There has been an absolutely spectacular change in the forecast for parts of the N Hemisphere.
In terms of N American weather regimes, this appears as a shift from a persistent Greenland High (with almost no chance of Pacific Ridge), to Alaskan Ridge followed by Pacific Ridge! 🧵 It's not just GEFS that has seen such a monumental change -- and it has significant ramifications for meteorological variables such as 2m temperature. Greenland High favours cold, which has disappeared from the forecast

https://x.com/VORTEXJeff/status/1757606595465249126?s=20

I created an NAO index and timeseries figure to mimic cpc.ncep.noaa.gov/products/preci…, but using ERA5 MSLP with a simpler method, and updated the EOF computation (and timeseries standardization) to cover 1959-2022. 🧵 Despite using different methods, different datasets, computed over different time periods, there is very good agreement (r = 0.92) with the multidecadal variability well-captured. However, ERA5's SLP NAO pegs 2018 as negative (-0.32), whereas CPC have it at +1...

Following on from what was the driest July on record for parts of England [metoffice.gov.uk/about-us/press…], it is staggering & very concerning to see 16-day accumulated precipitation forecasts showing zero or no meaningful accumulations for large parts of the country. 🧵 The above is just a single GFS run, but is representative of many recent forecasts. Furthermore, weekly-mean anomalies out to week 3 from ECMWF also show a signal for continued below-normal precipitation. [apps.ecmwf.int/webapps/opench…]

Historic, unbelievable chart.

GFS 18Z with widespread temperatures of at least 41°C in England on 16 July. The current UK record is 38.7°C.

It is possible that the model has runaway warming in its surface layer. It is also possible that an unprecedented heatwave may develop. This scenario would obliterate maximum temperature records across western Europe.

The NAO is heading negative. But what does that mean in reality? It is just one component of the overall anomalous flow -- meaning that whilst the NAO has a characteristic pattern, the overall field you see in the anomaly maps varies hugely depending on what else is happening. 🧵 This is where clustering-based regimes can be helpful, because they are not dealing with individual components. Instead, they aim to capture recurrent patterns that you would actually see in the full anomaly maps (typically 500 hPa height).

December update to the 8 seasonal prediction systems contributing to @CopernicusECMWF. All the models have now converged on a positive Arctic Oscillation pattern for JFM 2022. These are very strong ensemble mean anomalies, similar in amplitude to those from Dec 2019. Seasonal-mean maps at climate.copernicus.eu/charts/c3s_sea… where you can also compare with previous initialisation dates.

With forecasts suggesting a significant early-season disruption to the stratospheric polar vortex may occur soon, it’s worth thinking about how the timing of a vortex disruption can influence the effects – short and long term. 🧵 [1/9] Rossby waves cannot propagate into easterlies, and they also tend to propagate best along sharpened potential vorticity gradients. Significant disruptions to the vortex (like major SSWs) can thus limit or stop wave activity reaching the mid-upper stratosphere. [2/9]

There is now a significant deceleration of the Arctic stratospheric polar vortex in the medium-range GEFS & GFS forecasts -- which could set new date-record minima for 10 hPa 60°N zonal-mean zonal winds. 🧵 This vortex weakening event is forecast to be a wavenumber-1 disturbance, with the stratospheric vortex shunted toward Eurasia. In some ways this is a "Canadian warming" (doi.org/10.1175/1520-0…)

The big difference between the heatwaves of 1976 and 2018.
June 1976: the UK was one of the warmest places relative to normal across the globe, with most areas cooler than average.
June 2018: the UK was just another warm blob in a mostly warmer than normal world.
#GlobalHeatwave. Wow, this got far more exposure than I was anticipating! I’m glad this has made the global warming picture make sense for so many. Data and maps from the excellent @NASAGISS website, where you can plot similar maps (with whatever base period you like!).