BLIPMAP Prediction Parameters and Description

 BLIPMAP = Boundary Layer Information Prediction MAP
The atmospheric Boundary Layer (BL) is the region mixed by thermal eddies


To access the forecasts of parameters described here, see the appropriate regional BLIPMAP link


THERMAL PARAMETER FORECASTS:
Thermal Updraft Velocity (W*)     [FREE]       CurrentDay      Current+1      Current+2     
Average dry thermal updraft strength near mid-BL height.  Subtract glider descent rate to get average vario reading for cloudless thermals.  Thermal strengths will be stronger if convective clouds are present.  W* depends upon both the BL depth and the surface heating.  MoreInfo
Height of Boundary Layer Top (TI=0 height)      CurrentDay      Current+1      Current+2     
Height of the average dry thermal tops, or Thermal Index TI=0 height.  Over flat terrain maximum thermalling heights will be lower due to the glider descent rate and other factors.  However, thermal tops will be higher over small-scale topography not resolved by the model and some pilots have reported that in elevated terrain the heights they can reach over local terrain features correspond better with the TI=0 height than with Hcrit.  In the presence of clouds the thermal top will increase, but the maximum thermalling height will then be limited by the cloud base (see the "Cloud prediction parameters" section below).  [This parameter is truncated at 22,000 for plotting.]  MoreInfo
Height of Critical Updraft Strength (Hcrit)      CurrentDay      Current+1      Current+2     
This parameter estimates the height at which the average dry updraft strength drops below 225 fpm and over flat terrain is expected to give better quantitative numbers for the maximum cloudless thermalling height than is the traditional TI=0 height given above, although the qualitative patterns should be similar for both parameters.  (Note: the present assumptions tend to underpredict the max. thermalling height.) In the presence of clouds the maximum thermalling height may instead be limited by the cloud base (see the "Cloud prediction parameters" section below).  [This parameter is truncated at 22,000 for plotting.]  MoreInfo
Thermal Height Variability      CurrentDay      Current+1      Current+2     
This parameter estimates the variability (uncertainty) of the BL top (TI=0) height prediction which can result from meteorological variations.  Larger values indicate greater variability and thus better thermalling over local "hot spots" or small-scale topography not resolved by the model.  But larger values also indicate greater sensitivity to error in the predicted surface temperature, so actual conditions have a greater likelihood of differing from those predicted.  MoreInfo
Buoyancy/Shear Ratio (B/S)     [FREE]       CurrentDay      Current+1      Current+2     
Dry thermals may be broken up by wind shear and unworkable if B/S ratio is 5 or less.  If convective clouds are present, the actual B/S ratio will be larger than calculated here.  [This parameter is truncated at 20 for plotting.]  MoreInfo

WIND PARAMETER FORECASTS:
Wind Speed in the Boundary Layer      CurrentDay      Current+1      Current+2     
The speed of the vector-averaged wind in the BL.  This prediction can be misleading if there is a large change in wind spped or direction through the BL (for a complex wind profile, any single number is not an adequate descriptor!).  MoreInfo
Wind Direction in the Boundary Layer      CurrentDay      Current+1      Current+2     
The direction of the vector-averaged wind in the BL.  This prediction can be misleading if there is a large change in wind direction through the BL (for a complex wind profile, any single number is not an adequate descriptor!).  Note that there will be a abrupt artificial gradient at the "cross-over" between 0 and 360 degrees.  MoreInfo
Wind Shear in the Boundary Layer      CurrentDay      Current+1      Current+2     
The magnitude of the vector wind difference between the top and bottom of the BL.  Note that this represents vertical wind shear and does not indicate "shear lines" (which are horizontal changes of wind speed/direction).  MoreInfo
BL Max. Up/Down Motion (BL Convergence)      CurrentDay      Current+1      Current+2     
Maximum grid-area-averaged extensive upward or downward motion within the BL as created by horizontal wind convergence.  Positive convergence is associated with local small-scale convergence lines (often called "shear lines" by pilots) - however, the actual size of such features is much smaller than can be resolved by the model so only stronger ones will be forecast and their predictions are subject to much error.  If CAPE is also large, thunderstorms can be triggered.  Negative convergence (divergence) produces subsiding vertical motion, creating low-level inversions which limit thermalling heights.  This parameter can be noisy, so users should be wary.  MoreInfo

