May 2023 update SA
May 2023 update SA
*Forecast prepared between May 17th-19th 2023
Review:
G’Day everyone, I know some of you have been eager to
receive the Mid-Month update and I do realise that 2 weeks is a long time
between drinks so to speak at this critical time of
the year. I should stress that when this becomes an official subscription-based
business, I will be doing weekly instead of fortnightly updates to cut down the
gaps as well as a lot more other content in between. A few personal things have halted the
progress of that happening sooner, but I still at least want to keep the
current 2-week updates rolling for free in the meantime to maintain continuity
and accountability through the period. Inconsistent updates won’t serve much of a purpose and things can quickly
get out of hand if you take your eye off the weather.
Now so far this month we have seen a dominant area of
high pressure move in with widespread frost and morning fog patches after some
initial moisture and showers earlier in the month. Pressures have been hovering
near 1030hPa for quite some time and the overall pressure anomaly so far this
month is well up on the mean but as you all know, this was expected for many
months and is certainly here with interest now. Frost was the most widespread
on the morning of the 7th followed by more on the 8th
although cloud did temper some areas here. More recently however, the current
large high has again delivered frost with similar slight to moderate frost
being reported on mornings of both 17th and 18th as I
type this.
Overall, the month has run as expected with the dominant
high pressure, light winds and mostly fine conditions and certainly below
average rainfall to date.
Meteorological Discussion:
An important period ahead looms and there is a mixture of
good and bad news. The bad news is, there is going to be more high pressure,
more frost and a bigger battle for moisture east and north of the
ranges/goyders line respectively. The good news is, we are pushing past the mid
point in our broadscale dry pattern and to break up the second half, we will see
a little burst of onshore moisture starting on the 19th through till
about the 21st before another strong high from below WA ridges in.
Now this high has been holding back the longwave trough
that tried to lift up near WA a few days ago. As a result the Longwave will now
burst out further east at around the Tasmania/VIC longitude towards the end of
the month and will do so with interest.
One thing I always find is the greater the atmosphere
pushes in one direction, the greater the counteractive move is when there is a
change in pattern. Such a large dominant period of high pressure with repeat
1030-1036hPa highs will be replaced by powerful Southern Ocean lows, albeit
generally keeping well south of the mainland. However with the brief weakening
of the high pressure, fronts will have an easier path up from the SW and this
period should occur from approx 25th May through till the first week
of June. Very large and powerful swells will be generated out of the longwave
in the order of 7-10m off the South Central and Lower SE coasts. Tasmania
especially will feel the full force of this burst of low pressure with repeat
squally fronts, heavy showers and widespread snow with the west coast copping a
repeat battering classic of an El Nino pattern. We will also see a wet burst
over Southern VIC, extending especially up into the Alpine region with the
first proper burst of snow of the season.
This all ties in to expectations that despite the drier
period from April to September, the fronts as a whole will be quite vigorous at
times. I expect a better window to line up SA through mid to late June however
early June won’t be too bad in the SE of the state but once again, these will
be primarily frontal with a lack of NW cloudband infeed ahead of them. Only a
very powerful longwave standing up in the Bight will be able to find some
moisture out of an Indian that is working against us right now. Vigourous
fronts will bring more severe wind than usual so outside of the high pressure,
expect a fair bit of wind and a greater chance of cold air tornadoes along the
frontal zones on any of the strongest of cold fronts.
For the outlook/forecast period though weather wise we
will see a SW flow with couple of weak fronts slide up from the SW, the first
due in the morning of the 19th. This will drive scattered showers,
across the southern agricultural areas, but due to slack steer these will move
fairly slowly and are likely to form into streams, especially at night off
bodies of water such as the two gulfs and Lake Alexandrina and Albert. A second
front due into Kangaroo Island at around
1am on Sat 20th will bring a slightly better band of showers with it
and some slightly colder air aloft thus resulting in more beefy shower activity
which will once again form up into streams for a short while. A third weak
trough will advance more south to north up ahead of a large 1035hPa high early
on Sunday 21st bringing isolated showers once again but working some
streams up the Spencer Gulf in particular into the Melrose region.
