diff --git a/docs/internal/facts.md b/docs/internal/facts.md
index eb78270..ac313c1 100644
--- a/docs/internal/facts.md
+++ b/docs/internal/facts.md
@@ -26,6 +26,8 @@ uses half-open period overlap to select hourly, narrative, daily, and alert
 data; it also derives precipitation timing. Convective outlooks are retained
 only when their valid interval overlaps the report period, with discussions
 kept for represented outlook days. Both collections are sorted deterministically.
+It rejects collected hourly data with a precipitation probability outside the
+finite 0 through 100 percentage domain before constructing derived facts.
 
 Report identity controls the summary shape:
 
diff --git a/docs/internal/forecast-derivation.md b/docs/internal/forecast-derivation.md
index 4413180..90ebf82 100644
--- a/docs/internal/forecast-derivation.md
+++ b/docs/internal/forecast-derivation.md
@@ -26,12 +26,18 @@ its selected hourly periods and derives temperature and apparent-temperature
 ranges, timed precipitation and wind maxima, dominant and notable conditions,
 and weather indicators.
 
+Hourly precipitation probabilities must be finite percentages from 0 through
+100. Daily-summary construction rejects invalid values before they can affect
+timed maxima or precipitation windows.
+
 Indicators are deterministic checks over normalized values and condition text:
 heat, cold, and wind use package-owned numeric cutoffs; snow, ice, fog, and
 wind text are detected from the forecast description. `BuildPrecipTiming`
 sorts periods, records the maximum and first precipitation, groups contiguous
 periods at or above its package-owned probability threshold, and records
 thunder mentions.
+Ice detection uses the bounded condition vocabulary `ice`, `icy`, `freezing`,
+and `sleet` as whole words.
 
 Daypart temperature and apparent-temperature ranges are Fahrenheit values, and
 timed wind maxima are mph values. When only metric source fields are present,
diff --git a/internal/adapters/weatherapi/client.go b/internal/adapters/weatherapi/client.go
index 28461bc..9f4c8bc 100644
--- a/internal/adapters/weatherapi/client.go
+++ b/internal/adapters/weatherapi/client.go
@@ -228,6 +228,9 @@ func (b *bundleBuilder) fetchHourly(ctx context.Context) error {
 		if !period.HasUsableTimeBounds() {
 			return fmt.Errorf("hourly forecast from %s has unusable time bounds for period %d", source.Endpoint, i+1)
 		}
+		if !period.HasValidPrecipitationProbability() {
+			return fmt.Errorf("hourly forecast from %s has invalid precipitation probability for period %d", source.Endpoint, i+1)
+		}
 	}
 	source.IssuedAt = &hourly.IssuedAt
 	source.UpdatedAt = hourly.UpdatedAt
diff --git a/internal/adapters/weatherapi/client_test.go b/internal/adapters/weatherapi/client_test.go
index fda6401..751537c 100644
--- a/internal/adapters/weatherapi/client_test.go
+++ b/internal/adapters/weatherapi/client_test.go
@@ -112,6 +112,22 @@ func TestFetchBundleFromFixtures(t *testing.T) {
 	}
 }
 
+func TestFetchBundleRejectsInvalidHourlyPrecipitationProbability(t *testing.T) {
+	for _, probability := range []string{"-1", "101"} {
+		t.Run(probability, func(t *testing.T) {
+			server := fixtureServer(t, map[string]handlerOverride{
+				"/forecast/hourly": {status: http.StatusOK, body: `{"data":{"periods":[{"startTime":"2026-05-29T13:00:00Z","endTime":"2026-05-29T14:00:00Z","probabilityOfPrecipitationPercent":` + probability + `}]}}`},
+			}, nil)
+			client := newTestClient(t, server.URL+"/", nil)
+
+			_, err := client.FetchBundle(context.Background())
+			if err == nil || !strings.Contains(err.Error(), "invalid precipitation probability") {
+				t.Fatalf("FetchBundle() error = %v, want invalid precipitation probability", err)
+			}
+		})
+	}
+}
+
 func TestFetchBundleBuildsExpectedQueries(t *testing.T) {
 	var requested []string
 	server := fixtureServer(t, nil, &requested)
diff --git a/internal/facts/facts.go b/internal/facts/facts.go
index 0977014..dfd5b89 100644
--- a/internal/facts/facts.go
+++ b/internal/facts/facts.go
@@ -101,6 +101,11 @@ func BuildDerived(req BuildDerivedRequest) (DerivedFacts, error) {
 	}
 	bundle := req.Collected.Bundle()
 	period := req.Resolved.ValidPeriod
+	if req.Collected.Hourly != nil {
+		if err := forecast.ValidatePrecipitationProbabilities(req.Collected.Hourly.Periods); err != nil {
+			return DerivedFacts{}, err
+		}
+	}
 	spcOutlooks, spcDiscussions := selectSPCConvectiveOutlooks(req.Collected.SPCConvectiveOutlooks, period)
 	derived := DerivedFacts{
 		ValidPeriodHourlyPeriods:    forecast.SelectHourlyPeriods(req.Collected.Hourly, period),
diff --git a/internal/facts/facts_test.go b/internal/facts/facts_test.go
index 9696e65..81c3dba 100644
--- a/internal/facts/facts_test.go
+++ b/internal/facts/facts_test.go
@@ -127,6 +127,21 @@ func TestBuildDerivedDailyIncludesNextDayOvernightAlerts(t *testing.T) {
 	}
 }
 
