August update 2024 SA
August update 2024 SA
*Forecast
prepared between Aug 17th-19th 2024
Review:
G’Day one and all, the mid-month is here, and it is
most certainly time for an update on proceedings and even though it has been a
short while since the last update, a lot has happened with the most recent
system and the expectations of what is to come as we move into this uptick in
mobile cold fronts.
A recap on the 15th to 16th event and
firstly apologies for not getting a quick update out for that event, for the
initial August outlook it showed a thundery signature and that certainly played
out and upgraded as we got closer to the event. Rainfall wise, it turned out
almost exactly as expected with some good and welcome falls across the Lower to
Upper North and northern Yorke Peninsula especially however I did think the
Mallee to Upper SE as well as Lower Yorke Peninsula came in with unders for the
event. Essentially what we saw was an initial pre-frontal trough the night before with a modest area of showers,
and the odd isolated thunderstorm tending to patchy rain areas before petering
out. Now this expanded a little more than first thought giving some handy 4-8mm
totals before the main cold front came through later on
the 15th. The rain the night before assisted in wetting the lower
levels and surface a bit to essentially ‘pave the way’ for the cold front to
maximise rainfall with what moisture it had to work with. At least, some
assistance from Q LD was noted after
the strong upper trough delivered large rainfall there and then this moisture
advected SW into SA before the cold front.
As far as rainfall recording go from the system and the
onshore colder air stream showers, there were some useful ones. Notable totals include: Riverton 42mm, Naracoorte 33mm, Kies Estate
29mm, Clare 28mm, Mintaro 28mm, Stockport 26mm, Keith West 25mm, Hoyleton 23mm,
Wirrabara PO 22mm, Auburn 22mm, Georgetown 22mm, Yacka 22mm and a handy 20mm at
Maitland which had been battling a bit. The Upper North which had been in all
sorts of strife generally managed 10-20mm as well as northern portions of the
Lower North like Jamestown with 21mm there. Good rain just in time. The Mallee
saw general 6-12mm totals which was about 4-5mm short of initial expectations.
Follow up is certainly needed there.
Meteorological
Discussion:
Onto current proceedings and we have a strong cold
front and associated low moving across WA with some areas of rain as well as
general showers and thunderstorms, especially in the pre-frontal trough ahead
of it further east. This system is supported by an upper trough and together
these features will move east into SA during the day, with isolated
mid-level-showers and thunderstorms developing over the Far West later on the 18th. This system will weaken
as it moves east however, moisture levels ahead of it should be sufficient enough to keep general scattered mid-level
showers and isolated thunderstorms across most districts on-going on the 19th,
especially later afternoon and early evening over Central to Eastern Districts.
A secondary band is likely to form on the back side of the first but perhaps a
little more isolated in nature with a weak trough located there. This will keep
some isolated mid-level showers and possible a thunderstorm going into the
early hours of the 20th. During the daytime hours of the 20th,
the main low-pressure centre will affect the coastline driving cooler air and
showery conditions into the general southern coastline, reaching Eyre Peninsula
just before daybreak and then further E to SE thereafter. Showers will be more
frequent about Kangaroo Island and the Southern Fleurieu as well as the SE
Districts during the afternoon where in the latter, some isolated thunderstorms
may develop due to the assistance of solar heating.
The NW flow aloft will remain strong with a zonal pattern
emerging from the westerly belt. Fast moving fronts will generally pass below
the state, only skimming the southern coastline however with each front, the
jet aloft will be squeeze and so some brief mid-level cloudbands with areas of
patchy rain may develop, the first being later on the 21st. Patchy
rain and showers moving over the Lower SE from late afternoon on 21st
till early 22nd. Most other districts north will see patchy light
rain in a secondary band, perhaps a little more consistent across the Eyre
Peninsula to the Lower and Upper North. Some on shore moisture behind the weak
front may also throw up some light shower activity across southern agricultural
areas later in the day.
