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First Alert5 Storm Impact Scale cheat sheet

SIS 1
SIS 2
SIS 3
SIS 4
SIS 5
Posted at 12:28 PM, Apr 15, 2021
and last updated 2021-04-15 19:28:53-04

You may have noticed on the First Alert5 7 day forecasts an "impact" numbered 1-5. This is the Storm Impact Scale or SIS for short.

News 5 developed the SIS to categorize the severity of an incoming weather event, allowing you to gauge 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.

We will categorize this per fall/winter and spring/summer to gauge cold weather or warm weather events. But as we know in Colorado, snow and thunderstorms can extend outside of these seasonal categories, so take this with a grain of salt.

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.

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.

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: Severe thunderstorms possible including hail 1 inch or larger and winds gusts over 58 mph. Flooding is possible.

Impact 4

General: Stay indoors if you can.

Fall/Winter: Snow accumulations 8-12 inches. Schools and businesses closed. Blizzard conditions are possible.

Spring/Summer: Several severe, damaging thunderstorms expected with large hail, strong winds, flooding, and/or isolated tornadoes possible.

Impact 5

General: Dangerous storm, remain sheltered.

Fall/Winter: Snow accumulations over 1 foot. Blizzard conditions expected. Travel becomes impossible. Power outages expected.

Spring/Summer: Major, widespread severe weather. Very large hail, damaging wind or tornadoes likely, and flooding expected.