University of Arizona, Department of Atmospheric Sciences



500 mb Height Anomaly Maps /Example Write-up for section 2The 500 mb maps used for this project during last spring are located at . This year’s maps will be placed in similar tables, with the forecast maps in the top table and the verification (or true) maps in the bottom table. We will refer to the maps from last spring in this document.Reading the Maps produced by Pivotal WeatherWe will look at some maps from last spring. The 240 hour (10 day) forecast map from the GFS model is shown below.The maps show contours of 500 mb height (the black lines, labeled with numbers). The 500 mb heights are labeled in decameters (dam), where 1 dam = 10 meters. To get the 500 mb height in meters, just add a zero on to the end of the number labeled on the contours. The labeled height lines show the 500 mb pattern of troughs, ridges, and closed contours. The color shading on the map shows the 500 mb height anomaly, which is the difference between the 500 mb height and the average 500 mb height. The key at the bottom is in units of decameters, so a height anomaly of -20 dam means the 500 mb height in that region is 200 meters below average (and you would expect well below average temperature for that region). The time label is the second line in the upper left. “Fttt” gives the forecast time in hours, for example F240 on the map above indicates that the map is a forecast 240 hours into the future. The time after “Valid” tells you the time and date of the forecast, for example, the map above indicates the forecast map is for 12Z on Monday, February 1 (5 AM in Tucson). The label on top right of the image tells you the time the model forecast was initialized or when it began running, and which weather forecast model is displayed. The map above shows the initial time was for this model run 12Z on January 22 and that it is the GFS model. Example Write-up for Section 2 of 500 mb projectWe just looked at a 240 hour forecast map from GFS. Below is the corresponding 240 hour forecast map from ECMWF.In sections 2a and 2b of the project report, you are asked to describe the 500 mb height patterns over the continental US and to make specific weather forecasts for Tucson based on forecasts from both the GFS and ECMWF. To give you an idea of the type of analysis expected, a sample write-up for sections 2a and 2b for this example 240 hour forecast is provided. Keep in mind that this would be just one paragraph of the three expected in each section of your report. The information in bold is most important. If necessary, you may need to refer to a map of the US that shows state names and abbreviations.2a. Description/Comparison of the forecasted 500 mb height pattern over the Continental US.(sample paragraph for the 240 hour forecast based on maps shown above). The GFS forecast has a deep, broad trough over the southwestern quarter of the country. Heights from central CA east through the 4 corners regions are as much as 250 meters below average. The most favorable areas for precipitation look to be across New Mexico, west TX, OK, and NE, and CO, though there appear to be shortwave features at the base of the trough over southern CA and AZ, which could bring precipitation to these areas. Zonal flow or a slight ridge over the eastern US with heights near average most places. In comparison, the ECMWF forecast shows a sharper and deeper trough in the west with a large 5340 closed low centered over ID. Some areas have 500 mb heights more than 300 meters below average. The ECWF has a more pronounced ridge over the great lakes region with above average heights for much of the northeastern one quarter of the country. 2b. Specific Weather Forecasts for Tucson(sample paragraph for the 240 hour forecast based on maps shown above). The 500 mb height over Tucson from the GFS is about 5520 meters. 500 mb height from ECMWF is about 5430 meters. These are well below the average of 5690 meters(*), so expect well below average temperature, even more so based on the ECMWF forecast. Tucson is in a location favorable for rain on the ECMWF forecast as it is just downwind of a strong trough. It is a more difficult precipitation forecast using GFS. The trough is right over Tucson and the main precipitation region could be east of Tucson. However, the wiggles in the 5640 and 5520 meter contour lines over AZ indicate that perhaps there are shortwaves in the flow, which could bring precipitation. (*Note the average 500 mb height for February 1 over Tucson is 5690 meters. For your project, the average 500 mb height for early November is 5770 meters over Tucson).To complete sections 2c and 2d, you need to compare the forecasts maps from each model (examples shown above) with the verification (or true) 500 mb map for the corresponding day. The true 500 mb map corresponding to the forecasts above is shown below. This is the map that is used to judge the accuracy of the forecasts discussed in parts 2a and 2b. A sample paragraph is provided for both sections 2c and 2d of the 500 mb project below. Keep in mind that these represent one of the three required paragraphs for sections 2c and 2d.2c. Evaluation of the Accuracy of the Forecasts of the 500 mb Height Pattern(sample paragraph for the 240 hour forecast based on maps shown in this section. Here we are comparing the 240 hour forecasts for 12 Z Feb 1 from the GFS [1st map in document] and ECMWF [2nd map in document] with the true 500 mb height pattern for 12 Z Feb 1 [3rd map in document above]).Both models forecasted a trough in the western US. The ECMWF was better with the trough, but both have errors. The trough forecasted by GFS was too broad (wide) and not cold enough. For the ECMWF, the 5340 meter closed low over Idaho was much further south (over northwestern Arizona) than forecasted, and it was not as cold in the northwest as forecasted. For the eastern part of the US, the GFS forecasted near to slightly below average heights, while the true map has near to slightly above average heights. It was actually well above average in the far northeast, which was not forecasted. The ECMWF forecast for the eastern US was too warm with a pronounced ridge over the great lakes, which was much weaker and shifted to the east in the true map. Overall, the ECMWF was slightly more accurate, though both model forecasts had significant errors.2d. Evaluation of the Forecasts for Tucson(sample paragraph for the 240 hour forecast based on maps shown in this document). The true 500 mb height over Tucson based on the verification map (directly above) is about 5460 meters (hard to read precisely, since height lines so packed over region). The ECMWF forecast for Tucson was better (5430), while the GFS heights were too high (5520). Both correctly forecasted a trough in the area, with a chance of precipitation and cold. Overall, the 240 hour ECMWF forecast for Tucson was better than the 240 hour GFS forecast. ................
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