National Weather Service United States Department of Commerce

Apparent Temperature

is the perceived temperature derived from either a combination of temperature and wind (Wind Chill) or temperature and humidity (Heat Index) for the indicated hour. When the temperature at a particular grid point falls to 50 F or less, wind chill will be used for that point for the Apparent Temperature. When the temperature at a grid point rises above 80 F, the heat index will be used for Apparent Temperature. Between 51 and 80 F, the Apparent Temperature will be the ambient air temperature.

Convective Hazard Outlook

is a categorical forecast (marginal, slight, enhanced, moderate, or high risk) that specifies the perceived level of threat of thunderstorms, severe thunderstorms, hail, damaging winds, and tornadoes.

Critical Fire Weather

delineates areas where pre-existing fuel conditions, combined with forecast weather conditions will result in a significant threat for wildfires. Two types of areas are specified: critical fire weather areas for wind and relative humidity, and extremely critical fire weather areas for extreme conditions of wind and relative humidity.

Damaging Thunderstorm Wind Probability

is the probability (in percent) of winds greater than 58 miles per hour occurring within 25 miles of any point during the outlook period. The higher the probability, the higher the threat of severe thunderstorm winds occurring.

Dew Point

is the expected dew point temperature for the indicated hour. Dew point temperature is a measure of atmospheric moisture. It is the temperature to which air must be cooled in order to reach saturation (assuming air pressure and moisture content are constant).

Dry Thunderstorms

delineates areas expecting widespread or numerous thunderstorms that produce little wetting rain (<0.10 in) where dried fuels exist.

Extreme Hail Probability

is the probability (in percent) of hail greater than 2 inches in diameter within 25 miles of any point during the outlook period. The higher the probability, the higher the threat of extreme hail occurring.

Extreme Thunderstorm Wind Probability

is the probability (in percent) of winds greater than 75 miles per hour occurring within 25 miles of any point during the outlook period. The higher the probability, the higher the threat of extreme thunderstorm winds occurring.

Extreme Tornado Probability

is the probability (in percent) of Enhanced Fujita scale 2 (EF2) tornadoes occurring within 25 miles of any point during the outlook period. The higher the probability, the higher the threat of extreme tornadoes.

Forecast of Reference Crop Evapotranspiration (FRET)

is the expected depth of water that would evaporate and transpire from a reference crop under the forecast weather conditions on a daily and weekly basis over the next 7 days. The FRET is for short crops with an approximate height of 12 cm similar to full cover grasses.

Hail Probability

is the probability (in percent) of hail greater than three-quarters of an inch in diameter (size of a penny) occurring within 25 miles of any point during the outlook period. The higher the probability, the higher the threat of severe hail.

Hazards

are weather or hydrologic hazardous events issued for the protection of life and property and the enhancement of the national economy. The hazards grid includes long duration coastal, marine, non-precipitation, tropical, convective, winter weather and some hydrological hazards. It does not include some short duration watches and warnings such as Tornado, Severe Thunderstorm, Flash Flood, Gale, and Special Marine Warnings. See the latest Hazards Product Description Document at: https://products.weather.gov/PDD/HazardsGrid.pdf for the latest listing of included hazards and definitions.

National Forecast Digital Database

Watch, Warning, & Advisory Legend

Color Label Hazard / Weather Event
    None
  Wind
 
LW.Y, WI.Y Lake Wind Advisory, Wind Advisory
 
HW.A High Wind Watch
 
HW.W High Wind Warning
  Flood
 
FF.A Flash Flood Watch
 
CF.Y, LS.Y Coastal Flood Advisory, Lakeshore Flood Advisory
 
CF.A, LS.A Coastal Flood Watch, Lakeshore Flood Watch
 
CF.W, LS.W Coastal Flood Warning, Lakeshore Flood Warning
 
FA.A Flood Watch (Area)
 
