Mmmm... This will be a tough call. Northern VA and also Southeastern VA will expect thunderstorms during Saturday morning. The reasons to justify your protective spray could be: 1) vines has been grown quite a bit over this week due to warm weather; and 2) you might have sprayed more than week or 10 days ago and it is due anyway. One the other hand, there are reasons to hold off the spray: 1) the expected thunderstorm is in the morning hour and the forecast of Saturday is sunny with high wind, i.e., leaves may not stay wet for a long time; 2) the chance of rain is 30-40% as of this morning; and 3) there are rain forecasted early next week (20-40% during Tuesday and Wednesday for many parts of VA) and you may want to aim for that rain.
Please check your local weather, fungicide record, and disease history of your vineyard to make a decision the best for you.