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#1224811 - 12/12/2013 18:37 Re: Streamflow Observations [Re: Seina]
Seina Offline
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Registered: 27/08/2003
Posts: 7770
Loc: Adelaide Hills
Originally Posted By: -Cosmic- (naz)
Possible revision of likely totals to week ending 9th Dec 2013 (20-40 mm) if specific humidity reaches 0.01 kg/kg by early tomorrow morning and diurnal temp range sharpens enough smile . See BoM Adelaide forecast 4.10 pm CDT.

Probabilities/thresholds not reached. Slight falls recorded - 6 mm total recorded to the 12th (from 8th).
The 5th of December was the last time the minimum temperature dropped to 5 or less.

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#1225547 - 17/12/2013 16:35 Re: Streamflow Observations [Re: Seina]
Seina Offline
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Loc: Adelaide Hills
GFS suggesting the previously-mentioned specific humidity threshold could be reached and exceeded from up to 48 hours from about mid morning on the 20th. Also suggesting precipitable water could reach above 40 mm for a similar period. Interesting times ahead smile .

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#1225788 - 19/12/2013 17:02 Re: Streamflow Observations [Re: Seina]
Seina Offline
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Registered: 27/08/2003
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Based on official forecasts and an interpretation of models:

Originally Posted By: -Cosmic- (naz)
GFS suggesting the previously-mentioned specific humidity threshold could be reached and exceeded from up to 48 hours from about mid morning on the 20th. Also suggesting precipitable water could reach above 40 mm for a similar period. Interesting times ahead smile .

Written 9:35 pm, 18th of December 2013:

Temperature likely to start falling more appreciably Friday afternoon (20th) [GFS, BoM]. Possible falls in the 15-30 mm range could affect soil moisture for the period 20th to 23rd. Thundery showers possible over the Northern Hills in the 24-36 hours prior to the change in temperature [BoM].

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#1226210 - 23/12/2013 10:50 Re: Streamflow Observations [Re: Seina]
Seina Offline
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Registered: 27/08/2003
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Loc: Adelaide Hills
Originally Posted By: bd bucketingdown
why is moist air less dense than dry air?

METEOROLOGIST JEFF HABY

The units of density are mass divided by volume (m/V). Density will increase if either mass increases while the volume remains constant or if volume decreases while mass remains constant.

Density of air will vary as the temperature and moisture content in the air varies. When the temperature increases, the higher molecular motion results in an expansion of volume and thus a decrease in density.

The amount of water vapor in the air also effects the density. Water vapor is a relatively light gas when compared to diatomic Oxygen and diatomic Nitrogen. Thus, when water vapor increases, the amount of Oxygen and Nitrogen decrease per unit volume and thus density decreases because mass is decreasing.

The two most abundant elements in the troposphere are Oxygen and Nitrogen. Oxygen has an 16 atomic unit mass while Nitrogen has a 14 atomic units mass. Since both these elements are diatomic in the troposphere (O2 and N2), the atomic mass of diatomic Oxygen is 32 and the diatomic mass of Nitrogen is 28.

Water vapor (H2O) is composed of one Oxygen atom and two Hydrogen atoms. Hydrogen is the lightest element at 1 atomic unit while Oxygen is 16 atomic units. Thus the water vapor atom has an atomic mass of 1 + 1 + 16 = 18 atomic units. At 18 atomic units, water vapor is lighter than diatomic Oxygen (32 units) and diatomic Nitrogen (28 units). Thus at a constant temperature, the more water vapor that displaces the other gases, the less dense that air will become.

You may be familiar with the concept that moist air is less dense than dry air. This is true when both have the same temperature or when the moist air is warmer. Said in another way, air with a greater percentage of water vapor will be less dense than air with a lesser percentage of water vapor at the same temperature. Often people erroneously believe that moist air is denser than dry air because very moist air is more difficult to breathe than dry air.