Now after this things get a
little interesting. A high will move eastwards and strengthen allowing warmer
air to pool over the interior. This will then have another day or so to heat up
further to hot air causing temperatures to rise with the potential for 35-38C
temperatures across the Far North of the state and 30-35C temperatures over the
Southern interior and possibly even the outer Northern agricultural areas ahead
of the next system. Initially however, a trough will slide SE out of WA on the
23rd with perhaps a few mid-level showers, tending to patchy light
rain over the Eastern/SE Districts but not a lot with mild to warm conditions
and freshening NE to N winds. On the 24th a more substantial upper
trough and associated cold front will stand up over WA. Ahead of this warm to
very warm conditions (especially so for late August) will extend throughout
with the potential for near record August temperatures in the mid to high 20’s
over agricultural areas and near 30C for Eyre Peninsula and place like Port Augusta.
By about the 25th, the strong upper trough and associated cold front
will be advancing eastwards so initially it is likely to still be quite warm
but with freshening N winds ahead of showers and isolated thunderstorms over
the West Coast, thickening a little to patchy rain along a very tight jetstream
gradient. Specifics are still hard to nail but it looks like most rain will
have a southern bias once more with better falls with southern extent and
patchier falls further north as the system does slide SE as it comes over us.
Beyond this system I expect the westerly belt to be very
fluid with fast moving cold fronts, the next likely around the 27th-28th
period with potential for strong winds and squally showers, especially over
southern agricultural areas. Cold air is likely behind this front or the one
due after from the 29th to 30th period as the flow is
likely to take on some more SW in it as the broader longwave shifts eastwards. I may need to issue one or maybe two special/quick
updates to nail down these systems with greater detail for timing and rainfall.
Overall though, most of this is a west to east flow with a lack of deeper SW-NE
or S-N which will hamper rainfall totals to some degree.
So overall, a slightly better finish to the month which has
been expected (feels like it was never going to come at one point) but
certainly not a big finish by any stretch with some areas still getting light
rainfall only. Volatility likely into September with big temperature
fluctuations and fast-moving systems. It would be fantastic if we could squeeze
out average rainfall for August with the systems remaining and that is indeed possible I think for some areas, but certainly not
all. The rest of spring looks at least about average at this stage rainfall
wise but certainly warmer and windier than the mean (and spring is windy in any
given year). Thunderstorm levels are expected to be pretty much as per normal,
but with a slightly higher based flavour due to the faster moving systems.
Climate
Drivers:
A quick look at the Pacific now and we have some small
movement on our expected La Nina. This and the coming month or so is when I
expect a ‘move’ to be made with the increased trade winds allowing some of that
colder than normal water in the sub-surface to reach the surface. At the same
time, I expect the Humboldt Current to make a return to what it was doing a few
months ago. Figure 1 shows the current situation and Figure 2 shows just how
cold the water is in the sub-surface. I still don’t expect a strong or major La
Nina but a weak one, possibly closing on moderate is a fair estimation now.
Figure 1: Current SST anomalies – source Tropical
Tidbits https://www.tropicaltidbits.com/analysis/ocean/cdas-sflux_ssta_global_1.png
Moving on to the Indian Ocean we see that the little cold
area off Java and Sumatra persists. It hasn’t changed a great deal in strength
however has expanded in size somewhat. This was expected but more so expected
in July. I think the main impacts of it have already occurred in the first half
of August as looking at the current satellite, we do see significant convection
over and just west of that region (see Figure 3). Overall, the index is
neutral, and I expect it to remain that way as we move into Spring, but with a
negative tendency. There will be one final attempt at making an official weak
negative IOD but that’s about as far as we will go there.
The SAM has been strongly negative thanks to the Sudden
Stratospheric Warming (SSW). On closer inspection, there have been two main
pulses of this, the first affected us badly and now the second has started to
help us with a more active westerly belt closer to home. Modelling now shows a
third event is possible and if that does occur, expect a wildly volatile,
mobile and fluctuating pattern as we move through September with fast moving
spring fronts, increased thunderstorm risk but also increased odds of getting
warm to even hot air right down into agricultural areas ahead of those systems
and potentially cold behind should there be more SW in things. The squeeze of
the Jetstream aloft will certainly make for dynamic events and a pattern free
of blocking. The SAM should remain negative for quite some time with only brief
moves back to neutral.
