Snow Forecasts Are Difficult
There was a blizzard in the northeast this week. Or maybe this alleged storm was just light dust. It depends on who you ask. Growing up in the Colorado mountains, I am well aware of how dire these predictions are. This is because snow is especially difficult to predict.
As you know, the weather is difficult to predict, and even the terminology can be incomprehensible to understand. In the event of snowfall, the task of forecasters is not only to predict when and for how long it will snow, but also how much snow will accumulate on the ground. All these factors make it difficult to calculate the amount of snow.
So let’s start from the very beginning. The BBC points to one of the main reasons for whether it will snow or rain is difficult to predict in certain regions:
Snow is more likely in high altitude areas as temperatures are lower, which means rain can fall as snow in mountainous areas, causing snow dust on peaks, which is usually seen at this time of year.
Similarly, the likelihood of snowfall in urban areas is lower due to warmer surroundings. It rains occasionally in central London, but it is snowing heavily for a relatively short distance off the M25.
“One tenth of a degree is enough to be either rain or snow …”
The difference between what becomes rain and snow can be small, and atmospheric humidity and surface temperature all play a role in what happens.
Overall, we’re pretty good at predicting whether precipitation will fall, but in which they form, and where it is often a bit more complicated. The vertical structure of temperatures in the atmosphere is such that precipitation may eventually hit the ground in the form of snow, sleet, freezing rain, or just plain old rain. There are many differences here and it skews how much snow falls, where it falls and how much accumulates.
This is all the more confusing when you consider the regional climatic zones that could damage forecasts. The Washington Post uses Washington, DC as an example:
Making forecasting even more difficult, Washington sits between two regional climatic zones: the Atlantic Ocean and the Gulf Stream in the east and the Blue Ridge and Appalachian Mountains in the west. This creates a special transition zone with warm air on one side and cold air on the other.
The difference in elevation between low-lying areas east of Washington and high elevations to the west also plays a decisive role in snowfall forecasts. Higher elevations are more likely to snow, simply because it is colder. Sometimes even a small deviation in temperature can predict or disprove.
Many major US cities are surrounded by strange regional climates that make weather forecasting difficult, such as Denver’s Rocky Mountains or Seattle’s Puget Sound.
In the case of the last storm in the northeast, all the “inaccuracies” boiled down to where the storm landed. In an interview with the Wall Street Journal, meteorologist Patrick Burke notes that snow accumulation is already a challenge because temperatures must be accurate and any change can cause dramatic shifts in accumulation totals.
The boundary of a storm can change throughout the day, so a 6 a.m. forecast is not the same as an 11 p.m. forecast. In the case of the last northeast storm, the storm moved slightly north and west, causing it to hit harder 50 miles north of New York. That is, during the storm, a lot of snow still fell, the meteorologists just missed a little.
As a rule, high accuracy cannot be expected on snow. A storm can miss a large city, but dump cargo in nearby mountains, or the warmer air from a nearby climate zone can quickly turn that snow into rainy slush. Many factors can change what happens, but it’s always best to prepare for the worst .
I would also like to mention one personal theory about why we talk so much about snow predictions: you can go out and measure snow accumulation with a ruler. This means that all of a sudden, everyone becomes a snow expert. Most of us do not collect rainfall data or measure exactly how much sun is visible on a given day, so we have nothing to do with how accurate this data can be.