The Troubles Of Forecasting Snow
Whether It's A Flake Or A Blizzard, It's Hard
When the winter season begins, a new challenge begins for meteorologists – forecasting snow. Enough of it can call off school, cause cities and towns to issue snow alerts and insulate crops during the winter months but whether it’s a tenth of an inch or a couple of feet, forecasting it is difficult.
First and foremost, meteorology is not an exact science. Your local meteorologists, like your KDLT meteorologists, are trying to predict the future using data from weather models. One of those models update hourly, the High Resolution Rapid Refresh, one every three hours, the Rapid Refresh while others update every six hours, both the North American Model and the Global Forecast System. Not only do the hours vary, so do their biases; some of the models tend to exaggerate precipitation, have systems move quicker, tend to be less on snow fall, etc. and your local meteorologist has to keep those biases in mind while using their own knowledge of the area each time they build a forecast.
NAM depicting possible snow Tuesday morning
GFS depicting possible snow Tuesday morning
Which is why forecasting snow and snow totals accurately is very hard. But we’ll explain why it’s hard so you, at home, the next time snow is in the forecast, can understand what your local meteorologists go through!
Monday 12z GFS showing low-pressure across north Thursday AM
The first thing that meteorologists look at, during both the winter and severe weather season, is the path that the incoming low will take. This begins days before the actual event, snow or severe. The possible path will start large and ambiguous, with multiple paths possible the low could take. It is also during this time that there are other uncertainties; along with the track, temperature trends and whether or not there will be precipitation. It’s at this point that your local meteorologist will say that we are looking at the possibility at an incoming system that could bring us some precipitation to the area. Your local meteorologist knows that from the time we first see the incoming system to the day of the event, the path, temperature trends and precipitation trends will change.
Sunday 00z GFS showing same low Thursday AM
Sunday 18z GFS showing same low Thursday AM
Changes will continue to happen between each run but slowly, and surely, it’ll become clearer. The track of the system will become clearer from possibly tracking through multiple states to tracking through, say, southern parts of the Sioux Empire. Who will be under the warm sector, or who will feel warmer temperatures, and who will be behind the cold front and when becomes clearer. Also, precipitation totals like snow and rain become clearer but, as the Sioux Empire saw with the last system, there’s still the knowledge in the back of your local meteorologists mind that this could change at the last minute because, as stated above, meteorology isn’t an exact science.
While factors are becoming clearer, forecasting snow and total accumulation still remains one of the hard even during the event. But why? In short, there are a lot of factors that are needed to not only needed to produce snow but also maintain snow.
Forecasted temperatures for Monday, February 27th
Temperature is one of the most important factors. Days leading up to the event and during the event, meteorologists are constantly looking at the temperatures at the surface and the layers above it. The surface of the ground has to be cold enough for falling snow to accumulate otherwise, the snow will continue to melt upon contact until the surface is cold enough. But that’s if the column of air above the surface is cold enough to produce snow. The air above the surface has to be cold enough to allow snow to produce and stay cold enough, if layers below where the snow is forming are too warm… the snow will turn to rain and will fall to the surface as rain until the column of air is cold enough.
Forecasted 850mb Temperatures Wednesday AM
The track of the system will also determine where the snow will fall and who will see the most snow. If you are along the freezing line, where the temperature above the surface is hinder on that 32 degree mark, you may receive heavy snow if the column can cool quickly or you may receive a rain/snow mix until the column of air becomes cool enough to maintain snow formation. Another place you want to be is on the northwest side of a low pressure system, that’s where wrap around moisture from the warm sector will meet the cooler air and become snow however, it typically isn’t as intense as snow fall rates along the fronts.
Should the track change, like it did with this last system, those who are located along the freezing line, warm front, cold front or on the northwest side could all be changed. With the latest system, the track of the low went south meaning less of the forecast area was on the northwest side of the low and the fronts were not as far north.
Radar loop of heavy snow band over Valentine, Thursday, February 23rd
Lastly, bands of heavier snow will cause local areas to possibly see more snow that initially forecasted. We have seen an event like this across the Sioux Empire. If you recall November 20th, 2015, northern parts of Sioux Falls recorded around a half a foot of snow while on the other side of town, more than double that amount of snow fell. These heavy bands are very hard to predict, nearly impossible, before the day of the event because of the size of snow. Smaller weather phenomenons like snow and rain are harder to predict by models as opposed to large scale systems like low/high pressure systems.
We hope that this clarifies a little bit about why forecasting snow is hard. From temperatures to the track of the low pressure system and where the heavier bands are going to develop, your local meteorologist has to look at a lot and decipher a lot of data when it comes to making a snow accumulation graphic for the next big snow storm.
Forecasted snow totals from February 23rd/24th low pressure system