AccuWeather's 2021 Europe autumn forecast

Summer has brought opposite and disastrous extremes across Europe. Parts of north-central Europe were dealt deadly and historic flooding, with communities still facing lengthy recovery. Farther south, dangerous heat and destructive wildfires have left their mark in southeastern Europe. This week AccuWeather forecasters released their 2021 autumn forecast, and meteorologists say that the season may offer a glimmer of hope for these hard-hit regions.

Members of AccuWeather's international long-range forecasting team Tyler Roys and Alan Reppert say that the La Niña pattern developing in the Pacific, which will affect weather patterns across the globe, will be the driving force behind the conditions throughout the fall. The season officially begins with the autumnal equinox on Sept. 22. The winter of 2020-2021 also featured a La Niña pattern, so the transition season this year may hold some similar highlights to last year.

A couple takes a selfie photo in front of autumnally colored trees on a bridge over Brouwersgracht canal in the centre of Amsterdam, Sunday, Oct. 16, 2016. (AP Photos/Margriet Faber)

Some relief from stormy weather in central Europe

As the La Niña pattern ramps up across Europe during the months of September, October and November, the storm track that has been dominating over the northern portion of the continent will gradually shift south across the continent.

This will come as a relief to portions of Germany, Belgium, Netherlands, Luxembourg and northeastern France, where torrential rainfall produced historic flooding during the month of July. A drier pattern expected for the autumn months will be favorable for areas still recovering from flooding.

Fewer storms in the coming months over north-central Europe, especially during the second half of autumn, will bring favorable harvest conditions from southern England to Poland as the region dries out, according to AccuWeather forecasters. Drier soil will allow heavy machinery to navigate fields better during harvest.

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Temperatures across the region will also be near normal throughout autumn as this will be one of two sections of the continent where heat waves aren't expected during the autumn months, said Roys. Normal high temperatures typically range from 70-73 F (21-23 C) in early autumn to 39-47 F (4-8 C) late in the season across the region.

Western Europe faces storm concerns

Before the storm track fully shifts south in autumn, some storminess will linger over Ireland and the United Kingdom to start the season -- and some of these storms may pack a tropical punch.

Despite a record-setting hurricane season in 2020, none of the tropical storms survived the trek across the northern Atlantic Ocean, with all of the systems dissipating before reaching Europe. AccuWeather forecasters warn that this year could be a different story.

With sea-surface temperatures about 2-5 degrees Fahrenheit (1-3 degrees Celsius) above normal this year, there is plenty of energy to sustain any tropical features that track through the region, said Reppert.

"That is why one to three tropical entities are expected to reach Europe in the coming months," said Roys, adding that tropical storms or tropical rainstorms may survive the trip across the Atlantic and reach the continent.

During the first half of the season a tropical system would be most likely to strike in the region of Ireland, Northern Ireland, Scotland, Wales and northern England. But as the storm track shifts south into late October and November, this threat would also shift south into northwestern Spain and Portugal.

The tropics won't be the only source of stormy weather across western Europe in autumn.

A La Niña pattern ramping up during the autumn and taking hold during the winter would bring a windstorm season similar to the 2020-2021 season in western Europe. The number of windstorms during last year's season was right around normal, and AccuWeather forecasters expect the same this year. Similar to the tropical storm threat, the areas mostly likely to be impacted by windstorms will shift from northwestern Europe to southwestern Europe as the season progresses, bringing more storms into France and Spain than normal.

All of that stormy weather will help to keep temperatures in check across the region. Overall, temperatures throughout autumn are expected to be right around normal with no heat waves anticipated, explained Roys. He added that the mercury may even struggle to reach normal levels some days.

Normal high temperatures from the beginning of September to the end of November go from the 60s F (16-21 C) to the upper 40s F (7-9 C) across the U.K.

Flood threat to develop in central Europe

While the southerly storm track will offer a break for parts of Europe, others may not benefit from the seasonal pattern change.

A weather pattern with some storminess in western Europe combined with a blocking pattern over Scandinavia and Russia can lead to the development of cut-off low pressure areas, said Roys. That could set the stage for potent storms across southern Europe this fall.

