Wincanton and Leicester to look
Thursday's meetings at Wincanton and Leicester must pass 8am precautionary inspections on raceday morning to go ahead.
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Although both circuits were reported as raceable on Wednesday morning, course officials are taking no chances.
Overnight temperatures are set to drop to -2C at Wincanton, but the inspection has been called in case the frost is sharper than has been forecast.
Clerk of the course Barry Johnson said: "I'm very confident we'll be fine - we'll be OK with -2C - but some forecasts are predicting -4C.
"There's no frost in the ground now, but the ground is quite wet. It's very much a precautionary inspection."
Temperatures at Leicester are set to drop to as low as -5C. A maximum daytime temperature of 2C on Thursday has also been forecast.
Clerk of the course Jimmy Stevenson said on Wednesday morning: "There's no frost whatsoever this morning - we are perfectly raceable - but there is quite a severe frost forecast tonight.
"If we think we have a chance at 8am, we'll probably call another inspection later in the morning and then, possibly, look again.
"We've got to be patient."