Severe/Winter Weather Guides
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The National Weather Service authors Severe and Winter Weather guides annually. The guides serve to educate the public about hazardous weather and how to prepare for severe and winter storms.
 
Winter Guide (Winter Terminology)
 
Snow
 
Heavy Snow - In general, this is 4 to 6 inches of accumulation in 12 to 24 hours.  However, there are slight variations to this rule across Arkansas. Five offices issue forecasts for the state (for areas of responsibility, click here), with their heavy snow criteria as follows: 
 
Heavy Snow Criteria Across Arkansas
Office 12 hours 24 hours
Little Rock 4" 6"
Tulsa 4" 6"
Memphis (Clay, Greene, Lawrence, and Randolph Cos) 4" 6"
Memphis (All Other Cos) 3" 4"
Shreveport 4" 4"
Jackson 2-4" 4"

In observations ("current conditions"), "heavy snow" is used for visibilities of 1/4 mile or less, "moderate snow" for visibilities greater than 1/4 mile but less than or equal to 1/2 mile, and "light snow" for visibilities greater than 1/2 mile.

Snow Shower - A fall of snow of increased intensity over a short period of time. In this sense, they are much like rain showers in the warmer parts of the year -- intensities can vary quite a bit over small distances, and intensities at any given location can change quickly. While snow showers are more common in other parts of the country, they do occur in Arkansas. They can present a hazard to motorists due to rapid changes in visibility and because they can quickly coat roads with a thin layer of snow.

"Thunder Snow" - While this is not a "technical" weather term, it commonly is used to refer to the occurrence of lightning and thunder while snow is falling. Although this does not occur frequently in Arkansas, there are usually a few occasions each winter when it occurs somewhere in the state. For thunder to occur with snow, conditions in the atmosphere must be unstable, usually a few thousand feet above the ground. Because of the unstable conditions, snow that occurs with thunder can accumulate very quickly -- often, several inches in one hour. In fact, some of the largest snow accumulations in Arkansas history have been caused by thunder snow.

Snow Flurries - Intermittent light snowfall of short duration with no measurable accumulation. Since, by definition, no measurable amounts are expected, forecasts that call for snow flurries normally do not specifically mention such things as "no accumulation expected."

Blowing Snow - Fallen snow that is picked up by the wind, causing visibilities to be reduced. Drifting Snow - The uneven distribution of snow depth due to strong winds. Blowing and drifting snow often occur together, and they do occur in Arkansas from time to time.

Blizzard - The following conditions occur for 3 hours or longer: Sustained winds or frequent gusts of 35 mph or greater, and considerable falling and/or blowing snow which frequently reduces visibility to less than 1/4 mile. There are no set temperature requirements; however, temperatures below 20 degrees greatly increase the threat to life. Blizzards are extremely rare events in Arkansas.

 

Ice
 
Freezing Rain and Freezing Drizzle - Rain or drizzle that freezes on surfaces, such as the ground, trees, power lines, cars, roadways, etc. For freezing precipitation to form, snow falls from the clouds, but goes through a layer of warmer air above the ground, causing the snowflake to melt and become a drop of liquid rain or drizzle. Then, the rain or drizzle reaches the ground, where temperatures are below freezing and freezes on contact with an exposed surface. An older term for freezing rain is "glaze" because of the glaze of ice that it forms on objects. For people who get freezing precipitation confused with sleet, the idea of the "glaze" can help them remember how to differentiate between the two. Freezing rain is reported in observations when ice begins to form on any exposed object. Normally, this is on metal objects such as cars and flagpoles, and on taller objects such as treetops. Thus, freezing rain may be reported even though ice has not yet begun to form on roadways.

Ice Storm - Freezing rain produces significant accumulations of ice, usually at least 1/4 inch thick in 12 to 24 hours. Such accumulations damage trees and pull down power lines. On a few occasions in Arkansas, significant ice storms have occurred but have produced little in the way of hazardous driving conditions. This is due to the ice forming on taller objects (trees and power lines), at a height where the temperature is a few degrees colder than at ground level.

Sleet - This is the common name for "ice pellets." Sleet forms when snow falls from the clouds, goes through a warmer layer, causing the snowflakes to melt into raindrops, then goes through another cold layer above the ground, causing the raindrops to freeze into translucent pellets of ice. The pellets usually bounce upon hitting the ground or other hard objects. For people who get the definitions of freezing rain and sleet confused, the idea of the "ice pellets" can help them remember how to differentiate between the two. Heavy sleet refers to sleet covering the ground to a depth of 1/2 inch or more in 12 to 24 hours.

