Severe/Winter Weather Guides
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The National Weather Service authors Severe and Winter Weather guides annually. The guides serve to educate the public about hazardous weather and how to prepare for severe and winter storms.
 
Winter Guide (The Groundhog Story)
 
An annual feature weather story for news people occurs on February 2 when debate swirls over the groundhog's shadow, and whether it produces a forecast of additional winter.

 

We checked the statistics. We took a quick look at some weather data during the month of February to see what happened if a groundhog saw a shadow in Little Rock. It turned up a couple of interesting facts, but as a predictor of additional winter weather it doesn't work.

 

To calculate this study, we defined a winter event as having minimun temperatures below freezing, above average number of days during the month that had sleet, freezing rain or snow, and above average amounts of snowfall during the month.

Here's what we found:

From a dataset of the past 40+ years, there were an average of 12 days during the month of February with minimum temperatures at or below freezing. If it was cloudy at sunrise, the month produced 11.2 days below freezing. If it was clear at sunrise, the month produced 13 days below freezing. If he saw his shadow, there would be one more day of minimum temperatures below freezing. More winter? Not very significant.

We averaged 2.2 frozen events (sleet, snow) during February. A cloudy sunrise on Groundhog's Day produced 2.4 events during the month, while a clear sunrise produced 1.9 events. Fewer events happened if there was a shadow. Less winter? Again, not very significant.

 

How often is the groundhog right? Flip a coin.

Here is the interesting weather fact:

We average about one inch (1.04) of snow during February. If it was cloudy on February 2 at sunrise, the month averaged 0.6 inches of snow. If it was clear and you could see a shadow, the average snowfall for the month was 1.9 inches. Looks like we could be onto something! If there is a shadow, does that mean a heavier snow event during February? Well, 8 of the 16 times (50%) there was a shadow, no measurable snow fell in the month. Same thing with cloudy days: 18 of the 30 times it was cloudy, no snow was measured during the month. That means as a predictor, you would do just about as well flipping a coin.

 

One thing of meteorological significance: if the groundhog sees a shadow and it snows during the month, the data suggests there is a greater likelihood the snow will be heavier than if it had been a non-shadow Groundhog Day. Why?

Weather systems tend to be rather persistent. If it is cold now, there is a strong possibility that it will be cold for the next 30 days if we are in a relatively stable flow pattern. In winter, we usually get larger, slower moving weather systems than in spring and fall. A large high pressure system moving down across the lower plains would produce clear sunshine and is usually quite cold. When the warm air returns, many times large bands of heavy snow are produced as the cold air retreats. Since it is cloudy on February 2 two-thirds of the time, the data suggests more shallow cool air is usually in place and which will produce lighter, but frequent winter weather.

 

We are still looking for the right animal to predict the end of Winter. Perhaps studying the effects of a groundhog and his shadow is significant in Pennsylvania. In Arkansas, we're still looking for the right animal.

 


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