After doing some research of my own I mostly agree with the quote you presented. I would however be (on my own part) cautious to differentiate between more surface and upper-level pressure features (in the troposphere) as these dynamics may have a significant impact on where the air is less dense or more dense due to humidity or lack thereof.

------------------------------------------------

Written 17th of December 2013 [revision]:
Temperature statistics:
Max: 36; Min: 6. Diurnal Range, DR: 30.
[18th – 39/8 (DR: 31); 19th – 42/12 (DR: 30); 20th – 37/14 (DR: 23).
18th – first time the diurnal range exceeded 30 since the 7th of January. A diurnal range of 2 degrees was recorded on the 9th of this month (16/14).]

Overnight low of zero on the 22nd of November broke an apparently near-linear run of lows hovering around 5 degrees since the 18th of March, including frosts during the winter months when the low dropped below zero degrees.

The variance (trend) in the maximum temperature between approximately early September and now has been gradually increasing – only since the 22nd has the minimum begun to follow.

Update 22nd December (views expressed are opinion and not fact):
As the specific humidity (in kg/kg) and actual vapour pressure (in hPa) have also risen quite sharply since the 22nd [Nov](from about 0.003 kg/kg and 6 hPa to around 0.008 kg/kg and 13 hPa on the 20th [this month]), this seems to imply a greater availability of moisture. The fluctuations in surface barometric pressure (hPa) over the last 4 days (996, 1007, and 999 since) seems to suggest troughs are becoming more prevalent, with lows mostly further south. This appears to support north-west in-feeds of tropical moisture however the nature of the cloud-cover (with mild changes in temperature and pressure) indicates the cyclic trough-like patterns which are prevalent have a high degree of variability (are therefore characteristically not very robust). This variability seems unlikely to support or facilitate significant rainfall activity because the conditions required to sustain such rainfall (whether convective or not) are not in themselves maintained at such a level (above a threshold) for long enough. For this reason I used the words “possible” in my last post, so those criteria are maintained. Only with adequate conditions for long enough (sustained) will this change – this is something higher precipitable water values can support as the temperature is affected by vapour pressure and thus humidity. High-humidity air is facilitated by evaporation.

The partial consequence of the above is a general depletion in stream flow since early September; however the river is still flowing.


Edited by -Cosmic- (naz) (23/12/2013 10:55)

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#1229000 - 04/01/2014 13:05 Re: Streamflow Observations [Re: Seina]
Seina Offline
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Registered: 27/08/2003
Posts: 7770
Loc: Adelaide Hills
3rd January 2014:
During the last couple of days 9.2 mm has fallen (to 9 am this morning). This appears to be a consequence of relatively high precipitable-water (45-50 mm, GFS), lower pressure (990-1000 hPa sfc), lower density (more water molecules in the lower atmosphere and a greater 500-1000 mb thickness, GFS), and a sharpened diurnal temperature range (19/14 just yesterday). The cut-off for the effects of a narrow, moist cloud-band in the wake of TC/Ex TC Christine appeared sharp, with negative dew points over the Northern (Mount Lofty) Ranges, moderate Central Ranges, and higher on the Southern Fleurieu Peninsula (BoM obs). The higher precipitable-water along with the drop in pressure appeared to facilitate greater likelihood of rain (enough to settle the dust). If, in the next 2-3 days, the maximum temperature falls slightly more (from 23 today), this may increase the chances of rain or showers in the wake of recent days, further.

Streamflow now appears to be borderline base-flow (groundwater only).

Update 4th January 2014: little 988 hPa low barrelling northeast in the Bight 00:00 UTC today on the BoM synoptic chart!?

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#1229641 - 06/01/2014 15:41 Re: Streamflow Observations [Re: Seina]
Seina Offline
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Registered: 27/08/2003
Posts: 7770
Loc: Adelaide Hills
2 mm from the low to add to the 9.2 mm earlier.

