With a few exceptions, most operational public forecasts only go out to 7-10 days. This is due in large part to the fact that the skill of models gets progressively worse the further out you go. However, a recent article in the Journal of Climate suggests that models may have sufficient skill for forecasts in the 3-4 week range.
I have not read the full paper (yet), but the abstract suggests that this isn’t as great as it might sound at first. The resolution, according to the abstract, is one degree of latitude at 38 degrees. By my calculations, that’s a grid spacing of about 69 miles or 111 kilometers. The grid used by the Global Forecast System (GFS) model is 28 kilometers. Shorter-range and regional models use even smaller grids.
Grid size matters because it effects the scale of weather phenomena that can be modeled. Larger-scale features such as low pressure systems can be captured at this scale, but much gets lost. So you wouldn’t use this forecast to schedule the exact time of your picnic four weeks from now, but it might at least help you pin down a day.
Of course, this is all dependent on the NOAA budget. The Weather Research and Forecasting Innovation Act of 2017, which was passed by Congress and signed by the President, requires additional efforts to improve forecasts at this range. However, the proposed budget from the White House cuts this effort.
Terminate Investment in Mid-Range Weather Outlooks: NOAA requests a decrease of $5,000,000 to terminate all development, testing, and implementation of experimental products to extend operational weather outlooks, including temperature and precipitation outlooks, from 16 days to 30 days.”
The Washington Post‘s Capital Weather Gang blog and Professor Cliff Mass both have excellent writeups on the misalignment between the Weather Research and Forecasting Innovation Act and the proposed budget. The Weather Research and Forecasting Innovation Act of 2017 was a good bill, hopefully the budget adjusts to meet it. If not, the state of American weather forecasting is set to take a dramatic hit.