Now the large high will pass a little further north
keeping the southern coastal fringes of the state in a slightly moister WSW to
W flow from the 22nd to 23rd allowing occasional showers to pass in
this districts but the further north one ventures the drier and finer it will
become.
Beyond this we will start to see the influence of the
lows/longwave that will begin to expand over waters well to our SE below Tassie
etc. Cold fronts will begin to rotate up around the backside of this, in
varying strengths, though none at this stage look strong, weak to maybe perhaps
moderate at best but at least something. The first is due roughly around the 25th
but mostly favour southern districts with a lack of penetration past anywhere
north of Adelaide. A broad WSW onshore flow will traverse into the SE corner of
the state over the coming days after this which should at least keep occasional
showers going ahead of a potentially moderate strength system due somewhere
around the 28th though I’m not entirely convinced on the strength of
this one just yet. Showers and possible isolated thunderstorms south of about
Kangaroo Island to Kingston SE are likely with this system but once again, a
lack of deep inland penetration.
The trend beyond May is still for a comeback in June to
closer to average falls in some southern ag areas but still a battle inland.
Better than May however. July at this stage looks to be lining up for another
downwelling window but not as powerful as the May one, though mcuh colder
nights will ensue with frost due to the time of the year. August and September
will have their fronts again but a lack of cut off lows and rain as opposed to
showers is the most likely scenario so it all plays to the tag line of south of
Goyders line, there will be enough rain to make something of it but by no means
an easy ride. North of that, it will be very hard work over a sustained period
unfortunately.
Finally, frost risk is slight to moderate for the Upper
North and Riverland on 23rd and 24th but decreases with
Southern extent due to westerly flow. Another period right near the end of the
month or start of June looms with some lower temperatures.
Climate Drivers
The Pacific has continued it’s move towards an El Nino
with a consolidation of the warmer anomalies off Peru and the extension of that
warm tongue or spike along the equator towards the west which is the signature
move of El Nino. In the sub-surface the warmth in the far east is very clear
but also we can see that there is a complete connection across the basin from
the east to the west (refer to Figure 1). The cooler water much deeper down
whilst broad, is not relevant at this stage given we have re-enforcing Westerly
Wind Bursts emerging from the Indian and the latest is now just NE of PNG and
heading east to further enhance the developing El Nino.
Figure 1: Sub-surface SST anomalies across Pacific
showing intense warmth in east - source
BoM http://www.bom.gov.au/climate/enso/#tabs=Pacific-Ocean&pacific=Sea-sub%E2%80%93surface
Figure 2: Current SST anomalies – source Tropical Tidbits
- https://www.tropicaltidbits.com/analysis/ocean/cdas-sflux_ssta_global_1.png
Note in Figure 2 the warm wateer still broadly over the
Western Pacific. This will re-enforce the entire basin and add to the total El
Nino affect but also be a soup for onshore moisture into the eastern seaboard
to fuel any showers there as well as East Coast Low’s (ECL’s).
The Indian is still showing signs of moving towards the
Positive IOD with some cooler anomalies appearing in the nothern and eastern
basin. Oner thing to note is the impact of the powerful Tropical Cyclone that
moves over the Bay of Bengal and slammed Bangladesh recently. The cold water
upwelling from the depths has really hammered the warmth and now is driving
colder anomalies to push south and further halt any warmth. This is what
happened in 2020 when we were setting up for a negative IOD and instead a
monster TC in this very area introduced a large cold spot that didn’t go away
for the entire spring period. Warmth is also appearing in areas off the horn of
Africa and certainly increasing east of Madagascar.
