Definitions, Thresholds, Criteria for Warnings, Watches and Advisories

Extreme Heat Warning:

  Heat Index values will attain or exceed 105°F.

Extreme Heat Watch:

  Possibility that Extreme Heat Warning Criteria may be met at longer ranges (12 to 48 hrs out).

Map of Extreme Heat Warning Criteria

Heat Advisory:

  Heat Index will be equal to or greater than 100°F, but less than 105°F.

Map of Heat Advisory Threshold

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).

Wind Advisory

  Sustained winds 31 to 39 mph, for better than 1 hour - and/or wind gusts 46 to 57 mph for any duration.

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.

Freeze Watch 

  Possibility that Freeze Warning Criteria may be met at longer ranges (24 to 48 hrs out).

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.

See a map of the Average Date of the Last Freeze in Spring

We will issue Frost Advisories and Freeze Warnings between the dates on the map to the left and the actual first freeze of that autumn.

As you might expect, the growing season is much shorter in the Northern Mountains than in the Lower Susquehanna Valley.

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.

Median Date of the First Autumn Freeze

Dense Fog Advisory

  widespread areas of fog reducing the visibility to less than 1/4 mile.

Extreme Cold Warning 

  Values of -20 F or colder along and south of I-80, and -25 F or colder for areas north of I-80

Cold Weather Advisory

  Values of -10 F or colder along and south of I-80, and -15 F or colder for areas north of I-80

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.

Winter Storm Warning (Heavy Snow) threshold map

(5 or 6 inches depending on location)

When the threat is predominantly snow or a mix of snow and sleet.

Ice Storm Warning map

When the threat is predominantly Freezing Rain (Ice)

Snow Advisory threshold map 

(2 or 3 inches depending on location)

When the threat is predominantly snow or a mix of snow and sleet.

Freezing Rain threshold map

When the threat is predominantly Freezing Rain (Ice) = Trace 

Blizzard threshold (map) 

  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.