Flows on the Yellowstone and Boulder Rivers are currently nosediving due to drastically below normal temps. This is setting these rivers up for a brief window of clear (clear enough) water over the next week to ten days, particularly during the early to middle part of next week. These mid-snowmelt windows of fishable conditions only occur about one year in three and can produce the best fishing for large trout of the season. This is true for experienced anglers, anyway.
See for yourself.
I do not suggest booking a float trip at this time unless you are prepared to drive to the lower Madison River if things turn out to not follow the predicted flows noted above. That said, keep an eye on things. If the above forecasts do pan out and you can book a float on short notice, I strongly encourage you to consider it for Tuesday or Wednesday, the 28th or 29th.
Note that this same cooldown should make the Yellowstone in its Black and Grand Canyons, Slough Creek, the Gardner, and the Firehole/Gibbon/Madison ALL fishable for the opening weekend of the Yellowstone season, and the few days thereafter. I will post more details on this late in the week.