Why Do Snowfall Forecasts Change as a Potential Storm Gets Closer?

Linda Lam
Published: December 2, 2019

Forecasts change as details become clearer when any weather event approaches, and this is certainly the case with snowstorms.

Atmospheric changes, timing and temperatures can make big differences in snowfall amounts. That's why it's important to check the forecast daily if a storm looms.

Here are a few factors that play important roles in forecasting snow.

(PODCAST: What You Should Know About Your Local Snow Forecast)

Changes in Forecast Models Due to Additional Data

Forecast details become clearer and more certain as a storm gets closer due to additional and more accurate information.

For systems that track from the Pacific Ocean across the Lower 48, the data available before a storm reaches the West Coast is limited. But once it moves into the West, additional data about the system in ingested into models improve the forecast.

Several days out, the information a computer model has may lead to one possible outcome. However, there are many variables that included in the model, and some of these change over time.

These small changes can make a big difference, as shown in this graphic from the National Weather Service.

This can also be thought of as the butterfly effect, where a small change in one area can result in a big change somewhere else.

For instance, it can be tough to determine how much sleet and freezing rain, if any, may impact snowfall totals.

Details can be sparse for for a specific area. Forecast models cannot determine far in advance where heavier bands of snow may develop. These details typically don't become clearer until a day or so ahead of time when short-range models are available. Heavier bands of snow can bring 2 or more inches of snow per hour and can result in pockets of greater snowfall totals.

There are times when the models are in fairly good agreement and are consistent over time. When this happens, there is a greater confidence in the forecast, sometimes a few days ahead of an event.

Variations in Track and Strength of System

So what details in the computer models lead to changes in the snowfall forecasts?

One important factor is the track of the low pressure system. Small changes of less than 50 miles in the track can have big implications in terms of how much snow will fall in a given location

The track of a storm is influenced by the jet stream, or upper-level winds, so changes not just at the surface but aloft can make a difference. Changes in the upper-level pattern even at a great distance from where impacts from a particular event are expected can play a role in how a storm evolves.

The graphics below show an example of how a relatively small change in the track of an area of low pressure can make a big difference for Northeast. The rain/snow line can be tricky to forecast in the Interstate 95 corridor. A track close to the coast can bring in warmer air and result in mainly rain. A track farther offshore can bring hefty snowfall totals. A track away from the coast can mean just light snow near the coast or no precipitation at all.

Examples of two possible scenarios where small shifts in track and amount of cold air available can result in different impacts.

The strength of a system, as well as how much moisture is available, help determine how much precipitation will fall. How intense a system is also is a factor in the path it may take.

Small changes in any of these variables can result in a very different snowfall forecast from the week before.

Amount of Cold Air Available

Another important factor in forecasting snow is how much cold air will be in place.

Air temperatures often vary as the system can bring both warmer air ahead of it and colder air behind it, which can lead to a variety of precipitation. Warmer water temperature near the coast can also sometimes play a role in whether snow or rain falls.

The temperature at different levels in the atmosphere determine the precipitation type that reaches the ground, and this can be difficult to determine several days before a winter storm. Additional data closer to the event allows the forecast to be fine-tuned.

(The National Weather Service/NOAA)

Snow forms when all layers of the atmosphere are below freezing or when above freezing temperatures are in a relatively shallow layer at the surface.

(MORE: Winter Weather 101

Sleet forms when snow aloft passes through a warm layer that melts it into rain but then refreezes into ice pellets as it falls into a deep and/or very cold layer of below-freezing air near or just above the earth's surface.

The setup for freezing rain is similar to sleet except the warm layer is closer to the ground. As a result, raindrops do not have time to refreeze into ice pellets and freezes when it makes contact with objects and the ground.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.