Four-week yes-or-no drought call for about 3,200 river stations in the lower 48. The interesting part is not beating the U.S. Geological Survey. It is a rule, written before later years were scored, that is allowed to say “keep this model” or “retrain.”
The mix of high and low flows is close enough to 1980–1999 that we are not flagged to check inputs.
Drought here means flow at or below the 10th percentile for that station and time of year, four weeks ahead. We call drought when the model’s 0–1 score is at least 0.70. That is a decision line, not “a 70% chance.” The simple check we have to beat is: if it is already dry this week, call drought four weeks out.
Each point is the last 52 weeks we already know the answer for. The dashed line is 10 percentage points. Crossing below it is when we retrain, then wait 52 more known weeks.