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#1231122 - 12/01/2014 12:05 Re: Streamflow Observations [Re: Seina]
Seina Offline
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Registered: 27/08/2003
Posts: 7770
Loc: Adelaide Hills
Possible shower(s) circa Wednesday 15th of January smile [Adelaide Hills, BoM].

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#1231131 - 12/01/2014 12:50 Re: Streamflow Observations [Re: Seina]
Seina Offline
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Registered: 27/08/2003
Posts: 7770
Loc: Adelaide Hills
Originally Posted By: -Cosmic- (naz)
Possible shower(s) circa Wednesday 15th of January smile [Adelaide Hills, BoM].

Streamflow is now depleted to the condition where, at various points along the river, there is no flow. With day-time maximum temperatures now well into the lower-to-mid 30s, sometimes approaching 40, and overnight lows starting to rising only slightly in the last 7 to 10 days, this seems to suggest the diurnal range is increasing. However, the dew point temperature and corresponding vapour pressure remain mostly steady.

Given the tendency of the surface pressure to follow a pattern of 5 to 7 days of rising-and-falling, then deepen to a trough or low-pressure system, this seems to lend support to another prevailing phenomenon. The high-pressure ridge near-adjacent the West Coast of Western Australia has been sitting there, near-stationary for some time now (days, if not a week or more, facilitating trough-like and low-pressure conditions along the west coast, conducive to cyclogenesis), and appears forecast to do so for yet another week. To the south-east of this prevailing feature is a zone (for lack of better terminology) in the lower-to-mid Bight conducive to the passage of low-pressure systems or assisting in the formation of tropical in-feeds across the continental interior. These in-feeds also subsequently appear to be blocked [in the east] by another ridge or similar feature extending into the eastern continental interior. If one then follows the pattern of ridging around to the east, there is yet another zone of higher pressure, more subject to change, yet also near-stationary with prevailing trough-like conditions along the east coast. There, south-easterlies are directed onto more northern coasts, with another quite dominant ridge focused on the continental interior [mentioned earlier] directing similar winds inland.

In both cases (ACCESS-G and GFS) the gap down the middle (a zone of some kind) along with the ridge on the east coast and the trough near the west, are facilitating the thickness to move further south. With the possibility of cyclogenesis along the west coast, the intensity of this may affect wind speeds as far south as 30 degrees latitude (leading them to become lighter). As the [500-1000 mb] thickness is proportional to temperature, and the winds along the eastern continental interior allow heat to be drawn inland, losing latent heat along the way, it may be possible, in my view, the region of instability in the Bight may lead a front to interact with a west-coast low (all else remaining the same).

The region of instability, with south-easterlies at the surface [Central Hills], and a much higher zonal thickness across 30 degrees latitude [looking at Wednesday 15th of January] could provide the conditions for a narrow band of precipitable water to drag across from either a low or region of cyclogenesis, or, follow straight down the continental interior. Narrow because the influence of the sea-breeze would have a narrow coastal cut-off. The former scenario seems more likely at the moment.

Although only hypothetical, the consequences of such a phenomenon could be a rapid increase in dew point, and, given the number of days with mostly-clear if-not clear skies in recent times, a change in the pressure vertical velocity to facilitate rapid cloud formation. The likelihood of specific-humidity thresholds being exceeded in also increased with lower summer-time pressures.


Edited by -Cosmic- (naz) (12/01/2014 12:56)

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#1232184 - 16/01/2014 10:29 Re: Streamflow Observations [Re: Seina]
Seina Offline
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Registered: 27/08/2003
Posts: 7770
Loc: Adelaide Hills
5.1 mm 9 am 14th to 9 am 15th from thundery showers/thunderstorms. Enough to wet the ground before it started to evaporate again.

