What Does It All Mean?

| January 8, 2011 @ 10:00 am | 99 Replies

I get many questions about the terms we use in our daily discussions… and we should mention, we do these every day throughout the year, not just during snow or severe weather. Here is a look at the terms you might read here…

Critical Thickness (540 line): Thickness is the vertical distance between two isobaric surfaces, which is often proportional to the temperature of that layer. As such, empirical studies have shown that certain values of differing thickness have shown some skill in differentiating between rain or snow:
1000-500 mb (5400 m)
1000-700 mb (2840 m)
1000-850 mb (1300 m)
850-700 mb (1540 m)

GFS: Global Forecast System model. Run four times a day (00, 06, 12, 18 UTC) and produces forecasts up to 16 days in advance, but with decreasing spatial and temporal resolution over time. The model is run in two parts: the first part has a higher resolution and goes out to 180 hours (7 days) in the future, the second part runs from 180 to 384 hours (16 days) at a lower resolution. The resolution of the model varies in each part of the model: horizontally, it divides the surface of the earth into 35 or 70 kilometre grid squares; vertically, it divides the atmosphere into 64 layers and temporally, it produces a forecast for every 3rd hour for the first 180 hours, after that they are produced for every 12th hour.

NAM: North American Mesoscale model. It technically is the Weather Research and Forecasting Non-hydrostatic Mesoscale Model (WRF-NMM), but it is run as the NAM. The model is run four times a day (00, 06, 12, 18 UTC) out to 84 hours. It is currently run with 12 km horizontal resolution and with 1 hour temporal resolution, providing finer detail than other operational forecast models.

RPM: Rapid Precision Mesoscale model. A numerical weather prediction system based on the Advanced Research Weather Research and Forecast system (WRF-ARW). RPM generates forecasts out to 24 hours with updates every 3 hours in the United States and every 6 hours outside the United States. Precipitation forecasts are calculated from half-hourly instantaneous precipitation forecasts output by RPM. RPM model output is not available to the pubic; but we have full access and use it extensively in our forecast products.

Z: When you see a time like 12Z, 00Z, 06Z, etc… Z simply stands for Universal Coordinated Time, the time standard by which the world regulates clocks and time. It is equivalent to GMT, or Greenwich Mean Time. This is six hours ahead of local time, but when we are on Daylight Saving Time is is 5 hours ahead of us. For example, at this time of the year, 00Z is 6:00 p.m. Alabama time. 12Z is 6:00 a.m. Alabama time.

500 mb chart: This is one of the mandatory levels in the atmosphere; they are constant-pressure levels for which a complete evaluation of data derived from upper-air observations is required. Currently the mandatory pressure levels are 1000 mb, 850 mb, 700 mb, 500 mb, 400 mb, 300 mb, 200 mb, 150 mb, 100 mb, 50 mb, 30 mb, 20 mb, 10 mb, 7 mb, 5 mb, 3 mb, 2 mb, and 1 mb. The radiosonde code has specific blocks reserved for these data. 500 mb is usually about 18,000-20,000 feet off the ground. When we show a 500 mb chart, the height contours you see are in decameters, and represent the altitude where the pressure is 500 mb.

I will add more to this list when time allows in coming weeks… and we will keep this in the category “Meteorology 101″… so you can access it when you need it!

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About the Author ()

James Spann is one of the most recognized and trusted television meteorologists in the industry. He holds the AMS CCM designation and television seals from the AMS and NWA. He is a past winner of the Broadcast Meteorologist of the Year from both professional organizations.

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