Weather Bug

Thursday, December 23, 2010

Today's Weather Outlook

Posted by ILIAS at 12:41 AM

Today's Weather Outlook 12 AM EST, December 23, 2010 By WeatherBug Sr. Meteorologist, James West
	Click to Enlarge Related Content Current Temperature Current Radar Highs Today United States Alert Map Your Photos The last work day for many Americans before the Christmas holiday will be a quiet, albeit chilly one, to do last-minute shopping. The Mountain West and the northern Plains will be the exception, where snow could trip travel plans. After an active several days, California and the Southwest will finally get a chance to clean up and get ready for Christmas. The early-week downpours, flooding and mountain snow will finally slide east into the Rockies, allowing skies to finally clear out and for even some late-day sunshine to develop. The Northwest will see a few showers as the next Pacific storm edges closer to the coast. However, the Rockies and Wasatch, next in line for the major winter storm, will be buried under feet of new snow today. Travel between Salt Lake City and Denver will be difficult to nearly impossible as the storm cranks up high winds, reducing visibility. Snow, not on the scale of the Rockies level, will fall across the Dakotas eastward into the Upper Midwest this afternoon and evening. A dusting to a few inches of snow will be all that will fall. However, it could just be enough to trigger a few minor traffic delays in Minneapolis. Outside of scattered snow showers downwind of the Great Lakes, the rest of the U.S. will have smooth sailing as a clear sky produces bright December sunshine. High temperatures will be cold for much of the northern tier from the Mountain West to the Northeast, with highs generally in the 20s and 30s. The northern Plains will be coldest, with the mercury only rising managing the teens and 20s. More moderate temperatures, generally 40s and 50s, will be found across the West Coast and throughout the Southwest, southern Rockies, Texas, South and Southeast. The warmest temperatures will be in sunny Florida, with daytime readings in the 70s. Be sure to keep WeatherBug active to receive the latest weather in your neighborhood and get the latest updates anywhere on Twitter at WeatherBug What do you think of this story? Click here for comments or suggestions.

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Cause

On Earth, common weather phenomena include wind, cloud, rain, snow, fog and dust storms. Less common events include natural disasters such as tornadoes, hurricanes, typhoons and ice storms. Almost all familiar weather phenomena occur in the troposphere (the lower part of the atmosphere).[3] Weather does occur in the stratosphere and can affect weather lower down in the troposphere, but the exact mechanisms are poorly understood.[5]

Weather occurs primarily due to density (temperature and moisture) differences between one place to another. These differences can occur due to the sun angle at any particular spot, which varies by latitude from the tropics. In other words, the farther from the tropics you lie, the lower the sun angle is, which causes those locations to be cooler due to the indirect sunlight.[6] The strong temperature contrast between polar and tropical air gives rise to the jet stream.[7] Weather systems in the mid-latitudes, such as extratropical cyclones, are caused by instabilities of the jet stream flow (see baroclinity).[8] Weather systems in the tropics, such as monsoons or organized thunderstorm systems, are caused by different processes.

Because the Earth's axis is tilted relative to its orbital plane, sunlight is incident at different angles at different times of the year. In June the Northern Hemisphere is tilted towards the sun, so at any given Northern Hemisphere latitude sunlight falls more directly on that spot than in December (see Effect of sun angle on climate).[9] This effect causes seasons. Over thousands to hundreds of thousands of years, changes in Earth's orbital parameters affect the amount and distribution of solar energy received by the Earth and influence long-term climate. (see Milankovitch cycles).[10]

The uneven solar heating (the formation of zones of temperature and moisture gradients, or frontogenesis) can also be due to the weather itself in the form of cloudiness and precipitation.[11] Higher altitudes are cooler than lower altitudes, which is explained by the lapse rate.[12][13] On local scales, temperature differences can occur because different surfaces (such as oceans, forests, ice sheets, or man-made objects) have differing physical characteristics such as reflectivity, roughness, or moisture content.

