The forecasts are based on the best available data generated by the National Weather Service(weather forecast), National Resource Conservation Service(snowpack and soil moisture data) and the BLM/BoR and other dam maintainers. There are additional providers of significant information such as state agencies.
The data is then aggregated and processed by River Forecast Centers(RFC) of the National Weather Service to generate forecasts for individual rivers.
As such, there is no better, no alternate source for river forecasts to my knowledge. The forecasts are as accurate as the data entered that the models process. Generally, the true question is, how accurate are weather forecasts especially toward the end of the forward looking interval of time, say 5 to 10 days out.
If you go to the RFC forecast website and for most rivers there is a graph that can be accessed that illustrates previous daily forecasts vs current actual. It does show that nothing is perfect when predicting the future.
Regarding the graphs posted here, I poach RFC data for the individual rivers and by means of painstaking manipulation collect the forecast data and feed it to the large graph that I post.
Regarding the San Juan, I'm sorry that the data does not provide a more favorable outlook. Weather patterns did not favor snow pack for the southern Rocky Mountains and to further reduce San Juan flows the person (Susan Novak Behery) who controls the valves releasing water from Navajo Dam posted notice here on mountain buzz stating that there would be no supplemental "Spring Peak Release" due to inadequate water levels in Navajo Lake.
The numbers come from the flow report from river basin lines.
The flow report is generally organized by state and river basin. States are gray filled lines. Basins are green filled lines. The green filled lines for river basins contain three numbers in each dates cell. The numbers are the river basins percent of average SWE(snowpack)(green), percent of average precipitation(blue) and percent of average of select rivers(black). Select rivers are the significant rivers of interest that I include in the flow report for each basin.
Frequently the numbers you mention from the Forecast are followed by a plus or minus value. This number is also from the flow report and is the percent of average daily trend. For example, a green value of SWE "57+3" translates to a snow water equivalent in the basin's headwaters of 57 percent of average with a two week trend increasing by 3 percent a day.
The numbers are sometimes scrambled by issues concerning the formatting of numbers in different software that I have not taken the time to address. If the number is funky, that is, not a one or two or three digit value then it is likely an error. Dates are errors also.
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