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#1232819 - 18/01/2014 11:54 Re: Streamflow Observations [Re: Seina]
Seina Offline
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Registered: 27/08/2003
Posts: 7770
Loc: Adelaide Hills
Another 4.9 mm to 9 am this morning, mostly from moderate rain and some sharp downpours. Very welcome change in conditions.

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#1233609 - 22/01/2014 21:08 Re: Streamflow Observations [Re: Seina]
Seina Offline
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Registered: 27/08/2003
Posts: 7770
Loc: Adelaide Hills
Where no references, views expressed are opinion:

It is likely rain will be developing late afternoon/evening tomorrow (23rd) [BoM, 4.32 pm CDT forecast], setting in in the following 12-24 hours. I would like to see whether a possible emerging trend over the past few years will eventuate, namely:

Written 19th of April 2011 [Environmental Observation, edited for clarity]:
  • More abrupt flooding associated with conditions conducive to heavy rainfall (thundery showers); locally sporadic (e.g. over 500 metres in 5-10 minutes; sultry conditions with a sudden light shower and 500 metres away, no rain/sultry conditions)…sub-tropical “heat waves” – altered prevailing winds (e.g. easterlies and south-easterlies during summer/autumn rather than southerlies or westerlies, or north-westerlies). Floods and sub-tropical “heat waves” could almost go together when there are sultry conditions.
In a recent heatwave (without quotations “”) the 500-1000 mb thickness [volume] did indeed prevail, however because the temperature was very stagnant [and high] the inverse proportionality between volume and pressure probably applied, meaning lower pressure emerged later on during the heatwave [apparent from the Bureau's Nuriootpa daily observations for the period].

Therefore: “Heatwaves” – increase in sensible heat flux associated with convection, troughs and enhanced CAPE in the absence of cloud-cover [increased humidity appears to have the characteristic of inhibiting photosynthetic processes at a given threshold of concentration]:

[Written 16th of January 2014:
NB: with more latent, and particularly sensible heat, retained in the lower atmosphere during a period of higher 500-1000 mb thickness and moderately-low pressure, hence higher temperatures, it seems the opacity of the atmosphere with respect to water-vapour (specifically, the quantity of heat retained per unit convective available potential energy in Joules per Kilogram) is heightened with higher dew point temperatures (witnessed on the 14th into the 15th January 2014). This could have enhanced convection within the short hourly period thunderstorm activity was mostly likely (which seemed very apparent), however it also means evaporation is strong, giving a possible sharp cut-off between the effects of cloud and no cloud.]

Meaning there may be a potential for flooding in the wake of a heatwave. There is, however, no guarantee of this if there is a sharp cut-off of the abovementioned factor under humid sub-tropical conditions.

Minor flooding at this time may require a quick 10-20 mm from a thunderstorm, or 20-40 from a rainband. It is possible [though remote at the moment], but will have to wait and see smile .


Edited by -Cosmic- (naz) (22/01/2014 21:16)

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#1234852 - 27/01/2014 14:42 Re: Streamflow Observations [Re: Seina]
Seina Offline
Meteorological Motor Mouth

Registered: 27/08/2003
Posts: 7770
Loc: Adelaide Hills
At least one of the environmental observations over the last 2-3 years was falsified (in this occasion) in the last week with the idea that flooding might follow heatwave conditions, probably (again, as an observation) due to a relatively dry lower troposphere, and some parameters which were not sufficiently high-enough to provide a mechanism for significant rainfall (23rd – 24th) (e.g. specific humidity, dew point, precipitable water).

N.B.:

A recent drop in surface pressure circa the 16th-17th suggested the possibility the pressure drop acted as a mechanism in a 20-degree temperature drop over the space of a few hours (near the end of a recent heatwave). The trough seemed to affect both the temperature and thickness.

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#1239863 - 06/02/2014 17:55 Re: Streamflow Observations [Re: Seina]
Seina Offline
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Registered: 27/08/2003
Posts: 7770
Loc: Adelaide Hills
A possible dramatic local increase in the 500-1000 mb thickness (from current values) within the next 1-2 weeks could actually assist the formation of a substantial temperature-pressure gradient across the southeast (ACCESS-G, GFS).


