In late fall and winter, attention is always focused on the stratospheric polar vortex. Its characteristics and behavior also have an influence on the actual weather layer – the troposphere – with a time lag. In addition, the latest long-term forecasts for the winter months were published by ECMWF and NOAA on November 5th.
Intact polar vortex in the coming weeks
The polar vortex is currently "healthy". In its center, temperatures at the 10 hPa level are currently around -76 degrees, and current forecasts predict that they will remain in this range in the coming weeks.
Fig. 1: Currently intact polar vortex, temperature at approx. 30 km altitude (10 hPa level, GFS); Source: meteociel.fr
The band of westerly winds surrounding the polar vortex is also well developed. This is unlikely to change much until well into December. The vast majority of model members (see below) show persistent westerly winds, with only a few outliers. Sudden stratospheric warming is therefore very unlikely in the near future.
Fig. 2: Forecast index of the mean zonal wind at the 10 hPa level. Positive for westerly winds, negative for easterly winds; Source: ECMWF
This means that the coldest air is also concentrated one level lower in the troposphere in the high latitudes, making extreme outbreaks of cold air far to the south less likely. The polar jet stream will remain relatively strong and the weather along the polar front dynamic. For Western and Central Europe, this would mean Atlantic westerly weather on average (changing flow patterns between southwest and northwest). New strong areas of low pressure have been forming over the North Atlantic recently, and this low-pressure-dominated weather seems set to continue in the medium term.
The new seasonal forecasts from ECMWF and NOAA
The new seasonal forecasts were published the day before yesterday, and they also seem to confirm this trend. To clarify, this is not a specific weather forecast, but a large-scale and long-term assessment of flow and pressure patterns. This is done on a global and continental scale for the deviation of various parameters from the climatic mean. This is only a rough trend! So here is a comparison of the latest data from ECMWF and NOAA for the anomalies in air pressure, precipitation and temperature for the months of December, January and February (i.e. the meteorological winter).
Low pressure determined
The persistently brisk low-pressure activity over the North Atlantic is also reflected in the long-term seasonal forecasts. The air pressure in this 3-month period therefore tends to remain lower than the long-term average. Both models see the strongest negative deviation in the northwest. (The comparison period is slightly different: basis for ECMWF 1993-2016, for NOAA data 1984-2009).
Fig. 3: Deviation of air pressure from the long-term average in Europe for the months of December, January and February (ECMWF); Source: ECMWF
Fig. 4: Deviation of air pressure from the long-term average in Europe for the months of December, January and February (CFSv2, NOAA)); Source: tropicaltidbits.com
More precipitation
On average, more frequent low-pressure weather also tends to mean more precipitation, and this is also reflected in the long-term forecasts. The months of December, January and February will be somewhat too wet. These figures do not say whether it will be rain or snow. However, above-average amounts of snow are expected, at least in the high Alps.
Fig. 5: Deviation of precipitation from the long-term average in Europe for the months of December, January and February (ECMWF); Source: ECMWF
Fig. 6: Deviation of precipitation from the long-term average in Europe for the months of December, January and February (CFSv2, NOAA)); Source: meteociel.fr
Too mild
ECMWF and NOAA's CFSv2 show a similar picture for temperatures, meaning that winter in Europe will be too mild. The positive deviation in the south-west and north-west is comparatively small, but otherwise very pronounced.
Fig. 7: Temperature deviation from the long-term average in Europe for the months of December, January and February (ECMWF); Source: ECMWF
Fig. 8: Temperature deviation from the long-term average in Europe for the months of December, January and February (CFSv2, NOAA)); Source: tropicaltidbits.com
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