The image below shows the average position of the jet stream during the La Nina seasons and the corresponding weather development over North America. The twisted jet stream brings colder air and storms down from Canada into northern and the northwestern United States, and warmer and drier weather to the southern parts.
The La Nina is already past its coldest point and is currently expected to go into a neutral phase in spring. But a neutral phase looks to be dominant as we go further into Summer The actual ocean temperature forecast for Spring from ECMWF, shows the La Nina being present, but it is starting to get eroded in its eastern parts, and replaced by neutral to warmer waters. We now know what ENSO is, and how it impacts the pressure patterns.
Now we will take a look at the global long-range models, and how they forecast the developing La Nina Spring. Looking at the general La Nina correlation, we generated an expected pressure pattern image below, for the spring season Mar-Apr-May , based on the data from In Europe, we have lower pressure over western Europe and higher pressure towards the east.
For the spring forecast, we decided to focus on the 2 main or most used seasonal models. All these forecasts are an average picture over the course of 3 months March-April-May and show the general prevailing weather pattern forecast.
This does not mean that such weather conditions would last for 3 months straight. The ECMWF model is most often referred to as the most reliable model, at least in the long-range category. In reality, a lot depends on the individual situation and individual seasons. We are only looking at trends and how the weather patterns might evolve over the entire continents or the planet. A big part of the weather story lately is the breakdown of the Polar Vortex, due to a sudden stratospheric warming event.
To keep it simple, this can have a profound impact on the pressure patterns and the developing weather. At the beginning of the forecast in January, we can see the graph goes into negative numbers. That means the Polar Vortex has lost control and was broken apart or displaced. Why is this important? ECMWF is the only model that accurately sees this breakdown event at the beginning of its forecast.
This makes it more likely for the model to get a better longer-term picture, as it correctly sees the developing weather dynamics. Other models did not capture these strong events well enough, which can be a limiting factor. We will drop a link to an article at the bottom, which explains the Polar Vortex and these events in more detail.
Now, looking at the pressure pattern forecast for Spring Of course, we can see right away that the signature high-pressure system of the La Nina is present in the North Pacific. But we can see a neutral zone, which does mean in this context that a low-pressure zone will be present over western Canada and the northwestern United States. We can also see a blocked North Atlantic, with a hint of lower pressure over western Europe, like it was hinted at in the correlation graphic above.
However, looking at the surface pressure anomaly, we can see the low-pressure system over the northwestern United States and western Canada. Here we can also see another high pressure in the North Atlantic, which is likely somewhat related to the Polar Vortex breakdown in early January We use the temperature anomaly at mb level around 1. Most of the United States is warmer than normal, as is eastern Canada. This is a likely response to the lower pressure over western Canada, which promotes southerly flow over central and eastern United States and also eastern Canada.
Looking closer at Europe, we see an interesting progression from west to east. It is evident that cooler airmass from the Atlantic will be present for western Europe. Further east, warmer conditions prevail. This can change when the pattern re-adjusts and a cold outbreak can slip through from the north or the northeast.
Over North America, we see quite a warmer than normal pattern allover. Colder than normal conditions will prevail likely over western Canada and parts of Alaska.
Given the jet stream position during a typical La Nina season, the colder temperatures should extend further towards the northwestern and northern United States. We can see mostly above-normal temperatures over the continental United States and the Midwest into eastern Canada. The precipitation anomaly forecast below shows a typical La Nina type pattern over Canada and the United States.
Northern parts are under wetter conditions, while drier conditions prevail in the southern United States. For central Europe where the cold was the most pronounced , the picture is the same. Such cold springs — which used to be common — are becoming ever rarer. Temperatures varied considerably over Europe during both March and April , but on average were not extreme over the continent during either month.
During May , most of Europe experienced slightly below-average temperatures. March was milder than usual over northwestern Europe, colder than the — average over southeastern and far eastern Europe, and close to average over the southwest of the continent. April temperatures were below average across an area sweeping from Iceland to the Mediterranean and Black Seas, with — for example — Germany seeing its coldest April in 40 years , but above average over the west of the Iberian Peninsula and over the far east of the continent.
The month began with unusually mild conditions in many places, but temperatures plummeted over an extensive region during the first week, reaching record lows over parts of western and central Europe, for example over Slovenia. As for May, Germany experienced its coldest since and maximum temperatures for the first three weeks of May in the United Kingdom were some of the lowest on record. There were, however, above-average temperatures in eastern Europe and in southern Spain, Greece, Turkey, and western Norway.
Despite Europe itself being colder than average, the parts of the globe surrounding the continent were much warmer than the — average, especially in April and May.
The Atlantic Ocean, North Africa, the Middle East and parts of Russia all saw high temperatures, with records broken in many locations. Areas surrounding Europe saw large variations in temperature from month to month, especially northern Russia. Surface temperature anomalies are often directly influenced by large-scale wind patterns, which are themselves linked to the distribution of areas of low and high atmospheric pressure.
The models predict normal to warm temperatures and that it will be slightly drier than usual. Southern Europe especially has a high chance of above-normal temperatures meaning above the long-term average of , while Britain and northern Europe should have normal temperatures, but drier conditions. Instead, the forecast warm weather is part of a large trend of warmer summers and more droughts thanks to climate change.
So since the world is warming, on average most forecasts will predict normal to warm summers. If you need to make an educated guess, this would be the best choice. Seasonal forecasts definitely provide an added value, if you know how to use them. But we need to be aware of their limitations and see them as a general prediction or sense of direction.
We expect the summer to be normal to warm, but this is just an increased chance given the information we now have based on the current conditions and forecast — any one prediction can of course still be wrong. After all, often nothing is as unpredictable as the weather. Festival of Social Science — Aberdeen, Aberdeenshire. Edition: Available editions United Kingdom.
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