Things have changed a bit since the previous post so a quick update is needed for race day. Expect a final update Saturday night!
The unseasonable humidity and thundery showers for the end of the week and Saturday are set to be “blown away” on Sunday and replaced by ideal weather conditions for runners. The charts below show a big difference in Tmax temps between
26C 22C (updated) for parts of the SE on Saturday (itself depending on cloud cover and showers) and a cooler 16C on Sunday. The reason for this marked fall in temperature is a re-building HIGH pressure to the north which is introducing cooler air from a northerly direction to replace the humid continental air flow of the previous week. This change was expected but has arrived a day earlier than modeled previously. So Sunday is drying up and cooling down, perfect weather for the day! More details below:
Race day is likely to start with lingering cloud but it will brighten up slowly, albeit remaining cloudy through the morning. Some models keep light rain for a while but any remaining lingering showers should move away early. Humidity will also fall, from 90% RH at race start to around 70% RH by midday,as a drier cooler clearer northerly air mass quickly replaces the humid continental air of previous days. The fresher upper air will be wrapping around the HIGH pressure building to the north and arriving in Reigate as a Northeasterly breeze.
Early on, temperatures of 13C will make for a fresher start for runners. Sedentary early-bird spectators might begin to feel a little cool at the start after the relative heat of the previous days, especially in places open to the light NE breeze. An extra layer might be a good idea for spectators. The day is set to warm up steadily to a pleasant Tmax 16-17C, ideal running conditions.
Though never a real nuisance inland, a gentle breeze is set to pick up to 11-12mph from the NE by mid-morning and there might be low “gusts” reaching 15-20mph for open parts of the route away from trees and exposed to the NE. Reigate is sheltered from northerly / NE winds by the North Downs and any drying breeze is more likely to be welcome by sweaty runners! Another benefit is the air quality will be excellent compared with previous days of high pollution before the northerly moved in. So a healthy run too!
Overall, the day is set to be great weather conditions for runners: a cool start, cloudy but drying out and warming up pleasantly with a drying breeze.
A great way of illustrating the air mass change this weekend is to look at Theta e and CAPE graphs below. Theta E basically measures air mass instability (the higher the temp value the more “unstable” showery the air mass can be, similar with CAPE (convective potential energy)… spot how these two dramatically change on Sunday: from relatively unstable, energetic and showery conditions, to much more stable cooler air mass in a short space of time.