Whatever is causing all the rain over Sydney/Canberra region is a pretty
weak system:  From the charts (surface and upper) I can spy on the web
from here in Head Office, it is difficult to see anything much, not even
any upper low to speak of.

Are there any comments from those out there with some combination of
better insight and better analysis tools?

JMcB

Phil Davill

Hi John,
 Looking at both the LAPS and US (NCEP) runs on Monday both had a
significant upper low (from 700 to at least 400hPa) over NSW at 12Z
(confirmed eventually by the 12Z wind flights). Some convergence was
forecast by the models below 700 and with divergence above 400 over
eastern NSW.
Both models therfore had good upmotion over eastern NSW from 700 to 300
and consequently were forecasting significant rainfall Monday night.
What is interesting is the  US moves the action to near Coffs Harbour
Tuesday with rainfall totals for the period 18Z 4/02/02 to 06Z 5/02/02 in
excess of 75mm. The Laps does a similar thing but 6 hours later and the
meso laps goes for even more. All interesting stuff unless you are a
forecaster in Sydney.

John McB
Thanks mate.... I don't think my question is as stupid as it looks... I
just looked at the charts on the web; and indeed there is a good closed
low at 500hPa right now... e.g.  Go to DIFACS, charts, small tropics.. 50
hPa...

However, the chart we had available yesterday morning (i.e the one for 12Z
3 February) had no sign of any cut-off, and in fact had anticyclonic
curvature at that level.  Even now, the current LAPS run has no sign of
any low in the thickness patterns on the "all-Australia" display.

Nevertheless, as you say the models all picked it up nicely... I still
find it interesting though: going by the charts we had at the time,
this upper cutoff suddenly appeared from nowhere. However, the information
must ahve been there in the analysed fields as the GASP prog got it on
the 48-72 hour prog (see Beth Ebert's rainfall verification page... Under
BMRC internal web, Model Development Group, Experimental, Daily NWP
Rainfall verification.

cheers

John McB

Noel Davidson

 HEAVY RAIN : An interesting aspect of heavy rain situations is that
they often develop in relatively innocuous
looking environments. The reason for this is not absolutely
clear but based on some work we have done in the past, it seems
that while the large scale acts to de-stabilse in the vertical
and horizontal (conditional and inertial instability), the convection
/ moist processes act to stabilise (upper level warming, low level
cyclonic tendencies). Because of this cancellation, the net result
(what you see on the weather maps) can be very little. Models can
provide very useful forecast guidance for these unusual events
(not the DETAILS of rain amounts and locations), provided the large
scale is depicted reasonably accurately. Forecasting the details
needs work on mesoscale assimilation and initialisation
(and moist processes).

John McBride

Noel,

Thanks for the vote of confidence...????

Anyway, not sure you are right about heavy rain events in the tropics.
What you say seems right for mid-latitude upslide situations, and even
for some heavy rain from slow-moving MCC developments.  My feeling, though,
in the tropics is that there is usually some synoptic feature: e.g. the
100 km by 800 km 850 hPa jet we can see in the Jakarta flood situation....
Comments?

JMcB