CLOUD PARAMETER FORECASTS:
Cumulus Potential      CurrentDay      Current+1      Current+2     
This evaluates the potential for small, non-extensive "puffy cloud" formation in the BL, being the height difference between the surface-based LCL (see below) and the BL top.  Small cumulus clouds are (simply) predicted when the parameter positive, but it is quite possible that the threshold value is actually greater than zero for your location so empirical evaluation is advised.  I would be interested in receiving end-of-season reports on what threshold value worked for your site.  Clouds can also occur with negative values if the air is lifted up the indicated vertical distance by flow up a small-scale ridge not resolved by the model's smoothed topography.  [This parameter is truncated at -10,000 for plotting.]  MoreInfo
Cumulus Cloudbase (Sfc. LCL)      CurrentDay      Current+1      Current+2     
This height estimates the cloudbase for small, non-extensive "puffy" clouds in the BL, if such exist i.e. if the Cumulus Potential parameter (above) is positive or greater than the threshold Cumulus Potential empirically determined for your site.  The surface LCL (Lifting Condensation Level) is the level to which humid air must ascend before it cools enough to reach a dew point temperature based on the surface mixing ratio and is therefore relevant only to small clouds - unlike the below BL-based CL which uses a BL-averaged humidity.  However, this parameter has a theoretical difficulty (see "MoreInfo" link below) and quite possibly that the actual cloudbase will be higher than given here - so perhaps this should be considered a minimum possible cloudbase.  I would be interested in receiving end-of-season reports on how well this parameter worked for your site.  [This parameter is truncated at 22,000 for plotting.]  MoreInfo
OvercastDevelopment Potential      CurrentDay      Current+1      Current+2     
This evaluates the potential for extensive cloud formation (OvercastDevelopment) at the BL top, being the height difference between the BL CL (see below) and the BL top.  Extensive clouds and likely overcast development are predicted when the parameter is positive, with overcast development being increasingly more likely with higher positive values.  OvercastDevelopment can also occur with negative values if the air is lifted up the indicated vertical distance by flow up a small-scale ridge not resolved by the model's smoothed topography.  [This parameter is truncated at -10,000 for plotting.]  MoreInfo
OvercastDevelopment Cloudbase (BL CL)      CurrentDay      Current+1      Current+2     
This height estimates the cloudbase for extensive BL clouds (OvercastDevelopment), if such exist, i.e. if the OvercastDevelopment Potential parameter (above) is positive.  The BL CL (Condensation Level) is based upon the humidity averaged through the BL and is therefore relevant only to extensive clouds (OvercastDevelopment) - unlike the above surface-based LCL which uses a surface humidity.  [This parameter is truncated at 22,000 for plotting.]  MoreInfo
BL Max. Relative Humidity      CurrentDay      Current+1      Current+2     
This parameter provides an additional means of evaluating the formation of clouds within the BL and might be used either in conjunction with or instead of the other cloud prediction parameters.  Larger values indicate greater cloud probability, but use of this parameter must be empirical since no theoretical guidance is available - for example, pilots must determine by actual experience the percentage that correlates with formation of clouds above local mountains.  The cloud base height is not predicted, but is expected to be below the TI=0 height.  MoreInfo
CAPE      CurrentDay      Current+1      Current+2     
Convective Available Potential Energy indicates the atmospheric stability affecting deep convective cloud formation above the BL.  A higher value indicates greater potential instability, larger updraft velocities within deep convective clouds, and greater potential for thunderstorm development (since a trigger is needed to release that potential).  Note that thunderstorms may develop in regions of high CAPE and then get transported downwind to regions of lower CAPE.  Also, locations where both convergence and CAPE values are high can be subject to explosive thunderstorm development.   MoreInfo
The following experimental cloud parameters are available only from NAM model forecasts:
Total Cloud Cover      CurrentDay      Current+1      Current+2     
Total cloud cover in percent.  DrJack has his doubts about the usefulness of this parameter since very thin high clouds can produce a "100% cloud cover" even though considerable solar radiation still reaches the surface (see the following parameter).  But I will include it for now.
Surface Sun      CurrentDay      Current+1      Current+2     
Solar radiation reaching the surface.  This indicates a "degree of cloudiness", which is dependent upon cloud depth as well as sky cover fraction, since clouds are principally responsible for radiation not reaching the surface.  Comparison to the "Surface Heating" parameter below indicates the solar radiation fraction which is transferred into heating the atmosphere.

