From:
John McBride <jmb@BoM.GOV.AU>
To: synoptic_discussion@bom.gov.au
Subject: [synoptic_discussion]
Yesterday's Gold coast severe storms
We had a quick discussion
of these, alerted by John Colquhoun, while they
were happening; and I put
the radar images and some sounding on my web
page.
I saw on this morning's news
a NSW State Emergency Service Spokesman
saying there was very little
warning, about 1 hour: Does anyone have any
comment on this.... Did
the MesoLaps progs predict the storm environment?
JMcB
Terry Hart
Hello all,
I just had a quick look at
the +24 hour decision tree forecasts for 00
and 12 UTC yesterday. The
guidance has indicated severe TS and
flash-flooding at both times,
and micro-bursts at 12 UTC. I have not
looked at intermediate times,
but at first glance, the guidance seems to
have given a good indication
of the event.
Cheers,
Terry
Rob Webb
Hi John
I'm interested in the very
little warning comment and have contacted our SES
to find out who said what.
We put our first advice out for nearby areas at
10:30am at extended it to
the area in question at 1:23pm. Given the storm
did its worst damage just
before 4pm and we rang the SES division in
question once we could track
an individual storm updating them on its
position and severity for
the 2 hours leading up to it doing it's worst
damage I actually thought
we had given them a pretty good service as far as
TS go.
There was good discussion
across the border re severe weather reports and
warning/advice products.
(Thanks to Tony Wedd of QLD RO)
I'm not sure how the Queenslanders
saw the synoptic situation but everything
started to blow strongly
from the ESE ahead of the storms even to the point
where some damage was reported
in the inflow. There seemed to be some kind
of small low there helping
things along but I can't be sure. Maybe we'll get
a chance to look at it again
sometime.
Thanks
Rob Webb
Ted Williams
G'day John, Rob and others,
I too heard the SES commenting on
the warning period. It certainly
wasn't the main thrust of the interview
(which was more like how
many small babies were torn apart and/or battered
with hail).
I do recall mention of the
SES knowing something in the morning but followed
by the comment about liitle
warning (possibly this was regarding specific
storms).
Cheers
Ted
John Colquhoun
Terry et al.,
The decision tree severe
thunderstorm guidance for the afternoon covered
the storm area well over
NE NSW. However, there were no forecasts of
supercell thunderstorms
produced by the decision tree guidance. The
radar PPI evolution was
consistent with the occurrence of a high
precipitation supercell.
Graham Mills has indicated that the model
output showed low storm
relative helicity values. Lower in magnitude
than needed for a supercell
forecast.
There may have been several
factors contributing to the probable
underestimate.
- The mesolaps near surface
winds were too light. The wind at Casino, to
the west of the storm centre,
just before the storm was ENE 9 kt. On the
coast to the east of the
centre, they were ESE at 25 kt at Ballina
and ESE 46 kt (no typo)
at Cape Byron. A band of strong to gale force
winds moved northward along
the coast to the east of the storm. The near
surface winds feeding the
storm are very difficult to estimate.
- The model midtropospheric
winds may have been too light. They dropped
(in LAPS, I cannot access
meso laps) from more than 30 kt at the 00
analysis time to less than
30 kt at 06 UTC. In the storm environment
they were 30-40 kt in the
mid-troposphere.
- The decision tree storm
velocity was probably in error.
Press reports this morning
indicate the occurrence of a "mini tornado"
and "hail the size of a
tenpin bowling ball".
Sincerely,
John Colquhoun
Harald
Richter
John (C., not MB) et al.
(includes MB),
> ...The wind at Casino,
to
> the west of the storm
centre, just before the storm was ENE 9 kt. On the
> coast to the east of the
centre, they were ESE at 25 kt at Ballina
> and ESE 46 kt (no typo)
at Cape Byron. A band of strong to gale force
> winds moved northward
along the coast to the east of the storm. The near
> surface winds feeding
the storm are very difficult to estimate.
I suppose these strong winds
are associated with a convectively generated
surface low rather than
being a pre-existing environmental flow?
I think numerical models
aren't quite capable of modelling inflow band
strength and orientation
yet. We would have to get the storm scale convection
intensity, timing and placement
right first.
But from what I remember,
even the pre-storm environment seemed to suggest
sufficient deep-layer shear
for supercells (see one of my previous posts).
> Press reports this morning
indicate the occurrence of a "mini tornado"
> and "hail the size of
a tenpin bowling ball".
Geee, I am glad they didn't
have to go through a dangerous real tornado ... :)
A "tenpin bowling ball"
hailstone would surely set a world record, would it
not?
Did the "[del] tornado" occur early in the lifecycle of the storms?
Harald
John Colquhoun
> I suppose these strong
winds are associated with a convectively generated
> surface low rather than
being a pre-existing environmental flow?
One suspects that it was.
However, Ballina was about 35-40 km to the
east of the centre of the
storm and a convectively generated low this
scale seems rather large.
> I think numerical models
aren't quite capable of modelling inflow band
> strength and orientation
yet. We would have to get the storm scale
convection
> intensity, timing and
placement right first.
> But from what I remember,
even the pre-storm environment seemed to suggest
> sufficient deep-layer
shear for supercells (see one of my previous posts).
>
You are right. However, is
shear alone sufficient? The decision tree
incorporates BRi, which
includes shear, and storm relative helicity,
thought necessary to produce
updraft rotation.
> > Press reports this morning
indicate the occurrence of a "mini tornado"
> > and "hail the size of
a tenpin bowling ball".
>
> Geee, I am glad they didn't
have to go through a dangerous real tornado ...
:)
> A "tenpin bowling ball"
hailstone would surely set a world record, would it
not?
>
Perhaps, if the report is
accurate. The Australian on April 22-23 1995
has a report of a tornadic
storm in China that killed 37 people, some
killed by hailstones. It
reads in part ""In Huizhou , we had hailstones
as big as basketballs,"
the official said. The heaviest was found in
Zhaoqing City weighing 15
kg." Ludlam (Clouds and storms, p 292) records
the observation of an English
missionary at Yuwu in the Chinese provence
of Shansi of two hailstones
that weighed 7.5 catties (4.5 kg) each.
> Did the "[del] tornado" occur early in the lifecycle of the storms?
This seems to have been near
Cudgen, which is near the coast. So it was
late in the storm's life
over land.