The Weather Risk Matrix and examples of weather situations for the five impact levels are below for you to use as a reference.
The Weather Risk Matrix
Impact Levels
IMPACT LEVEL | Wind | Snow | Ice | Cold | Urban Flooding and Debris Flows | River Flooding | Heat | Thunderstorms | Marine and Coastal Hazards |
---|---|---|---|---|---|---|---|---|---|
1 No Impactful Weather. |
• No threat of wind damage | • No snow | • No ice | • No threat of cold | • No threat of urban flooding or debris flows | • No threat of river flooding | • Green (0) HeatRisk; Little to no heat risk Do NOT use impact/likelihood table. Use NWS HeatRisk |
• No threat of thunderstorms | Marine: • Small Craft Advisory |
2 Potential threat to lives & property if no or incorrect actions are taken. Localized impacts on commerce are possible. |
• Wind gusts 35-45 mph inland, 40-50 mph for inland coastal communities, 50-60 mph for beaches and headlands | • Snow below 1000ft with little to no accumulation on roads • Sub-advisory level snow at pass level |
• Freezing rain that is occurring when road temps are above freezing (e.g. little to no travel impacts) • Patchy road ice not associated with freezing rain |
• Low temperatures near 32F for elevations below 1000ft • Apparent temperatures 25-32F for elevations below 1000ft • Apparent temperatures between -9F and 0F from the Cascade foothills to the Cascade passes |
• Hourly rain rates < 0.25"/hr (winter) or < 0.5"/hr (summer), producing minor and isolated urban flooding in low-lying flood prone areas • 2" in 24 hours for elevations in urban areas |
• Isolated minor river flooding | • Yellow (1) HeatRisk; Minor heat risk; focus on levels in population centers & valleys Do NOT use impact/likelihood table. Use NWS HeatRisk *Email briefing: Include in Matrix but not in Details section |
• SPC General Thunder risk (set your thunderstorm risk level to MINOR in your email briefing) | Marine: • High-end Small Craft Advisory Coastal Areas: • Enhanced threat of sneaker waves • High Surf Advisory |
3 Potential threat to lives & property if no or incorrect actions are taken. Impacts on commerce are possible. |
• Wind gusts 45-55 mph inland, 50-60 mph inland coastal communities, 60-70 mph for beaches and headlands | • Advisory level snow amounts | • Freezing rain occurring when road temps are below freezing and road impacts are expected: Widespread ice accumulation of 0.01-0.24" • Widespread road ice not associated with freezing rain |
• Low temperatures 25-30F for elevations below 1000ft • Apparent temperatures 15-25F for elevations below 1000ft • Apparent temperatures between -19F and -10F from the Cascade foothills to the Cascade passes NOTE: If during growing season (Apr 1-Oct 31), add hard freeze/agricultural impacts to your email briefing details |
• 0.25"/hr for 1-2 hours (winter) or 0.5"/hr for 1-2 hours (summer) • 0.5"/hr for 1 hour (winter) or 1.0"/hr for 1 hour (summer) • 3" in 24 hours for elevations in urban areas |
• Widespread minor to moderate river flooding | • Orange (2) HeatRisk; Moderate heat risk; focus on levels in population centers & valleys Do NOT use impact/likelihood table. Use NWS HeatRisk |
• SPC Marginal risk (set your thunderstorm risk level to MODERATE in your email briefing) | Marine: • Gales and/or Hazardous Seas of 16-19 ft • Isolated waterspouts (1 or 2 expected) Coastal Areas: • High Surf Warning • Coastal Flood Advisory for tidal overflow |
4 Actions likely needed to save lives and/or property. Significant impacts to commerce possible. |
• Wind gusts of 55-65 mph inland, 60-70 mph inland coastal communities, 70-80 mph beaches and headlands | • Warning level mountain snow • 1 to 3" of snow in urban areas below 1000 ft during the middle of a week day (impacts to commutes) • 3 to 6" of snow in urban areas below 1000 ft overnight or on weekends • Near blizzard conditions, but visibilities are above 1/4 mile so no Blizzard Warning is expected |
• Ice storms with widespread ice accumulation between 0.25-0.5" or 0.1-0.25" of ice accumulation in populated areas along the I-5 corridor (Kelso to Eugene) | • Low temperatures 15-25F for elevations below 1000ft for two days or more • Any time apparent temperatures reach 5-15F for elevations below 1000 ft • Long duration extreme cold (at least 3 days in a row), but not historic • Apparent temperatures between -29F and -20F from the Cascade foothills to the Cascade passes NOTE: If during growing season (Apr 1-Oct 31), mention hard freeze/agricultural impacts in email briefing |
• 0.25"/hr for 3-4 hours (winter) or 0.5"/hr for 3-4 hours (summer) • 0.5"/hr for 2 hours (winter) or 1.0"/hr for 2 hours (summer) • Debris flow criteria met, Flash Flood Watches/Warnings may be needed over burn areas • 4" in 24 hours for urban areas |
• Widespread moderate and major river flooding | • Red (3) HeatRisk; Major heat risk; focus on levels in population centers & valleys Do NOT use impact/likelihood table. Use NWS HeatRisk |
• SPC Slight risk (set your thunderstorm risk level to MAJOR in your email briefing) | Marine: • Storm Force winds and/or hazardous seas at or over 20 ft • Freezing Spray Advisory • Several waterspouts expected (3 or more) Coastal Areas: • Coastal Flood Warning |
5 Actions required to save lives and/or property. Significant and extended impacts to commerce. |
• Wind gusts over 65 mph inland, over 70 mph inland coastal communities, over 80 mph beaches and headlands • Extreme fire weather conditions and critically dry fuels warranting a Particularly Dangerous Situation (PDS) Red Flag Watch or Warning |
• Blizzard conditions with Blizzard Warnings expected • High-end snowstorms for elevations below 1000ft with hourly snow rates over 1"/hr during the morning and/or evening commute and snow totals around 6" or more • Historic mountain snow amounts |
• High end ice storms with widespread tree damage/power outages expected due to ice accumulations of 0.5" or higher | • Low temperatures below 15F for elevations below 1000ft for 2 days or more • Any time apparent temperatures reach below 5F for elevations below 1000ft • Historic long duration extreme cold event • Apparent temperatures of -30F or colder from the Cascade foothills to the Cascade passes NOTE: If during growing season (Apr 1-Oct 31), mention hard freeze/agricultural impacts in email briefing |
• 0.5"/hr or more for over 3 hours or more (winter) or 1.0"/hr for 3 hours or more (summer) • 5" in 24 hours for urban areas |
• Widespread major or record flooding with impacts to major population areas • Imminent dam failure |
• Magenta (4) HeatRisk; Extreme heat risk; focus on levels in population centers & valleys Do NOT use impact/likelihood table. Use NWS HeatRisk |
• SPC Enhanced, Moderate, or High risk (set your thunderstorm risk level to EXTREME in your email briefing) | Marine: • Hurricane force winds • Heavy freezing spray Coastal Areas: • Significant coastal flooding >3 ft (high-end Coastal Flood Warning) |
Portland Weather Matrix