I weigh in in the morning, so I usually do not eat or drink anything until after I step on the scale.
Since you usually WI in the morning and are going to do it in the afternoon this week, I think you need to mentally prepare yourself for your number to be skewed. I am not sure what time you are going, but if there was any way to eat lunch after, it might help your number be closer to the actual number.
One more thing, do you have a scale at home? Do you know if your scale and your official scale are fairly close? If you have a scale, I would weigh yourself at home before eating or drinking so that in your mind you will have close to your actual number. That way when you WI in the afternoon and it doesn't show what you want / think / hope it will be, you will know in the back of your head what it really is.
The total upside of this is you know NEXT week when you go for your official WI at your normal time, you will probably show a huge loss