Fair Weather Event Aster: Unseasonably Warm Weather In The Western US


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warm forecast

The high temperature forecast for the next five days shows the warmth centered in Kansas. But the cold in the northeast is headed that way. Forecast courtesy of NOAA.

It’s an early spring from the western plains to the Sierras. Temperatures this weekend will run 10 to 30 degrees above normal for early February. Could this be a long-lasting warm spell, or is it just a pre-spring tease by Mother Nature?

It Isn’t Snowing Or Freezing Everywhere

Weather never makes the news unless it’s brutally cold, hot, snowy, rainy, or smoggy. If you only read the headlines, you’d think it’s never sunny and warm — or even pleasantly overcast.

But the fact is that the weather is more often nice than not. Though wind chills are now below zero in New England, this weekend will feature fabulous weather for any activity from Salt Lake City to Salina and from the Grand Tetons to the Grand Canyon.

Without sounding like a travel brochure, Decoded Science will try to point you towards the finest weekend weather every Friday. We’re naming this fair weather event Aster, after the flower.

Modern transportation (air travel) makes it easy to go from putrid to pleasant weather in a few hours. So if you live in Boston, catch a flight to Denver, Las Vegas, Cheyenne, or Salt lake City.

Why So Nice In The American West?

Last winter, the polar vortex drove into most of the continental US, while generally good weather covered the west coast. This year, the polar vortex is farther east, so that Nebraska, Colorado, and Wyoming share in the warm-weather bounty. As the ridge has built north, even parts of the west that normally don’t see decent weather until April have been pleasant.

The Jet Stream Is Contorted By Events In The Tropics And The Far North

Let’s start with the north pole. The temperature record clearly shows a dramatic warming (about twice that of the average over the globe), especially pronounced in the winter the past few years. As a result of the warming, polar pressures have risen and the jet stream has been pushed south and caused to wrinkle. The wrinkles bring extreme weather, both warm and cold.

jet stream this weekend

This jet stream analysis for Friday morning shows the ridge that’s causing very warm weather centered over the Rockies and western plains. Analysis courtesy of NOAA.

Now let’s look at the tropics, where El Niño Eggplant is inducing a low-latitude jet stream. For the most part, this jet has stayed over the Pacific Ocean, but in December it impacted Southern California with heavy rain.

And there’s a third player influencing US weather: the warm water (relatively) over the Gulf of Alaska. This warm water pool created high pressure that drove the polar vortex into the center of North America last year.

The jet stream over the Pacific has shoved the Alaskan ridge eastward, and the whole system has shifted to the east — to the disgruntlement of Bostonians but the approval of westerners.

The Adiabatic Process

Adiabatic means without the addition of external heat. In the atmosphere, the sun has an effect in a 24 hour cycle, and proximity to land or ocean surfaces can add heat. But meteorologists like to see what happens to air that is not affected by the addition of heat, and it turns out that a simple process that involves no adding of heat from the outside can create a heat wave.

When a flow of air encounters a mountain, the air is forced to rise. The air has a certain moisture content, measured by the dew point, which is the temperature at which the water vapor would condense.

The amount of water vapor that air can hold decreases as the temperature decreases. As you ascend in the atmosphere, the pressure decreases, and so by the ideal gas law, the temperature decreases.

Now consider a mythical ‘parcel’ of air going over a mountain. As it ascends, it cools. Once it reaches the dew point, water vapor condenses. The condensation process releases heat, known as latent heat — meaning it was there all time, but the energy was stored in the molecules.

When air climbs a mountain, it cools at first at the dry-adiabatic rate — the moisture content has no effect. After it reaches the dew point and continues to rise, the air is cooled at the wet-adiabatic rate — the dry-adiabatic temperature decrease is now somewhat compensated for by the heat from the condensation of water vapor.

The dry-adiabatic rate in the atmosphere is about 5.5 degrees per 1,000 feet and the wet-adiabatic rate is around 3 degrees per 1,000 feet.

When the air descends the mountain, it warms at the dry=adiabatic rate all the way to the bottom.

Let’s use some numbers: If forty-degree, saturated air is going over a twelve-thousand-foot-high mountain, the air will cool at the wet-adiabatic rate on the way up, and warm at the dry-adiabatic rate on the way down. So when it reaches the bottom, the air will be warmer by two and a half degrees for each thousand feet of the mountain. For a twelve thousand foot mountain, the air will have warmed by thirty degrees, and the cold 40 degrees will have become a balmy 70 degrees.

Who’s In The Hot Seat With Aster?

Sometimes the cold, dense air near the ground is hard to dislodge, and air crossing the Rockies overrides the colder air over the plains but doesn’t penetrate to the ground. This time, it will be warm at the surface and the warmest temperatures will be on the east slopes of the Rockies and in the western and central plains.

Record temperatures could be set on Saturday in Casper, Wyoming; Salina, Kansas; and Denver, Colorado. But for the best chance of witnessing a record high temperature, head for Pueblo, Colorado, where temperatures today and tomorrow in the mid-70s will almost surely eclipse the old marks.

How Long Will The Warmth Last?

Colder air is on the march from central Canada, and by the middle of next week the warmth in the plains will be known as the good old days.

The Rocky Mountains will act as a barrier to the westward progress of the cold air, so temperatures in the intermountain region will remain above normal for at least another week.

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