National Weather Service United States Department of Commerce
Graphical HWO - Primary Weather Threat

This graphic depicts the primary threat of any potential hazardous weather. It includes, but is not limited to, any threat areas that are illustrated in the other graphical HWO graphics.

For example, dense fog would not be reflected in any of the other threat graphics. However, since it could be hazardous to motorists, or other individuals, it would be shown in the Primary Weather Threat graphic on an otherwise quiet weather day.

Click the graphic to return to the graphical HWO page.


Most of the abbreviations in this product are designed to be self-explanatory (i.e., Lrg Hail means large hail, or hail above 3/4" in diameter). A table of abbreviations and their specific meanings follows.
None No hazardous weather is expected across the area.
Lrg Hail
Hail greater than or equal to 3/4" in diameter.
Sml Hail Hail less than 3/4" in diameter.
Dmg Winds Wind gusts in excess of 50 kts, or 58 mph.
Gusty Winds Wind gusts 35-55 mph.
Lrg Hail/Dmg Winds

Wind gusts higher than 58 mph and hail larger than 3/4" in diameter.

Isold Tor Isolated tornadoes.
Lcl Flooding Rainfall may be heavy enough to produce localized flooding.
Fqt Lightning Storms will be capable of producing frequent cloud-to-ground lightning.
Lcly Heavy Rain Locally heavy rain is possible, but not enough to produce any flooding.
Dense Fog

Fog will reduce visibilities below 1/4 mile.

Heat Indices The heat index is forecast to reach 105ºF or higher.
Excessive Heat

Prolonged heat indices of 110ºF or higher are expected.

Wind Chills Wind chills below 10ºF are expected.
Extreme Cold Wind chills below -10ºF are expected.
Lt Snow Snow accumulations below 1/2".
Mdt Snow Snow accumulations 1/2-2".
Hvy Snow More than 2" snow in 12 hours, or more than 4" snow in 24 hours.
Snow/Ice Accumulating mix of snow and ice expected.
Sleet Sleet/ice pellets are expected to accumulate.
Wintery Mix Mix of precipitation anticipated.
Frzg Rain Ice accumulations below 0.25" are expected.
Frzg Drizzle A light coating of ice is expected.
Ice Storm Significant accumulations (more than 0.25") of ice are expected.
Freeze Temperatures are forecast to be at or below 32ºF for 3+ hours during a climatologically significant time of year.
Hard Freeze Temperatures are forecast to be at or below 28ºF for 3+ hours during a climatologically significant time of year.
Frost Frost is expected to form on vegetation and exposed surfaces. Tender vegetation may be damaged or killed.
Fire Risk

Weather conditions are becoming more conducive to wildfires or rapid fire growth.

Cool Season Criteria (Nov 1st - April 30th): The relative humidity is less than or equal to 25% for four or more consecutive hours, or relative humidity less than 25% concurrent with sustained 20ft winds of 20 mph or more. 

Warm Season Criteria (May 1st - October 31st): The relative humidity is less than 30%, concurrent with sustained surface winds equal to or greater than 10 mph, and Keetch-Byram Drought Index (KBDI) greater than 500. The KBDI criteria requirement is suspended if a county is under a Fire Alert.