Criteria for Weather Watch, Warnings and Advisories issued by NWS New Orleans/Baton Rouge

Winter Storm Outlook

Issued when there is a 30% or greater chance of a winter weather event exceeding local warning criteria 3-7 days in advance of the event.

Winter Storm Watch

A Winter Storm Watch is issued when there is a 50% or greater chance of hazardous winter weather conditions exceeding warning criteria. Winter Storm Watches are issued 12 to 48 hours before a winter weather event.

Winter Storm Warning

A Winter Storm Warning is issued when a mix of wintry precipitation is occurring,is imminent, or has a very high probability of occurrence...80% or greater chance of warning criteria being exceeded.

Heavy Snow Warning

A Heavy Snow Warning is issued when heavy snow is occurring,is imminent, or has a very high probability of occurrence...80% or greater chance of warning criteria being exceeded.

A heavy snow event is defined as:
Greater than or equal to 2 inches of snow per event .

Ice Storm Warning

An Ice Storm Warning is issued when heavy freezing rain is occurring, is imminent, or has a very high probability of occurrence...80% or greater chance of warning criteria being exceeded.

Warning Critera: 0.25" or more of ice accumulation.

Winter Weather Advisory

A Winter Weather Advisory will be issued for winter weather situations such as light snow, and light icing conditions that cause significant inconveniences but do not meet warning criteria and, if caution is not exercised, could lead to life‑threatening situations. The Winter Weather Advisories should be used for mixed precipitation such as sleet and snow.

Freezing Rain Advisory

Ice accumulations of less than 1/4 inch.

Freeze Warning/Hard Freeze Warning

A freeze is when the surface air temperature is expected to be 32 degrees F or lower over a widespread area for a climatologically significant period of time, mainly during the growing season. A freeze may or may not be accompanied by frost. A hard freeze is generally meant to describe conditions when the temperature is maintained at 25 degrees F or below or long enough to freeze water pipes. A light freeze is generally defined as 26 to 32 degrees even if it lasts for several hours.

Freeze Warning Criteria - North of Donaldsonville to Lake Pontchartrain to Pascagoula Line

a) Freeze Warning for the first two light freezes of the season

b) Freeze Warning for any hard freeze.

c) Freeze Warning for freezing temperatures that occur after an extended warm spell, especially when vegetation begins to grow, in latter February and March.

Freeze Warning Criteria - South of a Donaldsonville to Lake Pontchartrain Line

Freeze Warning issued anytime freezing temperatures are forecast. Highlight those rare events when pipe bursting minimum temperatures are forecast.

Wind Chill Advisory

A Wind Chill Advisory is issued when wind chills of 13F or colder are expected to last for two hours or longer.

Dense Fog Advisory

A Dense Fog Advisory is issued when widespread or localized fog reduces visibilities to 1/4 mile or less.

Wind Advisory

A Wind Advisory is issued when the following conditions are expected for 1 hours or longer.

1) sustained winds of 26 to 39 mph

1) gusts 40 mph or greater

High Wind Warning

A High Wind Warning is issued when the following conditions are occuring or imminent:

1) sustained winds of 40 mph or higher for one hour or more

OR

2) wind gusts of 58 mph or higher for one hour or more.

Small Craft Advisory

Small Craft Advisories are issued for the coastal waters within 60 NM of the coast from the Mississippi Alabama border to the Atchafalaya River...including Lake Pontchartrain and Lake Maurepas...when one or both of the following conditions is expected to begin within 36 hours:

1) sustained winds of 21 knots to 33 knots, or frequent gusts between 21 knots and 33 knots.

OR

2) waves of 7 feet or higher

Gale Warning

Gale Warnings are issued for the coastal waters within 60 NM of the coast from the Mississippi Alabama border to the Atchafalaya River...including Lake Pontchartrain and Lake Maurepas...when one or both of the following conditions is expected to begin within 36 hours:

1)  sustained winds of 34 knots to 47 knots

OR

2) frequent gusts between 34 knots and 47 knots.

Storm Warning

Storm Warnings are issued for the coastal waters within 60 NM of the coast from the Mississippi Alabama border to the Atchafalaya River...including Lake Pontchartrain and Lake Maurepas...when one or both of the following conditions is expected to begin within 36 hours:

1)  sustained winds of 48 knots to 63 knots

OR

2) frequent gusts of 48 knots to 63 knots

Special Marine Warning

A warning of potentially hazardous weather conditions of short duration (up to 2 hours) affecting areas included in a CWF that are not adequately covered by existing marine warnings and producing one or more of the following:

• Sustained marine convective winds (showers/thunderstorms) or associated gusts of 34 knots or greater
• Hail three quarters of an inch or more in diameter
• Waterspouts

 Tropical Storm Watch

A Tropical Storm Watch is issued when a tropical cyclone containing winds of 34 to 63 kt (39 to 73 mph) or higher poses a possible threat, generally within 48 hours. These winds may be accompanied by storm surge, coastal flooding, and/or river flooding.

The watch may be issued for the entire forecast area.

The watch does not mean that tropical storm conditions will occur. It only means that these conditions are possible.

