fix(parser): probabilistic LABEL is a fraction, not integer percent (100x scale bug)

CHANGELOG.md

@@ -6,6 +6,23 @@ uses [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
 
+## [0.1.1] - 2026-06-06
+
+### Fixed
+
+- **`parse_probabilistic` probability scale (100x bug).** The day-1/2/3
+  probabilistic-outlook parser divided every isopleth value by 100
+  unconditionally, assuming `LABEL`/`dn` is an integer percent (`"2"`,
+  `"5"`). The live `www.spc.noaa.gov` and ArcGIS GeoJSON feeds actually
+  ship the value as an already-normalized fraction (`"0.02"`, `"0.30"`),
+  so a 2% tornado risk parsed to `0.0002` instead of `0.02`. Now
+  normalized like the day4-8 path (`pct > 1.0 ? pct / 100.0 : pct`):
+  integer percents are divided, fractions pass through. Both forms are
+  pinned by regression tests (`Parser.ProbabilisticFractionalLabel`,
+  `Corpus.ProbabilisticParsesAndScales`). Consumers that persist
+  probabilities (e.g. `spc-data`'s `spc.prob_outlooks`) get correct
+  `[0,1]` values after upgrading.
+
 ## [0.1.0] - 2026-05-17
 
 Initial public release. Open-source C++23 SDK for NOAA Storm Prediction

CMakeLists.txt

@@ -1,5 +1,5 @@
 cmake_minimum_required(VERSION 3.20)
-project(spc-cpp VERSION 0.1.0 LANGUAGES CXX)
+project(spc-cpp VERSION 0.1.1 LANGUAGES CXX)
 
 set(CMAKE_CXX_STANDARD 23)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)

src/models/outlook.cpp

@@ -118,16 +118,22 @@ ProbOutlookPayload parse_probabilistic(std::string_view body, std::int32_t day_o
 		}
 		ProbOutlookFeature pf;
 		pf.hazard = hazard;
-		// SPC prob isopleths store percentage as LABEL ("2", "5", "10", ...) or
-		// as DN, occasionally as a numeric.
+		// SPC prob isopleths express the risk percentage two incompatible ways
+		// depending on the source: as an integer-percent (`LABEL`/`dn` = "2",
+		// "5", "30") or as an already-normalized fraction (`LABEL` = "0.02",
+		// "0.05", "0.30" — the form the live www.spc.noaa.gov + ArcGIS GeoJSON
+		// actually ship). Normalize like the day4-8 path (convective.cpp): only
+		// values > 1 are percents to divide by 100; fractions pass through. A
+		// bare `/ 100.0` here silently produced 100x-too-small probabilities
+		// (0.02 -> 0.0002) for every real feed.
 		double pct = detail::json_number_or_numeric_string(*props, "LABEL");
 		if (pct == 0.0) {
 			pct = detail::json_number_or_numeric_string(*props, "label");
 		}
 		if (pct == 0.0) {
 			pct = detail::json_number_or_numeric_string(*props, "dn");
 		}
-		pf.probability = pct / 100.0;
+		pf.probability = pct > 1.0 ? pct / 100.0 : pct;
 		pf.issued_at = detail::as_spc_ts(*props, "ISSUE");
 		if (pf.issued_at.empty()) {
 			pf.issued_at = detail::as_spc_ts(*props, "issue");

tests/test_corpus.cpp

@@ -62,16 +62,18 @@ TEST(Corpus, ArcGISGeoJsonCategoricalParses) {
 }
 
 TEST(Corpus, ProbabilisticParsesAndScales) {
-	// ArcGIS f=geojson probabilistic tornado: label is "0.02"/"0.05"/"0.10".
-	// parse_probabilistic divides by 100 (verbatim spc-data semantics), so a
-	// "0.02" label -> 0.0002 probability.
+	// ArcGIS f=geojson probabilistic tornado: label is the already-normalized
+	// fraction "0.02"/"0.05"/"0.10". parse_probabilistic passes fractions
+	// through (only integer-percent forms are /100'd), so a "0.02" label ->
+	// 0.02 probability — NOT the 0.0002 a bare /100 used to produce. The
+	// >= 0.01 floor pins that fix (every real SPC isopleth is >= 2%).
 	const ProbOutlookPayload p =
 		parse_probabilistic(slurp("arcgis_day1_prob_tornado.geojson"), 1, "tornado");
 	ASSERT_GT(p.features.size(), 0u);
 	for (const ProbOutlookFeature& f : p.features) {
 		EXPECT_EQ(f.hazard, "tornado");
-		EXPECT_GT(f.probability, 0.0);
-		EXPECT_LT(f.probability, 1.0);
+		EXPECT_GE(f.probability, 0.01);
+		EXPECT_LE(f.probability, 1.0);
 		EXPECT_FALSE(f.rings.empty());
 	}
 }

tests/test_parser.cpp

@@ -92,6 +92,26 @@ TEST(Parser, ProbabilisticTornado) {
 	EXPECT_DOUBLE_EQ(p.features[1].probability, 0.05);
 }
 
+// Regression: the live www.spc.noaa.gov + ArcGIS GeoJSON ship the probability
+// as an already-normalized fraction ("0.02", "0.30") — NOT an integer percent.
+// A bare `/ 100.0` turned a 2% risk into 0.0002. Both label forms must map to
+// the same [0,1] probability.
+TEST(Parser, ProbabilisticFractionalLabel) {
+	const ProbOutlookPayload p = parse_probabilistic(R"({
+		"type": "FeatureCollection",
+		"features": [
+			{"properties": {"LABEL": "0.02", "LABEL2": "2% Tornado Risk"},
+			 "geometry": {"type":"Polygon","coordinates":[[[-100,34],[-95,34],[-95,38],[-100,38],[-100,34]]]}},
+			{"properties": {"LABEL": "0.30", "LABEL2": "30% Tornado Risk"},
+			 "geometry": {"type":"Polygon","coordinates":[[[-99,35],[-96,35],[-96,37],[-99,37],[-99,35]]]}}
+		]
+	})",
+													 1, "tornado");
+	ASSERT_EQ(p.features.size(), 2u);
+	EXPECT_DOUBLE_EQ(p.features[0].probability, 0.02);
+	EXPECT_DOUBLE_EQ(p.features[1].probability, 0.30);
+}
+
 TEST(Parser, UnknownLabelSkipped) {
 	const CategoricalOutlookPayload p = parse_categorical(R"({
 		"type": "FeatureCollection",