Edited by -Cosmic- (naz) (06/02/2014 17:56)

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#1240631 - 09/02/2014 13:00 Re: Streamflow Observations [Re: Seina]
Seina Offline
Meteorological Motor Mouth

Registered: 27/08/2003
Posts: 7770
Loc: Adelaide Hills
Originally Posted By: -Cosmic- (naz)
A possible dramatic local increase in the 500-1000 mb thickness (from current values) within the next 1-2 weeks could actually assist the formation of a substantial temperature-pressure gradient across the southeast (ACCESS-G, GFS).

Revision from yesterday: two [temperature-pressure] gradients developing in the time frame indicated – one was overnight (a front expected to clip the coast with minor or trace rainfall totals), the other potentially mid next week.

Note-of-benefit: There is (per the model mentioned) an increasing probability the 2nd gradient change will bring with it conditions conducive to a major rainfall event over 2-3, maybe 4 days. The temperature (GFS) is forecast to fall for the period 13th-16th of February from around 40 degrees C to 30, then maybe the mid-20s. This could be quite a contrast from recent shallow falls in temperature. A reasonably low pressure region is also forecast to cross the state from the west-to-north-westerly direction across WA (both models). A recent post this year [in this thread] highlighted the potential consequences [dynamics] of such a hypothetical scenario unfolding, and made reference (in the last paragraph) to the number and frequency of clear- or mostly-clear-sky days. Other GFS parameters (e.g. precipitable water, specific humidity, 500-1000 mb thickness) could be described as either relatively high or excessive for the period in question.

20-40 mm for the period might be a reasonable estimate however depending on how model scenarios unfold over the next week it might be 4 times that. It seems more reasonable to underestimate and be prepared.

The river as dropped significantly below no-flow levels in all places observed.

Gradient change No.1:
Based on ACCESS-R, GFS forecasts (written 7th of February 2014):

Strahan in Tasmania could be due for a significant rainfall event this weekend (compared to the year to date). On the afternoon of Sunday the 9th of February, GFS is forecasting a steep, temporary change in the 500-1000 mb thickness which may lead near-surface air temperatures to fall significantly towards conditions possibly conducive to hail (approaching 10 degrees Celsius from approximately 22-24 with a similarly sharp drop over a period of 24-36 hours from today [revision 9th – 12-24 hours]). This is supported by the ACCESS-R +48 hour forecast (valid 00Z Sunday 9th of February), which suggest[s] a pocket of near-5400 metre thickness approaching from the west coast [9th - less likely!?]. Combined, GFS and ACCESS-R seem suggest 20-40 mm for the period to early on Monday morning. The mean sea-level pressure may also reach 1000 hPa in this region as the low passes. Slight-to-moderate change in runoff may occur.

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#1240904 - 10/02/2014 12:15 Re: Streamflow Observations [Re: Seina]
Seina Offline
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Registered: 27/08/2003
Posts: 7770
Loc: Adelaide Hills
Originally Posted By: -Cosmic- (naz)
Gradient change No.1:
Based on ACCESS-R, GFS forecasts (written 7th of February 2014):

Strahan in Tasmania could be due for a significant rainfall event this weekend (compared to the year to date). On the afternoon of Sunday the 9th of February, GFS is forecasting a steep, temporary change in the 500-1000 mb thickness which may lead near-surface air temperatures to fall significantly towards conditions possibly conducive to hail (approaching 10 degrees Celsius from approximately 22-24 with a similarly sharp drop over a period of 24-36 hours from today [revision 9th – 12-24 hours]). This is supported by the ACCESS-R +48 hour forecast (valid 00Z Sunday 9th of February), which suggest[s] a pocket of near-5400 metre thickness approaching from the west coast [9th - less likely!?]. Combined, GFS and ACCESS-R seem suggest 20-40 mm for the period to early on Monday morning. The mean sea-level pressure may also reach 1000 hPa in this region as the low passes. Slight-to-moderate change in runoff may occur.

