Definitions, Thresholds, Criteria for Warnings, Watches and Advisories

Winter Precipitation

For Snow & Ice Threats:

Winter Storm Watches are issued at longer ranges when confidence is lower in meeting the warning criteria. Warnings and advisories are issued when confidence is high that widespread accumulations will meet these criteria. 

Cold Temperatures and/or Wind Chills (NEW CRITERIA FOR 2024-2025):

Either wind chill or temperatures can meet these criteria (i.e. if there is light/no wind, temperatures alone can trigger these products):

Thunderstorm Threats

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 1 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.

Non Precipitation Phenomena Warning Terminology

The State College NWS Office will issue Advisories and Warnings under the header "PHLNPWCTP (WWUS71 KCTP)" 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.
• High Wind Watch: Possibility that High Wind Warning Criteria may be met at longer ranges (24 to 48 hrs out).

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 occur with mainly clear skies and light winds, and low temperatures near or slightly above freezing (33°F to 36°F).
• Freeze Warning: low temperatures across the whole county/zone 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 has reached 32°F or below) which usually occurs in late Sep to late Oct in Central PA. See a map of the Average Date of the First Autumn Freeze. As you might expect, the growing season is much shorter in the Northern Mountains than in the Lower Susquehanna Valley.
• Freeze Watch: Possibility that Freeze Warning Criteria may be met at longer ranges (24 to 48 hrs out).

For High Heat Indicies:

• Excessive Heat Warning: Heat indicies will attain or exceed 105°F. (see map of Excessive Heat Warning Threshold)
• Heat Advisory: Issued when the Heat Index will be equal to or greater than 100°F, but less than 105°F. (see map of Heat Advisory Threshold)
• Excessive Heat Watch: Possibility that Excessive Heat Warning Criteria may be met at longer ranges (12 to 48 hrs out).

For Widespread Dense Fog:

• Dense Fog Advisory: widespread areas of fog reducing the visibility to less than 1/4 mile.

   

Fire Weather Products

The State College NWS Office will issue these two Fire Weather related products under the header "PHLRFWCTP (WWUS81 KCTP)" for situations when the following criteria are met:

1. Criteria for rapid initiation and spread of wildfires in PA:
   • Winds must be sustained at 20 mph (17 knots) for two hours or more, and
   • Minimum Relative Humidities (which usually occur in the afternoon) must be 30 percent or less, and
   • 10-hour Fuel Moistures must be 10 percent or less (and expected to remain there for two or more days).

• Fire Weather Watch: All three of the above criteria are expected to develop simultaneously within 12-24 hours.
• Red Flag Warning: All three of the above criteria are being met currently, or are imminent.