There is a theory that the long range forecasts, those are forecasts for more than 3-5 days, are not as good as they were because of the reduced number of aircraft in the air. Most modern aircraft relay temperature, humidity and wind data back to the various meteorological agencies, and these extra data improve the accuracy of the forecasts, especially those in the longer ranges.
I was curious about this. My son is working in northern Alberta, and each day I send him a ten day forecast which I get from the
Norwegian Weather Service which are based on the European Centre for Medium Range Weather Forecasts. This is the gold standard for weather forecasts. I have noticed that the total rainfall amounts for the ten days really jumps around. Here are the data for the past 13 days:
8.7
41.7
23.6
24.8
5.0
60.6
3.9
15.6
18.7
49.8
13.2
98.9
38.9
Each forecast has nine days in common with the ones on either side, so the rainfall amounts should vary slightly, but not as wildly as these.