Highest falls were scattered across the state in a range from about 15-20 mm. Minor changes in flow. See BoM website.


Edited by -Cosmic- (naz) (10/02/2014 12:18)

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#1241735 - 13/02/2014 13:12 Re: Streamflow Observations [Re: Seina]
Seina Offline
Meteorological Motor Mouth

Registered: 27/08/2003
Posts: 7770
Loc: Adelaide Hills
Steady moderate-to-heavy rain, 12 mm and climbing smile .

Edit:

Don't know what infiltrating rate the upper soil can handle, but puddles are forming.


Edited by -Cosmic- (naz) (13/02/2014 13:16)

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#1241769 - 13/02/2014 14:00 Re: Streamflow Observations [Re: Seina]
Seina Offline
Meteorological Motor Mouth

Registered: 27/08/2003
Posts: 7770
Loc: Adelaide Hills
Originally Posted By: -Cosmic- (naz)
Steady moderate-to-heavy rain, 12 mm and climbing smile .

Edit:

Don't know what infiltrating rate the upper soil can handle, but puddles are forming.

Estimated rain rate 10 mm/hr at 1 pm.

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#1241988 - 14/02/2014 08:15 Re: Streamflow Observations [Re: Seina]
Seina Offline
Meteorological Motor Mouth

Registered: 27/08/2003
Posts: 7770
Loc: Adelaide Hills
Over 3 inches [system total] to 7 am and climbing.

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#1242018 - 14/02/2014 09:42 Re: Streamflow Observations [Re: Seina]
Seina Offline
Meteorological Motor Mouth

Registered: 27/08/2003
Posts: 7770
Loc: Adelaide Hills
94 mm to 9 am since the first rain band - the river is flowing moderately (has risen more than a 1 metre judging from where the river bank is). Is still raining.


Edited by -Cosmic- (naz) (14/02/2014 09:43)

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#1242662 - 16/02/2014 18:09 Re: Streamflow Observations [Re: Seina]
Seina Offline
Meteorological Motor Mouth

Registered: 27/08/2003
Posts: 7770
Loc: Adelaide Hills
[] Indicates correction or additional info.

#1198291 - 10-06-2013 01:37 PM:

Originally Posted By: -Cosmic- (naz)
2nd Temporary Flash-flood Event, Witnessed 31st May 2013, Upper Onkaparinga Catchment [1st in December 2010]:
See #897948 - 13-11-2010 10:19 PM (very similar):

3rd Temporary Flash-flood Event, Witnessed 14th of February 2014, Upper Onkaparinga Catchment:

Gradient change No.2, written 12th of February 2014:

Originally Posted By: -Cosmic- (naz)
Originally Posted By: -Cosmic- (naz)
A possible dramatic local increase in the 500-1000 mb thickness (from current values) within the next 1-2 weeks could actually assist the formation of a substantial temperature-pressure gradient across the southeast (ACCESS-G, GFS).

Revision from yesterday: two [temperature-pressure] gradients developing in the time frame indicated – one was overnight (a front expected to clip the coast with minor or trace rainfall totals), the other potentially mid next week.

Note-of-benefit: There is (per the model mentioned) an increasing probability the 2nd gradient change will bring with it conditions conducive to a major rainfall event over 2-3, maybe 4 days. The temperature (GFS) is forecast to fall for the period 13th-16th of February from around 40 degrees C to 30, then maybe the mid-20s. This could be quite a contrast from recent shallow falls in temperature. A reasonably low pressure region is also forecast to cross the state from the west-to-north-westerly direction across WA (both models). A recent post this year [in this thread] highlighted the potential consequences [dynamics] of such a hypothetical scenario unfolding, and made reference (in the last paragraph) to the number and frequency of clear- or mostly-clear-sky days. Other GFS parameters (e.g. precipitable water, specific humidity, 500-1000 mb thickness) could be described as either relatively high or excessive for the period in question.

