RAP Grid Orientation Information


The following maps are provided to aid orientation with respect to the smoothed model topography:

Smoothed Topography Warning:   Use of an exact lat/long can be misleading in complex terrain

      Pilots are used to having a specific lat/long be associated with a real-world surface elevation, but a model uses smoothed topography and so can have a significantly different elevation at that point - and it will predict conditions based upon that elevation, not upon the real-world elevation.  In the presence of complex topography the model topography can differ significantly from the actual topography and when that occurs the model predictions should be interpreted relative to the model terrain, not with respect to the actual terrain.  This is the reason why the model topography, not the actual topography, is depicted on all BLIPMAP plots - this is a feature to help produce better forecast interpretations.

      One consequence of smoothed topography is that the influence of small real-world features is absent from the model predictions.  For example, pilots flying out of Minden are familiar with the Pine Nut Mountains to the east, which are often used a jumping off point.  But the Pine Nuts are too narrow to be represented in the smoothed topography so conditions over their ridgeline are simply not predicted.  The figures below compare the actual and model topography for the Minden area, where the Pine Nuts lie beneath the "M" in the actual topography image.

      Another consequence of smoothed topography is that there can be a prediction location "fuzziness" when the model surface elevation differs significantly from the real-world elevation.  Because the atmosphere varies much more rapidly in the vertical than in the horizontal, the model prediction which "best" fits a given real-world location will then not be exactly at the model lat/long identical to the real-world lat/long but instead at some nearby location where the model surface elevation more closely agrees with the surface elevation of the real-world location.  This "fuzziness" is negligible for relatively flat regions but can be significant where there is a rapid change in topographic height - in complex topography a displacement of 11 nm (1 grid point) is not unusual.  For example, the figure below illustrates the model topography for the Los Angeles region with the actual location of Hemet located at the "M" of "HMT".  This model location has a surface elevation of 2700 ft but the actual elevation of Hemet is 1512 ftMSL.  Hemet lies in LA's eastern basin just to the west of the San Jacinto Mts and what has happened is that the smoothing of those mountains has increased their width artificially, raising the surface elevation at Hemet's location.  In this case a better forecast would be obtained by using a location in what the model recognizes as the LA basin, i.e. just to the left of the "H" where the model surface elevation is much closer to the actual Hemet elevation. 

      Similarly, a model terrain ridgeline can be displaced from the location of the actual ridgeline, particularly if the ridge is very asymmetric as the Sierras are.  In such cases predictions are best made relative to the model ridgeline, not from the lat/long of the actual ridgeline.

      In short, one must be cautious when examining weather predictions at some exact lat/long, and this is the reason why the smoothed model terrain contours, not real-world terrain contours, are displayed on BLIPMAPs.  The "identified locations" map places identifiers at the actual lat/long associated with each location, but pilots are expected to have the sophistication to recognize that model predictions are obtained from smoothed topography and appropriately evaluate those predictions in light of that topography.  These model limitations are reduced when topography resolution is increased, as in the ETA model, but will always be there to some extent. 

      The sophistication required to use BLIPMAP predictions properly in the presence of smoothed topography effects can be likened to trying to center a thermal using a variometer - there is a valuable signal in the variometer readings, but they are best interpreted not from the instantaneous readings directly but by also considering the instrument's lag time and associating each reading with where the glider was at some time previously, not where it is now.  In other words, the values must be interpreted in light of other knowledge - which in this case is knowledge of the model topography - not simply read and used without further evaluation.

      Also note that BLIPSPOTs are plotted at the grid location actually utilized for their predictions which, also due to the smoothed topography, may differ by up to one grid point from the lat/long of the location after which they are named.


If an "Identified Locations" map do not exist for your region or if you would like to add plotted locations for your area:

To get location IDs from your state or area included on your regions's "Identified Locations" map, do the following:

  1. Let me know of the area you plan to cover (hopefully at least a quarter of the regional map), so that there will be no duplication of effort

  2. Create a text file with each line containing the location ID, name, latitude, longitude, and your name as per the following example.  Do not include locations which are very close to one another, since the identifiers will then overlap - to see the size of the plotted IDs, see this location plot example which is based on this input data example .  

          ID   LOCATION (with state)    LATITUDE LONGITUDE  YOUR_NAME (AND ANY OTHER USEFUL INFO) 
    
      MEV  Minden NV                39.0003  -119.7508  Jack Glendening                       
    
      TRK  Truckee CA               39.3200  -120.1396  Jack Glendening
  3. Send me your list by email - it's best to send a text file with your data as an attachment rather than as part of the email body, since the latter often garbages up the text columns (MS Outlook Express does, at least).  I will then create a plot from your data, post that map, and send you an email when that is done so you can view the resulting plot on the website.  If you are not satisfied with that map, send back an edited list and I will do a replot - but please do not expect me to do re-plot after re-plot.