Some I would like to make a proper note of:

At a glance, the pressure could have dropped on the 17th, heading into late spring-early summer. As it stands, it appears to have been the highest-recorded pressure since late-August last year (2010). In other words, it looks like it was higher than the winter max. If anything this possibly represents a departure from what might have been expected to occur (the pressure continuing to fall, steadily, as it had been doing since the 23rd of August, this year). This little 1029 hPa on the 17th is a little out of sequence unless one assumes that (a) the pressure is going to remain relatively high for perhaps a few weeks to a month, or (b) the trend of the pattern of the last two years will not continue as envisaged into summer – that is a seasonal variation are per usual (summer, autumn, winter, spring).

But then again there was also another blip in the time series, circa 7th-12th March 2010, when the pressure decided to buck the prevailing trend and first head south (1002), then north (1029, again). This one (17th) is a little more dramatic, but the “principal” of bucking to trend is similar. Assuming the current trend continues and the 1029 is statistical aberration (not sure about that), we’ll be at summer-type pressure by December (per usual). Assuming the 1029 is not a statistical aberration (quite possibly), we could be looking at a relatively mild-to-cool period of perhaps 2-3 months (that is moderated, sultry conditions) (nearing the end of January).

Update:

The pressure is now around 1005. The maximum pressure last summer was about 1018, so well below the summer maximum. However, this is not the middle of summer, even if winter has officially passed. Another interesting observation – yesterday’s max: 30 degrees with sultry conditions…that is, warm and extremely (I would go so far as to say possibly unusually) humid. The surface-heating was immense, even though other factors such as convection were somewhat less.

This is the middle of spring, and apart from a few blips here and there, one does not necessarily expect to see blips of this kind (pressure, humidity) on a regular basis.

The ants (same species) are continuing to build their nests (they have increased in size) in different places…some hundreds of metres apart.

Ants will do what ants do (find food and build nests) .

...some forecasts are suggestiing rather heavy falls in Central SA over the next 24 hours, and more so the next 48 hours.

Make that 72 hours in the Mallee...severe storm(s) went through causing considerable damage nearer the border, made the news last night (ABC).

Streamflow has remained about stagnant for the past few weeks, barely changing at all (neither significantly rising nor falling). The humidity has also been reasonably high with some atmospheric instability. There was an electrically-active thunderstorm yesterday which dumped half a mm in the area in about 5 mins late morning, but it was enough to affect runoff noticeably. Conditions have been conducive to rainfall on and off, with some thundery showers during the afternoon on the day of the wild weather in the mallee, but still any rainfall to date (this month) has been mediocre. The upper soil is still quite wet and dark in texture (I suspect clay-loam), the lower layers a little drier and lighter in colour (sandy).

I’m not sure that this summer is going to be particularly dry and hot, as while temperatures have been quite mild (hovering around 30 degrees of late), the fact the river level has not changed much and the high humidity with those temperatures (as well as the instability) seems to mean there’s still a lot of antecedent moisture remaining in the catchment – antecedent meaning from previous rainfall events. This period leading into summer normally starts to get a little wetter. Although that is not a guarantee of a wet start to summer, it happens often enough on an annual basis to have some confidence in it.

The green growth in the area is also prolific – grass has/is been cut and paddocks readied for summer. It’s a little reminiscent of this time last year.

Ok, the barometric pressure has risen about 10 hPa overnight (from 992 to 1002 - it had been well above 1000 hPa for some time before dropping), so there is now an increased chance of thundery activity or rain. Temperature is currently 16 degrees, with 82% RH and light drizzle.

Make that a flat 16 degrees all day Saturday until around dusk, with light to fine drizzle/rain (trace recorded). Vegetation growth is still strong.

Could have put a date on that rainfall probability, but given many projections are indicating many different things, I figured give it a week from the time of the post...at least then it will include the expected coming period of mild-to-cool conditions (3-4 days).

Re: GFS 00Z 25th of November 2011.

Sub-Note:

Is reminiscent of (if there is anyone who can recall) of the morning of the 8th of December 2004.

http://nomad3.ncep.noaa.gov/ncep_data/index.html (under forecasts)

GFS High Resolution (0.5 degree, 1 week): plot.