20-40 mm for the period might be a reasonable estimate however depending on how model scenarios unfold over the next week it might be 4 times that. It seems more reasonable to underestimate and be prepared.

The river as dropped significantly below no-flow levels in all places observed.

Precipitable water and thickness have moderated somewhat – specific humidity remains relatively high to excessive for the 2-to-3-day period 13th-15th (GFS). Given the increased cloud-cover today, expecting [maximum] temperatures to be in the 35-to-40-degree range, dropping towards 20-to-25 degrees, 13th-15th, increasing slightly in the following week. Relative humidity staying in the 85-to-95 % range from about Thursday night, with much higher dew points, bordering on or leading to super-saturated conditions. Flooding after about the first 12-24 hours from Thursday night may be a genuine possibility, with in excess of a 50% change in upper soil moisture (GFS). The impression I get is that 20-40 mm may be a conservative estimate given factors such as the higher dew point, sustained relative humidity, orographic effects and a rather higher specific humidity. This does not appear to be a fickle system.

Flood[ing] is likely (my view) with falls of between 50 and 120 mm locally in the 24-36 hours from Thursday night.

-------------------------------------------------------------

Written yesterday (15th) and today (16th):

After checking figures, stream flow reached a flow volume not seen since the 31st of August 2013 – volume up approximately 6.62 fold in magnitude from Wednesday, soil moisture (estimated) changed from ~ 0.18 to 0.247 as a fraction of 1.

129.5 mm to 9 am Saturday morning.

The rain rate lead to moderate surface land run-off (although run-off was clearly visible when it peaked), lower (modelled) soil layers probably being affected (due to the rain rate being on or around 3-4 mm/hour, at times 5-10 mm/hour). Some modelled indicators have suggested moisture reached deeper retention layers of the soil (up to 1 to 1.5 months lagging), however, these effects probably didn’t become apparent until about midday on Friday, as the river started to rise more rapidly (towards approximately 1.5 metres in height, above levels on Wednesday).

Although low-lying areas were slightly affected, soil infiltration was the primary response apparent (meaning more moisture soaked into the ground than ran off paddocks). The actual prior dry and hot-to-very-hot heatwave conditions probably contributed to this situation because the humidity was quite low in the lower atmosphere to begin with (20-30%) before reaching well above 80% sustained on Friday.

I might add that this is the kind of thing that might seem likely after several weeks (not exactly sure how many, but a lot) with mostly clear-sky or near-clear sky days. Evaporation becomes minimal after a while and sensible heat flux takes over because of the lack of moisture anywhere near the surface. The mechanism seems to be a change in humidity in the lower troposphere brought about by advection of copious quantities of moisture. In theory, the higher temperatures prior to this period facilitated this moisture advection (because the atmosphere could hold more of it). Then all is needed is a pressure gradient (change in pressure with time) and the moisture gradually rains out (in this a case a ridge separating a tropical in-feed from frontal activity to the south).

With some land-surface run-off there has also been mild soil erosion in areas (particularly in parts where the soil couldn’t deal with the rain rate). This morning (Sunday) it was apparent, after closer observations of debris heights, that the river reached varying levels depending on topography (between about 1 and 1.5 metres deeper and 1-4 metres wider).

No doubt this is an historical event (not just in the Adelaide Hills), however sometimes (perhaps) it can help to appreciate what actually does happen (in this case mostly to soil moisture, with some run-off) when we move from one extreme to another (in terms of weather conditions), so we might get an idea of how to better manage impacts to the land in the future. While this rain period may not have been the same as in early December 2010 or late May 2013, the outcome was roughly the same.

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