Sometimes local AVA (American Viticultural Area) groups will have data available on their websites or emails available to members (the latter is the case with, for example, Napa Valley Vintners). For the most part, however, home winemakers won’t be members of these kinds of groups (they can cost thousands of dollars to join) so they will have to seek out data independently.
Many viticulturalists and winemakers I know subscribe to newsletters and data services like those from Western Weather Group (westernweathergroup.com) or Weather Bell (weatherbell.com). You can look up forecasts for a given area, current conditions as well as browse searchable and downloadable historic data in order to get an idea of how an area is progressing. Sadly, these are not wine-specific in any way, though most will provide an agricultural-based data set like degree days from certain dates, etc., which can help predict when grapes will ripen.
This kind of forecasting and historical weather information isn’t free, however. For example, Weather Bell costs $270/year for a personal/non-professional subscription and $660/year for a professional/business subscription. It might be worth poking around the NOAA (National Oceanic and Atmospheric Administration) website at www.ncei.noaa.gov/cdo-web/ because it does seem to offer current and historical data and is searchable by state, county, or zip code. It’s a big clunky website, however, so it might take a little digging to find things that will be of specific interest to you. In the few minutes I spent on the website, I was able to find degree-day data as well as precipitation data for the Napa area over certain date ranges.
Numbers are only numbers, however. Actually getting in the vineyard and checking out the fruit and canopy, even two or three months before harvest, can give you a great idea about what the year has done so far. For example, this year, as I was touring our Russian River vineyards, I could visually see that in some cases the winter hail damage we had caused a lot of berries to not set, and the vines were setting a bunch of second crop. If you can possibly go to visit a vineyard yourself that’s always best, but if you can’t, if you’ve got connections in the area or if you can have the vineyard owner send you pictures or just talk you through what’s going on in an area, it all will help you to make a decision.