The Challenges of Forecasting Snow and Other Winter Precipitation in the South

Section 1: Is it Going to be a Big Deal?

One of the more difficult forecasts to make is the winter precipitation forecast. The complexity is with determining precipitation type, areal coverage, and whether any winter precipitation will accumulate at the ground surface in the first place. The public is very sensitive to winter forecasts primarily due to the travel headaches they cause. The winter precipitation forecast is particularly challenging in the “transition zone” region. The rain/sleet/mix/snow line has the ultimate impact in determining which locations will receive winter precipitation and how much. A winter weather situation requires much more time looking at the analysis and model charts than on a typical day. The following questions need to be addressed in a winter precipitation event: What will be the precipitation type(s)? What will be the accumulation? How intense and at what duration will the precipitation be? Is a forecaster overdoing or underdoing the event? Will the temperatures be cold enough to support an accumulation? What evidence is there that winter precipitation will occur at all? There are three categories that will be discussed for a winter precipitation event. They are:

(1) no event
(2) the “no big deal” event
(3) the “big” event

It is up to the forecaster  to determine which category the event will entail. Calling for a no event when “decent” winter precipitation occurs or calling for a big event and having nothing happen are clearly busted forecasts. Whether an event is big or not is regionally dependent. Three inches of snow in South Bend, Indiana is a “no big deal” event and is only casually noticed while 3 inches of snow in Atlanta, GA will be a “huge event” making front cover news and people frantically buying groceries and preparing late into the night the day before the big snowfall. It is up to the forecaster to determine what accumulation entails a big event.

Generally, freezing rain or drizzle on a below-freezing road surface is a “big event” even in small amount because it can cause traffic accidents with only a minor accumulation. Sleet is a big deal if it accumulates sufficiently on the road. Snow is extremely regionally dependent. The Northern U.S. generally needs 6 or more inches to be a big deal while anything over an inch in the South is a cause for concern. After the forecaster has determined the expected winter precipitation and whether it will be a big deal or not, it is up to the forecaster to inform the public of how much preparation they need to accomplish for the event.

The ultimate embarrassment is to forecast a non-event and a big event occurs or vice-versa. It is common for some forecasters to blindly go along with someone else’s forecast. This is not a wise habit and will not make one a better forecaster. It is critical that the forecaster successfully predict the winter precipitation situation as a no event, a “no big deal” event, or a “big” event.

Section 2: Why is Winter Weather Forecasting in the South a Big Challenge?

Winter weather can be very difficult to forecast in the Southern United States. Examples include Texas, Arkansas, Mississippi, Alabama, Georgia and several others. Of course, this is a generalization since winter weather as a whole can be difficult to forecast anywhere. However, there are some reasons why it is particularly difficult to forecast winter weather in the South. This section will go over these reasons.

One reason is due to the forecast having to cover a range of different precipitation types. In the North, the precipitation type is often snow in the winter while in the South the precipitation type is often rain. This is another generalization. However, in the South there are often difficult situations in which the precipitation type could either be a cold rain, snow, sleet, freezing rain, or a combination of 2 or more of these. It is difficult enough to forecast if precipitation will occur but predicting the precipitation type adds another level of complexity to the forecast.

Another reason is because winter precipitation is less common in the South. Forecasters in the North often have more experience with forecasting snow and other winter weather types. Going further South, these experiences are less. This is another generalization since a forecaster in the South can be great with winter weather forecasts. On the whole though, having fewer opportunities to forecast winter precipitation types for the local area can make it more difficult to forecast winter weather when it does happen.

A third reason is due to public attention. Winter weather forecasts for snow and ice often get more attention in the South than in the North. Big winter storms can grab headlines in the North also but they tend to be particularly newsworthy in the South, even when relatively small accumulations are expected. Reasons for this include winter precipitation being less common, less road preparations for winter weather and less overall preparation people have in general. Thus, in the South, the forecast will be monitored very closely. With such a strong scrutiny, any forecast mistake on precipitation occurrence, precipitation type and precipitation amount will be strongly noticed.

A fourth reason is due to hype. Having a winter weather forecast that is “overdone” will often receive considerable attention. Just the mentioning of a winter weather precipitation chance (even if it is really just 10%) can bias the public into thinking a winter weather event will occur. In this age of social media abundance from many sources, forecasts can be found that overdo the winter weather potential. It is important to know the source that a forecast originates from.