The MJO has moved across the top end and into the Western
Pacific at a moderate to strong magnitude and is currently in phase 7 and will
be increasing in strength as it moves into phase 8 over the other side of the
world – this is not what Australia wants to see if it is to break the grip of a
broadscale dry pattern, certainly at least inland. Having said that, the MJO
has less relevance at this time of the year than it does in the summer months
however having this remaining strong over the Pacific and beyond will only keep
those westerly wind bursts going with that smaller wave near PNG atm pushing up
behind the previous larger one.
Now the SAM is probably the most impressive of all of the
drivers atm with a massive positive burst induced by the large area of high
pressure that we have seen and as expected this has come right in the window of
expectation of this brodscale dry pattern with massive downwelling/subsidence
induced by the expanding Hadley cell – these aren’t local changes, they are
large scale changes and are in turn having an impact on a large scale which
reduces the chance of even the local wetter spots breaking that trend. Onshore
moisture is all we can hope for this season in the form of cold fronts and when
they get crushed, it gets very dry and frosty.
Districts:
Eyre Peninsula
Weak front due 19th with scattered showers in
moist onshore flow. Secondary front with a re-invigoration of showers due
around around 4am or so on 20th, with showers more frequent and heavier about
Lower Eyre Peninsula early morning. Weak trough with further isolated showers
on 21st but not a lot in it, easing later in the day. Strong 1035hPa
high to move in from west from 22nd to 25th however
perhaps isolated coastal showers about Lower Eyre Peninsula may persist in
westerly flow with high positioned to north. Slight frost risk 23rd
inland. Another weak front due 25th
with isolated showers, chiefly about Lower Eyre Peninsula with winds likely to
be fresh to strong near the front. Moist onshore WSW flow to persist till 28th
bringing further isolated showers, less likely inland before a moderate front
with showers possible on 28th. Rainfall for period 5-10mm about
Lower Eyre Pen and the West Coast of Eyre Pen, decreasing to 2-3mm inland and
less than 2mm in the NE and N.
Upper North
Weak front due 19th with isolated showers,
scattered in the south and about higher terrain in a moist SW flow. Secondary
front with a re-invigoration of showers due around around 10am or so on 20th,
with showers more frequent and heavier in any streams off the Spencer Gulf.
Weak trough with further isolated showers on 21st but not a lot in
it, mostly confiend to a stream into the Melrose area off the Spencer gulf once
again. Strong 1035hPa high to move in from west from 22nd to 25th with a slight to moderate frost risk on 23rd
to 24th inland. Another weak
front due 25th with isolated showers, but only in the far south.
Moist onshore WSW flow to persist till 28th bringing further
isolated showers, less likely with northern and eastern extent before a
moderate front with showers possible on 28th. Rainfall for period mm
about 4-8mm generally however localised total in streams towards the Melrose
area may reach 15-20mm and windward slopes and higher terrain closer to 8-15mm.
Totals diminishing in far N and towards backs of ranges less than 2mm.
Lower North
Weak front due 19th with scattered showers in
a moist SW flow though more isolated with N and E extent. Secondary front with
a re-invigoration of showers due around around 8am or so on 20th, with showers
more frequent and heavier in any streams off the gulfs. Weak trough with
further isolated showers on 21st but not a lot in it, more likely in
the south and higher terrains. Strong 1035hPa high to move in from west from 22nd
to 25th with a slight frost
risk on 23rd to 24th inland and in low lying valleys.
Another weak front due 25th with isolated showers, but mostly in the
far south. Moist onshore WSW flow to persist till 28th bringing
further isolated showers, less likely with northern and eastern extent before a
moderate front with showers possible on 28th. Rainfall for period mm
about 5-10mm generally however localised total in streams off the gulfs may
reach 12-18mm. Totals diminishing in far east off backs of ranges to less than
3-5mm.
