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Why Do Forecasts Change So Much?

We’ve all seen it, the long-range forecast plastered all over Facebook that says a world-ending blizzard will hit some time next week!

Even though this current winter storm is still a whopper, it is a prime example of exactly what I mean.

Long Range Forecasts

It all starts with a simple post. Someone looks at the 300 hour GFS (forecast for almost 2 weeks out) and posts all over social media that their area is going to get hit with feet of snow.
A local radio station in the Twin Cities was talking about this very storm dumping 2 feet of snow on Minneapolis– bringing about the worry of our next ‘snowpocalypse’ or ‘snowmageddon’.

But here’s the problem. Long range forecasts don’t often pan out. (Verify to us meteorologists)

Just look at day 7 on this Minneapolis forecast and see if this actually happens…

Plain and simple, weather is complicated. Even with modern technology at our fingertips, there are so many factors that go into a forecast– we just can’t accurately put them together weeks out.

While calm conditions and a steady pattern can be easier to forecast, it’s a different story when talking about storms.
Upper-atmospheric winds play a major role, but small local geography or temperature differences can change the path or amount of precipitation from one town to the next.

Medium Range Forecasts

Just look at the map below, a few days ago the National Weather Service issues winter storm watches in advance of our massive winter storm.


Now we are getting closer.
Typically ranging 3 to 7 days, things start to get a little more accurate.

In my lifetime the accuracy of a 5-day forecast has improved drastically. In fact, a 5-day forecast today has about the same accuracy as a 2-day forecast from the 1990’s.
When you stop and think about it, that’s actually fascinating!

Realistically, even with modern computing and more than 11,000 weather stations around the world, being off by a few degrees on a 7 day forecast is a decent forecast.
And when it comes to measuring snowfall, things tend to get a bit trickier…
If we can tell the moisture equivalent, how much rain would fall if it were warmer, that would be a great start. Even then, things get a bit tricky when it comes to forecasting snowfall.

A few days ago snowfall totals were much more impressive for some– you might notice a change between the snow forecast then (below), and our current one… (wait for it)

We nerds talk about snow in ratios. You may even hear us say something like, “This is a heavier snow, about an 8 to 1 ratio.”
What this means is for every 8 inches of snow, it would melt to the equivalent of 1 inch of rain.
And with snow ratios ranging from 4 to 1 all the way up to almost 30 to 1, even knowing the water equivalent exact totals remain tricky.

Short Term Forecasts

This is the basket you want to put your eggs in.

Typically around 48 hours or less.
Models pick up the small changes, and we can take the time to compare what we know to the different models. Picking and choosing which might be the most accurate or even adding in a bit of our own knowledge.
For example, Meteorologist Joel Barnes keeps record of which storm tracks bring drastically different snowfall totals to the Front Range of Colorado.
Going over his notes, he can often give an extremely accurate forecast in an area that is known for its difficulty in hitting exact snowfall totals.

Just as an example, only a day later and look how much the above snowfall forecast has changed for the same storm!
If you look close, we’ve even changed the end time for the snow and it still doesn’t look nearly as impressive!

Within 48 hours, predicting a storm’s path is much more accurate. And with atmospheric conditions known, it also becomes much easier to nail down snow ratios.
Now if we could only predict when your backyard seems to get more snow than the neighbors’!

But as you may have noticed with this latest storm, forecasts are changing right down to the minute– which just goes to prove the old adage true…
The Most Accurate Forecast is When the Weather is Already Happening

For WeatherNation — Meteorologist Jeremy LaGoo

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