FL.Y, FA.Y Flood Advisory (Point), Flood Advisory
  Thunderstorm
 
SV.A Severe Thunderstorm Watch
  Tornado
 
TO.A Tornado Watch
  Tropical
 
SS.A Storm Surge Watch
 
SS.W Storm Surge Warning
 
SS.A Hurricane Wind Watch
 
SS.W Hurricane Wind Warning
 
HU.A, TY.A Hurricane Watch, Typhoon Watch
 
HU.W, TY.W Hurricane Warning, Typhoon Warning
 
TI.A, TR.A Tropical Storm Wind Watch, Tropical Storm Watch
 
TI.W, TR.W Tropical Storm Wind Warning, Tropical Storm Warning
  Marine
 
HF.A Hurricane Force Wind Watch
 
HF.W Hurricane Force Wind Warning
 
SR.A Storm Watch
 
SR.W Storm Warning
 
GL.A Gale Watch
 
GL.W Gale Warning
 
SU.Y High Surf Advisory
 
SU.W High Surf Warning
 
SW.Y, RB.Y, SI.Y, SC.Y Small Craft Advisory
 
SE.A Hazardous Seas Watch
 
SE.W Hazardous Seas Warning
 
UP.Y Freezing Spray Advisory
 
UP.A Heavy Freezing Spray Watch
 
UP.W Heavy Freezing Spray Warning
 
BW.Y Brisk Wind Advisory
 
LO.Y Low Water Advisory
 
BH.S, RP.S Beach Hazard Statement, Rip Current Statement
  Winter Weather
 
WW.Y Winter Weather Advisory
 
WS.A Winter Storm Watch
 
WS.W Winter Storm Warning
  Snow
 
LE.A Lake Effect Snow Watch
 
LE.W Lake Effect Snow Warning
 
LE.Y, LB.Y Lake Effect Snow Advisory, Lake Effect & Blowing Snow Advisory
  Blizzard
 
BZ.A Blizzard Watch
 
BZ.W Blizzard Warning
  Ice
 
IS.W Ice Storm Warning
 
ZR.Y Freezing Rain Advisory
 
ZF.Y Freezing Fog Advisory
  Cold
 
WC.Y Wind Chill Advisory
 
WC.A Wind Chill Watch
 
WC.W Wind Chill Warning
 
EC.A, EC.W Extreme Cold Watch, Extreme Cold Warning
 
FR.Y Frost Advisory
 
FZ.A Freeze Watch
 
FZ.W Freeze Warning
 
HZ.A Hard Freeze Watch
 
HZ.W Hard Freeze Warning
  Heat
 
HT.Y Heat Advisory
 
EH.A Excessive Heat Watch
 
EH.W Excesssive Heat Warning
  Miscellaneous
 
FW.A Fire Weather Watch
 
FW.W Red Flag Warning
 
DU.Y Blowing Dust Advisory
 
SM.Y Dense Smoke Advisory
 
DS.W Dust Storm Warning
 
FG.Y Dense Fog Advisory
 
AS.Y Air Stagnation Advisory
 
AF.Y Ashfall Advisory
 
AF.W Ashfall Warning


Hurricane Threats

are the worst-case plausible scenario or threat associated with each of four tropical cyclone hazards: wind, storm surge, flooding rain, and tornadoes.   Threats take into account the forecast magnitude and the associated forecast uncertainty of the hazard.   There are five levels which describe each threat grid; Little to None, Elevated, Moderate, High, and Extreme.   Threats are valid whenever tropical cyclone watches and warnings are in effect along the Atlantic and Gulf Coasts, as well as Puerto Rico.   They convey the threat for the duration of the event, not specific timing.  See the latest Tropical Cyclone Threats Product Description Document at: https://products.weather.gov/PDD/TCThreatGridsNDFD.pdf for the listing and description of the included threats.

Ice Accumulation

is the expected new ice accretion on all exposed surfaces during a 6 hour period.

Maximum/Minimum Temperature

is the daytime maximum or the overnight minimum temperature.

Maximum/Minimum Relative Humidity

is the highest or lowest relative humidity (%) expected for the indicated 12-hour period.

Precipitation Amount

is the expected quantity of liquid precipitation accumulated over a six hourly period. A quantitative precipitation forecast (QPF) will be specified when a measurable ( 1/100th of an inch or more) precipitation type is forecast for any hour during a QPF valid period. NDFD valid periods for QPF are 6 hours long beginning and ending at 0000, 0600, 1200 and 1800 UTC. QPF includes the liquid equivalent amount for snow and ice.