Finally, these forecasts are very sensitive to slight changes in the weather analysis. A small change in temperature can mean the difference between snow, rain and another precipitation type. A slight change in the low pressure track will change the areas that are expected to receive a certain precipitation types. A slight change in lifting can mean the difference between precipitation that produces icing and no precipitation. This sensitivity from many weather variables makes it difficult to pin point a winter weather precipitation forecast in the South.

The Snow Event

Section 3: When the Winter Weather Threat “Busts”

A bust is a situation in which an anticipated weather event does not occur. This is especially true if there was a high probability of the weather event occurring. Winter weather and severe weather busts are two categories that are anticipated weather events. By anticipated it means that a large percentage of the general public is aware of the threat. Typically when a watch is issued then more people will be aware of the situation. When a warning is issued, then even a higher percentage of the general public will be aware of the situation. If the weather changes people’s behaviors then this is another way the weather event is anticipated. Stocking up on groceries before a winter storm is one way of anticipating a winter weather event. This section looks at some reasons why a winter weather event may bust:

1. Ground is too warm: This situation can occur when the temperature is very close to freezing and the winter weather event requires a ground surface that is at or below freezing. For example, a freezing rain event can be in the forecast but it fails to occur  if the ground is just above freezing, resulting in a cold rain event instead.

The More You Know: the “ground is too warm” argument only goes so far before becoming mere hand-waving.  When surface temperatures fall into the 20s, and especially if precipitation is heavy, the notion that a recent warm spell will preclude a significant winter storm enters “myth” territory.  Be sure to read Chris Robbins’ piece on the topic of significant winter storms that followed extended warm spells in the South.

2. Dry air below cloud base:  Snow that falls from the clouds and through a very dry layer of air will sublimate before reaching the ground.  This is called virga. Forecasters usually know ahead of time if there will be very dry air near the surface and they must adjust the forecast accordingly.  Ice microphysics tells us that sublimating snow or evaporating rain causes the vapor content to increase (phase change from solid/liquid to vapor).  If the precipitation is heavy enough and lasts for several hours, as we often see with winter storms, the dry layer will eventually become saturated from the top down and the precipitation will eventually reach the ground (especially in higher elevations — hence the primary reason that the higher terrain elevations get larger snow accumulations).  

The failure of one forecast element effects all dependent elements; if the forecaster miscalculates the duration and/or intensity of precipitation, and the sublimating snow struggles to erode the near-surface dry layer, much less snow will be observed at the surface.  Many weather enthusiasts and amateur forecasters begin to spread unwarranted snow hype during the winter when they see winter precipitation generated in the model-simulated reflectivity products (these are simulations of what the radar should look like at a given place and time), but they fail to compare these data to the model-generated quantitative precipitation estimate (qpf) to confirm that precipitation is actually reaching the ground in the model simulation.  

 The More You Know: When there is virga, snow sublimates (i.e., phase change from solid directly to vapor without melting); rain falling through a dry layer evaporates (phase change from liquid to vapor).

 3. Track of low pressure is different from forecast: The track of the low pressure will determine where winter weather occurs in many winter weather situations. If the track is a little too far to the North in the U.S. for example, then it can end up that temperatures are too warm and the winter precipitation ends up being more to the North than the original forecast called for. The best winter weather tends to occur under the low and near the low on the cold side of the low pressure system.

4. Potential event is too far into the future: Forecasts are more likely to be incorrect if the forecast is made too far in advance. There should be caution with forecasts that are made 3 days out or more. The uncertainty is much higher in these longer range forecasts. Forecasts for 1 to 2 days before the event are more likely to occur if based on sound data and judgment. Confidence that a winter weather event will occur is going to the best the day and hours leading up to the event. Forecasts beyond 2 days are prone to significant and multiple revisions.

5. Weaker-than-expected accumulation: This is especially true for snow forecasts. In some situations, the winter weather will still occur but accumulations may fail to reach the predicted amounts. An example of this is a forecast of 6 to 8 inches, while only 2 or 3 inches actually accumulates.  Another example is a forecast of 6 inches of snow, but much of the precipitation falling as rain before finally changing over to snow that accumulates to only an inch. These busts can occur when a slight warming in the temperature profile can result in lower accumulations of snow.