21st of November 2011, gfs_master_18z.ctl, Map

PWATclm, 00Z 25th of November 2011, Aust

Plot (opens a separate window)

Mid-to-late Spring, 2011:

Taken 17th of October (~ 9 am), relatively high pressure.



Taken 17th October (~ 3 pm), looking north-northwest on the eastern side of the Onkaparinga River.



Taken 21st of October (~2.50 pm), looking upstream…strong vegetation growing in riverbed.



Taken 21st of October (~2.50 pm), looking downstream.



Taken 21st of October (~3 pm), looking southwest. Some long grass in surrounding paddocks. Dense vegetation growth near river.



Taken 28th of October (~ midday), looking northwest. Turbulent skies, something might be developing.



Taken 28th of October (~ midday), looking northwest. Turbulent skies getting closer.



Taken 28th of October (~ midday), looking southwest. Looks like it’s sliding southwest.



Taken 5th of November, looking upstream…even stronger vegetation growing in riverbed.



Taken 5th of November (mid afternoon), looking southwest…virga.



To be continued...

Looks like it's going to cool down a bit over the next few days (below ~20 C). Pressure is currently 1020. Precipitable water could well peak in that time .

If anyone gets a chance, see if the precipitable water from 00Z 18th of November 2011 to more recent runs for time: 00Z 25th of November (map of Australia) can be compared in the link provided above. It looks pretty solid from here.

The heaviest rain (so far – this system appears to be petering out circa 1:30 pm) was either early in the morning yesterday when the humidity peaked, or this morning at around 11:50 am, when we recorded 1 mm in 5 mins. Pressure is still relatively low (1006, 12:30 pm)…

Depending on the conditions, a moderate-to-significant rise in the atmospheric pressure following a fall to reasonably low pressure (say between 1000 and 990) would constitute a change in regional patterns (that is this area of the Adelaide Hills, where rainfall within say 1 km has a very sharp variation – just yesterday around 1.5 mm fell in a short period just 2 km east (BoM), we barely got 0.3 mm). A change in pattern usually means either the onset of heat-wave conditions, or rain. In this case it was rain. It didn’t stop raining yesterday…apart from a brief spell around midday. The humidity picked up, temperatures between say 15 and 25 degrees, maybe a little lower yesterday (18)…currently 20 degrees, mildly humid (65%), 2 pm.

All up, 15.1 mm to 1 pm today.

Rain can also occur directly in a pressure drop, particularly the more dramatic ones (to less than say 985), or with a trough present and much near-surface heating, however those changes are more associated with the passage of fronts and the development of the north-west cloud band.

Near-surface heating is particularly important when the humidity is lower (say below 70-80%) as it provides the local latent heat flux necessarily to kick off sporadic showers (1 mm in 5 mins).

In terms of the effects on runoff, there was enough rain yesterday that puddles did begin forming, but while vegetation growth is retaining some moisture, the soil is becoming more and more visibility parched – whether this is due to drier conditions or more solar heating and sultry conditions I would say is open the question. The river is certainly higher than I would have thought for this time of year, still flowing. The humidity is definitely palpable.

In terms of streamflow we’re talking weeks to months for things to eventuate – that’s about the span of the antecedent conditions for soil moisture from each rainfall event (that is, assuming it doesn’t evaporation too much or the humidity fall to 10-20%).

Immediate, significant changes in streamflow will only happen if these antecedent conditions have been significant enough for long enough, or if there is a significant thunderstorm dropping say 20-30 mm in an hour or so.

Three keys variables to keep an eye on every now and then could be precipitable water, latent heat flux and 1000-500 mb thickness.

The first gives the potential moisture for rain, the second relates to evaporation and convective activity and the third is more associated with fronts and cut-off lows.

In this region it is possible the actual evaporation could be estimated from the local maximum and minimum temperatures, the precipitable water, from that pressure and the atmospheric pressure…the thickness relating the scale height of the atmosphere through the pressure and temperature. The regional wind direction (observations, not measured) also has an impact on the chance of rain, and in which season.

