Weather Forecasting using 500 mb Height Maps

One of the nice things about being able to interpret 500 mb height maps is that one can can easily visualize the large-scale weather patterns that are forecasted by computer-generated numerical weather prediction models. There are dozens of groups worldwide that run different numerical models for weather forecasting and hundreds of web sites that display this data. The fact that each model produces different forecasts indicates that there is uncertainty in making numerical weather forecasts. You probably realize from your own experiences that weather forecasts are not exact and can sometimes be way off. When different forecast models make different predictions of future weather, your local weather forecaster has to decide which to believe in making his personal forecast. Generally numerical weather forecast models in the winter season are quite accurate for 1-5 days into the future, somewhat less accurate for 6-9 days into the future, sometimes helpful for 10-12 day forecasts, and have little skill in forecasting more than 12 days into the future. We will cover material discussing limitations on the accuracy of numerical weather forecasting in later lectures.

On this page you will learn how to get numerical weather forecasts from two different sources and how to interpret the 500 mb height maps produced. We will also use maps from these sources for a homework set.

University of Wyoming's Model Forecast Page

This site was mentioned on the introductory lecture page on 500 mb maps. You may wish to open this link http://weather.uwyo.edu/models/ in a new tab or browser window so that you can follow these instructions. You must first select a model. In class we will use either the GFS (Global Forecast System) or Medium Range Forecast (MRF) both run in the US by NOAA. You must fill in the drop-down menus and dots to select the maps you want to display. For now select the latest possible initial time (default value) and set the other items as shown below

After setting these values, click the "Get Data" button at the bottom. If all works well you should get a movie of 500 mb height images. GFS images are produced every 6 hours and run from the initial time out to 180 hours (7.5 days) into the future, while MRF images are produced every 12 hours and run from the initial time out to 240 hours (10 days) into the future. Notice the time stamp located at the bottom of each image. The first part "__ Hour" indicates how many hours have passed in the forecast since the initial time, where 00 Hour is not really a forecast at all, but the 500 mb pattern based on observed data around the world. The next part "500 hPa" tells us that the height map is at the 500 hectoPascal pressure level (hPa and mb can be used interchangeably, hPa is the International standard unit, mb are favored in the US). The last part gives the time in which the forecast map is valid in Grenwich Mean Time (recall that Tucson local time is 7 hours earlier than Z time).

If you are interested you can use this site to produce other forecast images. We will not have time to do much of this as part of the class, but feel free to explore and ask questions. One item that I sometimes choose to add to the 500 mb maps is to select precipitation as one of the color fills to see where and how much precipitation is forecast by the model.

El Dorado Weather

This site will be used to get 500 mb height forecasts from the ECMWF (European Centre for Medium Range Forecasting) model. Again you may wish to open this link in a new tab so that you can follow the instructions http://www.eldoradocountyweather.com/current/models/North-America/ecmwf-listings.html. There are two tables within which you can select various model forecast maps. The ECMWF model is started from measured data twice per day at 00Z and 12Z. The only maps we are going to discuss in this class are the 500mb Height Anomalies for the Northern Hemisphere and North America (these are in the 3rd and 4th rows of the tables). If you select loop you will get a movie of the 500 mb height forecast, otherwise you will get a single frame. The individual images have time stamps at the bottom. You need only look at the first set of numbers (before the first colon) to get the time of the forecast. For example "Fri 120113/0000V024" means the forecast is valid for Friday, January 13, 2012 / at 0000Z and is a 024 hour forecast.

The contour lines on these maps are the 500 mb heights, which are labeled in meters above sea level. The color shading is of the 500 mb height anomaly, which is defined at the difference between the forecasted 500 mb height and the average 500 mb height. The color scale for the height anomaly is shown to the left of the image. The H's and L's labeled with numbers are point values of the 500 mb height anomaly. This is meant to show centers of high and low 500 mb height anomalies. The nice thing about these maps is that you can immediately see how the 500 mb height compares to average, which makes it much easier to interpret the maps in terms of expected temperature relative to average. Places where the 500 mb height is forecasted to be below average (blue and purple shading) are places where below average temerature is expected, places where it is above average (green, yellow, and red) shading are places where above average temperature is expected. If the height anomaly over a region is within 40 meters of average, expect near average temperatures for that region; if the height anomaly is 40-100 meters above (or below) average, expect temperatures to be moderately above (or below) average for that region; and if the height anomaly is 100 or more meters above (or below) average, expect well above (or below) average temperatures for that region. The H's and L's indicate local regions of maximum and minimum height anamolies.