Severe Weather Watch / Warning Guidelines
The following information is gathered from The National Weather Service in State College, Pa and serves to define the conditions associated with the various watch and warning statements issued by their office during predicted bad weather.
Severe Thunderstorm RISK Terminology
(used in Hazardous Weather Outlooks)
SLIGHT RISK: Severe thunderstorms are expected to be few or isolated.
MODERATE RISK: Severe thunderstorms are expected to be more organized, numerous, or widespread.
HIGH RISK: Severe thunderstorms with the potential for tornadoes, damaging windstorms, and/or large hail are expected
SEVERE THUNDERSTORM or TORNADO WATCH Severe thunderstorms with large hail, damaging winds, and/or tornadoes are possible, but the exact time and location of storm development is still uncertain. A watch means be prepared for storms.
SEVERE THUNDERSTORM WARNING A severe thunderstorm is imminent or occurring; it is either detected by weather radar or reported by storm spotters. A severe thunderstorm is one that produces winds 58 mph or stronger and/or hail 3/4 inch in diameter or larger. A warning means to take shelter.
TORNADO WARNING A tornado is imminent or occurring; it is either detected by weather radar or reported by storm spotters. A warning means to take shelter.
FLOOD WATCH Conditions are favorable for flash flooding. A watch means to get prepared for possible flooding.
FLASH FLOOD WARNING Life-threatening flooding is imminent or occurring; it is either detected by weather radar, indicated by stream gauges, or reported by storm spotters. A flash flood is a flood that occurs very quickly; it is caused by heavy rainfall over a short period of time or from a dam break. A warning means to leave low-lying or flood prone areas.
For Snow alone:
Winter Storm Warning for Heavy Snow: An Average of 6" or 7" of snowfall in 12 hrs., or An average of
8" to 9" of snowfall in 24 hrs.
(See a map of 12 hr Snow Warning thresholds for all of PA)
(See a map of 24 hr Snow Warning thresholds for all of PA)
Winter Weather Advisory for Snow: An Average of 3" to 4" of snowfall in 12 hrs.
(See a map of 12 hr Snow Advisory thresholds for all of PA)
Winter Storm Watch: Possibility that Warning Criteria may be met at longer ranges (24 to 48 hrs out).
For Lake Effect Snows (ONLY FOR Northwestern PA):
Lake Effect Snow Warning: An Average of 6" or more in 12 hrs.
Lake Effect Snow Advisory: An average of 3" to 5" in 12 hrs.
Lake Effect Snow Watch: Possibility that Warning Criteria may be met at longer ranges (24 to 48 hrs out).
For Ice alone:
Winter Storm Warning for Freezing Rain: 1/4" or more of ice accumulation
(See a map of Freezing Rain Warning thresholds for all of PA)
Winter Weather Advisory for Freezing Rain: <1/4" of ice accumulation
(See a map of Freezing Rain Advisory thresholds for all of PA)
Combinations of Winter Weather Hazards:
BLIZZARD WARNING: Sustained or frequent gusts to 35 mph or greater AND considerable falling and/or blowing snow. These conditions must last for AT LEAST 3 hrs. (Usually associated w/6+ inches of snowfall in central PA)
WINTER STORM WARNING: either 6 inches or more of snow w/some ice accum. or 1/4" ice + any snow/sleet.
Winter Weather Advisory: A bothersome - but not damaging - combination of snow, sleet, and/or freezing rain/drizzle.
Non Precipitation Phenomena Warning Terminology
The State College NWS Office will issue Advisories and Warnings for situations where the following non-precipitation phenomena are expected:
Winds NOT associated with a specific Thunderstorm:
Wind Advisory: sustained winds 31 to 39 mph, for better than 1 hour - and/or wind gusts 46 to 57 mph for any duration.
High Wind Warning: sustained winds 40 mph or greater, for better than 1 hour - and/or wind gusts 58 mph or greater, for any duration.
Cold Wind Chills:
Wind Chill Warning: wind chills values of minus 25°F or lower.
(See a map of Wind Chill Warning thresholds for all of PA)
Wind Chill Advisory: wind chill values of minus 15°F to minus 24°F.
(See a map of Wind Chill Advisory thresholds for all of PA)
Widespread Frosts/Freezes, after the beginning (and before the end) of the "growing season":
The BEGINNING of the growing season is set by the average date of the last freeze in spring, and normally happens during very late April to mid May in Central PA.
The NWS will then issue Frost Advisories and Freeze Warnings when appropriate, until the end of the growing season, according to the following rules:
Frost Advisory: a widespread frost, which typically with mainly clear skies and light winds, and low temperatures near or slightly above freezing (33°F to 36°F).
Freeze Warning: low temperatures at or below 32°F.
The END of the growing season occurs when most of the county/NWS zone has had it's first freeze in the autumn (1/2 or more of the zone got to 32°F or below), and usually occurs in late Sep to late Oct in Central PA. As you might expect, the growing season is much shorter in the Northern Mountains than in the Lower Susquehanna Valley.
Heat Advisory: heat index greater than or equal to 105°F and less than 115°F for less than 3 hrs/day. Also, nighttime heat indicies should remain above 80°F
.
Excessive Heat Warning: Heat indicies 105°F > 115°F for more than 3hrs per day for at least 2 consecutive days, or eclipse 115°F for any length of time.
Dense Fog Advisory: widespread areas of fog reducing the visibility to less than 1/4 mile