7 July update: As torrential showers descend upon Kennedy Space Center, concerns linger as to whether conditions tomorrow at this time will permit a launch to occur. As has been the case in the past bad weather can suddenly yield a favorable window. At this time NASA seems to be intent upon going ahead with the countdown in case this opportunity arises. If weather forces a scrub odds are that there will be 48 hour delay with an attempt on Sunday. A GPS launch currently has the range reserved between 11-5 July and NASA is looking at ways to possibly get 11 July from them.
As things stand right now weather is the only thing standing in the way of an otherwise flawless countdown for the last Space Shuttle mission. No technical issues are being tracked except for some air conditioner problems in a building in Houston. In other words, this has been "textbook" thus far from the hardware point of view.
Right now weather forecasters predict that there is better than a 60% chance that conditions at launch time on Friday will violate launch constraints. Things look a little better on Saturday and get even better on Sunday. This is due, in part, to the fact that the launch window moves earlier in the day with each successive day - and with it, better weather.
The expectation here at KSC is that NASA will go ahead with launching preparations and tanking until or unless conditions warrant a halt. If there is a scrub on Friday informed odds suggest that the next attempt would be on Sunday when conditions are better and launch team has had a chance to rest. Traffic in the area due to half a million or more launch viewers is expected to be brutal. This influx of out of ton traffic would make it hard for KSC folks to get home only to turn around and come back to support another launch attempt hours later.
If a launch does not occur on Sunday NASA will have to stand down until the end of next week so that a previously-planned GPS satellite launch can take place.
There is a strong move to try and launch on Friday as planed. "Friday is game day. We do not want to play on Saturday - we want to play on Friday" said Shuttle Launch Director Mike Leinbach.
More than one inference was made by NASA that "forecasts are just forecasts". Unless some blatantly bad weather that would preclude tanking were to appear NASA will move ahead with launch preparations and then see what the weather does. Leinbach and Moses were quick to remind reporters that there had been times in the past when NASA had proceeded toward launch with bad weather forecasts - and then managed to catch a brief moment of good weather and launch. There have also been countdowns that went smoothly until the last minute when weather did something unexpected.