Thread by @althecat

This thread is about global weather. As solstice approaches the cold is coming in from the north, the monsoon belt is moving south and weather is changing.

But:
- The Nth Hemis. remains rather wet.
- & Indian/WestPac tropics are continuing to produce lots of moisture.

The first animation (above) is a 16 day PWAT (Water/Energy) forecast for the North Atlantic, Nth Africa and Europe from 21st November.

During COP26 Tropical Atlantic atmospheric rivers kept Glasgow both warm and wet. And this has continued through November.

This is the rainfall forecast [@NOAA gfs1 model] with lots of rain over Europe. and across Algeria and Morroco. The first animation also showed significant water transport over the Sahara. And this satellite animation from today shows that happening.

These two observation graphs from @Meteoblue show another view of the current weather over Europe. The warm temperatures you can see over the Agean Turkey and the Eastern Med. are due to that warm wet Atlantic air.

Another view of the weather pattern today, a circulation which arches north up around the North Atlantic has been bringing chilly northerlies into Western Europe for the past few days. But in the East the dominant weather pattern is coming from the Atlantic/African monsoon belt.

Here's a current long range weather forecast, and those Tropical Atmospheric water transport events are set to continue, but decrease in their intensity through the first week of December.

The combination of northern cold and incoming moisture means snow in the forecast, in large enough quantities to accumulate on and around the Alps with significant falls in Switzerland, Austria, Croatia and Hungary in the offing.

This is a seven day rainfall anomaly forecast for the Northern Hemisphere. It begins Tuesday 6th December and runs through December 14th. It tells us the models are expecting northern hemisphere rainfall to return towards climate norms over the forecast period.

And here is the corresponding graph from a week earlier. which does not show as great a return to normal. We would need some deeper analysis to confirm this but my impression is this data is telling us the Northern Hemisphere is remaining warmer and wetter for longer than normal.

So then the question becomes what about the Southern Hemisphere. Is it about to experience something similar to the Northern Hemisphere summer in terms of extreme weather events.

Last Feburary East Australia experienced a massive flooding event, will there be a repeat?

But first up... lets look at the global picture (as forecast 21-30/11/21.

This is what the world's water system looks like in total. Heat in the tropics causes evaporation and aerial water flow from East to West around the globe.

Atmospheric rivers shoot back in the opposite direction from West to East in the northern and southern hemispheres. These tend to be stronger where the water builds up as it gathers over land, South America, Africa, India and South East Asia /West Pacific.

The PWAT anomaly view of atmospheric water is my favourite ways to visualise this as it strips out the noise and shows you the largest concentrations of water formed by these atmospheric rivers. Here we are looking at a period beginning 21st Nov. (above Nth Hem - below Sth Hem)

And again as we did before we can now compare what the models are showing now vs what they were showing a week earlier. The volume of atmospheric moisture activity ought to be moving south around now.

[21st Nov >>

https://twitter.com/althecat/status/1465338368275922945?s=20

]

These global fluid dynamics simulations from the major global models are surprisingly accurate about the general trends over the 16 day period of the forecasts.

Here we see a 21st November simulation of the Southern Hemisphere.

And its 29th November update.

While so far we can only see an initial snapshot of the change of seasons here it is apparent even in these two plots that the amount of tropical water heading south is building. Australia, Patagonia and and South Africa are the populated areas on the right and bottom.

Here are 21st November 16 day rainfall forecasts for Australia, South America, Oceania (incl NZ) and Southern Africa.

And 29th November 16 day rainfall forecasts for the same regions. [These rainfall forecasts are all from the @NOAA GFS model.]

Looking at these forecasts two things stand out to me.

1. Australia's forecast rain (left 21 Nov - right 29 Nov) in the east is already indicating flooding potential, and is extremely widespread over the continent.

2. The forecast rain in the east of Southern Africa (left (21 Nov - right 29 Nov) is also very significant - and notably that forecast for southern Madagascar which is in a very long drought - is very encouraging.

And finally.

Ordinarily the fact we are in a La Nina period ought to mean that dry conditions would be expected in the southern hemisphere. However appears to not be the case yet. It will be interesting to see what happens as the southern summer gets fully into gear.

/ENDS

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