London is experiencing its warmest days of the year so far late this week. Whilst temperatures will be trimmed slightly by Sunday, this year's London Marathon will be one of the warmest on record.

The 38th running of the London Marathon will be a stark contrast to the Boston Marathon which took place on Monday amid heavy rain, strong winds and temperatures near 5 C (41 F).

A high temperature of 21 C (70 F) is forecast for Sunday in London with a temperature near 17 C (62 F) at the start of the day’s events. The all-time record for the marathon is 22.2 C (72 F).

This unseasonable warmth will occur as air from Africa and the Mediterranean settles over the United Kingdom through this weekend.

While spectators will enjoy the warmth and sunshine, racers would rather have more seasonable temperatures for the event. The normal high in London on April 22 is 14 C (57 F).

Racers will have to take the necessary precautions to not overexert themselves amid the warmth.

At least, temperatures will be trimmed from Thursday's and Friday's highs of 26-27 C (around 80 F) that are having many in London wondering if the calendar has been flipped ahead to summer.

Cloudy spells will offer some relief from the strong April sun.

Another concern will be for a thundery shower during the afternoon hours.

"Any shower or thunderstorm could create a slick running surface," said AccuWeather Meteorologist Tyler Roys. Lightning would pose a greater risk to spectators and runners for a brief time.

Wind is not expected to play a significant role in the race as southwest winds of 7-14 mph (11-22 km/h) are forecast. Occasional gusts to 20 mph (32 km/h) are possible during the afternoon and during any thunderstorm.

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Events will get underway at 8:55 a.m., local time, with the elite wheelchair class. The last wave of runners start at 10 a.m.; this includes the elite men and mass start.

Her Majesty The Queen will be the official starter for the race this year, according to the London Marathon’s official website. Prince Harry will be at the finish line.

Approximately 40,000 runners are expected to take part in the event that runs from Blackheath to The Mall alongside St. James’s Park.

A pollution band of moderate was issued on Friday. An import of polluted air from the near-continent is expected, while dispersion of local emissions will be poor in light winds.

Alert for #LondonMarathon | Multiple sources confirm particles including #SaharanDust expected around @WHO guideline levels on Sunday morning. Check health advice for MODERATE 4/10 #airpollution https://t.co/Xs2vhczST5. @LondonMarathon should advise you cc @H_S_E. Thread pic.twitter.com/JIOHTa2wVE
— Clean Air in London (@CleanAirLondon) April 21, 2018