Yorke Peninsula
Weak front due 19th with scattered showers in
moist onshore flow. Secondary front with a re-invigoration of showers due
around around 3-4am or so on 20th, with showers more frequent and heavier about
the foot early morning. Weak trough with further isolated showers on 21st
but not a lot in it, easing later in the day. Strong 1035hPa high to move in
from west from 22nd to 25th however perhaps isolated
showers about the foot of the Peninsula may persist in westerly flow with high
positioned to north. Slight frost risk 23rd in the far N. Another weak front due 25th with
isolated showers, chiefly about the foot and winds likely to be fresh to strong
near the front. Moist onshore WSW flow to persist till 28th bringing
further isolated showers, less likely inland before a moderate front with
showers possible on 28th. Rainfall for period generally 5-8mm with
isolated totals about the foot to 15-18mm. Streams also in the N off the
Spencer may reach 10-12mm. Lighter isolated totals about the east coast of
3-5mm.
Kangaroo Island
Weak front due 19th with scattered showers in
moist onshore flow. Secondary front with a re-invigoration of showers due
around around 1am or so on 20th, with showers more frequent and heavier within
the band early morning. Weak trough with further isolated showers on 21st
but not a lot in it, easing by lunchtime. Strong 1035hPa high to move in from
west from 22nd to 25th however occasional showers will persist in a moist
westerly flow with high positioned to north.
Another weak front due 25th with isolated to scattered
showers, and winds likely to be fresh to strong near the front. Moist onshore
WSW flow to persist till 28th bringing further isolated showers
before a moderate front with showers possible on 28th. Rainfall for
period generally 10-15mm with isolated totals about the western end of 20-25mm.
Totals diminishing towards the E end of 5-10mm.
Adelaide Plains
Weak front due 19th with scattered showers in
moist onshore flow. Secondary front with a re-invigoration of showers due
around around 6-7am or so on 20th, with showers more frequent and heavier about
near the frontal band early morning, especially closer to the foothills. Weak
trough with further isolated showers on 21st but not a lot in it,
easing later in the day. Strong 1035hPa high to move in from west from 22nd
to 25th however perhaps isolated showers about the southern suburbs
may persist in westerly flow with high positioned to north. Slight frost risk
23rd and potentially early 24th about outer plains. Another weak front due 25th with
isolated showers, chiefly south of about Adelaide and winds likely to be fresh
to strong near the front. Moist onshore WSW flow to persist till 28th
bringing further isolated showers, less likely north of Adelaide before a
moderate front with showers possible on 28th. Rainfall for period
generally 10-15mm with isolated totals about the foothills to 20mm. Streams
also off the gulf may cause local 15-20mm totals. Total generally lighter about
the western suburbs down to 5-10mm.
Mt.Lofty Ranges
Weak front due 19th with scattered showers in
moist onshore flow. Secondary front with a re-invigoration of showers due
around around 7-8am or so on 20th, with showers more frequent and heavier about
near the frontal band early morning, especially about the central and southern
ranges. Weak trough with further isolated showers on 21st but not a
lot in it, easing later in the day. Strong 1035hPa high to move in from west
from 22nd to 25th however perhaps isolated showers about
the southern Fleurieu may persist in westerly flow with high positioned to
north. Slight frost risk 23rd and potentially early 24th
about low lying valleys. Another weak
front due 25th with isolated showers, chiefly south of about
Adelaide and winds likely to be fresh to strong near the front. Moist onshore
WSW flow to persist till 28th bringing further isolated showers,
scattered south of about Mt.Lofty before a moderate front with showers possible
on 28th. Rainfall for period generally 15-25mm with isolated totals
about known wet spots and the Southern Fleurieu to 30mm. Streams also off the
gulf may cause local 25-30mm totals with terrain lift. Total generally lighter
about the northern ranges down to 8-15mm
Riverland/Mallee
Weak front due 19th with isolated showers,
scattered in the south of the Mallee in a moist SW flow. Secondary front with a
re-invigoration of showers due around around 8am or so in the Mallee and 10am
or so in the RIverland on 20th, with showers more frequent and heavier in the
SW Mallee and in any streams that may occur off the Lakes. Weak trough with
further isolated showers on 21st but not a lot in it, mostly
confiend to the back side of the ranges and S Mallee. Strong 1035hPa high to
move in from west from 22nd to 25th with a slight to moderate frost risk on 23rd
to 24th inland. Another weak
front due 25th with isolated showers, but chiefly for the far S of
the Riverland and Mallee. Moist onshore WSW flow to persist till 28th
bringing further isolated showers to the SW/S Mallee though remaining largely
dry north of this before a moderate front with showers possible on 28th.