 

Cold
 
Freeze - The surface air temperature drops to 32 degrees or lower over a widespread area for a climatologically significant period of time. A freeze may or may not be accompanied by frost. The term "hard freeze" may be used when the temperature will drop well below freezing, but there is no set definition for the exact temperature which must be reached to constitute a hard freeze. (In Arkansas and other parts of the southern U.S., "Freeze Warnings" are issued for the first few freezes in the fall (until a killing freeze puts an end to the growing season) and in the spring when plants have begun to blossom. In the South, the term "Freeze Advisory" is not used.)

Frost - The formation of thin ice crystals on the ground or other surfaces. Frost develops under conditions similar to dew, except that the temperature is 32 degrees or lower. There are frequently questions as to how frost can occur when the reported temperature is in the mid or even upper 30s. This is because the official thermometers are always about 5 feet above the ground. On clear, calm nights, the temperature at the ground can be several degrees lower, thus allowing the formation of frost.

(In Arkansas and other parts of the southern U.S., Advisories and Warnings for frost are not issued. Instead, the frost is simply mentioned in the body of the forecast when there is a threat to vegetation in the fall and early spring.)

Wind Chill - The effect of temperature and wind on the body; specifically, the heat loss from exposed skin. Experiments have shown that wind chills become dangerous at values of -19 degrees or so and lower...with exposed skin suffering frostbite in 30 minutes or less. 

 

Watches, Warnings and Advisories
 
A Watch is issued when the risk of hazardous winter weather has increased significantly, but its occurrence, location, and/or timing is uncertain. In general, Watches are issued about 12 to 24 hours in advance of the threat, but they may occasionally be issued 24 to 36 hours in advance.

Watches (specifically "Winter Storm Watches") are issued when blizzards, heavy snow and/or significant icing (freezing rain/sleet) become concerns.

Winter Storm Watches evolve into Warnings or Advisories, or are cancelled.

Warnings - A Warning is issued when hazardous winter weather is occurring, imminent, or has a very high probability of occurring. Warnings are used for conditions posing a threat to life or property. In general, Warnings are issued for events occurring during the first 12 hours of the forecast, but they can be continued into the 12 to 24 hour time frame if the forecaster is very confident that the event will last that long.

Warnings (specifically "Winter Storm Warnings") are issued for blizzards, heavy snow, and/or significant icing (freezing rain/sleet).

During the growing season, "Freeze Warnings" are issued when temperatures are expected to drop below freezing...with the cold adversely affecting vegetation. "Wind Chill Warnings" are posted when wind chill indices drop below -15 degrees for at least one hour. 

 

Heavy snow blanketed much of central Arkansas during the morning hours of February 6, 2002. In the picture: Heavy snow blanketed much of central Arkansas during the morning hours of February 6, 2002. Four to six inches of snow was reported in the Little Rock (Pulaski County) area.

 

It is important to note that a Warning is stronger wording than an Advisory and indicates that worse conditions are expected. Thus, the mention of a Warning automatically implies that hazardous driving conditions are expected.

Advisories - Advisories are issued for less serious conditions than Warnings, but these conditions cause significant inconvenience and, if caution is not exercised, could lead to situations that may threaten life or property.

Advisories are issued for conditions that are occurring, imminent, or have a very high probability of occurring. In general, Advisories are issued for the first 12 hours of the forecast, but on occasion may be issued for the 12 to 24 hour time frame if the forecaster is very confident the event will occur.

Advisories (specifically "Winter Weather Advisories") are issued for amounts of snow and ice (freezing rain, freezing drizzle and/or sleet) that are less than Warning criteria.

There are also non-precipitation advisories issued in the Winter. "Wind Chill Advisories" are posted when wind chill indices drop below 0 degrees for at least three hours. "Freezing Fog Advisories" cover episodes when dense fog occurs with temperatures below freezing. This often leads to icy deposits on roadways (mainly bridges and overpasses).

Terminating Headlines -Normally, Warnings and Advisories are terminated when the weather event on which the Warning or Advisory was based ends or tapers off so that the criteria are no longer met.

In the case of precipitation, this means the Warnings or Advisories are terminated when the precipitation comes to an end. This procedure is designed so that the forecaster deals with forecasting the weather event, not road conditions. Of course, hazardous driving conditions often exist for a number of hours, or even days, after the precipitation ends...but the extent of the hazard depends on a number of non-weather related conditions, such as pavement type, amount of traffic on the road, road maintenance procedures, sunny versus shady areas in the daytime, etc.

 


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