I am sure any number of people could find the data of use.

May not be all that useful for a hay grower. Though it is difficult to fully asses a service from a more or less demonstration package.

What is of use for me is a good 7 day PoP. I use Ag Weather Information Service and they have a 7 day PoP. Initially it looks to be an 8 day forecast (S,S,M,T,W,T,F,S) but day one is 19/0600 to 18/1800 Central Time, and the last day is 25/1800 to 26/0600. The Probability of Precipitation (percentage) works well, using a spread sheet to track and evaluate trends.
As with any and most rain fall forecast the first 48 hours is reasonably accurate. From there it becomes a science based guessing game. Their preprioritory mathomatics take a good bit of the guessing out of the equation.

For primary hay production period here in Central Texas we really need a good forecast out 10 or 14 days. A few days for the ground to dry enough to drive on without rutting the ground, and 5 good drying days for each field cut.

Our friends at www.awis.com also provide a 6-10 day and 8-14 day forecast. Useful but can not be taken at face value. I like to use the spread sheet to evaluate and weigh these and apply them to the trends depicted in the 7 day forecast.
They also provide a local forecast, of which I find the hours of sunshine, Pan evaporation, predicted rainfall amounts and coverage, to be the most use for hay production, HERE.

On a day to day basis I also look at their forecast time for 65% RH & 55% RH and Growing Degree Days.

When haying is getting exciting or when I am planning to spray I also refer to the NOAA http://forecast.weather.gov/MapClick.php?TempBox=1&WindBox=1&am... forecast.
This is reasonably site specific. As with any of these, this also has it's own strengths and weaknesses.

For short term trending I look at their National Forecasting Weather Maps. Usually out up to 36 or 48 hours.

Some of the weakness of the UNISYS forecast maps, to me the color coding is enough different to be a irritant. With much data the maps become rather busy or cluttered. It would be a help if the subscriber could zoom in, in increments to keep from getting lost, to a fairly large scale.

I look at their 65% & 55% RH information as a GUIDE for timing of the raking and the baling of hay.
I want to cut the hay on a long sunny day so the hay, spread out in a wide swath, can receive as much direct sunlight as possible. As long as there is direct sunlight the stomata on the leaves stay open and this helps speed the first day drying.
For raking or if necessary using a tedder I want to accomplish these operations when the humidity, at the windrow, is well above 65%. (There can be and usually is a large difference between the humidity at shoulder height and at ground level)
For baling I want to bale when the humidity, at the windrow, is between 65% and 55%, Then the hay will be dry enough to bale but not too dry & shattrer the leaves.
Operator discretion is important.

Probably a whole lot more than you wanted to know.