John McBride:

I  believe there were some very heavy daily and/or six-hourly totals along
the Queensland coast yesterday.  Anything to report or any interesting
meteorology to discuss?

Was it widespread or very localised?

John McB

Andrew Watkins

Always good to check out the hydrology pages... they're tops, and allow you to zoom in and get point totals etc (at least
for Qld). Been very handy for monitoring recent storms etc:

http://www.bom.gov.au/hydro/flood/

From that page:

Jeff Callaghan

We really have had an amazing week of severe wx. up here. It started with the establishment of the block over SE Oz on Sat 24 placing us in mongrel  hot humid N to NW winds for a week. This  gave us our own groundhog day  weather pattern with dewpoints reaching up to 25C prior to our daily severe thunderstorm in Brisbane and surroundings. There were 3 storm related fatalities with one child in a critical condition. Tornadoes were sighted  on the 29th and 30th while 400 hectares of state forest were destroyed by a storm on Australia Day at Toolara (near Gympie). The heavy rain developed  overnight Monday/Tuesday with most problems in the Bundaberg Hervey Bay region. This rain developed in a region of very strong (dare I sat it) warm air advection ahead of an upper trough and a small low resulted  just east  of Brisbane early yesterday morning. The low was well located east of 160 last night with gales evident so it really rocked along. The models picked
this low development well and gales warnings were in place well before it developed.
Jeff

 Barry Hanstrum
John

The new thunderstorm diagnostics have been providing useful guidance on the severe weather up in Brisbane and also on the heavy convective rain potential, see

http://web.bom.gov.au/cgi-bin/nmoc/srod/tsdiag/thunder.pl

then go to view archive and select the date you want to look at.  By  clicking on the map you can zoom in on the state,  by clicking on the arrow button above the maps you can see what type of severe weather is being  forecast, by clicking on the arrow button above the weather elements you can see a time series of the particular element.

Let me know if you want me to talk you through it.  Given this is National  and web based it provides a useful place to start a discussion because everyone has easy access.

John McBride

In response to my note from earlier today, a few people have alerted me to
the mesoviewer.

This is actually terrific.  By playing with this, you can produce nice
maps of CAPE (for example) for each period of the forecast, overlay
surface dewpoint on the CAPE map (thus showing in the tropics, they are
largely rescaling of the same parameter) and so on.

By playing around with such maps (e.g of CAPE, of each day, we can
possibly learn a lot about how tropical convection and rainfall evolve,
what are the controlling parmeters etc.

If anyone makes much progress with this, and/or sees anything interesting
as they play with the meso-viewer each day, please inform the rest of us
in real-time; so we can evolve together in our understanding of tropical
convection

On a related issue,  the "National Thunderstorm Forecasting Guidance Page"
(  http://web.bom.gov.au/cgi-bin/nmoc/srod/tsdiag/thunder.pl  ) includes
an archive.  I went back and looked at the guidance for the 1/100 year
convective rainfall event for Melbourne on the evening of 2 December.  The
heavy convective rainfall maps for Victoria were blank.  Did anyone look
at or think about this failure of the Meso-viewer algorithms at the time
any insights?  (Though I know December 2 was a while ago.. "Bridge under
the water", as a friend of mine says)

Kevin Parkyn
Hi John,

Re: Flash Flood event Melbourne 2/3 Dec 2003

Because the Mesoviewer has been designed around an ingredients based
approach, certain thresholds need to be reached before it starts to show
areas (pixels) of severe weather. Nevertheless, there are many positive
aspects to the Mesoviewer, one of which is how Mesoviewer was developed
(a sensational collaborative approach!) which allows forecasters to
examine the diagnostics and appreciate why or why not thresholds have
been breached.

Your assessment of the Melbourne Flash Flood event is correct -
Mesoviewer failed to highlight flash flooding as a possibility. We're
aware of why this is the case, which is largely due to the mixing ratio
needing to be greater than or equal to 12 g/kg (over the lowest 50hPa).
This criteria might be suitable for other parts of Australia, but it not
appropriate for Victoria. Part of the developmental process of
Mesoviewer will involve a workshop to assess its performance since it
being introduced operationally in November.

For what its worth, in VRO we examine precipitable water values (amongst
other things) for flash flooding potential from deep convection, which
has been a highly reliable indicator in Victoria over the years.

Cheers........Kevin

 Barry Hanstrum
Hi John

You wrote..
 