+func TestBuildDerivedRejectsInvalidPrecipitationProbability(t *testing.T) {
+	location := testLocation()
+	bundle := testBundle(location)
+	invalid := -1.0
+	bundle.Hourly.Periods[0].ProbabilityOfPrecipitationPercent = &invalid
+	_, err := BuildDerived(BuildDerivedRequest{
+		Resolved:  resolveForTest(t, report.Hourly, mustParse("2026-05-29T08:00:00-05:00"), location),
+		Timezone:  location.String(),
+		Collected: BuildCollected(bundle),
+	})
+	if err == nil || !strings.Contains(err.Error(), "invalid precipitation probability") {
+		t.Fatalf("BuildDerived() error = %v, want invalid precipitation probability", err)
+	}
+}
+
 func TestBuildDerivedTomorrow(t *testing.T) {
 	location := testLocation()
 	for _, id := range []report.ID{report.Tomorrow} {
diff --git a/internal/forecast/derive.go b/internal/forecast/derive.go
index 64ff819..9be4a23 100644
--- a/internal/forecast/derive.go
+++ b/internal/forecast/derive.go
@@ -192,6 +192,9 @@ func BuildDailySummary(bundle *weatherdata.Bundle, date time.Time, location *tim
 	if bundle.Hourly == nil || len(bundle.Hourly.Periods) == 0 {
 		return nil, fmt.Errorf("hourly forecast data is required")
 	}
+	if err := ValidatePrecipitationProbabilities(bundle.Hourly.Periods); err != nil {
+		return nil, err
+	}
 	day := timeutil.CivilDay(date, location)
 	windows, err := ResolveDayparts(date, location, dayparts)
 	if err != nil {
@@ -247,6 +250,15 @@ func SelectDiscussion(bundle *weatherdata.Bundle) *weatherdata.Discussion {
 	return bundle.Discussion
 }
 
+func ValidatePrecipitationProbabilities(periods []weatherdata.ForecastPeriod) error {
+	for i, period := range periods {
+		if !period.HasValidPrecipitationProbability() {
+			return fmt.Errorf("hourly forecast period %d has an invalid precipitation probability", i+1)
+		}
+	}
+	return nil
+}
+
 func SummarizeDaypart(name string, period timeutil.Period, periods []weatherdata.ForecastPeriod) DaypartSummary {
 	summary := DaypartSummary{
 		Name:          name,
@@ -357,12 +369,24 @@ func indicatorsForText(text string) Indicators {
 	lower := strings.ToLower(text)
 	return Indicators{
 		Snow: strings.Contains(lower, "snow"),
-		Ice:  strings.Contains(lower, "ice") || strings.Contains(lower, "freezing") || strings.Contains(lower, "sleet"),
+		Ice:  mentionsIce(lower),
 		Fog:  strings.Contains(lower, "fog"),
 		Wind: strings.Contains(lower, "wind") || strings.Contains(lower, "gust"),
 	}
 }
 
+func mentionsIce(text string) bool {
+	for _, token := range strings.FieldsFunc(text, func(r rune) bool {
+		return !('a' <= r && r <= 'z')
+	}) {
+		switch token {
+		case "ice", "icy", "freezing", "sleet":
+			return true
+		}
+	}
+	return false
+}
+
 func mentionsThunder(text string) bool {
 	return strings.Contains(strings.ToLower(text), "thunder")
 }
diff --git a/internal/forecast/derive_test.go b/internal/forecast/derive_test.go
index f526cd6..433eb80 100644
--- a/internal/forecast/derive_test.go
+++ b/internal/forecast/derive_test.go
@@ -297,6 +297,41 @@ func TestBuildDailySummaryRequiresHourlyData(t *testing.T) {
 	}
 }
 