Assuming all else remains the same (fixed), especially the pressure (Force per square metre of area) as the temperature falls, the density (kilograms per cubic metre) increases (due to humidity, I think I’ve got that right?), and the precipitable water then increases and the air can retain less moisture. This forces the humidity up further. The trigger for this fall in the temperature is a drop in the latent heat flux (less evaporation), increased cloud cover, and greater water vapour concentration. The trigger for all of these appears to be more moisture…which is what we seem to be getting more of atm.

There has been a rather dramatic turn around in the pressure over a period of 24-48 hours – 29th of November to 1st December. After remaining at 988 hPa from a few hours, in the following 24 to 48-hours period it rose to be in excess of 1020 hPa.

It’s currently sitting on 1013 hPa (~ 8 pm).

Continuing…

Taken 9th of November (~ midday), convection to the north-northwest.



Taken 9th of November (~ midday), same convection, shifted east.



A photo I took on Friday the 11th of November 2011 looking north-west towards Lobethal has led me to wonder about the medium-term prospects of significant falls. The photo shows what appears to be high altocumulus clouds, a very fine, extensive deck with the checker pattern.



A chance to see what the soil was like underground…12th of November (~ midday).



Streamflow…21st of November (~ 7.30 pm)



Sunset, 21st of November…



Rough Sunset Panorama, November 2011

The next week or so looks interesting in terms of the quantities of moisture forecast to be present in the atmosphere…and runoff.

Temperature doesn’t seem to be increasing much (to maybe 30-35)…followed by a decline.

Humid, sultry…maybe wet.

Written 9th December 2011:


No significant visual or audible change in flow. Temperatures more-or-less up to 35 degrees by Thursday. Mostly below 20 degrees leading into Friday, and Saturday.

One thing I will note about the pressure though:

I don’t know whether it is thought pressure changes occur consistently at the same rate or not…but from what I’ve witnessed, it certainly doesn’t seems to be the case.

The pressure can change quite considerably and abruptly, within a very short period of time (minutes), or it can take several hours to days to change. It is that dynamic. For the period in question (9th-11th) the [surface] pressure has remained relatively steady at around 995 hPa. The pressure is probably going to start rising sooner rather than later (within a day or two, maybe even hours given this phenomenon).

Update (11th): Pressure now rising gradually.

[A yet-to-be validated low-resolution experimental rainfall model (specific to this region) I’d been looking into didn’t show much rain for this period (9th-11th)…when I checked on the 5th. 5 days later (10th) there is a slightly higher peak around the 18th-19th, maybe 20-30 mm, which I honestly didn’t believe to start with. The actual amount (for that peak) varied from about 200 mm down to about 5 mm, and shifted form the 25th to the above dates. It indicates the pressure initially starting to drop by about the 15th, and then rising with 2-3 days before a major rainfall peak around the dates shown. For now the pressure is slightly rising].

Update (13th): Latest short-term forecast (00Z13DEC2011 to 12Z20DEC2011) for this area has about 235 mm for the Saturday afternoon, with a major change in runoff (up to ~ 3.5 kg/m2). Pressure (mean sea-level) is expected to fall quite a lot (down to perhaps 1000 hPa (surface perhaps 980-990)). Precipitable water is forecast 35-40 kg/m2.

As always, time will tell with these things. Yesterday (12th) we had a brief period of intense drizzle where the temperature dropped from 17 to 15 degrees from about 6:25 pm onwards. While there was no runoff, it seems to be an indication that there is or has been a lot of moisture in the atmosphere.

Looks like moisture could well be penetrating down into the lower layers of the soil...not sure, but perhaps 50-60 mm. Precipitable water is beginning to become significant - 40-45 kg/m2, mean sea-level pressure barely dropping below 1000 hPa.

One thing about models: don’t expect a trend to follow from one year/month/week/day to the next in a linear fashion…the system behaves non-linearly.

Forecast appears to be steady, expect much rain ! Looks like the soil might be a benefactor this time...

Some may notice in the above I did not indicate how much I thought was going to fall. Conditions first need to become conducive to rain, and then rain must actually fall at or above a certain rate for a given period of time.

Sticking with my original thoughts (#1043773 and #1044246).

If anyone is interested:

Streamflow, 21st November 2011:



Streamflow, 18th December 2011:



A lot of debris around the place today (small fallen branches).

34.1 mm system total.