FUNDAMENTAL BL PARAMETER FORECASTS:
Boundary Layer Depth      CurrentDay      Current+1      Current+2     
Depth of the layer mixed by thermals.  This parameter can be useful in determining which flight direction allows better thermalling conditions when average surface elevations vary greatly in differing directions.  MoreInfo
Surface Heating      CurrentDay      Current+1      Current+2     
Heat transferred into the atmosphere due to solar heating of the ground, i.e. the heating that creates thermals.   [This parameter is truncated at -100 and +1000 for plotting.]  MoreInfo
Surface Temperature      CurrentDay      Current+1      Current+2     
This model-predicted surface temperature can be compared to the actual temperature at 2m during the day to evaluate the accuracy of model heating predictions.
Experimental      CurrentDay      Current+1      Current+2     
The parameter displayed here varies, often being one used for testing. 

INFORMATION
FORECAST DESCRIPTION:

    BLIPMAPs predict thermal soaring conditions resulting from surface heating of the Boundary Layer (BL), the scientific term for the turbulent atmospheric region mixed by surface-based thermals (so thermal tops occur at the top of the BL).  The BLIPMAP program post-processes numerical weather model predictions to provide parameters suited to the needs of soaring pilots and presents them in graphical format, as in this sample BLIPMAP.  Relative differences, both in location and in time, are expected to be more reliable indicators of soaring differences than are the precise numerical values.
    A continuing sequence of forecasts, all for the same validation time of that on the regional BLIPMAP webpage, are produced as new observational data becomes available (each updated forecast is for a shorter forecast period).  The "CurrentDay" forecasts are based upon the current GMT day given above, which changes at 0Z, with "Current+1" and "Current+2" giving predictions for the following two days.  The last link, "PreviousDay", gives the last forecast from the previous day's prediction sequence, useful for those who want to compare their experiences from yesterday's flight to the NAM BLIPMAP predictions.  At the top of each forecast plot is the name of the parameter, the validation date and time, the forecast period, and the data source. 
    The parameters are averages over 12 km grid squares as forecast by the NOAA's National Centers for Environmental Prediction (NCEP) NAM model. Other NAM forecasts are also available from various sources, including Unisys NAM forecast maps, , and .  NAM BLIPMAPs are updated as new NAM forecasts become available.
    The parameter values are represented by color hues which increase in "warmness" as the value increases in magnitude.  When many gradations are required, two cycles are made through the color wheel with "light" and "intense" tints respectively representing ranges of lower and higher magnitude. A screen magnifying tool, such as the freeware Super Magnify for Windows machines or Xzoom for X11/Linux/Unix machines, helps when discrimination between adjacent contours is difficult.
    State outlines are depicted on each BLIPMAP in white.  The NAM topography is plotted as black contours, at 500 ft intervals with thicker lines at 2000 ft intervals, to assist in location identification but also to emphasize the smoothed nature of the model topography.  The BLIPMAP does not predict thermal lift created by small-scale terrain features which are not resolved by the model topography, which often give localized updrafts significantly stronger than those over the surrounding smoother terrain. 
    NAM BLIPMAPs can also be viewed using a regional BLIPMAP Viewer or a UniViewer - both can create a BLIPMAP image sequence. The viewers are most useful for quickly cycling between or through BLIPMAPs since the images exactly overlie - but some might just consider them simpler to use than this index page.  Alternate graphical display formats are also available using BLIP data display software. 


Notes:

    As with all weather products, users should check the date on each map for currency.  Small anomalous diamonds, the size of an individual model gridpoint, may appear in the plots, particularly for more sensitive parameters such as convergence or cloud parameters; these result from numerical noise and should be disregarded.
    NAM BLIPMAPs are still in development and there will likely be problems, changes, and tweaks.  Opinions on factors affecting its usability are solicited. 

Medal     Heros: I personally thank those whose support is listed on the Contributors webpage.