Tropical Storm Warning

A Tropical Storm Warning is issued when sustained winds of 34 to 63 kt (39 to 73 mph) or higher associated with a tropical cyclone are expected in 36 hours or less. These winds may be accompanied by storm surge, coastal flooding, and/or river flooding.

The warning may be issued for the entire forecast area.

Hurricane Watch

A Hurricane Watch is issued when a tropical cyclone containing winds of 64 kt (74 mph) or higher poses a possible threat, generally within 48 hours. These winds may be accompanied by storm surge, coastal flooding, and/or river flooding.

The watch may be issued for the entire forecast area.

The watch does not mean that hurricane conditions will occur. It only means that these conditions are possible.

Hurricane Warning

A Hurricane Warning is issued when sustained winds of 64 kt (74 mph) or higher associated with a tropical cyclone are expected in 36 hours or less. These winds may be accompanied by storm surge, coastal flooding, and/or river flooding. A hurricane warning can remain in effect when dangerously high water or a combination of dangerously high water and exceptionally high waves continue, even though winds may be less than hurricane force.

The warning may be issued for the entire forecast area.

Severe Thunderstorm Watch

A Severe Thunderstorm Watch is issued when severe thunderstorms are possible in and near the watch area. It does not mean that they will occur. It only means they are possible.

Severe thunderstorms are defined as follows:

Winds of 58 mph or higher

AND/OR

Hail one inch in diameter or larger.

Severe Thunderstorm Warning

A Severe Thunderstorm Warning is issued when severe thunderstorms are occurring or imminent in the warning area.

Severe thunderstorms are defined as follows:

Winds of 58 mph or higher

AND/OR

Hail one inch in diameter or larger.

Tornado Watch

A Tornado Watch is issued when severe thunderstorms and tornadoes are possible in and near the watch area. It does not mean that they will occur. It only means they are possible.

Severe thunderstorms are defined as follows:

Winds of 58 mph or higher

AND/OR

Hail one inch in diameter or larger.

Tornado Warning

A Tornado Warning is issued when a tornado is imminent. When a tornado warning is issued, seek safe shelter immediately.

Coastal Flood Watch

Coastal Flood Watches are issued for areas along the Gulf of Mexico and around Lakes Pontchartrain and Maurepas.

Coastal Flood Warning

A Coastal Flood Warning is issued when moderate to severe coastal flooding is occurring or imminent. This flooding will pose a serious risk to life and property.

Coastal Flood Warnings are issued for areas along the Gulf of Mexico and around Lakes Pontchartrain and Maurepas.

Coastal Flood Advisory/Statement

A Coastal Flood Statement is issued when minor or nuisance coastal flooding is occurring or imminent.

Coastal Flood Statements are issued for areas along the Gulf of Mexico and around Lakes Pontchartrain and Maurepas.

Flash Flood Watch

A Flash Flood Watch is issued when conditions are favorable for flash flooding. It does not mean that flash flooding will occur, but it is possible.

Flash Flood Warning

A Flash Flood Warning is issued when flash flooding is imminent or occurring.

Urban and Small Stream Flood Advisory

An Urban and Small Stream Flood Advisory is issued for nuisance flooding of streets, low lying areas, and poor drainage areas. Flood levels are expected to remain below levels that would prove a threat to life and property.

Flood Watch

A Flood Watch is issued when conditions are favorable for flooding. It does not mean flooding will occur, but it is possible.

Flood Warning

A Flood Warning is issued when flooding is imminent or occurring.

River Flood Watch

A River Flood Watch is issued when river flooding is possible at one or more forecast points along a river.

River Flood Warning

A River Flood Warning is issued when river flooding is occurring or imminent at one or more forecast points along a river.

Heat Products

Heat Advisory HI greater than or equal to 108°F.  OR Temperature greater than or equal to 103°F.
Excessive Heat Warning HI >= 113°F or Temp >= 105°F (Watch is issued if these conditions are expected in the next 24 to 72 hours)

Fire Weather Watch/Red Flag Warning

Fire Weather Watches/Red Flag Warnings are issued when the combination of dry fuels and weather conditions are such that extreme fire behavior or ignition is occurring or is expected to occur. Forecasters will coordinate with local fire and land managers(State Forestry from Louisiana and/or Mississippi/US Forest Service) to discuss the issuance of a Fire Weather Watch or Red Flag Warning. In addition, issuing offices should coordinate with adjacent weather offices.

Below are some of the criteria to consider when evaluating Red Flag Conditions:

MISSISSIPPI 

• Red Flag Conditions 

         Relative Humidity less than or equal to 25%

         Wind speed of greater than or equal to 15 mph 

• Fire Weather Watch Conditions 

  Relative Humidity less than or equal to 25%

  Wind speed of greater than or equal to 15 mph

  LOUISIANA 

  • Red Flag Conditions

  Keetch-Byram Drought Indices of greater than 700

  Relative Humidity less than 25%

  Wind speed of greater than or equal to 25 mph 

  • Fire Weather Watch Conditions 

  Keetch-Byram Drought Indices of greater than 700

  Relative Humidity 25 to 30%

  Wind speed of greater than 20 mph.