From Milton Speer (afternoon of 2 January)

Here's Hank's site for generating archived temp traces:
http://serva.sa.bom.gov.au/cgi-bin/extract_skewt_from_adam.pl

I think the vertical mixing down from mid-levels sums it up pretty well in this situation and in 94.
If you go to, for example, 2300UTC 29 Dec 2001 for Sydney from this site, (I  just happen to
have a hard copy in front of me) you'll see the dry adiabat extends above 700 hPa with potentially
negative dewpoints at the surface. A better one to look at would be an afternoon trace
(yesterday's,which would be a good example, doesn't look like it was done) when the
effect of turbulent mixing from the surface to above 700 should be evident. If the low
level winds are not strong enough, the surface inversion hangs around particularly near the coast as
seabreezes or weak changes affect that area.  If the synoptic scale low is too far west or
southwest of Tasmanian longitudes the pressure gradient opens out along the NSW coast and the
low level winds may not be as strong as expected. This may also allow Bass Strait ridging to
turn the corner and produce shallow, short-lived southerly changes. This occurred last
Sunday and kept temperatures down along the coastal fringe in Sydney. Even when the
main synoptic low moves through Tasmanian longitudes Bass Strait westerlies turn the
corner and affect the NSW south coast, as they have today and as they did in January 94
(Fig.4 of the ref. in my previous email).

M

From John McBride

I had another chat with Graham Mills (who, by the way, is becoming the
Guru_murrumbidgee of this discussion group).

We were speculating on the current situation in the context of what are the
conditions leading to major fire events in New South Wales/Sydney.  My feeling
is that the major Victorian fire events are relatively short-term situations
whereby we have very strong gradients with hot dry northerlies in advance of a
rapidly moving summertime front.

For the current Sydney fires situation, however, the extreme fire days seem to
be only one or two days through the sequence: e.g Christmas eve/Christmas day
and yesterday (2 January).  Those days have the extreme north-westerlies and
zero dewpoints.  They get the fires going over an enormous areas.

Once going the fires never stop as we have sustained strong winds that are dry
because they are westerly. These westerlies go on for day after day.  So, even
though the temperatures are not all that high, the winds never stop.  Thus the
Vic fire situations are 1-2 day events, whereas the current Sydney event (and
presumably the 94 event) seem to be much more sustained synoptic situations.
 

Any comment? Is this correct?  Does anyone out there have the knowledge or
skills to pull out a time series of surface winds over the past couple of
weeks over the fire region.

Cheers

John McB

From Milton Speer

John and all,
That's basically right.
There was also another acute fire danger period in early December 97 or 98. It was
shorter- I think about a total of 3 or 4 days but roughly similar synoptics to now
and 94.

There are some time series of winds floating around up here - I'll see if I can
locate them in due course.