The MJO has been weak of late and should continue to be
however there may be a slight uptick of strength as it moves through phases 4-5
which is right over the Australian mainland leading towards the end of the
month. This does roughly link up to the same time we have our uptick in cold
fronts and the mobile westerly pattern so whilst the influence will be small,
there should be an extra 5-10% of moisture available for use into those frontal
systems.
Figure 2: Current Sub-surface in the Pacific showing cold pool below surface at approx. 100-150m depth – source BoM http://www.bom.gov.au/climate/enso/wrap-up/archive/20240806.tao_sub_surface.png?popup
Figure
3: Current satellite imagery as of 18/08/24 noting strong convection west of
Sumatra but clearer skies to the south and off Java -Source Weatherzone
Districts:
Eyre
Peninsula
Weakening trough and cold front moving eastwards on 19th
causing freshening northerly winds ahead of some scattered mid-level showers
and isolated thunderstorms. Activity may tend to patchy light rain at times.
Low behind front early on 20th driving showers over the district,
more isolated in the north and NE. Weak front sliding below state on 21st
with patchy light rain developing although chiefly in the south. Activity
lingering into the 22nd before clearing by the mid to late afternoon
as a high strengthens to the east. Warm and potentially windy conditions
developing on the 23rd ahead of a trough sliding rapidly SE from
inland WA however only isolated to scattered mid-level showers are expected
here. Warm to very warm conditions then likely from 24th-25th
with temperatures in the mid to high 20’s with freshening N winds ahead of a
stronger upper trough and cold front with scattered showers and isolated
thunderstorms once again extending from the west during the 25th,
possibly tending to areas of rain over the Lower Eyre Peninsula. Follow up cold
fronts likely before the end of the month around the 27-28th and
again around the 29th-30th with potentially colder air to
follow bringing showery and windy conditions. Rainfall for the period generally
5-10mm, increasing to 10-15mm about Lower Eyre Peninsula. Totals decreasing
with northern extent down to 3-5mm about the N to NE portions.
Upper
North
Weakening trough and cold front moving eastwards on 19th
causing freshening northerly winds ahead of some Isolated mid-level showers and
thunderstorms. Low behind front early on 20th may bring some
isolated showers over the south of the district with nothing further north.
Weak front sliding below state on 21st with patchy light rain
developing although chiefly in the south at first. Activity lingering into the
22nd before clearing by the late afternoon as a high strengthens to
the east. Warm and potentially windy conditions developing on the 23rd
ahead of a trough sliding rapidly SE from inland WA however only isolated to
scattered mid-level showers are expected here. Warm to very warm conditions
then likely from 24th-25th with temperatures approaching
30C near Port Augusta with freshening N winds ahead of a stronger upper trough
and cold front with scattered showers and isolated thunderstorms once again
extending from the west during the 25th. Follow up cold fronts likely before
the end of the month around the 27-28th and again around the 29th-30th
with potentially colder air to follow bringing showery and windy conditions
though much more isolated shower activity in the north towards the Flinders.
Rainfall for the period generally 3-6mm, increasing to 6-12mm about higher
terrain in the south. Isolated 12-15mm falls possible here as well. Totals
decreasing with northern and eastern extent down to 1-3mm about the N to NE
portions.
Lower
North
Weakening trough and cold front moving eastwards on 19th
causing freshening northerly winds ahead of some Isolated mid-level showers and
thunderstorms. Activity may tend to patchy light rain through the far south.
