If we want to see what the current weather is like across the country or what weather that could be expected over the next day or two, we would consult the daily weather maps and the forecast charts. But what if we wanted to know about the atmospheric conditions during the just concluded winter or during this upcoming summer season? We soon realize that weather can be used to describe the atmospheric state not only at the current time, but also over relatively short intervals, say up to ten days out into the future. Description of atmospheric conditions for longer time spans, such as for a month or for a three-month season, typically falls into to the realm of climate analysis or forecasting. For such time scales, individual storms or migratory high pressure systems become less relevant, as the general atmospheric circulation scheme begins to become more important in deciding multi-day temperature and precipitation patterns.
Weather forecasts for the next several days are based upon numerical prediction models, such as those run at NOAA's Hydrometeorological Prediction Center. These models are predicated on sophisticated numerical weather prediction models that have a set of equations including those describing Newtonian motion, thermodynamics and mass conservation. Observational data from the surface and aloft describing current weather conditions are entered into these weather prediction models, and iterative calculations are made of the atmospheric state at computational small time-steps over the next several days. Output statistics are generated by these models for the next 60 hours (short-range forecasts) and the next 10 days (medium-range forecasts).
To determine what type of weather conditions that we could expect for the next month or the next season (typically a three-month span) would require consultation with the official climate outlook maps generated by NOAA's Climate Prediction Center (CPC). (A chart shows the types of outlook maps, graphs and tables produced by CPC). These maps show the probabilities of how the temperature, precipitation and sea surface temperatures (SSTs) will deviate from the long-term or normal state for the next month and three month periods, extending from one to thirteen months into the future. In addition to these one-month to three-month climate outlooks, CPC also issues 6-10 Day and 8-14 Day extended outlook maps. These maps also show the probabilities of temperature and precipitation departing from the long-term average or normal conditions. The long-lead forecasts that may range from two weeks to thirteen months are based upon a dynamical ocean-atmosphere model that incorporates oceanic and atmospheric data. Sophisticated statistical analysis predicts how the patterns of temperature and precipitation should evolve based upon recent patterns of temperature and precipitation across the national, along with global sea-surface temperatures and tropospheric flow patterns. Consideration is also made of the effects of trends associated with El Niño and La Niña effects (to be discussed next week).
STORMS & THEIR TRACKS
When we look at surface weather maps, we would soon realize that tranquil weather is usually associated with areas of high pressure, while inclement weather typically accompanies low-pressure systems. Midlatitudes are characterized by migratory weather systems, such as storms, that are carried along by the general westerly winds in the troposphere. These storms develop in a process called cyclogenesis and evolve as they are carried eastward, finally dissipate in a process called cyclolysis. A series of weather maps can show the complete sequence of cyclogenesis, travel and cyclolysis of individual storms. By tracking all the storms that develop over an extended time interval, say for the last 10, 30 and 90 days, we can get some indication of how atmospheric patterns are evolving not just on the time scale that we call weather, but on monthly and seasonal time scales that can be considered climate.
For example, CPC has a series of graphics that show the locations of cyclogenesis and cyclolysis across a large section of the Northern Hemisphere extending from eastern Asia across North America to western Europe. The current series of 10, 30 and 90-day maps show an active pattern, which should be expected as the time intervals cover a sizable portion of the winter season for the Northern Hemisphere. The movement of storms can also be tracked, with the resulting maps called "storm track maps." These storm tracks are studied by CPC, along with several other important fields, such as lower tropospheric winds, associated precipitation patterns, significant ocean wave height and sea ice. On the individual maps, the intensity of the storm (or cyclone) is indicated by a color code that is based upon the minimum central pressure (in millibars). When the storm tracks are overlaid upon precipitation maps, one can see that widespread precipitation can be found along the storm tracks, especially over the subpolar North Pacific and North Atlantic and adjacent land areas on the eastern sides of Eurasia and North America. Significant wave heights are also linked to the storm tracks, extending outward across the North Pacific and North Atlantic.
So how do the storm tracks vary by season of the year? A climatology of storm tracks has been assembled to allow CPC forecasters to monitor recent storminess across both the Northern and Southern Hemispheres. This storm-track climatology was developed by determining the location of low-pressure systems on a daily basis, running from 1950 through 2002. A four-panel map shows the seasonal frequency of storms and the distribution of storm tracks across the section of the Northern Hemisphere extending from eastern Asia to western Europe. The top panel (a) is for boreal winter (December, January and February) and shows most of the storms during this season develop and spend much of their time in preferred regions of cyclogenesis. These cyclogenesis regions are over the western North Pacific Ocean off Japan and the eastern coast of Asia, as well as over the North Atlantic off the Canadian Maritime Provinces on the eastern coast of North America. Seven to eight storms typically form between December and February over these waters. Other areas with high numbers of storms are across the Great Lakes in North America, and along the eastern slopes of the Rocky Mountains from southwestern Canada southward to the Panhandles of Oklahoma and Texas. The next panel (b) for northern spring (March, April and May) shows fewer storms and a northward displacement of the regions of cyclogenesis. This trend in reduced storminess and more northerly track of the storms is also noted in the third panel (c) for the boreal summer season (June, July and August). Most of the storms develop and are found across eastern Canada, in a region that corresponds to the summer position of the Icelandic low pressure system in the planetary scale pressure patterns. Some storms also are found over the North Pacific off Russia's Kamchatka Peninsula and along the western Aleutian Island chain. The last map (d) for northern autumn (September, October and November) shows an intensification of the storms across the Gulf of Alaska, the western North Pacific and over the Great Lakes of North America.
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Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 2011, The American Meteorological Society.
