29 November 2015

Some Thoughts About The Coming Winter: Part One

Posted by Dan Satterfield

My photo from last February in Salisbury, Md.

This time of year it’s the number one question that every meteorologist hears: “What’s the winter going to like??”

The correct answer is, “We can’t predict the weather 3 months in advance with any real accuracy.”. That said, we can make some decent guesses about the climate patterns that we may see, in some areas more than others.

First, let me show you why we really cannot use numerical weather models much beyond 7-10 days. The image below shows the output of several models, run with very slight changes of their starting points. We don’t know the pressure and exact temperature everywhere, so we give the models our best guess. Change that guess very slightly, and run the model again, and you will likely get much the same general answer over the 3-5 days. Look how closely the different runs agree:

This is a 96 hour forecast of pressures in the upper atmosphere, using slightly varied starting conditions. Notice they are in good agreement. (For weather geeks this is the 500mb height forecast.)

Now let’s check on these same models at 252 hours:

Now you know why we call this chart the spaghetti plot! (Really, we do!)

If all this does is convince you that 15 day forecasts on certain web sites are worthless, then I’ve done a good thing here!

Attacking the Problem from First Principles

We learned to make forecasts several days in advance by trying and understanding the physics of how the atmosphere works. So attempting to make a winter forecast in October is a good thing. Now, telling the public about it may not be a great idea, but we should try, because we learn by failing. Experimentation is indeed the heart of all science. Starting from what we know about the physics, we can use every clue that has value and build on it. Progress is being made, and we know that the most important piece of information is where the heat in the atmosphere is coming from. Yes, you can trace it back to the Sun, but the answer you should have yelled out is THE OCEANS!

Read this from NASA, and especially the part I put in bold red letters!

The ocean is a significant influence on Earth’s weather and climate. The ocean covers 70% of the global surface. This great reservoir continuously exchanges heat, moisture, and carbon with the atmosphere, driving our weather patterns and influencing the slow, subtle changes in our climate. The oceans influence climate by absorbing solar radiation and releasing heat needed to drive the atmospheric circulation, by releasing aerosols that influence cloud cover, by emitting most of the water that falls on land as rain, by absorbing carbon dioxide from the atmosphere and storing it for years to millions of years. The oceans absorb much of the solar energy that reaches earth, and thanks to the high heat capacity of water, the oceans can slowly release heat over many months or years. The oceans store more heat in the uppermost 3 meters (10 feet) than the entire atmosphere, the key to understanding global climate change is inextricably linked to the ocean.

The key to making a long-range winter forecast is understanding how the heat is distributed in the oceans, and that’s why an El Nino or La Nina event allow us to have extra confidence in a long-range forecast (for some areas). Since the three most important factors in the weather are ocean,ocean, and ocean, it’s not surprising that during a strongepisode, we can expect to see different weather patterns through the winter (ENSO episodes tend to peak during winter).

GUESS WHAT?

We have one of the strongest El Nino events on record underway!

So, what does this mean? Let’s look at the weather in strong El Nino years, but keep in mind that each El Nino is different, and we only have a very small sample size, so expect surprises!

This is winter precip. in strong El Nino winters. Notice much of the country sees little change from the winter averages.

Below is winter precip. in the two strongest El Nino winters, and notice how much stronger the signal is:

Beware! We only have a sample size of two here!

How about temperature during very strong El Nino winters:

and for strong El Nino winters:

Here is what NOAA’s winter outlook looks like, and you can see that it is based heavily on these charts above, as it should be.

NOAA Temp. outlook for this winter.

and the NOAA precipitation forecast:

From NOAA Climate Prediction Center

Snowfall is quite variable in the Mid-Atlantic during El Nino winters. The chart above is for cities in Va. and Md. There are other factors that have far greater influence (see part two for more about them.)

So there you have it right? We just look at what happened during the two strong El Nino events and that’s the winter forecast. Well, not so fast. Remember we have a sample size of less than 4 and we also know something very important: Every El Nino is different, and it actually has little impact on the weather during winter in much of the country, with a large impact in only a few regions. We must also remember the law of small numbers which states that small sample sizes are very poor predictors.This El Nino will certainly be different and there are actually other atmospheric patterns that have a far bigger impact on the weather. I’ll talk more about them in part two.

In Part Two THE ELEPHANT IN THE ROOM