Low behind front early on 20th may bring some isolated showers. Weak
front sliding below state on 21st with patchy light rain developing
although chiefly in the south at first. Activity lingering into the 22nd
before clearing by the late afternoon as a high strengthens to the east. Warm
and potentially windy conditions developing on the 23rd ahead of a
trough sliding rapidly SE from inland WA however only isolated to scattered
mid-level showers are expected here. Warm to very warm conditions then likely
from 24th-25th with temperatures in the mid to possibly
high 20’s with freshening N winds ahead of a stronger upper trough and cold
front with scattered showers and isolated thunderstorms once again extending
from the west during the 25th. Follow up cold fronts likely before the end of
the month around the 27-28th and again around the 29th-30th
with potentially colder air to follow bringing showery and windy conditions at
times. Rainfall for the period generally 5-10mm, increasing to 10-15mm about
higher terrain. Isolated 20mm falls possible here as well. Totals decreasing
with northern and eastern extent down to 3-5mm about the N to NE portions.
Yorke
Peninsula
Weakening trough and cold front moving eastwards on 19th
causing freshening northerly winds ahead of some Isolated mid-level showers and
thunderstorms. Activity may tend to patchy light rain about the lower half of
the Peninsula. Low behind front early on 20th may bring some
isolated showers. Weak front sliding below state on 21st with patchy
light rain developing although chiefly over the foot at first. Activity
lingering into the 22nd before clearing by the late afternoon as a
high strengthens to the east. Warm and potentially windy conditions developing
on the 23rd ahead of a trough sliding rapidly SE from inland WA
however only isolated to scattered mid-level showers are expected here. Warm to
very warm conditions then likely from 24th-25th with
temperatures in the mid to possibly high 20’s in the north with freshening N
winds ahead of a stronger upper trough and cold front with scattered showers
and isolated thunderstorms once again extending from the west during the 25th.
Follow up cold fronts likely before the end of the month around the 27-28th
and again around the 29th-30th with potentially colder
air to follow bringing showery and windy conditions at times. Rainfall for the
period generally 7-10mm, increasing to 10-15mm about the foot. Totals
decreasing with northern extent possibly down to 4-7mm about the N areas though
generally a fairly even spread down the first half of the Peninsula.
Kangaroo
Island
Weakening trough and cold front moving eastwards on 19th
causing freshening northerly winds ahead of some scattered mid-level showers
and possible isolated thunderstorms. Activity may tend to patchy light rain at
times. Low behind front early on 20th driving scattered showers over
the island. Weak front sliding below state on 21st with patchy light
rain developing. Activity lingering into the 22nd before clearing by
the mid to late afternoon as a high strengthens to the east. Mild to warm and
potentially windy conditions developing on the 23rd ahead of a
trough sliding rapidly SE from inland WA however only isolated to scattered
mid-level showers are expected here. Mild to warm conditions then likely again
from 24th-25th with freshening N winds ahead of a
stronger upper trough and cold front with scattered showers and isolated
thunderstorms once again extending from the west during the 25th,
possibly tending to areas of rain. Follow up cold fronts likely before the end
of the month around the 27-28th and again around the 29th-30th
with potentially colder air to follow bringing showery and windy conditions.
Rainfall for the period generally 12-20mm, increasing to 20-25mm about the
western end. Totals decreasing with eastern extent down to 8-12mm there.
Adelaide
Plains
Weakening trough and cold front moving eastwards on 19th
causing freshening northerly winds ahead of some Isolated mid-level showers and
thunderstorms. Activity may tend to patchy light rain at times. Low behind
front early on 20th may bring some isolated showers. Weak front
sliding below state on 21st with patchy light rain developing.
Activity lingering into the 22nd before clearing by the late
afternoon as a high strengthens to the east. Warm and potentially windy
conditions developing on the 23rd ahead of a trough sliding rapidly
SE from inland WA however only isolated to scattered mid-level showers are
expected here. Warm to very warm conditions then likely from 24th-25th
with temperatures in the mid to possibly high 20’s with freshening N winds
ahead of a stronger upper trough and cold front with scattered showers and
isolated thunderstorms once again extending from the west during the 25th.
