You can here it now! Climate Clown , Climate Change do it, Climate Change do it, " no it all atmospheric-river event.now"(What the Jet steam?) Yawn! California, Dry year may be in store
State forecasters are predicting a third dry year for California, the Department of Water Resources announced Wednesday.
The forecast for water year 2014 is of particular interest since water years 2012 and 2013 were both dry, and 2014 brings the possibility of a third dry year.
The experimental forecast, prepared for DWR by Dr. Klaus Wolter of the Cooperative Institute for Research in Environmental Sciences at the University of Colorado, makes the following predictions based on statistical models that consider global influences on California climate:
• Mostly dry conditions for most of California, with dry conditions being especially likely in Southern California.
• Near-normal to drier than normal for the Colorado River Basin, an important source of water supply for Southern California, although not as dry as in water year 2013.
• A small chance of a spring shift to El Niño conditions that could bring wetter weather for Southern California late in the season.
“Atmospheric river (AR) storms are a wild card in this forecast,” Wolter said. “My forecast last year for dry conditions in water year 2013 seemed destined for failure at first, since California experienced record wet conditions in late November/early December of last year courtesy of AR storms. However, the remainder of the season was record dry, producing an overall result of dry for the water year.”
California’s annual water supply is determined by a relatively small number of storms — only two or three storms or their absence can shift the balance between a wet year and a dry year.
On average, about half of California’s statewide precipitation occurs December through February, with three-quarters occurring November through March. Averages can mask great variability within the wet season, however.
Water years 2012 and 2013 were both dry, but their precipitation patterns were complete opposites. Water year 2012 began with record dry conditions, setting a record for the latest closing date for the Tioga Pass highway due to the absence of significant snow until January. Water year 2013 began record wet in Northern California, but then turned record dry from January on.
Atmospheric river (AR) storms are a wild card in this forecast,” Wolter said. “My forecast last year for dry conditions in water year 2013 
Animation of the atmospheric-river event. This animation shows an atmospheric river event over Dec. 18-20, 2010. High-altitude winds pull large amounts of water vapor (yellow and orange) from the tropical ocean near Hawaii and carry it straight to California. Image Credit: Anthony Wimmers and Chris Velden, University of Wisconsin-CI
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Jet Streams
Jets are fast moving ribbons of air high up in the atmosphere. They are responsible for transporting highs and lows. They affect precipitation and temperatures, and they mark boundaries between massive air masses.
Why do I care? Jet streams are important to the transport of highs and lows that affect our weather and lives on a daily basis.
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| Figure A: Jet Stream Wind Speeds |
| Image from NOAA |
Jet streams are the major means of transport for weather systems. A jet stream is an area of strong winds ranging from 120-250 mph that can be thousands of miles long, a couple of hundred miles across and a few miles deep. Jet streams usually sit at the boundary between the troposphere and the stratosphere at a level called the tropopause. This means most jet streams are about 6-9 miles off the ground. Figure A is a cross section of a jet stream.
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| Figure B: Location of Jet Streams |
| Image from NOAA |
The dynamics of jet streams are actually quite complicated, so this is a very simplified version of what creates jets. The basic idea that drives jet formation is this: a strong horizontal temperature contrast, like the one between the North Pole and the equator, causes a dramatic increase in horizontal wind speed with height. Therefore, a jet stream forms directly over the center of the strongest area of horizontal temperature difference, or the front. As a general rule, a strong front has a jet stream directly above it that is parallel to it. Figure B shows that jet streams are positioned just below the tropopause (the red lines) and above the fronts, in this case, the boundaries between two circulation cells carrying air of different temperatures.
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| Figure C: Typical Locations of Jet Streams Across North America |
| Image from NASA |
The two jet streams that directly affect our weather in the continental US are the polar jet and the subtropical jet. They are responsible for transporting the weather systems that affect us. The polar front is the boundary between the cold North Pole air and the warm equatorial air. The polar jet sits at roughly 60°N latitude because this is generally where the polar front sits. The subtropical jet is at roughly 30°N latitude. The subtropical jet is located at 30°N because of the temperature differences between air at mid-latitudes and the warmer equatorial air. The polar and subtropical jets are both westerly, meaning they come from the west and blow toward the east. Both jets move north and south with the seasons as the horizontal temperature fields across the globe shift with the areas of strongest sunlight.
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| Figure D: Typical Locations of Polar Jet Stream Throughout Summer and Winter |
| http://www.meted.ucar.edu/afwa/climo/intro/media/graphics/polar_front_summer_winter.jpg |
In the winter the polar jet moves south and becomes stronger because the North Pole gets colder but the equator stays about the same temperature. This increases the temperature contrast and moves the strengthened polar front jet farther south. As you probably have noticed, jet streams are not just straight across, but have a wavy pattern. The jets follow the contours of low and high pressure areas (troughs and ridges, respectively), which move like waves in the atmosphere across the earth. In winter when the polar jet dips into the US, the troughs and ridges affect what kind of weather an area will have. Figure D is a comparison of the strength and position of the polar jet in summer versus winter. If a trough is sitting over you, it is generally very cold and snowy or rainy. If a ridge is sitting over you, it is generally warm and dry. So if it’s winter and the jet stream looks like it does in the picture, Utah would be warmer than average and Tennessee would be colder than average, possibly experiencing some snow.
The subtropical jet also moves and evolves over time. The strongest effect of the subtropical jet in the southeastern US is in winters during years when El Niño is occuring in the eastern Pacific Ocean. The subtropical jet is then positioned in such a way that southern Georgia and Florida are right underneath the main jet, and experience colder and wetter conditions than years when there is no El Niño.
Links to National Science Education Standards:
5th grade science: 5.E.1.3 : Explain how global patterns such as the jet stream and water currents influence local weather in measurable terms such as temperature, wind direction and wind speed, and precipitation.
7th grade science: 7.E.1.5 : Explain the influence of convection, global winds, and the jet stream on weather and climatic conditions.
Earth Science: EEn.2.5 : Understand the structure of and processes within our atmosphere.
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