about two weeks (January 20th) since we'd seen a day so perfectly primed for severe
thunderstorm activity. Instability was greater (than on the 1st) due to slightly warmer and
more moist air wrapping around the intensifying low. Shears were greater as the upper system
was becoming cut off with the wrapping jet intensifying but probably of most significance was the
deep drier W/SW change expected to move through Sydney later in the afternoon which we
were confident would break the caps associated with the northeast winds along the coast. It is
within these east winds that the hodograph looked so perfect for supercell activity. The
results were magnificent from a storm nut point of view, an approximate pre storm environment
for the central Sydney basin given below:

   * temp ~ 30
   * dp ~ 20
   * wind ~ 040/10
   * CAPE ~ 1160 (sondtool)
   * LI  -3 (sondtool)
   * Cap 3C (sondtool)
   * RI 19 (sondool)
   * Shear surface - 850 21kts (Helindex)
   * Shear surface - 700 38kts (Helindex)
   * Shear surface - 600 42kts (Helindex)
   * Shear surface - 500 62kts (Helindex)
   * Helicity 0-3km - 153 (Helindex)

Initial convection went on the W/SW change and tracked through Sydney in the afternoon. There
was also a NW/NE convergent line that triggered thunderstorms on the northern ranges. This
environment produced the HP supercell inthe following radar image: (more good
photos can be seen at
<http://www.smh.com.au/photography/readerpics/index.html>http://www.smh.com.au/photography/rea
derpics/index.html).
 

The storm was moving at around 85 km/h and the potential for damagedoesn't need to be stated.
Hail to 6cm reported, multiple reports of around 4cm, destructive wind gusts flash flooding
and a possible tornado. The Hunter and Southern highlands also had good supercell activity.
The guidance for this day was on the whole very good. Apart from a fire alarm and preparations
to evacuate the RFC as the supercell was intensifying in the western suburbs, just about
everything unfolded as expected.

It was with a little relief that the media focus moved to mostly to Victoria on the 3rd,
although we were not completely spared as there were some good pictures of trees on cars and
people sitting on car roofs waiting to be rescued etc. Andrew Haigh and myself went for a
damage survey of the possible tornado the following day and we are still compiling the
evidence.  (note the NSW sevwx section has evidence of 4 possible tornadoes this season but at
this stage the evidence is not conclusive, except maybe for the Coonabarrabran event).

As I sit here polishing off some rather boring fire dangers the low is on its last legs and
moving into the southern Tasman. Gotta say that I wouldn't mind if we had more than a couple
of weeks before the next one,

Cheers,
JT.

John McBride (Replies by Blair Trewin and Kevin Tory)

MINIMUM SURFACE PRESSURE

I'll start with a few random thoughts: At lunch yesterday Harvey Stern quoted some figs on the
value of surface pressure reached in Melbourne in the middle of the event (around 984 hPa)
Harvey????...... which apparently was close to a record.
Blair: Lowest 3-hourly pressure was 985.2 (I'm using 3-hourly so that like can
be compared with like, since that's the data we've got on the historic
events). There have been three events with lower pressures in Melbourne,
all of them in winter/early spring (lowest 982.5 on 12/7/1964). It's the
first sub-995 (!) ever recorded in February.
Hmmmmm.... excuse my blissful ignorance... but what are "AEP stats?" and what does 1-2% mean
in this context?
Blair: Not being a hydrologist I've probably used the terminology wrongly, but what
it (Annual Expected Probablility) means is a rainfall with a 1-2% chance of being
exceeded in any year (or what gets referred to as a 50-100 year event).
 

WIND AND TREE DAMAGE

Does anyone in the Bureau keep stats on the magnitude of these events in terms of projected
insurance payouts, number of calls to the SES etc? One of the reasons I ask is because my
impression was  most of the emergency in Melbourne was related to trees falling down, rather
than rain and flooding...Is this correct?   If so, sure there were high wind gusts, but I
didn't think the wind was anything extraordinary compared with other major wind events.
Possibly it was the combination of rain (soft soil) and wind that brought the trees down....
but as I am a meteorologist and not a tree expert.. this is pure speculation.
Kevin Tory: Just a trivial comment regarding trees uprooted.  I came to the same
conclusion as you that the winds weren't particularly high, but the soil was
significantly softened.  At my place we didn't have any tree damage, but all
my garden stakes were leaning at 30 degrees from vertical when I got up on
Thursday morning.  The clay soil was saturated and responding as a liquid. I
noticed in my little survey down the Yarra near Rosanna, and on my way to
work via Heidelberg, Ivanhoe, Fairfield, Clifton Hill, Carlton etc. that it
was mostly the non-indigenous trees that had been uprooted (each very old
and well established).  The damage to eucalypts seemed to have been mostly
branches snapped off.
Obviously the snapping off of branches can not be blamed on soft soil.

