CLIMATE OUTLOOK - DECEMBER 2021

Lets have a look at December 2021 in greater detail, as we are coming off the back of a cold Spring, many out there, as from what I have seen on social media are shocked by the lack of warmth this winter....no shock here. We have been talking about this since July and it has verified well. December will see a warming up of the nation, it is Summer after all, but the humidity values are expected to increase dramatically in line with that.


That will lead to more rain and storms, the cooler than average weather is expected to continue for the remainder of 2021. The above average rainfall is also expected to continue thanks to the developing La Nina and the waning IOD, which is continuing to drive rainfall opportunities in the short and medium term, but the influence will be handed off to the Coral Sea and the ENSO in combination with the SAM phase.


FORECAST

%chance of exceeding median rainfall for December 2021

Excessive rainfall has been observed over large parts of central and eastern Australia with flooding across many states. That will persist for the next week. The rainfall has waned and I want to point out that periods of drier weather amongst a wet forecast is normal as the atmosphere resets. With that said, a wet end to November and start to December is expected for many areas of the nation. The weather over northern Australia is likely to turn monsoonal before Christmas. This will have an elevated risk of rainfall turning above average over eastern and northern Australia, dipping into Central Australia. The remainder of the nation has a leaning bias towards wetter than average, but more at the mercy of upper level winds dragging the moisture into troughs to bring the above average rainfall. That means the bias of above average rainfall is expected once again the further north and east you go throughout the nation.

%chance of exceeding median temperatures for December 2021

Below average temperatures continue for much of inland Australia, as observed through these past 4-6 weeks. The coastal areas, thanks to above average SSTs is expected to keep the temperatures near to above seasonal at times, but mitigated by onshore winds and rainfall. The weather is expected to remain fairly cloudy over inland areas, especially once the nation's north turns wetter, drawing in the deeper moisture profile supporting the development of cloud and rainfall. This pattern is not unexpected given the developing La Nina phase.

%chance of exceeding median humidity for December 2021

The humidity has been shunted out of the nation, but I suspect that will be the last time this happens over northern and eastern Australia until we get to April 2022. The humidity values will be above average as seen through late October and early November. This has lead to increased rainfall, severe weather events and below average temperatures. This will continue. The further south and west you go through WA, the more seasonal the weather will be, thanks in part to high pressure anchored over the region.

SEVERE WEATHER RISKS

Flash Flood Risk December 2021

This carries the highest concern for the month of December with extensive moisture being drawn south and east across the nation with the early onset wet season verifying and assisting in producing above average rainfall chances for the nation. The weather likely to e nearer to seasonal for the west and southwest of the nation but as we saw last week, one trough is all it takes to produce flash flooding and this is the case for the coming month. The north of the nation with enhanced rainfall chances and deeper moisture content in the atmosphere will likely see flash flooding frequently. Finally, with the saturated catchments of large parts of the east, we are likely to see flash flooding more easily for much of QLD, NSW and into eastern VIC.

Large Hail Risk December 2021

The large hail risk remains very high of the first half of the month but I do think with the saturation of the upper atmosphere, as we have seen with the previous storm outbreak, the risk may translate into heavy rainfall and damaging winds being the primary focus.

Damaging Winds Risk December 2021

Damaging winds risk following the same script as November, with a very high chance associated where the air is likely to be warmer and more unstable with troughs over the NT into QLD and NSW posing the greater threat. There are damaging winds threats further south but likely to be infrequent and associated with organised batches of showers and storms near troughs/low pressure passing through.

Riverine Flood Risk December 2021

Moderate to major flooding is occurring over the eastern inland and this is expected to remain in place for the remainder of November. With enhanced rainfall chances moving into December, the flooding is likely to remain in place for most of December and possibly becoming more widespread as we move through the month if the rainfall forecasts verify. You don't need the heavy rainfall overhead for flooding to occur in your part of the world. Everyone living along the Murray River needs to watch the flood warnings downstream from Lake Hume in the coming 6 weeks.

Fire Risk Outlook - December 2021

The risk of fires through the north is over during December for the tropics as is seasonally acceptable with the wet increasing. I am pulling the risk off for most of QLD at this time with a wet November and December diminishing the risks. Low end risk for the southeast and east with a wet and humid spring persisting into December with seasonal to below seasonal temperatures. Medium risk of fires for the SWLD with the season hotter and possibly drier which is seasonally normal for this region.

Crop Disease Risk December 2021

We have seen significant humidity and wet weather to kick off November, now we are seeing the signal turn humid and more unsettled for the back half of November with that pattern continuing into December. The risk for stone fruit, citrus and vingerons in particular is quite elevated with the climate signals.

DATA

Euro Seasonal - December Rainfall Anomalies

The Euro is a lot more conservative but again the seasonal to above seasonal rainfall continues for December. I am tipping this model will increase rainfall as it is now picking up on the La Nina chances, it has been slower to pivot the ENSO into a La Nina.

CANSIPS Seasonal - December Rainfall Anomalies

The wet bias continues on this particular model, the overall trend of a bumper start to the wet continues and will drive the above average rainfall chances for the remainder of the nation.

CFS - Rainfall Next 6 weeks - December 2021

The data set remains unchanged and now is picking up on the monsoonal activity developing in the north and sweeping southwards during the latter part of the month. This is one idea that many models support, early cyclone season, early monsoonal season, enhancing rainfall and keeping summer heat low.

Control Member - December 2021

The signal from the deterministic and ensemble data is for a wet December with above average rainfall continuing with the below average temperatures, it has been consistent for the past few months that the wet signal is expected to continue into the early new year.

Euro Temperature Anomalies - Next 6 Weeks - December

Seasonal to below seasonal values are expected in the same regions as per November. The above average rainfall driving the cooler bias for large areas of the inland. There is a low chance of searing dry heat through to the end of the year and longer hot dry spells allowing us to dry out.

CFS Temperature Anomalies - Next 6 Weeks - December

This is probably running a little too warm over northern and eastern Australia but will keep an eye on the forecasts. With the wetter forecast, I am tipping cooler weather for many.

More details on Summer and Autumn coming up this afternoon



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