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How risk bands
are defined.

Each beach is assigned one of four risk bands described as an estimated chance a lab test at this beach exceeds the EPA single-sample threshold of 104 CFU/100mL, from its past 12 months of tests.

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.

Estimated chance

The rating is each beach’s estimated chance that a lab test will exceed 104 CFU/100mL. It starts from the beach’s past 12 months of tests and is adjusted by a logistic model for its latest result (how far above or below the limit, and how recent), rain in the 72 hours before the morning update, and the season.

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
Usually tests clean.
About 3 in 100 lab tests at this level came back over the safe limit — still check posted signs.
typically < 35 CFU/100mL
Estimated exceedance chance < 20 %
Lower exceedance record and conditions, but still check county advisories and posted warnings.
An estimated chance a lab test at this beach exceeds the EPA STV (104 CFU/100mL) below 20 %, from its past 12 months of tests, its latest result and recent rain. Realized exceedance rate in testing, Apr–Sep 2026: 4 %.
Moderate
Sometimes tests high.
About 1 in 10 of this beach's lab tests in the past year came back over the safe limit.
typically 35–104 CFU/100mL
Estimated exceedance chance 20–30 %
Avoid swallowing water; rinse thoroughly after exit.
An estimated 20–30 % chance a lab test at this beach exceeds the 104 CFU/100mL STV, from its past 12 months of tests, its latest result and recent rain. Realized exceedance rate in testing, Apr–Sep 2026: 19 %.
High
Often tests high.
About 1 in 3 of this beach's lab tests in the past year came back over the safe limit — check county advisories.
typically 104–320 CFU/100mL
Estimated exceedance chance 30–70 %
Avoid water contact and check county advisories, especially for sensitive groups.
An estimated 30–70 % chance a lab test at this beach exceeds 104 CFU/100mL, from its past 12 months of tests, its latest result and recent rain; the county may post an advisory. Realized exceedance rate in testing, Apr–Sep 2026: 37 %.
Very High
Usually tests unsafe.
Most of this beach's lab tests in the past year came back over the safe limit.
typically > 320 CFU/100mL
Estimated exceedance chance ≥ 70 %
Avoid the water; expect closure-level concentrations based on recent samples and conditions.
An estimated ≥ 70 % chance a lab test at this beach exceeds the 104 CFU/100mL STV, from its past 12 months of tests, its latest result and recent rain. It does not predict a concentration or a specific CFU value. Realized exceedance rate in testing, Apr–Sep 2026: 87 %.
Advisory
Official advisory posted
County posting shown alongside the rating — set by the county, not our estimate.
n/a CFU/100mL
Set by the county — not our estimate
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 rating 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 estimate 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, runoff after the morning update) can override a beach's track record and recent rain. This is a beta estimate, not an official advisory.
  • 04The estimate uses 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.