Section 4: Influences on Snow and Ice Accumulation

This section examines several factors that can influence how much accumulation of winter precipitation occurs.

A. Grass vs. Concrete

Snow accumulates more efficiently on grass and elevated surfaces than pavement and concrete for several reasons.

(1) A road surface is connected directly to the Earth’s surface. Vegetation and grass is more exposed to the cold air. It takes time for the soil temperatures do adjust to colder air temperatures. Warmer soils will continuously conduct heat upward. A road will continue to be warmed by the warmer soil below. Bridges and overpasses will freeze before roads because there is cold air on both the top and bottom portion of a bridge or overpass.

(2) Roads are a good absorber of radiation and take longer to cool off than vegetation.

(3) The shadow effect in grass and vegetation allows for less melting of snow. A road surface is completely exposed to the sun’s radiation. Even on a cloudy day, radiation will still make it to the Earth’s surface. Places shaded from solar energy (within grass blades, behind bushes) keep radiation from sublimating the snow.

(4) Vehicles warm roads. A high volume of traffic on a roadway will effectively melt snow. This melting occurs from two sources, friction and vehicle heat / exhaust. The friction between car tires and the road/snow can warm the temperature up enough to melt the snow / ice.

(5) The smooth surface of a road makes it difficult for snow to accumulate on a road surface, especially when wind speeds are high. The grass blades and bushes slow the wind and provide traps for the snow to settle.

(6) Snow that falls into water will melt more quickly than snow falling on a dry surface. Vegetated regions are much better at absorbing excess water than roadways. When a ponding of water occurs on the road, snowflakes will melt quickly as they fall into the water.

(7) When roads are salted or other melting agents are applied to roads, the accumulation will be reduced or eliminated. Salt from one winter storm can linger on the roads and help melt winter precipitation that occurs in a future storm.

B. Influence of Previous Weather

The previous weather before a winter storm is an important consideration for accumulation potential. Soil and ground temperatures take much longer to adjust to air temperatures than elevated structures. Soil temperatures will be fairly warm if the winter storm occurs early in the winter season or the weather has been warm for several days previous to the colder weather.

The duration of below freezing air temperatures before a winter storm occurs has an influence on accumulation. Heat moves from warmer toward colder objects. When air temperatures are below freezing and soil temperatures are above freezing, heat will conduct from the ground toward the air. This heat has the capability of melting a significant amount of winter precipitation. If a winter storm occurs just after a warm period, there will be a significant buildup of ice or snow on vegetated surfaces but concrete and urban structures (such as ground-connected roads and parking lots) can melt a significant proportion of the ice or snow that falls on them.

If several days of below freezing temperatures precede the winter storm, very little of the precipitation will melt once reaching the surface. This is especially important in a freezing rain situation. Frozen soils along with below freezing surface temperatures will allow rain to freeze on both elevated and ground surfaces. Freezing rain is much more dangerous to transportation if the soil and ground temperatures allow for an accumulation of ice on all roads. 

On the other hand, as we discussed above, winter storms are often preceded by long-duration warm spells in the South, especially if the precipitation is very heavy and/or temperatures fall into the 20s. In the 20s, and when sleet or snow are falling on pavement, the temperature of underlying surfaces (including asphalt) can cool much faster than you might expect.

C. Snowfall During the Day vs. Night

The sun’s radiation is a powerful sublimation agent. You may have noticed that on a sunny day after a snowfall, snow remains in the shady places but quickly melts or sublimates in the regions exposed to direct sunlight. Overcast conditions do NOT prevent all the sun’s energy from reaching the surface. If none of the radiation reached the surface then it would look like night outside. Even in overcast conditions, the shady areas will have an advantage of retaining snow accumulation longer. Sublimation occurs when ice is converted directly to vapor without going through the liquid stage. Shortwave radiation from the sun is effective in forcing this sublimation process. This sublimation process is more powerful in the lower latitudes such as in the South since the sun angle is higher. When the temperatures climb above freezing, then melting and sublimation act together to remove the snow and ice accumulation. Even when the air temperature is below freezing, shortwave energy can warm objects adjacent to the snow above freezing and this will melt snow and ice. Objects include the metal of cars and dark roofs. After the ice melts, the water will freeze again when it moves to a shady region (forming icicles) when the ambient air temperature is below freezing.

