CLIMATE UPDATE - JULY 2021

HEADLINES


  • We have already achieved above average rainfall through parts of NSW and QLD during the past 24 to 36hrs with heavy rainfall expected to bring up rainfall totals well and truly above the July average for the month.

  • A wet signal continues for southwest and western parts of WA with an active westerly zonal pattern developing, that signal has been largely unchanged for weeks now.

  • Rainfall chances do pick up for eastern and southeastern areas as we go through the month.

  • Temperature outlook again largely unchanged from recent forecast packages, with the warmer than normal dry season continuing and pockets of cooler than average weather expected over the southern parts of the nation. Though the cooler signal is weakening in line with the IOD developing for southern areas.

  • Indian Ocean and SAM likely to be in phase with one another from next week for about 10-14 days bringing active weather for most of southern Australia, while the eastern inland returns dry after this wet start to the month.

OUTLOOKS


%chance of exceeding the median July Rainfall - 2021.

%chance of exceeding the median July Temperatures - 2021.

CLIMATE DRIVERS


SAM - Positive today but expected to trend neutral next week and the possibly negative in a week's time meaning that more frontal weather will emerge and wet weather returns to WA with enhanced rainfall chances, sweeping through coastal areas of southern Australia and into the southeast. The eastern inland after above average rainfall over the coming days turns dry under this climatic driver.


SAM Forecast - Next 2 weeks - Valid July 2nd 2021.

We can feel the impacts of the SAM in positive phase with generous inland rainfall for QLD and NSW. The southeast has been mostly dry for days now as a result but a front moving through today is quite anomalous during a positive phase so enjoy that rainfall in southeast SA and VIC. The pattern flips in about a week and that is being reflected in short term modelling....more here

IOD - Indian Ocean - Remaining negative and continue to strengthen that signal as we track into late winter, through spring and a chance that it may still be waning in early summer. That will trigger widespread rainfall across the nation, especially for southeast and southern inland areas, and then graduating through the spring to QLD and interior parts. For the short term we can see the impacts of the IOD negative phase introducing moisture into cold fronts from Sunday and watch the event in about a week, that could be the biggest cloud band of the year so far and bodes better for parts of WA and SA.


IOD Outlook - Valid July 2nd 2021. BoM Model

The negative phase promotes a warmer Indian Ocean, more moisture and more rainfall over Australia, especially southeastern and eastern inland parts as well as the outback of Australia. More can be found here

ENSO - Remains neutral for now, but climate agencies are using the La Nina term again as we track into Summer 2021/22. I am still not convinced of this but there are signs that we may be seeing a cool neutral at least for the Pacific Basin, which would have marginal impacts on the rainfall and temperature outlooks for the summer season, but worth keeping an eye on with the negative IOD in place. A La Nina and a negative IOD would drench the nation through spring and summer so it is not impossible but the probability of both being in place is low.


ENSO Forecast - Nino 3.4 outlook. BoM Model.

There is a sharp trend towards a La Nina on the BoM model and some other climate agencies, but the signal is not overwhelming, but as mentioned in previous posts I note that some members are suggesting a deep La Nina and none are suggesting an El Nino so at worst, a seasonal/average spring and summer is likely for the eastern inland. Though the IOD will bring up rainfall chances for many.

Taking the international data on the La Nina possibilities - and there are signals that it could be on this summer, but I am being more conservative than some based on the reality we are in a low confidence period where the climate models struggle statistically at this time of year.


Climate Forecast System - ENSO outlook.

On the cusp of a weak La Nina. Many models are lower than the mean so that is interesting and another model to watch.

IRI ENSO Outlook - Probabilistic forecasts based on the model data coming into this reputable agency, again tilting the overall forecast into a weak and short lived La Nina come summer time.

Model predications from the CPC - Most keep the spring and summer cool neutral but there is a few that develop a La Nina. Ignore the outlier that plunges us into a very deep La Nina.


THE RAW DATA


Lets start with the rainfall because that is really what everyone wants to know on the land.