Rounds of these storms can bring periods of rain, which can lead to above-normal precipitation for the season. With each passing storm, the risk for flooding across the Alps and northern Italy will increase as the rugged terrain can help to enhance rainfall.

This photo released by the French Securite Civile Monday, Oct.5, 2020, shows pile of trees on a bridge over La Vesubie river in Saint-Jean-la-Riviere, southeastern France, Saturday, Oct. 3, 2020. Flooding has devastated mountainous areas in France's southeastern region of Alpes-Maritimes and Italy's northwestern regions of Liguria and Piedmont, after a storm swept through the two countries on Friday and Saturday. (Sécurité Civile-UIISC via AP)

The development of these storms over southern areas may also lead to the development of medicanes. A medicane forms when a nontropical storm feeds off the warm waters of the Mediterranean and develops tropical storm characteristics. While they are often short-lived, they can bring heavy rainfall and locally strong wind gusts to the Mediterranean region.

As of the middle of August, sea-surface temperatures in the eastern Mediterranean are about 5-7 F (3-4 degrees C) above normal, explained Reppert. Temperatures are even about 2-4 F (1-2 degrees C) above normal in western portions of the body of water.

Periods of heavy rainfall can also spread into southern Italy and into the Balkans, especially during late October and November.

However, it may be a different story on the other side of the Mediterranean.

The near-normal temperatures that persisted across Spain during the summer months are predicted to continue throughout autumn. In September, normal high temperatures are in the 70s F (21-26 C) and drop generally into the lower 60s F (16-18 C) by the end of November.

According to Roys, Spain typically experiences stormy weather during the end of October and into November -- and this year will be no exception. As storms move across the country frequently, the risk for flash flooding can increase, especially if areas are repeatedly hit by rounds of heavy rainfall.

Summerlike warmth to linger in the east

A theme across much of Europe this autumn will be near- to above-normal temperatures, and eastern portions of the continent will be an area where the heat will persist past summer.

Conditions from Sweden to northern Ukraine and into western Russia will be warm through most of September, October and November. Temperatures will remain above normal, but normal temperatures also trend down throughout the season.

The average high temperature in eastern Europe is around the upper 60s F (18-21 C) in early September then falls into the lower 30s F (around zero C) by the end of November. In the Balkans and Greece, normal high temperatures drop from the upper 80s F (29-32 C) to the 60s F (16-21 C) over the same time frame.

Roys also added that when storms track into eastern Europe they can still bring enough of a cold blast to bring some early snow threats, especially later in the autumn.

Farther south, the more intense heat of the summer is expected to linger across southern Italy and the Balkans, especially early on, stated Reppert.

Flames burn on the mountain near Limni village on the island of Evia, about 160 kilometers (100 miles) north of Athens, Greece, Tuesday, Aug. 3, 2021. Greece Tuesday grappled with the worst heat wave in decades that strained the national power supply and fueled wildfires near Athens and elsewhere in southern Greece. As the heat wave scorching the eastern Mediterranean intensified, temperatures reached 42 degrees Celsius (107.6 Fahrenheit) in parts of the Greek capital. (AP Photo/Michael Pappas)

The core of heat will persist in the Balkans and into Greece where heat waves will be possible, mainly during the first half of autumn.

The World Meteorological Organization (WMO) defines a heat wave as a period of five or more days with average temperatures at least 9 degrees Fahrenheit (5 degrees Celsius) above normal.

The heat and dry conditions early in the season will allow the wildfires that have been ravaging villages across parts of the Balkan Peninsula to continue into autumn.

Even though the heat will be a theme through the end of the season, storms from the Mediterranean and along the southerly storm track can bring brief relief from the heat at times.

The arrival of more substantial rainfall arriving by late October and November cannot only bring an end to the wildfire threat, but it may also bring additional threats.

"A flash flood threat will spread from Italy into the Balkans, especially by November," said Roys, adding that heavy rainfall can also pose the risk of mudslides where burn scars are present.

Rounds of rain can also bring thunder, and any storms can bring downpours and gusty winds. Localized hail can't be completely ruled out.

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