Probability of Precipitation (PoP12)

is the likelihood, expressed as a percent, of a measurable precipitation event (1/100th of an inch or more) at a grid point during the 12-hour valid period. The 12-hour valid periods begin and end at 0000 and 1200 Coordinated Universal Time (UTC).

Precipitation Potential Index (PPI)

is a means to show confidence as to the location of precipitation at each forecast time across the forecast area. PPI values range from 0 to 100 and resemble PoP12 values in magnitude. The PoP12 for any 12- hour period can be derived by taking the maximum PPI value within the desired period.

Relative Humidity

is a ratio, expressed as a percent, of the amount of atmospheric moisture present relative to the amount that would be present if the air were saturated. Since the latter amount is dependent on temperature, relative humidity is a function of both moisture content and temperature.

Sky Cover

is the expected amount of opaque clouds (in percent) covering the sky valid for the indicated hour.

Snow Amount

is the expected total accumulation of new snow during a 6 hour period. A snow accumulation grid will be specified whenever a measurable snowfall is forecast for any hour during a valid period. Valid periods for the NDFD begin and end at 0600, 1200, 1800, and 0000 UTC.

Temperature

is the expected temperature valid for the indicated hour.

Tornado Probability

is the probability (in percent) of a tornado occurring within 25 miles of any point during the outlook period. The higher the probability, the higher the threat of tornadoes occurring.

Total New Ice

is the sum of all forecast six hourly ice accumulations prior to the indicated hour.

Total New Precipitation

is the sum of all forecast six hourly liquid precipitation amounts prior to the indicated hour. This total includes the liquid equivalent amount for snow and ice.

Total New Snow

is the sum of all forecast six hourly snow amounts prior to the indicated hour.

Total Probability of Extreme Severe Thunderstorms

is the probability in percent of EF2 (Enhanced Fujita scale 2) tornadoes, damaging winds with speeds greater than 75 miles per hour, or large hail two inches or greater in diameter occurring within 25 miles of any point during the outlook period.

Total Probability of Severe Thunderstorms

is the probability in percent of tornadoes, damaging winds with speeds greater than 58 miles per hour, or large hail three quarters of an inch in diameter (penny-size) occurring within 25 miles of any point during the outlook period.

Tropical Cyclone Surface Wind Speed Probability (Cumulative)

is the probability (in percent) of sustained surface wind speed greater than 34-, 50- and 64-knots (3 separate elements) sometime during the specified cumulative forecast period (0 - 6 hours, 0-12, 0-18, etc.) at each specific point. NOTE: This element is provided for coastal and inland points as well as offshore locations (e.g., buoys).

Tropical Cyclone Surface Wind Speed Probability (Incremental)

is the probability (in percent) of sustained surface wind speed greater than 34-, 50-, and 64-knots (3 separate elements) sometime during the specified forecast period (0 - 6 hours, 6 -12, 12 -18, etc.) at each specific point. These values are incremental since they can increase in value by accounting for the possibility the event might start in an earlier period and still be occurring in the specified period. NOTE: This element is provided for coastal and inland points as well as offshore locations (e.g., buoys).

Wave Height

is the average height (from trough to crest) of the one-third highest waves valid for the top of the designated hour. Wave Height is a combination of wind waves and swell.

Weather

is the expected weather (precipitating or non-precipitating) valid at the indicated hour. Precipitating weather includes type, probability, and intensity information. In cases of convective weather, coverage may be substituted for probability.

NDFD Wx Legend

Wind Direction & Speed

Wind direction is the expected sustained 10-meter wind direction for the indicated hour, using 36 points of a compass. Wind Speed is the expected sustained 10-meter sustained wind speed for the indicated hour. Wind barbs (shown below) are used to denote wind speed and direction.

How to read wind barbs

Wind Gust

is the maximum 3-second wind speed forecast to occur within a 2-minute interval at a height of 10 meters. Wind gust forecasts are valid at the top of the indicated hour.