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

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.

Allan Huffman's Model and Weather Data Page

This site will be used to get 500 mb height forecasts from the ECMWF (European Centre for Medium Range Forecasting) as well as the GFS model. Unfortunately, for the United States, our operation weather forecasting models generally do not perform as well the the ECMWF model. This was publicly discussed with regard to the forecast of hurricane and superstorm Sandy as the long range ECMWF forecasts were more accurate than the GFS forecasts. Thus, it is worth comparing the GFS forecasts with those made by ECMWF. While Allan Huffman's site can be difficult to navigate, it does provide some nice looking maps. I have found that the Internet Explorer browser sometimes works better than the Firefox browser with this site. Again you may wish to open this link in a new tab so that you can follow the instructions http://www.americanwx.com/raleighwx/models.html. There are a lot of weather links on the homepage. We are only going to discuss the GFS and ECMWF images links that fall under the column heading of "Operational Global Models."

Select the ECMWF Images Link. 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 plots and the 500 mb Height Anomalies plots for the Northern Hemisphere and North America (these are in the first four 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 130111/0000V072" means the forecast is valid for Friday, January 11, 2012 at 0000Z and is a 072 hour forecast.

On the 500 mb height maps, the color shading is the 500 mb height pattern. This is what we are focusing on right now. The color scale for the height contours is shown to the left of the image. The other contour lines on the map are the sea level pressure, which we will briefly cover later in the semester. The height anomaly maps are something we have not covered yet. 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.

These same maps, 500 mb height and 500 mb height anomalies, are provided for the GFS model. The GFS model is started four times per day (i.e., new forecast runs based on latest observations) at 00Z, 06Z, 12Z, and 18Z. You can acces this information from the first four rows of links under the column heading "Operational Global Models." There are many more weather fields that can be plotted in these tables than were available from the ECMWF tables. This is because all of the US weather data is freely distributed, while the Europeans charge money for their forecast data. Only the cheap or free ECMWF fields are readily accessible on the web. If you are interested please check out some of the other forecast fields. One that I often check is precipitation forecast data. For this class, you should take a look at the 500 mb height and height anomalies forecasts. These are found in first row under "Quick Plots" and the first three rows under "Standard Plots." The interpertation of these images is the same as described for the ECMWF above. This makes it relatively easy to compare the forecasts made by the two models.