The data sets continue to stregnthen the signal for raifnall being above average through July and increasing that signal in the back half of the month and as we enter August. The signals for August at moment are wet wet wet. I will have more on that next week.


Rainfall for the coming 6 weeks - Euro Ensemble. Valid 2nd of July 2021.

The rainfall anomalies are quite high for southern coastal areas of the nation, but I caution you that you may see a lot of blue and green on the map, that doesn't always mean you are in for wet weather. The model is expressing a higher chance of above average rainfall, but like QLD, one event is all it takes to bring numbers up with a decent break in between so read the below data in context of your own climate averages. For the outback, a shower is all it takes for rainfall to be above average in July. For the southwest of the nation, it will be very wet with persistent rainfall to get those values achieved.

Rainfall for the coming 5 weeks - GFS Ensemble. Valid 2nd of July 2021.

Similar to Euro, the rainfall spread of this model is less intensive than Euro, but is a week shorter in it's run. But there are some climate models that only bring an average July across the nation, but I think that is unlikely given some areas have already experienced above average rainfall so far this month and we are on day 2. But it is important to contrast.

The anomalies for each model paint the picture for the month of July where one is more bias to the dry signal and the other more reliable model suggesting the wet signal.


GFS Ensemble - Rainfall anomalies for July 2021. Valid Friday July 2nd 2021.

The dry signal I am continuing to throw out but I just wanted to show you this, in case that the dry signal does indeed verify, but I am suggesting that is very very low chance.

Euro Ensemble - Rainfall anomalies for July 2021. Valid Friday July 2nd 2021.

Much more realistic and has captured the monthly outlooks well all year and I do believe that it will capture this outlook quite well. Probably a tad too dry for the tropical coast of QLD, especially if the rainfall in the short term modelling verifies later next week about that region.

CFS Ensemble - Rainfall anomalies for July through September 2021. Valid Friday July 2nd 2021.

Tie breaker to see where the bias lay for the season and it is looking wet. The climate forecasting system suggesting quite a wet July and indeed a wet season ahead through to September. This should be encouraging for many on the land that the wet signal is extending further inland and over QLD as well as SA.

Euro Ensemble - Rainfall anomalies for July through September 2021. Valid Friday July 2nd 2021.

Continuing the wet signal inline with CFS, the theme will be it gets wetter as we go through this year with the negative IOD the main trigger for this and that influence increasing as we see the event peak in spring 2021. The Euro is much more conservative but seeing that level of green is quite anomalous for this model. And the northern tropics note that blue shading offshore - humidity could kick off early this year.

The SSTs observed during the past month also suggest the temperature spread for the region alongside rainfall looks good for July. Rainfall derived from the warmer waters off the northwest of the nation and also over the southern coast line will continue. And with the warmer SSTs over the northern parts of the nation, this will keep temperatures up over the northern tropics with higher humidity values.


Average SST Anomalies for the past 4 weeks - Valid July 2nd 2021.

Pacific may look cooler than normal but it has recovered from the La Nina of summer.

Temperature data is largely unchanged from the model sets as well.


Euro Ensemble - Temperature anomalies for July 2021. Valid Friday July 2nd 2021.

Warm bias continues for the north and northwest in line with the IOD phase and the cooler than average temperatures for July may continue for the west coast and across southern and southeast Australia with more rainfall, cloud and a long period of westerly winds and cold fronts. In the short term that is being picked up by modelling through to mid month.

GFS Ensemble - Temperature anomalies for July 2021. Valid Friday July 2nd 2021.

The warm bias a little too strong for inland areas but now starting to fall into line with Euro for the west coast with cloud and windy weather and rainfall frequent enough to keep temperatures below seasonal for a number of days. That may run across the southern and southeast coastal areas as well, in line with the IOD phase.

A quick look at August temperature wise from the CFS and this is expressing a lot of colder weather to come with widespread rainfall.


CFS - Temperature anomalies for August 2021. Valid Friday July 2nd 2021.

Will have more on this coming up next week.

Need more climate information for your part of the world? Get in touch about tailored forecasts for the medium to long term. karllijnders@weathermatters.org



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