RAIN RETURN PERIODS
Next: the Melbourne record rain of 129 mm for 24 hours is interesting in that , to me , its
not such a high amount.  We received over 100 mm at our place in a couple of hours during a
thunderstorm event back on 2 December 2003.  Admittedly this was in North Melbourne and not at
the official Melbourne site; but there must be some sort of spectral gap in rain-return periods:  the 1 in 100 year event for 1-2 hour rain is associated with convection and is of
the order of 100 mm or so.  The 1 in 100 year event for 24 hour rain is associated with
large-scale stratiform and is also of the order of 100 mm ........Do the hydrologists out
there have any comment on this?
Blair: Grant Beard's working on calculating daily district averages for
districts 86 and 87 which may tell us an interesting story.
Back-of-the-envelope estimates suggest that, in terms of broadscale
events, the February 1973 event was more extreme west of central
Melbourne and the December 1934 one more extreme to the east, but that
Melbourne City is in a relatively narrow window between the two, too far
west to get the best of 1934 and too far east for 1973. This may explain
the (relatively) unspectacular AEP stats (1-2% from what I've heard).
Also fair to say that the CBD's never got 'lucky' with a long-lived
thunderstorm (Laverton has had 188 mm in a day under such conditions).

Prior to this event, there had been three other thunderstorm events
which had produced 100mm+ daily falls in the Melbourne region in the
last 14 months - the one John refers to, one in late January 2004 (I'm
not sure if this one cracked three figures at any official gauge, but it
certainly did at Mike Coughlan's house) and one in Geelong a couple of
weeks ago.

We're going to try to get an AMOS Melbourne Centre meeting/workshop on
these events up and running, hopefully sooner rather than later - maybe
March or April.
 

NAMING OF THE STORM

Next:  What should we call the storm?  It was Groundhog day, it was World wetlands day.  It
was Claire Yeo's birthday.  We could call it the "Wet Claire Groundhog" storm; but surely
someone has a better suggestion.

WHERE's GEOFF

Last:  Where are the storm experts: Geoff ONE (Callaghan).  You love classifying storms...
What do you have to say about this super low? Geoff TWO (Feren):  Surely you have something to
say about where the development occurred with respect to the location of the jet maximum and
the eternal significance thereof.
 

As I said, I'll put up some charts later and try to understand the dynamics a bit. Someone
earlier mentioned this being a "Mills reverse shear situation"... What does this mean?

cheers

John McB

Jeff Callaghan: Reverse Shear event
Date: Tue, 08 Feb 2005 09:35:13 +1000
From: Jeff Callaghan
Subject: WHERE's GEOFF

John
I sent the reverse shear email on the 1st-see below
It turned out like they forecast i.e. the warm ridge over Tassie and the cold low over Vic.

All the xxra in Vic came with onshore warm air advection type flow between these thermal areas
eg winds at Melbourne 9pm 2nd backing from 200/51 at 943hPa 190/47 at 850hPa to175/47 at
700hPa 155/41 at 650hPa and 120/39 at 500hPa
At 3am 3rd still very strong WAA 220/46 at 943hPa 200/53 at 850hPa to 145/47 at 700hPa 140/62
at 650hPa

The pattern of development was very similar to the Syd/Hobart storm with a very strong
tropopause undulation over moving over Melbourne with Temps as warm as -41 at 200hPa I think.

But what about the explosive development of Harvey yesterday
Jeff
> Date: Tue, 01 Feb 2005 09:04:53 +1000
> To: synoptic_discussion@bom.gov.au
> From: Jeff Callaghan
> Subject: reverse shear event Bass Strait
>
> Looks like the EC and the UK are going for one of Graham Mills' reverse shear events i.e the
> 700hPa  ridge from the UK SE of Hobart and the cold low over western Vic.at 0000UTC Thursday
> Jeff

Gary Weymouth:  Rainfall Records, tree-damage, strength of barocline, reverse shear

Hi John,

Some quick comments hopefully not too obvious:
- 100 mm rain is not unusual over a small area.
  The area covered by ~100 mm totals last week was unusually large.
  Hence flooding of southbank, tram lines etc.

- dry (warm?) ground (& vegetation???) means less flooding,
  though melbourne water claimed the biggest 1 day rise in water storages
  (possibly a mistake or maybe they meant 'in February').
  Also rain rates were mostly fairly low (compared to a heavy thunderstorm),
  so soil could take up more moisture and drains had more time to cope.

- a lot of trees in my area lost branches; one had the trunk snapped.
  so not just soggy ground.  Interestingly a tree which survived the storm
  fell over last night.

- the barocline preceeding the surface low development was one of
  the more spectacular I've seen near Melbourne.
  Graham Mills says it was 'right out there' climatologically speaking.
  he mumbled something about wanting to recover similar events from
  a previous archive he has.

- I think the reverse shear thing goes something like this:
 warm low level air gets wrapped around the back of the low,
  underneath the 'warm' upper air (potential temperature terms).
  This tends to reduce pressure and hence pressure gradient
  immediately to the west of the low, thereby increasing the
  pressure gradient further west.
  So the low can have a somewhat broad centre and very strong
  low level winds to the rear (west).
  The mesoscale models went for this.
  I think the strong winds occurred a little earlier and further
  east than expected - but pretty good forecast nonetheless I think.

cheers,
Gary
 
 
 
 

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