Surface temperature differences in turn cause pressure differences. A hot surface heats the air above it and the air expands,lowering the air pressure and its density.[14] The resulting horizontal pressure gradient accelerates the air from high to low pressure, creating wind, and Earth's rotation then causes curvature of the flow via the Coriolis effect.[15] The simple systems thus formed can then display emergent behaviour to produce more complex systems and thus other weather phenomena. Large scale examples include the Hadley cell while a smaller scale example would be coastal breezes.

The atmosphere is a chaotic system, so small changes to one part of the system can grow to have large effects on the system as a whole.[16] This makes it difficult to accurately predict weather more than a few days in advance, though weather forecasters are continually working to extend this limit through the scientific study of weather, meteorology. It is theoretically impossible to make useful day-to-day predictions more than about two weeks ahead, imposing an upper limit to potential for improved prediction skill.[17]

Modification

The aspiration to control the weather is evident throughout human history: from ancient rituals intended to bring rain for crops to the U. S. Military Operation Popeye, an attempt to disrupt supply lines by lengthening the North Vietnamese monsoon. The most successful attempts at influencing weather involve cloud seeding; they include the fog- and low stratus dispersion techniques employed by major airports, techniques used to increase winter precipitation over mountains, and techniques to suppress hail.[41] A recent example of weather control was China's preparation for the 2008 Summer Olympic Games. China shot 1,104 rain dispersal rockets from 21 sites in the city of Beijing in an effort to keep rain away from the opening ceremony of the games on August 8, 2008. Guo Hu, head of the Beijing Municipal Meteorological Bureau (BMB), confirmed the success of the operation with 100 millimeters falling in Baoding City of Hebei Province, to the southwest and Beijing's Fangshan District recording a rainfall of 25 millimeters.[42]

Whereas there is inconclusive evidence for these techniques' efficacy, there is extensive evidence that human activity such as agriculture and industry results in inadvertent weather modification:[41]

* Acid rain, caused by industrial emission of sulfur dioxide and nitrogen oxides into the atmosphere, adversely effects freshwater lakes, vegetation, and structures.
* Anthropogenic pollutants reduce air quality and visibility.
* Climate change caused by human activities that emit greenhouse gases into the air is expected to affect the frequency of extreme weather events such as drought, extreme temperatures, flooding, high winds, and severe storms.[43]

The effects of inadvertent weather modification may pose serious threats to many aspects of civilization, including ecosystems, natural resources, food and fiber production, economic development, and human health.[44

Forecasting

Weather forecasting is the application of science and technology to predict the state of the atmosphere for a future time and a given location. Human beings have attempted to predict the weather informally for millennial, and formally since at least the nineteenth century.[25][26] Weather forecasts are made by collecting quantitative data about the current state of the atmosphere and using scientific understanding of atmospheric processes to project how the atmosphere will evolve.[27]

Once an all-human endeavor based mainly upon changes in barometric pressure, current weather conditions, and sky condition,[28][29] forecast models are now used to determine future conditions. Human input is still required to pick the best possible forecast model to base the forecast upon, which involves pattern recognition skills, teleconnections, knowledge of model performance, and knowledge of model biases. The chaotic nature of the atmosphere, the massive computational power required to solve the equations that describe the atmosphere, error involved in measuring the initial conditions, and an incomplete understanding of atmospheric processes mean that forecasts become less accurate as the difference in current time and the time for which the forecast is being made (the range of the forecast) increases. The use of ensembles and model consensus helps to narrow the error and pick the most likely outcome.[30][31][32]

There are a variety of end users to weather forecasts. Weather warnings are important forecasts because they are used to protect life and property.[33] Forecasts based on temperature and precipitation are important to agriculture,[34][35][36][37] and therefore to commodity traders within stock markets. Temperature forecasts are used by utility companies to estimate demand over coming days.[38][39][40] On an everyday basis, people use weather forecasts to determine what to wear on a given day. Since outdoor activities are severely curtailed by heavy rain, snow and the wind chill, forecasts can be used to plan activities around these events, and to plan ahead and survive them.

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Thursday, December 23, 2010

Today's Weather Outlook

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