Most weather products tell you the forecast. Forecast Fork tells you whether to trust it — how much the ensembles disagree, which competing scenarios are still unresolved, and how likely the forecast is to change in the next few cycles. It is the open, continuously-verified instability layer of Weather Trader — and the numbers below are pulled from it live.
See it live on the Forecast desk → How it's verifiedloading live from weathertrader.com…
This is not a mock-up. It is fetched, as you load the page, from Weather Trader's verification engine: bin every run by the ensemble spread it showed at issue time, plot the skill it actually verified. The down-slope is the proof that spread is a bust signal — known before the outcome. That is the Forecast Fork.
A forecast can be confident — every model and member converging on one outcome — or forked: split into distinct scenarios the atmosphere hasn't chosen between yet. The second kind busts. Forecast Fork is the probability that materially distinct scenarios remain unresolved — and, uniquely, that the forecast itself is about to move.
Being precise about status: the Fork is really three quantities. FFspread — current uncertainty — is live and verified (the chart above). FFbust — a calibrated P(a forecast fails) — and FFrevision — P(the forecast itself changes next cycles) — are the models we are building next. Spread predicts uncertainty; it is not yet a calibrated bust probability. We combine them into one headline index only after each is independently, out-of-sample calibrated. Method & status on weathertrader.com/method.
One number per region & lead — how forked the forecast is — on the Forecast desk's agreement heat.
The 50 members clustered into competing patterns per region; when a minority scenario beats the mean, the consensus underweighted the truth. On every verify-time page.
Spread↔skill correlation, published and continuously updated — the chart above. See the Method.
How the distribution moves between cycles — drift, spread acceleration, reversals — surfaced as the what's-changing strip.
The synoptic regime the flow is in — blocked, zonal, PNA− — and which model owns it. The named reason a forecast is unstable.
Tamper-evident, content-addressed record of every forecast as issued — the sha256 is the identity, so any change is detectable. Hash-chained + externally-anchored roots are on the roadmap.
Forecast Fork is not ensemble spread. For a fixed valid time the ensemble is a mixture of coherent scenarios, and we track those branches across successive model runs — not re-clustering and forgetting each cycle. The object is a branching graph: how forecast alternatives emerge, split, merge, and gain or lose probability as the forecast evolves toward reality.
The system runs in both directions. Forward Fork: from the current state, which outcomes are possible and how are their probabilities evolving? Reverse Fork: given a specified outcome — a station's daily high above a threshold — which branches carry it, what must stay true (trigger), and what would kill it? P(E) = Σ qk(E)·πk, decomposed by branch via Bayes P(Bk|E).
The current branches — how many, how separated, their probabilities and cross-model support.
Given an outcome: its probability, the branches that carry it, and the trigger / kill conditions.
Which branch is gaining, losing, stabilizing or reversing — Δπ tracked across runs.
A fitted, out-of-sample-scored probability the forecast fails — not merely spread.
The effective number of independent model families behind a scenario — AI models echo their IFS/ERA5 training.
The calibrated chance the distribution materially revises in the next cycles.
Propagate one atmospheric branch to every downstream outcome — temperature, HDD/CDD, load, wind, contracts — coherently.
After verification: when the winning branch appeared, how its probability evolved, what was knowable when.
Status is honest: live is running publicly, experimental is built and being out-of-sample verified, building is on the roadmap. All of it runs on our own multi-terabyte ensemble archive, computed near the data. Full method + component status: weathertrader.com/method.
Forecast Fork is the transparent, auditable layer of Weather Trader — a prediction market for the atmosphere. The maps and verification are free and public; the goal is for "the Forecast Fork is high" to become how people say a forecast is unsettled. Start on the live Forecast desk or read the method.