Aintree has covered over the most vulnerable areas of the course as Saturday's Becher Chase meeting came under threat from a winter blast that could bring possible sleet or snow.
Clerk of the course Andrew Tulloch said the forecast had altered slightly on Thursday but it remained a source of concern.
With the going on the Grand National and hurdles tracks heavy, soft in places, and soft, heavy in places on the Mildmay course, Tulloch said: "There was talk of snow tomorrow but now they are saying it's more likely going to be rain, although it's still on the cusp.
"The forecast is a bit touch and go, with almost anything possible, and it could swing either way, but we've got the vulnerable areas covered and we are on fresh ground obviously on the Grand National course and we've moved the running rail out on the other tracks as much as we can. We'll keep an eye on it and give it every opportunity."
Owing to the going Gas Line Boy, who had been an 8-1 chance for the Randox Health Becher Chase, was rerouted to the 188Bet Grand Sefton Handicap Chase on the same card leaving Blaklion heading 16 runners.
Due to falling temperatures a 7am inspection will be held before Friday's meeting at Sedgefield can go ahead and Sunday's card at Huntingdon will be the subject of a 7am inspection on the day of racing due to forecast frost.
Clerk of the course Jack Pryor said: "We are forecast back to back nights of quite sharp frosts. The inspection is purely precautionary. The track is raceable and I am still pretty confident it will go ahead."