All signs pointing to another hot summer across the US

Those hoping for a cool summer are probably going to be disappointed.

While any one summer may be cooler or warmer than average, that average continues to get hotter as the climate warms. Over the past few decades, smaller features that could nudge a summer hotter or cooler than average are getting overwhelmed by the planetary warming signal .

Of the ten hottest summers on record nationally, eight have come during the past 20 years. The other two came in the 1930s. Last summer was not in the top ten, but it was in the top 20.

The average summer temperature over the entire 20th century was 71.4 degrees. So far in the 21st century, it is 72.8 degrees, and there has not been a summer cooler than the 20th century average since 2009.

Finding a cool summer requires going back to 1992 — the first full summer after the massive eruption of Mt. Pinatubo — the second largest volcanic eruption of the 20th century . Sending phenomenal amounts of ash, dust, and particulates into the stratosphere for more than a year, it managed to temporarily cool the planet by about one degree Fahrenheit during the following year.

Other regular fluctuations in the atmosphere and oceans impact weather patterns, among the most important is El Niño — the periodic warming of the central and eastern Pacific Ocean near the equator.

When that water is cooler than normal, it is classified as La Niña, and the Pacific has been in a transition between the two states this spring. A full La Niña is expected later this summer, and that may be the key to the summer forecast.

Looking back several decades for historical analogies, the transition has often led to hotter and drier summers in the eastern half of the country. But if that transition does not happen as quickly in the next month or two, a hot and dry summer may not come to pass.

Jan Dutton, a meteorologist with the World Climate Service , which consults businesses involved in agriculture, energy, and insurance, sees that transition as important, and is equally concerned about its timing.

"Analog analysis is useful because we look back to see forward. By examining what has happened in the past in similar climatic regimes, we are able to glean useful information about what's likely to happen in the future. El Niño/La Niña cycles are a critical input to many analog analyses, but other less well known climate cycles can be important too, which is the case in this summer forecast,” explains Dutton.

He also sees correlations with a summer that is especially drier than normal in the Ohio Valley — and in another location that is prone to drought.

“The same pattern suggesting dry conditions across the Ohio Valley are also responsible for dry conditions across the Colorado River Basin/Rio Grande Valley,” continued Dutton.

On top of that, the water in the Gulf of Mexico and central Atlantic Ocean each remain especially warm , also correlating with a hot summer east of the Mississippi River.

The combination of the warm water and transition to La Niña also suggest an especially active hurricane season. It is too early to indicate a hurricane will hit a particular spot, but those areas that are typically at risk — Florida and the Gulf Coast — should be especially aware of the heightened threat this season.

Farther west, there is relatively cool water along the Pacific Coast from Washington State to Baja California, suggesting a cooler than normal summer in southern California. And as a general rule, the weather is dry every summer in California. The Pacific Ocean along the coast is much cooler than the Atlantic at the same latitude, keeping the atmosphere more stable, and severely limiting the chance of summertime rain there.

And expect the Canadian wildfire smoke to be back. Whether it is as bad as last year is still a question, but over the last month, snow cover in Canada has been below normal and April was much warmer than normal. Both of those mean drier conditions going into the summer. Wildfires have already broken out in the Canadian prairies, sending smoke into Minnesota, Wisconsin and the Dakotas.

Adding it up, most indications suggest this will be the 15th consecutive summer that is hotter than normal nationally.

It is not a new normal. It is normal that will get hotter every year.