Rainfall for period mm about 4-6mm about the SW/S Mallee decreasing to 2-4mm
for the remainder. The Riverland and eastern border areas are likely to see
less than 1-2mm. Localised totals off the Lakes in streams may boost some areas
to 10mm.
Upper SE
Weak front due 19th with isolated to scattered
showers in a moist SW flow. Secondary front with a re-invigoration of showers
due around around 5am or so on 20th, with showers more frequent and heavier
near the frontal band. Weak trough with further isolated showers on 21st
but not a lot in it, slightly more frequent towards the Lower SE. Strong
1035hPa high to move in from west from 22nd to 25th though given the westerly flow, frost risk
should be low and chiefly N of the district.
Another weak front due 25th with isolated showers with windy
conditions likely near the front. Moist onshore WSW flow to persist till 28th
bringing further isolated to scattered showers, more frequent near the Coorong
coastline. Rainfall for period mm about 4-6mm, decreasing to 2-4mm towards the
NE corner of the district. Localised totals off the Lakes and Coorong in
streams may boost some areas to 10-15mm.
Lower SE
Weak front due 19th with scattered showers in
a moist SW flow. Secondary front with a re-invigoration of showers due around
around 1-2am or so on 20th, with showers more frequent and heavier near the
frontal band. Weak trough with further isolated to scattered showers due very
late 20th and continuing into 21st with possible thick
drizzle patches at times. Strong 1035hPa high to move in from west from 22nd
to 25th though given the
moist westerly flow, frost risk should be low and chiefly N of the
district. Another weak front due 25th
with isolated showers with windy conditions likely near the front. Moist
onshore WSW flow to persist till 28th bringing further showers,
possibly tending to rain at times with colder air to follow potentially
bringing isolatd thunderstorms. Rainfall for period mm about 20-30mm,
decreasing to 7-15mm towards the NE corner of the district. Isolated lower
totals down to 5-7mm out that way. Localised totals towards the southernn
coastline to 40mm.
Western VIC
Weak front due 19th with scattered showers in
a moist SW flow. Secondary front with a re-invigoration of showers due around
around 2-3am in the SW district or so on 20th, with showers more frequent and
heavier near the frontal band. Weak trough with further isolated to scattered
showers due very late 20th and continuing into 21st with
possible thick drizzle patches or patchy rain areas at times near the coast.
All shower activity more isolated over the Wimmera and very isolated to
negligible over the Mallee District. Strong 1035hPa high to move in from west
from 22nd to 25th
Slight frost risk on 23rd, chiefly over the Mallee. Another weak front due 25th with
isolated showers with windy conditions likely near the front over the SW
District. Moist onshore WSW flow to persist till 28th bringing
further showers, possibly tending to rain at times with colder air to follow
potentially bringing isolatd thunderstorms, again only over the SW district
with drier conditons further north. Rainfall for period mm about 18-25mm,
decreasing to 3-10mm towards the Wimmera and less than 1-3mm over the Mallee.
Localised coastal totals to 40mm are possible.
Final Notes
·
El Nino and
positive IOD locked in, both looking at a moderate strength event
·
Drier than
average rainfall right through till September at least however there will be
good periods of vigourous cold fronts spaced through winter with areas south of
Goyders Line seeing most benefit
·
More wind than
usual with these fronts which may be an issue in dry areas
·
Onshore period
with a few fronts in the short term bringing scattered showers, with heavier
totals in localised streams off the gulfs and lakes
·
Large swells
over Southern Ocean through late May and into early June
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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