Spring has been very reluctant to lock in across the state this year, with a top-3 coldest March of the last 3 years, and now a very up-and-down April that has seen highs in the 70s in Des Moines, as well as the 40s — on back-to-back days no less!

In the meantime, it seems every time we show the daily almanac are temperatures are nowhere near “normal.” Or are they?

We commonly refer to the “Average High Temperature” as “normal,” but really is that accurate? It depends on the definition, but a lot of statisticians will say no. An average is just that. A single number. A more accurate way to define normal requires you to specify a range of temperatures.

For example: In March, we saw 19 days that were 10 or more degrees off of the average high! 19! We had 9 days 15 degrees off of the average! But did you know, that really isn’t unusual?

Did you know that, in fact, one might be able to say that in March we could typically expected to have 13 or 14 such extreme days?

Check this out. Say a particular day has an average high temperature over 5 years of 10 degrees. Say the temperature each of the 5 days was 9, 9, 10, 11, and 11. All pretty similar. How widely the numbers vary, called the “standard deviation” would be small.

But let’s consider something else. Here is another day with the same average of 10 over five years. -15, 4, 10, 16, and 35. See how it varies quite a bit? Even though it averages 10, the odds of picking one of those numbers and being close to ten isn’t very good, especially compared to the first group. This example is like our March high temperatures.

We’re used to wild swings in March. And since we have cold, snowy months in March (like this year) and 80 degree streaks in March (like last year) it is a wildly varying set of temperatures making up that average.

So back to that gross statistic, “standard deviation”. In March, most days are hovering somewhere around 12.8 for a standard deviation. Lets use March 15 as an example.

On March 15, the average high temperature is 49 with a standard deviation of 13 degrees. All this means, is there is a pretty good chance (about 68%) the high temperature will be 13 degrees, high or low, of 49. That means anywhere from 36 to 62 is “normal,” statistically speaking of course.

The chart below shows the actual number of occurrences of a certain temperature on March 15 (dashed line). The solid dark lines are the cumulative distribution (odds the temp gets at least to that temp).

In fact, based on just random chance you could expect a roughly one-in-three chance that the daily high would fall OUTSIDE of that range! That’s TWICE-A-WEEK.

And those 19 days that were 10 or more off of the average. Well in March, it would seem there is roughly a 44% that any given day will be that extreme! Almost a 50/50 chance, which is not that out of the ordinary.

So keep that in mind the next time I slip up and call the average “normal.”

And in case you’re curious….the variance/standard deviation does change throughout the year. By July and August, it shrinks to 6 or 7 degrees depending on the day. So on those days of an average high at 86, statistically “normal” highs would fall in the 80-92 range.

A 10 degree extreme off the average on a random day in July? Somewhere around a 13% chance.

A 15 degree extreme day? We had nine of those in March. In July, it’s somewhere between a 2% and 3% chance! Makes all those 100+ degree days last year at 15 or 20 degrees higher than average seem all the more unlikely!

You can play around an check out some of the statewide averages and deviations for any day you want here, at the Iowa Environmental Mesonet. And even if this lecture just put you to sleep, give the link a click anyway and help me thank them for providing the data for your sleeping aid!

–Brett

This is the 3rd earliest 80 degree day in Des Moines. The last time we made to 80 or above was 81 on March 3, 1983. Before that was March 11, 1972. That time we hit 82.

Those are impressive numbers. But just in case you think we are completely out of the winter woods…. ISU Climatologist Elwynn Taylor said there was only one spring… 1946…escaped a hard freeze after March 15th.

More records are going to fall tomorrow. Even the overnight lows will be the warmest on record.

Our state is not unfamiliar with the dangers of flooding. During summer of 1993 over 200 thousand people in Des Moines were without power and water as the worst flood of the last century brought world-wide attention to Iowa.

Since then many floods have left portions of Iowa under water and without basic services for weeks and months at a time. Ceder Rapids and Iowa City were left stranded as the Cedar River raced out if it’s banks in June 2008.

Make sure you have a plan for yourself and your family. Plan to be aware of severe weather by watching Channel 13. 1 and Iowa’s Weather Plus on 13.2. Our Weather Plus is the only 24/7 broadcast channel that is dedicated to our weather. You will find updated forecasts all day…not just at the top of the hour. Weather crawls with severe weather information is always on the bottom of the screen on 13.2.

Have a meeting place for your family. You should have your home and a separate location to meet in case your home is not safe after severe weather.

Purchase a severe weather radio that is programmed to your location. The weather radio should alarm you when there is severe weather for your county. It should be loud enough to wake you during overnight severe weather. Remember the sirens in our neighborhoods are for those who are outside. They are not designed to be loud enough to be heard indoors.

You can be safe. Be prepared. We will help.

Always follow along on Twitter @EdWilsonWX13HD

Many believe that brooms and eggs will balance on end during solar storms or during the vernal and autumnal equinox. I hate to burst anyone’s bubble…but this is a myth.

A broom or egg can be made to balance on any day of the year. Face Book was full of people posting their pictures of the balancing acts today.

We always have this same story from viewers at the start of spring and fall. There are rumors this same effect can be found when the planets in our solar system align.

Here are a few sites that have the science behind the myth.

Astronomy Picture of the Day
Snopes: Infernal Egguinox
Wired: Balancing Brooms

Have a great night.

Ed

More sunshine is on the way tomorrow. It will be a nice end to the workweek. Highs will be in the middle to upper 50s. There will be a light north wind in the morning. By the afternoon you will feel the wind turn to the south.

Warmer highs on Saturday will come with mostly skies and that south breeze. I think the highs will be around 60.

Enjoy.

The warmest average temperature for any winter was 32.3 degrees during the winter months of 1881-1882.

Our average temperature this year was 32.0. We tied with 1991-1992.

Highs were in the upper 60s and lower 70s by 2PM. The gusty south wind was clocking in around 25 to 30 mph. Lots of folks out walking, jogging, biking and getting spring fever.

We have a tie for the record high. At 3:14 we hit 74…that ties the 74 that was the record high on this date in 2005.

Wednesday will be cloudy and a bit cooler. Highs will be in the lower 60s. The clouds will increase with a chance for showers by the afternoon.

Warm Winter

This winter will go down as one of the warmest in Des Moines. The lowest low temperature was +1. That is the warmest low for any winter in Des Moines.