There hasn't been a big change in what I'm thinking the weather will be in Mexico City this weekend for the WEC event, but my confidence is rising that the racing will be impacted by rain each day. A low pressure system will be taking shape in the Gulf of Mexico, just off the east coast of Mexico.
This will produce an easterly flow through the low and mid levels of the atmosphere that will pump moisture toward the track all weekend, but especially Saturday and Sunday. With mountains all around the city, the moisture rising up the mountains slopes will produce showers and thunderstorms each day, and mainly in the afternoon hours. I've raised the chance of rain with the forecast update to reflect my higher confidence, and I've raised the Impact to High, as I think there's a good chance of the rain being heavy enough at times to require a stoppage in the action. Rain amounts are likely to be between a quarter and a half inch each day, but amounts could be higher than that if a storm moves directly over the track. The Weather Channel has a pretty good interactive radar display for Mexico here.