NWS IND Criteria and Definitions

Criteria and Definitions for Watch/Warning/Advisory Products Issued by NWS IND**

**The NWS is transitioning to impact based watches and warnings. This means the criteria are just one component for issuing watches, warnings and advisories (WWA). The NWS will also look at timing, impacts and duration of an event in addition to criteria for making WWA decisions. 

Definitions

TORNADO – A violently rotating column of air, usually forming a pendant from a cumulonimbus cloud, whose circulation reaches the ground. It nearly always starts as a funnel cloud and may be accompanied by a loud roaring noise. On a local scale, it is the most destructive of all atmospheric phenomena.

FUNNEL CLOUD – A rotating column of air, forming a pendant from a cumulus or cumulonimbus cloud, whose circulation does not reach the ground.

TORNADO WATCH – Conditions are favorable for tornado development. Remain alert for approaching storms.

TORNADO WARNING – Radar has indicated a tornado or a tornado has been spotted. Take cover now.

SEVERE THUNDERSTORM – A thunderstorm accompanied by winds (sustained or gusts) of 58 mph (50 knots) or more and hail 1 inch in diameter or larger. Structural wind damage may be used to infer the occurrence of a severe thunderstorm.

SQUALL LINE – A line of thunderstorms or squalls which may extend over several hundred miles.

SEVERE THUNDERSTORM WATCH – Conditions are favorable for tornado development. Remain alert for approaching storms.

SEVERE THUNDERSTORM WARNING – Radar has indicated a severe thunderstorm or severe weather has been reported. Take cover now.

WATERSPOUT – A rotating column of air, usually forming a pendant from a cumulus or cumulonimbus cloud, which forms over a body of water, and whose circulation reaches the water.

DOWNBURST – A strong downdraft from a cumulonimbus cloud which induces damaging winds on or near the ground.

MACROBURST – A large downburst with the diameter of outflow 2 1/2 miles or larger and damaging winds lasting 5 to 20 minutes. Intense macrobursts can cause tornado-force damage.

MICROBURST – A small downburst with the diameter of outflow less than 2 1/2 miles and peak winds lasting only 2 to 5 minutes. They may induce dangerous wind and downflow wind shears which can affect aircraft performance.

FLASH FLOOD – A flood which follows a heavy or excessive rainfall, a dam or levee failure, or sudden release of water impounded by an ice jam, within a few hours. There is nothing in the National Weather Service definition that says a flash flood must be a "wall of water."

WINTER STORM WARNING FOR HEAVY SNOW –

Northern Indiana:
6 inches or more in one event (48 hours or less)

Central Indiana:
5 inches or more in one event (48 hours or less)

Southern Indiana: 
4 inches or more in one event (48 hours or less)

WINTER WEATHER ADVISORY FOR SNOW –
Northern and Central Indiana: 3 to 4 inches in 12 hours, optional for 2 to 4 inches
Southern Indiana: Less than 4 inches in 12 hours

BLIZZARD WARNING – Sustained winds or frequent gusts to 35 mph or more for at least 3 hours and considerable falling and/or blowing snow reducing visibility frequently to less than 1/4 mile.

ICE STORM WARNING – Ice accumulation of 1/4 inch or more on all surfaces. Brings down wires.

SNOW SQUALL WARNING -- Usually:  1) visibility 1/4SM or less with sub-freezing ambient road temperatures, or 2) plunging temperatures along and behind an arctic front sufficient to produce flash freezes, gusty winds and blowing snow.  Warnings typically last 30 to 60 minutes, and are not issued when Winter Storm or Blizzard Warnings are in effect.

HIGH WINDS – Sustained winds of 40 mph or greater or winds gusting to 58 mph or greater.

WINTER WEATHER ADVISORY FOR BLOWING SNOW  – Visibilities intermittently at or below 1/4 mile due to blowing snow with winds less than 35 mph.

WIND CHILL ADVISORY – Values -15 to -24

WIND CHILL WARNING – Values -25 or below