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Summer 2021 weather: long range seasonal forecast

06/05/2021

2021 Season
François-Xavier Branthôme
California,
Middle East
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Early forecast shows a stronger monsoon in the United States, while Western Europe will likely face drought conditions.

Summer 2021 early forecast summary
There is a general agreement between the models, that a low pressure zone develops somewhere over northwestern and northern Europe. 

In Europe, mostly warmer than normal conditions are expected across much of central and southern Europe. 
Drought conditions are likely across Western Europe, with the risk currently being highest for Spain and France. Central Europe is also likely to experience drought conditions, but with fewer drought days than Western Europe.

Over North America, there is a prevailing idea of warmer than normal temperatures across the northwestern United States and Midwest, extending into southern Canada. Drier conditions are likely over these regions, while more rainfall is possible over the eastern United States and south eastern Canada.
Stronger summer monsoon conditions are likely over the southwestern United States, which would come as a relief to exceptional drought conditions currently ongoing in the region.
 

Three main (or most used) seasonal models
Summer 2021 is slowly and steadily approaching. But we still have over a month of springtime left, so we decided to take a look at the early summer trends for 2021 across Europe and North America.
April has so far shown a not-so-typical spring face, with unseasonably cold air moving down into central and southern United States. Europe has also experienced frequent cold fronts, with spring snowfall events and frost damage (See related articles below).
 
So far, 2021 is not really abnormally warm for this time of year. There are several areas across the world that stand out in cold anomalies. Siberia, Alaska, south-central United States, Europe, and Australia stand out with cold anomalies. On average the temperatures are still warmer than normal globally, but 2021 is perhaps starting surprisingly on the lower end of the past 10 years.

The graph below shows the global temperature anomaly from 2012 to 2021. You can see that so far, 2021 is starting off with the lowest global temperatures in the past 6-7 years. That is also partially due to the cold phase (La Nina) in the tropical Pacific Ocean. 

 
But will this trend continue for the rest of 2021? Weather experts checked out what the latest summer trends are. For the summer 2021 forecast, they decided to focus on the 3 main (or most used) seasonal models. The ECMWF and UKMO from Europe, and the CFSv2 from the United States.
All these forecasts show an average picture over the course of 3 months (June-July-August) and reveal the general prevailing weather pattern forecast. This does not mean that such weather conditions would last for 3 months straight. It only shows/implies how the weather patterns might look 40-60% of the time.
 
Summer 2021 ECMWF forecast 
What stands out with the ECMWF model is the higher pressure over central and southern Europe, and a low-pressure zone indicated over northern Europe. ECMWF also shows much of Canada and the northern United States under higher pressure, with a low-pressure zone over the western Arctic Ocean.
 
Europe ECMWF forecast
Looking closer at surface temperatures, first over Europe. We can see much of southern and south-central Europe being warmer than normal. This is due to the expanding higher pressure area from the south. Northern Europe and Scandinavia are closer to average temperature. These conditions imply that strong summer heatwaves are possible, perhaps lasting longer than normal, and with higher temperatures.
 
The rainfall anomalies also imply the presence of a strong high pressure system over southern and central Europe. Much drier than normal conditions are present over southwestern and central Europe. This is a direct sign of a high likelihood of drought conditions in Spain, France, and partially the northern Balkans.
 
 
North America ECMWF forecast
ECMWF surface temperature forecast shows a large area of warmer than normal temperatures over the northwestern United States and southern Canada. The warm anomalies extend into the central and southern parts of the country.
 
The rainfall anomalies show a drier than normal summer over northern and central United States and southern Canada. We can also see wetter than normal conditions over the southwestern United States.
 

The image below shows the current drought conditions across the United States. Exceptional drought conditions prevail over the southwestern United States, following a dry winter season. This would imply, based on the historic data above, that the summer monsoon might be stronger than normal, which is exactly what the ECMWF rainfall forecast suggests.

 
Summer 2021 UKMO forecast
The second model of choice is the UKMO model, from the United Kingdom Met Office. Just like the ECMWF, the UKMO also shows the low pressure zone over northern Europe. But it is pulled a bit more back towards the west. This second model shows warmer air mass over southern and central Europe, and over parts of the northern United States.
 
Europe UKMO forecast
The UKMO summer forecast for Europe also shows warmer than normal conditions over southern and central Europe. But the anomalies are not as strong as with the ECMWF forecast. 
The rainfall forecast supports that idea; normal conditions prevail over the British Isles, while drier summer conditions can be expected over Western Europe. The drought conditions are not as strong as in the ECMWF forecast, likely due to the less amplified high pressure zone extending from the south. But both models agree on drier and warmer than normal conditions in Spain and France.
 
North America UKMO forecast
The UKMO summer temperature forecast is quite uneventful over North America, as it has most of the continent under warmer than normal conditions. Stronger warmer anomalies are indicated over the Midwestern United States. Canada is also shown with mostly above average temperatures. 
The rainfall forecast does not suggest a stronger monsoon in the southwestern United States. Drier conditions are hinted over the Midwestern United States and southern Canada.

Summer 2021 CFSv2 forecast
As a counterbalance, meteorologists decided to use the main North American long range weather model, the CFS version 2 from the NOAA/NCEP in the United States. 

Looking at the air mass temperature anomaly forecast, it suggests much warmer conditions over Siberia and a warmer than normal zone over Europe and the northwestern United States.

 
 
Europe CFSv2 forecast
The CFS also has warmer temperatures over southern and central Europe, but no strong warm anomalies over Spain.
There is quite a difference between the CFS and the previous two European models when it comes to rainfall. CFS model actually shows wetter than normal conditions over Western Europe. Drought conditions are present further east towards central Europe, northern Balkans, and Eastern Europe. 
 
North America CFSv2 forecast
The temperature forecast for North America shows warmer than normal conditions for the western United States and south central Canada.
Much drier conditions prevail over the northwestern United States and southwestern Canada. There are no direct signs of a stronger than normal southwestern summer Monsoon.

Sources: severe weather.eu, ecmwf.int, climate.copernicus.eu, cpc.ncep.noaa.