The Manchester derby is due to start at 4.30pm.
But heavy snowfall in the north of England has led to fears the game could be postponed.
While the pitch at Old Trafford will be fine to play due to undersoil heating, the area around the stadium could be deemed unsafe for fans.
Manchester United's safety advisory group will make the decision, and the council and police will also have a say.
Bookies Paddy Power have priced up a postponment at 12/1.
Shay Given's combined Manchester United and Manchester City XI
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A spokesman said: “Apparently (White) Christmas has come early to the UK, and it could put today’s big fixtures on ice.
“Knowing Jose Mourinho, he’s probably confiscated all salt from the Old Trafford area, so to delay his side having to face runaway leaders Man City – who we’ve already paid out on winning the title.”
United have asked for fans to arrive at the stadium early for today's game, with a series of security measures in place along with other busy events in Manchester.
PA
PA
The club wrote on their website: "Adverse weather is also likely this weekend, with expected snowfall hitting the north of England and supporters with long distance journeys are advised to plan routes before travelling."
There are also roadworks around Old Trafford which could delay some fans' arrival.
Turnstiles are the ground will open from 2.30pm with drinks on sale at half-price until an hour before kick-off in an atempt to draw spectators in earlier than usual.
The fear of the game being called off has led to fans joking about long-distance supporters having a wasted trip north.
The match is due to kick-off at 4.30pm.