+func TestBuildDailySummaryRejectsInvalidPrecipitationProbability(t *testing.T) {
+	location := time.FixedZone("Test", -5*60*60)
+	for _, probability := range []float64{-1, 101, math.NaN()} {
+		bundle := &weatherdata.Bundle{Hourly: &weatherdata.ForecastRun{Periods: []weatherdata.ForecastPeriod{
+			hour(location, "2026-05-29T06:00:00-05:00", "2026-05-29T07:00:00-05:00", "Showers", 70, nil, ptr(probability), nil, nil),
+		}}}
+		_, err := BuildDailySummary(bundle, mustParse("2026-05-29T12:00:00-05:00"), location, []DaypartDefinition{{Name: "morning", Start: "06:00", End: "12:00"}})
+		if err == nil {
+			t.Fatalf("BuildDailySummary(%v) error = nil, want invalid precipitation probability", probability)
+		}
+	}
+}
+
+func TestIndicatorsForTextClassifyIceVocabulary(t *testing.T) {
+	for _, tt := range []struct {
+		name string
+		text string
+		want Indicators
+	}{
+		{name: "ice", text: "Ice likely", want: Indicators{Ice: true}},
+		{name: "icy mixed case", text: "ICY roads", want: Indicators{Ice: true}},
+		{name: "freezing punctuation", text: "Freezing-rain possible", want: Indicators{Ice: true}},
+		{name: "sleet punctuation", text: "Sleet, then rain", want: Indicators{Ice: true}},
+		{name: "unrelated nice", text: "Nice weather"},
+		{name: "unrelated spicy", text: "Spicy conditions"},
+		{name: "other conditions unchanged", text: "Snow and fog with gusts", want: Indicators{Snow: true, Fog: true, Wind: true}},
+	} {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := indicatorsForText(tt.text); got != tt.want {
+				t.Fatalf("indicatorsForText(%q) = %#v, want %#v", tt.text, got, tt.want)
+			}
+		})
+	}
+}
+
 func TestAlertOverlap(t *testing.T) {
 	location := time.FixedZone("Test", -5*60*60)
 	raw := json.RawMessage(`{"event":"Flood Watch","headline":"Flooding possible","severity":"Moderate","effective":"2026-05-29T07:00:00-05:00","expires":"2026-05-29T10:00:00-05:00"}`)
diff --git a/internal/weatherdata/bundle.go b/internal/weatherdata/bundle.go
index 3b8d1b3..4cd23e9 100644
--- a/internal/weatherdata/bundle.go
+++ b/internal/weatherdata/bundle.go
@@ -3,6 +3,7 @@ package weatherdata
 
 import (
 	"encoding/json"
+	"math"
 	"time"
 )
 
@@ -137,6 +138,12 @@ func (p ForecastPeriod) HasUsableTimeBounds() bool {
 	return !p.StartTime.IsZero() && !p.EndTime.IsZero() && p.EndTime.After(p.StartTime)
 }
 
+// HasValidPrecipitationProbability reports whether an optional probability is a finite percentage.
+func (p ForecastPeriod) HasValidPrecipitationProbability() bool {
+	value := p.ProbabilityOfPrecipitationPercent
+	return value == nil || !math.IsNaN(*value) && !math.IsInf(*value, 0) && *value >= 0 && *value <= 100
+}
+
 type AlertRun struct {
 	AsOf   *time.Time        `json:"asOf,omitempty"`
 	Alerts []json.RawMessage `json:"alerts,omitempty"`
diff --git a/internal/weatherdata/bundle_test.go b/internal/weatherdata/bundle_test.go
index e589984..bed0af6 100644
--- a/internal/weatherdata/bundle_test.go
+++ b/internal/weatherdata/bundle_test.go
@@ -2,6 +2,7 @@ package weatherdata
 
 import (
 	"encoding/json"
+	"math"
 	"testing"
 	"time"
 )
@@ -93,6 +94,29 @@ func TestForecastPeriodHasUsableTimeBounds(t *testing.T) {
 	}
 }
 
+func TestForecastPeriodHasValidPrecipitationProbability(t *testing.T) {
+	for _, tt := range []struct {
+		name  string
+		value *float64
+		want  bool
+	}{
+		{name: "missing", want: true},
+		{name: "zero", value: ptrProbability(0), want: true},
+		{name: "one hundred", value: ptrProbability(100), want: true},
+		{name: "negative", value: ptrProbability(-0.1)},
+		{name: "above one hundred", value: ptrProbability(100.1)},
+		{name: "not a number", value: ptrProbability(math.NaN())},
+		{name: "infinite", value: ptrProbability(math.Inf(1))},
+	} {
+		t.Run(tt.name, func(t *testing.T) {
+			period := ForecastPeriod{ProbabilityOfPrecipitationPercent: tt.value}
+			if got := period.HasValidPrecipitationProbability(); got != tt.want {
+				t.Fatalf("HasValidPrecipitationProbability() = %t, want %t", got, tt.want)
+			}
+		})
+	}
+}
+
 func mustParseBundleTime(value string) time.Time {
 	parsed, err := time.Parse(time.RFC3339, value)
 	if err != nil {
@@ -100,3 +124,7 @@ func mustParseBundleTime(value string) time.Time {
 	}
 	return parsed
 }
+
+func ptrProbability(value float64) *float64 {
+	return &value
+}