      On a related issue,  the "National Thunderstorm Forecasting Guidance Page"
      (  http://web.bom.gov.au/cgi-bin/nmoc/srod/tsdiag/thunder.pl ) includes
      an archive.  I went back and looked at the guidance for the 1/100 year
      convective rainfall event for Melbourne on the evening of 2 December.  The
      heavy convective rainfall maps for Victoria were blank.  Did anyone look
      at or think about this failure of the Meso-viewer algorithms at the time
      any insights?  (Though I know December 2 was a while ago.. "Bridge under
      the water", as a friend of mine says)
 
 

The thunderstorm decisions are in two parts:

Firstly we look at surface driven convection using the average temp and mixing ratio in the
lowest 50 hPa, assuming this to be representative of "surface" conditions.  This works fine for
situations when the near surface boundary layer is well mixed and coupled to the layers above.
As you might expect there were no signals in the surface driven ts for the 2 December overnight
event.

Secondly we look at the occasions when storms are kicked off by lifting above the surface.
This was the scenario for the 2 December flash flood.  Essentially when nocturnal cooling of
the boundary layer occurs or when there is some shallow inversion layer  the surface indicators
are more or less useless as predictors for TS development, so we need to look above the surface
for indications.  In the current ts diagnostics we look for upmotion between 850 and 700 hPa
and lift parcels from 850-650 hPa to see whether we can find some instability.  Using this
method the model did paint ts signals indicated for the 2 December event, my memory of the
detail is a bit scratchy but the signal was present over Melbourne in the overnight period.
(These are the yellow regions of ts potential, green areas represent sfc based storms).

With regard the flash flood criteria these are currently only triggered if conditions for
surface thunderstorms are met, so in the case here where storms were being picked up by lifting
above the surface no calculation of the relevant parameters for flash flood forecasting were
made.  I believe the same thing happened with the most recent overnight Melbourne flash flood.
When we set up this scheme we had in mind  mid-level ts as being  much higher based storm
activity that occurs fairly frequently over much of the interior of the country, typically
these events have a relatively dry boundary layer and flash flooding is not an issue.
On the strength of recent events we may well need to do some tuning.

We're holding a National Meeting in Melbourne in late May where all of the regions will be
coming to Melbourne to discuss the verification of the performance of the ts system over
summer.  One of the things we will do in the version for next summer will be to connect the
flash flood decisions to ts forced from above the surface.  We also need to have a good look at
the criteria we use for alerting for flash flooding which were taken from the Colquhoun
decision tree using criteria derived from US flash floods described  in a paper by Doswell.
Comments greatly appreciated.

Cheers
 

Baz

Helen Pearce
 I think they were also blank for last weeks floods in Hawthorn

These storms were very slow moving and the rainfall may not have been above
the threshold   ( i.e. not cause the extent of flooding ) set for elsewhere.

Regards
Helen

Milton Speer

The flash flood tropospheric environments should vary quite a bit even just around southeastern
OZ.
In Sydney the severest ffs occur in nearly saturated surface to midlevels with generally light
winds (Speer and Geerts AMM 1994). In areas of high topography near the coast such as
Wollongong ( e.g.  Tech. Report by Julie Evans et al.) and Coffs Harbour (Speer and Leslie IJC
2000) they are generally low level wind driven but also nearly saturated surface to midlevel
environments. West of the the ranges the lifting  mechanisms would be at different levels
again.

Milton

Jeff Callaghan
We also have flash flooding resulting from various effects. On one hand we
have the purely convectively driven events eg Cooyar Flash flood event (Bom
report Oct 1990) where winds were very light up to 600hPa  and on the other
end of the scale we have embedded convection in strong warm air advection
type wind profiles. eg The Townsville flood event of Jan 1998 the winds at
Townsville backed from easterlies at low levels to NNE at 700hPa to
northerlies at 500hPa. This event produced 120.6 mm in one hour and 205.2
mm in two hours An extreme example of this type was the Mackay 1958
tropical low where Mt Pelion recorded 292 mm fell in 2.5 hrs and 589 mm  in
6.75 hrs. Most of our disastrous flash flood events  are the warm air
advection types
Jeff

Jin Lee

Hello,

Just to add a few comments:

Barry was right that one of the surface based-convection criteria for
flash flooding wasn't met in this case. Specifically MLAPS produced too
much CIN at 12Z 2/12/03 (current threshold is 25 J/kg) because of rapid
low-level nocturnal cooling.

As others have pointed out during the postanalysis of the event a
southwesterly surge moved through Melbourne (plus other complicating
factors) and was enough to overcome low-level inversion. MLAPS captured
this change and produced very strong low-level upmotion (between 0.9988
sigma to 850 hPa). Unfortunately the TS decision currently used does not
couple upmotion and CIN, and so even if there is a strong enough low-level
forcing if the CIN is > 25 J/kg parcels won't be able to convect. This is
something that Barry and I have been discussing but haven't quite got
around to doing anything, yet.
Cheers,

Jin