Follow up cold fronts likely before the end of the month around the 27-28th
and again around the 29th-30th with potentially colder
air to follow bringing showery and windy conditions at times. Rainfall for the
period generally 10-15mm, increasing to 15-20mm about the foothills. Totals
decreasing with northern extent possibly down to 5-10mm about the outer plains.
Mt.Lofty
Ranges
Weakening trough and cold front moving eastwards on 19th
causing freshening northerly winds ahead of some Isolated mid-level showers and
thunderstorms. Activity may tend to patchy light rain at times, especially
towards the southern coasts. Low behind front early on 20th may
bring some isolated showers, scattered towards the southern coasts and ranges.
Weak front sliding below state on 21st with patchy light rain
developing, though mostly confined to the southern coasts and ranges initially.
Activity lingering into the 22nd before clearing by the late
afternoon as a high strengthens to the east. Warm and potentially windy
conditions developing on the 23rd ahead of a trough sliding rapidly
SE from inland WA however only isolated to scattered mid-level showers are
expected here. Warm to very warm conditions then likely from 24th-25th
with temperatures in the mid 20’s with freshening N winds ahead of a stronger
upper trough and cold front with scattered showers and isolated thunderstorms
once again extending from the west during the 25th. Follow up cold fronts
likely before the end of the month around the 27-28th and again
around the 29th-30th with potentially colder air to
follow bringing showery and windy conditions at times. Rainfall for the period
generally 15-25mm, increasing to 25-30mm towards the southern coasts and ranges
and higher peaks. Isolated 35mm totals possible about known wet spots. Totals
decreasing with northern extent possibly down to 8-15mm about the Northern
Ranges and Barossa.
Riverland/Mallee
Weakening trough and cold front moving eastwards on 19th
causing freshening northerly winds ahead of some Isolated mid-level showers and
thunderstorms. Low behind front early on 20th may bring some
isolated showers over the southern Mallee with nothing further north. Weak
front sliding below state on 21st with patchy light rain developing
although chiefly in the south at first before some moves in over the Riverland
later. Activity lingering into the 22nd before clearing by the late
afternoon as a high strengthens to the east. Warm and potentially windy
conditions developing on the 23rd ahead of a trough sliding rapidly
SE from inland WA however only isolated to scattered mid-level showers,
possibly tending to patchy rain again are expected here. Warm to very warm
conditions then likely from 24th-25th with temperatures
in the mid to high 20’s with freshening N winds ahead of a stronger upper
trough and cold front with scattered showers and isolated thunderstorms once
again extending from the west during the 25th. Follow up cold fronts likely
before the end of the month around the 27-28th and again around the
29th-30th with potentially colder air to follow bringing
showery and windy conditions though much more isolated shower activity in the
north anywhere north of the Southern Mallee. This may be a particularly windy
period with enhanced downslope westerly winds of the back side of the Mt.Lofty
Ranges. Rainfall for the period generally 3-5mm, increasing to 5-8mm in the
southern to SW Mallee. Isolated 10mm falls possible here as well. Totals
generally decreasing towards the Riverland however localised holes through the
western Mallee may appear with 1-3mm pockets possible.
Upper
SE
Weakening trough and cold front moving eastwards on 19th
causing freshening northerly winds ahead of some Isolated mid-level showers and
thunderstorms. Activity may tend to patchy light rain at times. Low behind
front early on 20th may bring some isolated to scattered showers.
Weak front sliding below state on 21st with patchy light rain
developing. Activity lingering into the 22nd before clearing by the
late afternoon as a high strengthens to the east. Warm and potentially windy
conditions developing on the 23rd ahead of a trough sliding rapidly
SE from inland WA however only isolated to scattered mid-level showers, tending
to patchy rain are expected here. Warm conditions then likely from 24th-25th
with temperatures in the mid 20’s with freshening N winds ahead of a stronger
upper trough and cold front with scattered showers and isolated thunderstorms
once again extending from the west during the 25th. Activity may tend to rain
in the south. Follow up cold fronts likely before the end of the month around the
27-28th and again around the 29th-30th with
potentially colder air to follow bringing showery and windy conditions at times
with potentially local moderate falls in the south. Rainfall for the period
generally 10-20mm, increasing to 20-30mm about the southern border of the
district. Totals decreasing with northern extent possibly down to 5-10mm about
the Mallee border.
