The Triple Threat Of Extreme Weather Event Yeti: Three Different Ways To Make Precipitation


Home / The Triple Threat Of Extreme Weather Event Yeti: Three Different Ways To Make Precipitation
Freezing weather

The forecast for Monday shows Yeti’s giant footprint. Arctic high pressure is centered over Iowa. Around the periphery is every kind of precipitation: upslope snow in Colorado; warm advection freezing rain in Texas; and snow in the Appalachians from a developing vorticity center off the Atlantic coast. Forecast courtesy of NOAA.

Yeti is still going strong. The polar vortex is holding its position over the eastern United States, with a powerful jet stream on its periphery. The result is bad weather of many kinds. And while rain is rain and snow is snow, the mechanisms for producing them can be different in each individual case; Yeti has them all.

Winter Storms Pandora and Quantum

For the sake of clarity, Decoded Science notes that The Weather Channel (TWC) has named winter storms Pandora And Quantum. TWC has thrown responsibility to the winds and is apparently attempting to attract attention by the sensationalism of sheer numbers of named storms.

There is nothing about either of these so-called storms that is quantifiable or even identifiable; there’s rain and snow and even some other stuff, but where are they going to draw the line? Will we have a name for every snow flurry?

Decoded Science identifies Extreme Weather events by cause. This one is ongoing Event Yeti, a pronounced deepening of a jet stream trough over the eastern US that is also categorized as part of the polar vortex.

Historically located at the pole, the polar vortex has recently frequently fragmented, with parts of it forcing the jet stream southward.

The Three Conditions That Produce Precipitation

Rising air is a necessary but not sufficient condition for precipitation. In addition, there must be enough water vapor in the air for a rising column to produce condensation. But no matter how humid the air, it must rise in order to get that moisture to condense and fall as rain, snow, sleet, or freezing rain.

There are three atmospheric conditions that produce rising air:

  • When wind blows up a mountain, the air is obviously forced to rise.
  • When warm air flows over cold air, the denser cold air holds its position and forces the warm air to rise over it.
  • When air swirls in a counterclockwise direction, it spirals into a low pressure area and is squeezed upward.

Air cools as it rises due to the decrease in pressure. Colder air cannot hold as much moisture as warmer air. In all of the above cases, when the air is saturated and rising, some of the water molecules condense to form small droplets or ice crystals — a cloud. If the process continues, the cloud droplets coalesce to form raindrops or snowflakes.

Yeti’s Upslope Rain And Snow

As the cold air associated with Yeti heads south and west, it bumps into the Rocky Mountains. The resulting rising current is producing heavy snow in the Colorado mountains, including about eight inches in Denver and up to two feet at higher elevations.

The snow should end on Monday, but another push of arctic air could bring more upslope snow later in the week.

Warm air advection

Warm air advection occurs when wind blows from warmer air to colder. The cold, dense air forces the warm air to rise. Graphic courtesy of NOAA.

Warm Advection Rain And Snow

The word ‘advection’ means movement in bulk of some property of the atmosphere. Warm air advection is movement of air from warmer to colder. In technical jargon, the wind blows across the isotherms (lines of equal temperature) from warm to cold.

Precipitation connected with warm advection tends to be steady and moderate.

The south central United Sates is a prime area for precipitation from warm air advection. Cold air penetrates the continent and warm air from the Gulf of Mexico rides over the cold. Currently this is occurring in a long path from Texas to the mid-Atlantic coast, and will be exacerbated by a southern branch of the jet stream coming across the Pacific into the southwestern states. The latter is consistent with the current weak El Niño Eggplant.

Though warm air advection frequently produces rain and snow over the eastern US, the most dangerous and damaging type of precipitation is freezing rain. When the cold air is below freezing and the warm air is above, freezing rain can result.

The cold layer must not be too thick; raindrops can stay liquid below freezing (a process known as supercooling), but after a while the drops freeze into sleet.

If the warm layer is too shallow, the snowflakes falling through it will not have time to melt and will fall as snow.

Freezing rain not only creates hazardous driving conditions; it can also break things by sheer weight. Ice-laden tree limbs can break and fall on power lines. In general, ice more than a quarter inch thick can break medium-sized branches; ice thicker than a half inch can bring down larger limbs and cause widespread electricity outages when they fall on power lines.

Serious icing conditions could occur today and tomorrow over northern and central Texas.

Vertical Motion From Vorticity

Vorticity is the amount of spin in the atmosphere. Clockwise spin is designated positive vorticity. When the vorticity is high, the air tends to rise in a tumultuous way, with powerful updrafts and downdrafts. The precipitation is fitful and sometimes intense. High vorticity is normally associated with low pressure systems.

There is currently no identifiable low pressure center of any magnitude, but during the week, one could form near the Atlantic coast.

Yeti’s Enduring Legacy

Yeti will certainly be remembered for its record New England snowfall. But it may be more remarkable for its cold. Low temperature records fell in the entire eastern part of the country last week; another push of arctic air (actually Siberian air) is on the way and is likely to set many more records.

jet stream forecast

This is the jet stream forecast averaged over March 1-7. The polar vortex will retreat north and the strength of the jet stream will diminish. But don’t rule out a return of Yeti. Forecast courtesy of NOAA.

How Much Longer Will We See Yeti’s Footprints?

Spring is currently seven – or twenty-six – days away, measured by either the meteorological definition or the position of the sun. Parts of the west coast have been under the influence of Fair Weather Event Crocus and its springlike conditions for some time. But Yeti trudges on in the east and is spreading westward.

Most long-range forecasts indicate a moderation of the cold and a weakening of the jet stream, but the persistence of weather systems the past two winters has been noteworthy, so even after the thaw and some spring-like days, don’t rule out another blast from Yeti.

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