A nocturnal snowfall has the potential of producing much more accumulation than a day snowfall, especially in the lower latitudes of the South. The advantages of a night snow include:

(1) more likely temperatures will be cooler and stay cooler without being increased gradually by daytime heating
(2) no shortwave energy to sublimate the snow or warm ambient air temperatures and surface objects
(3) given the same temperature profile of the troposphere, a night snow will have a lower liquid content since no shortwave radiation is present
(4) less traffic on the roads.

These four advantages apply most significantly to a snowfall that occurs when temperatures are near freezing (i.e. a 32 F snowfall at 2 am vs. a 32 F snowfall at 2 pm).

D. Influence of the Vertical Temperature Profile

The density of snow varies as a function of temperatures. The density of snow is heavily dependent upon the liquid content of snow. Two classifications for snow are “wet snow” and “dry snow”. Wet snow occurs at temperatures near freezing in the PBL and/or with soil surface temperatures above freezing. Wet snow is partially melted and is therefore denser. Wet snow typically has between a 10 to 1 and 5 to 1 liquid equivalence. This is the snow that makes great snowballs, is tough to shovel, and is not easily drifted by the wind after it reaches the surface. Flakes of wet snow more easily stick together in flight and are not broken apart as much by the wind.

Dry snow occurs when the warmest temperature in the PBL and aloft are less than 28 F and the soil surface temperature is below freezing. Dry snow is less dense and can therefore accumulate to a higher depth. Dry snow is easily drifted by the wind and is difficult to make snowballs with. The liquid equivalent of dry snow ranges from 15 to 1 to greater than 30 to 1. Dry snow is easier to shovel due to its lower density. The flakes from dry snow tend to be smaller but more numerous.

Warm soil will tend to compact snow over time by giving it a higher liquid content. Gravity also compacts snow over time. Wet snow will compact more significantly than dry snow. A dry snow falling onto a warm surface will allow it to take on characteristics of wet snow. The perfect combination for snow lovers is a wet snow falling on a below freezing soil surface.

E. Intensity and Duration

Heavy snow and sleet will accumulate on any surface no matter how warm it is. This is somewhat of an exaggeration but heavy snow and sleet can make up for the fact that air temperatures in the lowest part of the PBL and the ground surface are above freezing. Snow or sleet with heavy intensity promotes the greatest amount of melting and evaporative cooling. Heavy snow or sleet accumulation will easily exceed the melting rate AS LONG AS IT CONTINUES TO FALL HEAVY. Once the precipitation ends and the surface and ground temperatures are above freezing, the accumulation will quickly melt.

Duration of snow or sleet is also a consideration. Hours of heavy snow or sleet will be much more significant than a burst of heavy snow or sleet that lasts less than an hour. The greatest amount of melting occurs when the heavy snow or sleet begins when temperatures near the surface and at ground level are above freezing. It is typical for heavy snow or sleet to drop the temperature to 32 F within 15 to 30 minutes of the precipitation beginning when the temperature is above freezing when heavy snow and sleet begins. This is due to melting and evaporation being cooling processes. Heavy precipitation maximizes how quickly the temperature drops to 32 F when it is initially above freezing. After this 30-minute time period, accumulation will be more significant if the heavy intensity sustains itself.

When the temperature at ground level is below freezing as well as air temperatures being below freezing, snow and sleet accumulation will be a direct function of the intensity and duration of the precipitation.

Section 5: Conclusion

Forecasting winter precipitation can be difficult to predict in any part of the country but it is especially complex to forecast in the South since winter weather occurs less often and preparation for it can be lacking. The public in the South is especially sensitive to a winter weather threat, which makes it even more important to have an accurate forecast. Being able to accurately forecast a winter weather event can make or break the credibility of a forecaster. Because of this, much more time is put into these forecasts and care has to be taken when disseminating the forecasted winter weather threat potential. Since these forecasts often bust, it is important to emphasize that short term forecasts are going to be much more trustworthy than long range forecasts. Even in the short term forecast, significant changes can occur just hours before the event. There are many variables that influence how much accumulation could occur. Slight changes in conditions and temperatures can mean the difference between no accumulation and significant accumulation. Winter weather forecasts are one of the forecasts that weather predictors live for! The attention these forecasts get is typically only surpassed by severe storm and tornado forecasts. The next writing on winter weather will discuss the various tools that forecasters use to predict winter precipitation, including their utilities and pitfalls.

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