At the bottom of our 7-day forecasts, you might see a number from 1-5 from time to time. This is the FirstAlert5 Storm Impact Scale, SIS for short. We've recently updated the scale to better reflect your real-world impacts: how it will affect your plans.
We have put together a guide so you can learn what conditions merit each impact number, any time of year.
categorizing impacts into "cold weather" and "warm weather" so you know what the numbers mean in different storm setups. But, the general idea regardless of time of year is that a higher number warrants more planning and closer attention paid to upcoming weather conditions.
Impact 1
General: Don't cancel your plans, impacts will be minor.
Fall/Winter: Snow accumulations less than 2 inches. Slick roads are possible.
Spring/Summer: Minor showers with minimal lightning.
TRAVEL: Minor slowdowns
Impact 2
General: Outdoor plans could be impacted.
Fall/Winter: Snow accumulations 2-4 inches. Icy and snow-packed roads likely.
Spring/Summer: Moderate to heavy rain showers. Minor flooding and frequent lightning possible. Small hail to the size of a penny, isolated low-end severe storms.
TRAVEL: Delays are likely - travel will take longer than usual
Impact 3
General: Make alternate plans for outdoor activities and travel.
Fall/Winter: Snow accumulations 4-8 inches. Schools may close and travel will be difficult.
Spring/Summer: Scattered severe thunderstorms possible with hail 1 inch or larger and wind gusts of at least 58 mph. Significant flooding also qualifies for an SIS3.
All-year: Non-thunderstorm wind gusts of 50-60 mph
TRAVEL: Exercise extra caution - weather impacts may make some roads impassable.
Impact 4
General: Stay indoors if you can.
Fall/Winter: Snow accumulations 8-12 inches. Schools and businesses closed. Power outages possible.
Spring/Summer: Numerous damaging thunderstorms expected with large hail, strong winds, flooding, and/or isolated tornadoes expected.
Year-Round: Wind gusts of 60-80 mph. Weather conditions that pose significant threats to life and property.
TRAVEL: Dangerous.
Impact 5
General: Dangerous storm, potentially life threatening, remain sheltered.
Fall/Winter: Snow accumulations over 1 foot. Blizzard conditions expected. Power outages expected.
Spring/Summer: Major, widespread, destructive severe weather. Very large hail, damaging wind or tornadoes likely, and flooding expected.
Year-Round: Wind gusts >80 mph
TRAVEL: Impossible. Doing so would risk your life.
***
Storm Impact and Weather Alert are different. A Weather Alert is issued day-of, or in rare cases the day before, when significant and potentially dangerous weather is currently or about to impact southern Colorado. The Storm Impact Scale is a planning tool: What to expect, and when.
