Research · Labels

How risk bands
are defined.

Each beach is assigned one of four risk bands derived from the model's calibrated exceedance probability — the chance that enterococcus exceeds the EPA single-sample threshold of 104 CFU/100mL on a given day.

From probability to label.

EPA STV

The single-sample threshold for enterococcus in marine waters is 104 CFU/100mL (EPA Recreational Water Quality Criteria, 2012). Exceeding this value does not guarantee illness but indicates elevated risk.

Calibrated probability

The model outputs a calibrated p_exceed — the estimated probability that the true concentration will exceed 104 CFU/100mL on the forecast day, given environmental covariates.

Band cut-points

The cut-points are 20 %, 30 % and 70 %. They are fixed relative to the base rate rather than tuned by a loss function — High begins at roughly three times the average beach’s exceedance rate. A beach with an active county posting is floored to at least the High cut-point, so a posted beach can never display Low or Moderate.

County advisories

Shorelife bands are forecasts; official CDPH/county advisories are posted after a confirmed sample violation. A Very High forecast does not replace a posted advisory.

Four levels, calibrated for public health.

BandGuidanceProbability gateAction

Low

Low modeled risk

Lower modeled exceedance risk.

typically < 35 CFU/100mL

Predicted exceedance probability < 20 %

Lower modeled exceedance risk, but still check county advisories and posted warnings.

A modeled probability, NOT a measurement. Low means the model puts the chance of exceeding the EPA STV (104 CFU/100mL) below 20 % — it does not mean the water has been measured below it. Over the last 90 days roughly a third of true exceedances were displayed as Low.

Moderate

Moderate modeled risk

Elevated modeled risk.

typically 35–104 CFU/100mL

Predicted exceedance probability 20–30 %

Avoid swallowing water; rinse thoroughly after exit.

A 20–30 % modeled chance of exceeding the 104 CFU/100mL STV.

High

Elevated modeled risk

High modeled exceedance risk — check county advisories.

typically 104–320 CFU/100mL

Predicted exceedance probability 30–70 %

Avoid water contact and check county advisories, especially for sensitive groups.

A 30–70 % modeled chance of exceeding 104 CFU/100mL; the county may post an advisory.

Very High

Very high modeled risk

Model estimates closure-level exceedance probability.

typically > 320 CFU/100mL

Predicted exceedance probability ≥ 70 %

Avoid the water; expect closure-level concentrations based on recent samples and conditions.

A ≥ 70 % modeled chance of exceeding the 104 CFU/100mL STV. The model outputs only that probability — it does not predict a concentration, so this is not a forecast of any particular CFU value.

Advisory

Official advisory posted

County posting shown alongside the model band — not a model output.

n/a CFU/100mL

Set by the county — not a model output

Do not enter the water. The county has issued an active health advisory based on recent samples or an event (sewage spill, storm runoff). Follow the official county source for current status.

Authoritative posting under the county health department. It is shown as a badge ALONGSIDE the model band rather than replacing it, so each beach still displays its own result for the day; the band is simultaneously floored to at least High, so a posted beach can never read Low or Moderate.

What the model can and cannot do.

01Shorelife forecasts are probabilistic estimates, not certified sample results.
02Official beach closures and advisories are issued by county environmental health agencies under California AB 411.
03A Low band forecast does not mean safe water. Localised events (sewage spills, storm runoff) can override model predictions. This is a beta model estimate, not an official advisory.
04The model is trained on marine enterococcus only — freshwater E. coli and other analytes are outside its scope. Both culture methods (MPN/IDEXX, CFU) and molecular ddPCR are included; ddPCR results, about 4 % of samples and concentrated in San Diego, are reported in copies/100mL and judged against the 1413-copy molecular threshold rather than the 104 CFU culture STV.
05For confirmed regulatory status, always consult the current county health advisory for the specific beach.