Weather Around The World, 12/29: Omni Persists; Full Moon On Christmas; Argentina Floods; Europe Still Warm

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Home / Weather Around The World, 12/29: Omni Persists; Full Moon On Christmas; Argentina Floods; Europe Still Warm
The jet stream forecast for January 3 shows a trough in the east which will bring much colder weather, and a subtropical jet across the southern US. Forecast courtesy of NOAA.

The jet stream forecast for January 3 shows a trough in the east which will bring much colder weather, and a subtropical jet across the southern US. Forecast courtesy of NOAA.

Weather Pattern Omni persists with virtually every kind of weather, but its effects will now subside. A new, substantially different, pattern will take its place over the US.

In Europe, changes are also in store, while South America feels the most direct effect of El Niño.

And, everywhere, the Christmas full moon was high and bright.

Let’s go Around The World.

Weather Pattern Omni Continues To Cause Winter-Like And Summer-Like Conditions

The Weather Channel keeps giving it different winter-storm names, but the weather associated with Weather Pattern Omni has only one source: A deep trough in the west and a huge ridge in the east of the United States.

Significant snow has delighted skiers in the western mountains, while record-breaking high temperatures have equally delighted east-coasters weary of recent blustery winters. In between, the weather has featured tornadoes, flash floods, severe thunderstorms, and freezing rain.

The forecast for Wednesday shows Omni's last gasp: Some flooding in the southeast and a little freezing rain in New England. By Thursday, this round of Omni will be over. Forecast courtesy of NOAA.

The forecast for Wednesday shows Omni’s last gasp: Some flooding in the southeast and a little freezing rain in New England. By Thursday, this round of Omni will be over. Forecast courtesy of NOAA.

The Weather Channel could argue that this storm is taking place in winter so calling it a winter storm is justified. But this claim is derailed by the fact that The Weather Channel named two ‘winter storms’ in November — which is not winter by any definition. Furthermore, forty-three people have died in this round of Omni, all from tornadoes and flooding, neither of which is a winter-type occurrence.

The Weather Channel’s initial attempt to communicate more precisely concerning weather that occurs during the winter was laudable. But the way they carried it out has proved impractical. At various times TWC has based naming a storm on aerial coverage, or severity, or number of people affected — or some combination of factors. It simply hasn’t worked.

Last year Decoded Science began giving non-tropical weather systems and patterns informative, though sometimes whimsical, names – such as last year’s ‘Yeti,’ which brought massive snows to New England, and the recent ‘Coldilocks,’ which indicated the just-right conditions needed for freezing rain.

‘Omni’ implies the wide variety of weather associated with the current weather pattern, the effects of which have been felt from coast to coast.

Decoded Science invites The Weather Channel to give up its misguided naming system and join in a discussion of how non-tropical systems can better be dealt with in regard to identification and the communication of information.

The first round of Omni came two weeks ago, and the current one has lasted over a week. But things are about to change. A trough is setting up in the east, with cold and possibly snow, while a warm ridge develops in the west. Significantly, a low-latitude jet stream, a hallmark of El Niño, is forecast to affect all of the southern US, with the potential for useful amounts of rain in southern California, and un-useful floods along the Gulf Coast.

Looking ahead, El Niño is expected to subside during the late winter and spring, and its opposite, La Niña, could form for the summer. La Niña is associated with years that have high hurricane activity in the Atlantic Basin (Atlantic Ocean, Caribbean Sea, and Gulf of Mexico).

Tornadoes In December

Tornadoes tore through nine states over the weekend. Photo courtesy of NOAA.

Tornadoes tore through nine states over the weekend. Photo courtesy of NOAA.

The outbreak of tornadoes this weekend, from Texas to Minnesota, and Wyoming to Alabama, is unusual but not unprecedented; the three years with the most December tornadoes in the US are now all El Niño years.

With the Gulf of Mexico supplying low-level moisture, and the Rockies drying out the jet stream, the stage can be set for a tornado outbreak by an Omni-type pattern in any season. It happens less frequently in winter because low-level winds normally blow from the north across the tornado-prone region.

The central US is especially susceptible to tornadoes because of conditional instability. When the dry upper level flow overlies the moist surface flow, the air column is initially stable. But if it is lifted it becomes unstable. Here’s why:

Temperature drops with height in the atmosphere due to the change of pressure. A lifted parcel of unsaturated air cools at about five degrees per thousand feet. Most of the time it finds itself colder than surroundings and sinks back to where it came from — the air is stable. But if the parcel of air is lifted past the condensation level, it only cools three degrees per thousand feet because some of the water vapor condenses, releasing latent heat. Saturated parcels may find themselves warmer than their surroundings and keep rising. The atmosphere is then unstable and thunderstorms will form.

The final ingredient in tornado formation is wind shear — a change of wind with height. In the US’s tornado alley, southerly winds at the surface combined with westerly winds above create the twisting that starts a, yes, twister.

Christmas Moon: Full, Bright, And High

The moon appears 14% larger at perigee than at apogee. Image courtesy of NASA.

The moon appears 14% larger at perigee than at apogee. Image courtesy of NASA.

At 11:12 Universal time on Christmas Day, the moon was at its fullest.

Technically, the moon can never be full and bright at the same time because a totally full moon — which would occur when the moon, sun and earth are exactly in line — would be in the shadow of the earth. But we call the moon ‘full’ each month at the time when it is closest to completely full.

Not only was the moon full, it also appeared big — bigger than normal by about 5%. Perigee (the moon’s closest approach to the earth) occurred on December 21, so the moon was about 10,000 kilometers closer to the earth than when it is at its average distance.

And there was one more thing: At its zenith, the moon was as high in the sky as it ever gets. Here’s why:

The moon is approximately in the plane of the ecliptic — the plane defined by the earth’s orbit around the sun. In the winter, the full moon is at the opposite side of the ecliptic from the sun, which means it is on a summer trajectory. On the day of the winter solstice (December 22 this year), the moon is at its highest point for the year.

All winter full moons (December, January, February) are higher in the sky than those in the summer — scant compensation for the cold and snow.

Warm Winter In Europe May Change

As the jet stream pattern re-aligns over the United States, a similar change is taking place in Europe. The typical El Niño pattern with a low latitude jet stream is also shaping up for the continent. Unusually high pressure is building over Scandinavia and eastern Siberia, while the low latitude jet could bring precipitation to the Middle Eastern deserts.

Temperatures over much of Europe should return to more seasonable levels, with warmth under the high pressure and precipitation along the path of the sub-tropical jet stream.

The Direct Effect Of El Niño

El Niño is a warming of the equatorial Pacific Ocean by as much as three degrees Celsius above normal. The most direct effects are a collapse of the anchovy fishery on the coast of Ecuador and Peru, and increased rainfall over much of South America. In the past month, flooding has been most severe in northeast Argentina, but has extended into parts of Brazil, Uruguay, and Paraguay.

El Niño affects the hemispheres differently because it peaks during northern hemisphere winter, which is summer in the southern hemisphere. Climatologists expect more flooding in tropical and sub-tropical South America, especially south of the equator, in the next few months.

It’s Almost A New Year

As the hottest year in recorded history ends, weather continues to evolve. El Niño is cresting. Will it be followed, as many strong El Niño’s are, by a powerful La Niña?

The weather gets, as Alice in Wonderland would say, curiouser and curiouser. What’s it doing where you are?

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