Great Falls, MT Weather Warnings

When any of the following is expected within the next 12 to 36 hours.

More than one predominant hazard.

• Winter weather event having more than one predominant hazard {ie. heavy snow and blowing snow (below blizzard conditions), snow and ice, snow and sleet, sleet and ice, or snow, sleet and ice} meeting or exceeding warning criteria for at least one of the precipitation elements.

A Blizzard Warning means that the following conditions are occurring or expected within the next 12 to 18 hours.
1) Snow and/or blowing snow reducing visibility to ¼ mile/.4 km or less for 3 hours or longer
AND
2)  Sustained winds of 35 mph/56 km/h or greater or frequent gusts to 35 mph/56 km/h or greater.
There is no temperature requirement that must be met to achieve blizzard conditions.

½ inch/1.27 cm or greater accretion of freezing rain in any zone.

When any of the following is expected within the next 12 to 24 hours.

More than one predominant hazard.

Winter weather event having more than one predominant hazard (i.e., snow and ice, snow and sleet, or snow, ice & sleet) meeting or exceeding advisory criteria for at least one of the precipitation elements, but remaining below warning criteria.

• Snow, Ocean Effect Snow, and/or Sleet
  3 inches/7.6 cm averaged over a forecast zone in 12 hours.

• Snow and Blowing Snow
  Sustained or frequent gusts of 25 to 34 mph/40 to 54 km/h accompanied by falling and blowing snow occasionally reducing visibility to less than or equal to ¼ mi/.4 km for less than 3 hours.

• Widespread or localized blowing snow reducing visibility to less than or equal to ¼ mi/.4 km with winds less than 35 mph/56 km/h.

• Freezing Rain
  Any accretion of freezing rain or freezing drizzle on road surfaces.

• Black Ice
  A Special Weather Statement will usually be issued when sufficient moisture is expected to cause a thin layer of ice on road surfaces, typically on cloudless nights ("black ice"). At forecaster discretion a formal Winter Weather Advisory may be issued instead.

Sustained winds greater than or equal to 74 mph/120 km/h (greater than or equal to 64 kts) (no gust criteria) associated with a hurricane expected to affect a coastal or inland zone within 36 hours.

Sustained winds 39-73 mph/62-117 km/h (34-63 kts) (no gust criteria) associated with a tropical storm expected to affect a coastal or inland zone within 36 hours.

Short-fused warning for onset of extreme winds, defined as sustained at 111+ mph/178+ km/h (equivalent to a Category 3+ hurricane).
This is typically used for the approach of the eyewall of a major landfalling hurricane.

When the danger of life-threatening inundation from rising water moving inland from the shoreline somewhere within the specified area, generally within 36 hours, is in association with a tropical, subtropical, or post-tropical cyclone.

Rapid and extreme flow of high water into a normally dry area, or a rapid water level rise in a stream or creek above a predetermined flood level, beginning within a short timeframe from the onset of heavy rain.

A dam or levee failure, or water released from an ice jam is also considered.

Expected overflow or inundation by water which causes or will cause damage and/or a threat to life.

Water level at a River Forecast point along a main stem river is expected to reach or exceed flood stage.

Expected inundation by water of some low lying and poor drainage areas, resulting in a nuisance to the public but not a threat to life and property.

A Coastal Flood Warning is issued when moderate to major coastal flooding is occurring or imminent. This flooding will pose a serious risk to life and property.

Minor coastal flooding expected. Examples include: splashover that causes a few roads to be briefly impassable, standing water in parking lots, etc.

A Coastal Flood Advisory is issued when minor or nuisance coastal flooding is occurring or imminent.

Sustained winds greater than or equal to 40 mph/64 km/h (greater than or equal to 35 kts) for at least 1 hour; OR any gusts greater than or equal to 58 mph/93 km/h (greater than or equal to 50 kts).

Sustained winds 31-39 mph/49-62 km/h (27-34 kts) for at least 1 hour; OR any gusts to 46-57 mph/74-91 km/h (40-49 kts).

Daytime heat indices of greater than or equal to 105°F/40°C for 2 or more hours.

Daytime heat indices of 95°F-99°F/35°C-37°C for 2 or more hours over 2 consecutive days, or 100°F-104°F/37°C-40°C for 2 or more hours over 1 day.

Issued for non-criteria warning/advisory heat. A heat wave is defined as 3 or more days of greater than or equal to 90°F/32°C temperatures.

When minimum shelter temperature drops to less than or equal to 32°F/0°C during growing season.

Issued under clear, light wind conditions with forecast minimum shelter temperature 33-36°F/0-2°C during growing season.

Wind chill index less than or equal to -25°F/-31°C for at least 3 hours using only sustained wind.

Wind chill index between -15°F/-26°C and -24°F/-31°C for at least 3 hours using only the sustained wind.

Brief/sudden occurrence of sustained wind or frequent gusts greater than or equal to 34 kts/39 mph/63 km/h, usually associated with thunderstorms, and/or hail greater than or equal to ¾ inch/1.91 cm in diameter; also issued for waterspouts.

Sustained winds or frequent gusts greater than or equal to 64 kts/74 mph/119 km/h (greater than 2 hrs) within 24 hours from a non-tropical system (marine only).

Sustained winds or frequent gusts 48-63 kts/55-72 mph/88-116 km/h (greater than 2 hrs) within 24 hours from a non-tropical system (marine only).

Sustained winds or frequent gusts 34-47 kts/39-55 mph/63-88 km/h (greater than 2 hrs) within 24 hrs from a non-tropical system (marine only).

Sustained winds or frequent gusts 25-33 kts/29-38 mph/47-61 km/h (greater than 2 hrs) AND/OR Seas greater than or equal to 5 feet/1.52 meters within 24 hours (marine only).

When high surf poses a danger to life in the form of rip currents or breaking seas. Generally issued when 7+ foot/2.3+ meters incoming seas reported at the buoys from Memorial Day through September.

When conditions yield ice accumulation rates less than 2 cm/hr ( 0.8 inch/hour). Favorable conditions for ice accumulation occur when air temperatures over the waters are less than or equal to 23°F/-5°C and sustained winds greater than or equal to 25 kts/29 mph/47 km/h and sea surface temperatures less than or equal to 43°F/6°C and seas greater than or equal to 4 feet/1.22 meters.

When conditions yield ice accumulation rates greater than or equal to 2 cm/hr ( 0.8 inch/hr ). Favorable conditions for ice accumulation occur when air temperatures over the waters are less than or equal to 23°F/-5°C and sustained winds greater than or equal to 25 kts/29 mph/47 km/h and sea surface temperatures less than or equal to 43°F/6°C and seas greater than or equal to 4 feet/1.22 meters.

High degree of confidence that dry fuels and weather conditions support extreme fire danger within 24 hours using the following criteria as a guide:

• Winds sustained or with frequent gusts greater than or equal to 25 mph/40 km/h.
• Relative Humidity at or below 30% anytime during the day.
• Rainfall amounts for the previous 5 days less than ¼ inch/.64 cm (except 3 days in pre-greenup).
• Lightning after an extended dry period.
• Significant dry frontal passage.
• Dry thunderstorms.
• Keetch-Byram Drought Index values of 300 or greater (summer only).

Widespread visibility less than or equal to ¼ mile/.4 km for at least 3 hours.

Very light ice accumulation from predominantly fog at or below freezing temperatures.

Widespread or localized smoke reducing visibilities to ¼ mile/.4 km or less.

Atmospheric conditions stable enough to cause air pollutants to accumulate in a given area. Criteria developed in conjunction with the local or state EPA and the product issued at their request.