Lower
SE
Weakening trough and cold front moving eastwards on 19th
causing freshening northerly winds ahead of some Isolated mid-level showers and
thunderstorms. Activity may tend to patchy light rain at times. Low behind
front early on 20th may bring some isolated to scattered showers.
Weak front sliding below state on 21st with patchy light rain
developing. Activity lingering into the 22nd before clearing by the
late afternoon as a high strengthens to the east. Mild to warm and potentially
windy conditions developing on the 23rd ahead of a trough sliding
rapidly SE from inland WA however only isolated to scattered mid-level showers,
tending to patchy rain are expected here. Warm conditions then likely from 24th-25th
with temperatures in the mid to possibly high 20’s with freshening N winds
ahead of a stronger upper trough and cold front with scattered showers and
isolated thunderstorms once again extending from the west during the 25th.
Activity may tend to rain in the south. Follow up cold fronts likely before the
end of the month around the 27-28th and again around the 29th-30th
with potentially colder air to follow bringing showery and windy conditions at
times with potentially local heavy falls, especially with southern extent.
Rainfall for the period generally 15-25mm, increasing to 25-35mm about the
southern coastline areas. Isolated 40mm totals possible. Totals decreasing with
northern extent possibly down to 10-15mm.
Western
VIC
Weakening trough and cold front moving eastwards late
on 19th and early 20th causing freshening northerly winds
ahead of some Isolated mid-level showers and thunderstorms. Activity may tend
to patchy light rain at times. Low behind front early on 20th may
bring some isolated to scattered showers over the SW District. Weak front
sliding below state on 21st with patchy light rain developing.
Activity lingering into the 22nd before clearing by the late
afternoon as a high strengthens to the east. Mild to warm and potentially windy
conditions developing on the 23rd to 24th ahead of a
trough sliding rapidly SE from inland WA however only isolated to scattered
mid-level showers, tending to patchy rain are expected here. Slight chance of
an isolated thunderstorm. Warm conditions then likely from 24th-25th
with temperatures in the mid to possibly high 20’s with freshening N winds
ahead of a stronger upper trough and cold front with scattered showers and
isolated thunderstorms once again extending from the west during the 25th.
Activity may tend to rain in the south. Follow up cold fronts likely before the
end of the month around the 27-28th and again around the 29th-30th
with potentially colder air to follow bringing showery and windy conditions
over SW District and southern portions of the Wimmera at times with potentially
local heavy falls, especially with southern extent over the SW District.
Rainfall for the period generally 15-25mm, increasing to 25-35mm about the
southern coastline areas of the SW District and the Grampians. Falls decreasing
towards the Wimmera of 8-15mm and Mallee 3-8mm generally.
Final
Notes
·
Complex pattern of troughs, fast moving
fronts and lows to evolve over the remainder of the month
·
Mobile and fast-moving pattern will bring
increased wind and warmth, especially ahead of the systems and some of these
winds will be very curing
·
Rainfall should finish off the month fairly well over the SE Districts and SW VIC although
the speed of the systems will mean many lighter falls for remaining areas,
especially those anywhere north of about Adelaide’s latitude
·
Windy September with fronts continuing and above
average temperatures though rainfall should be closer to normal at this stage
·
Weak La Nina and weak negative IOD to neutral
IOD for spring and early summer with average rainfall though moisture will
increase later in spring
·
Summer outlook is for average rainfall, possibly
above average over the interior and above average temperatures although not the
worst we have seen temperature wise
Link to
temporary blog website: https://farmweathersa.blogspot.com/
Happy
Farming and Cheers from The Weatherman
*Note –
For interest only, all thoughts are of The Weatherman and may not be reproduced
without my consent.
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