Collaborate_PoP_SnowAmt_QPF

Introduction

With the introduction of IFPS 11, the PoP, SnowAmt, and QPF grids are now floating, i.e., they are no longer fixed to specific 6 or 12 hour intervals. While this change allows for more flexibility in forecast preparation, it also creates two key problems for forecasters. First, the duration of the PoP, SnowAmt, and QPF grids now varies considerably from office to office, making the intersite coordination of these elements much more difficult. Second, the NDFD Integrated Work Team has recommended that the graphics for these elements still need to be valid for fixed durations (12 hours for the PoP graphics and 6 hours for the SnowAmt and QPF graphics).

The Collaborate_PoP_SnowAmt_QPF procedure addresses both of these forecast issues by creating fixed 12-hour PoP (PoP12hr), 6-hour SnowAmt (SnowAmt6hr), and 6-hour QPF (QPF6hr) grids based on the values contained in the floating PoP, SnowAmt, and QPF grids. These new grids may then be used for the purposes of intersite coordination as well as web graphic generation.

In accordance with the NDFD, PoP12hr grids are created at 12-hour intervals through 7 days, QPF6hr grids are created at 6-hour intervals through 72 hours, and the SnowAmt6hr grids are created at 6-hour intervals through 36 hours.

The procedure calls several Smart Tools which populate the PoP12hr, SnowAmt6hr, and QPF6hr grids with data from the floating PoP, SnowAmt, and QPF grids. The getMaxGrid Smart Tool reads the floating PoP grids and assigns the maximum PoP value for each 12-hr period to the PoP12hr grids. The getSumGrids Smart Tool is used to sum the SnowAmt and QPF grids into the SnowAmt6hr and QPF6hr grids. Note that these two Smart Tools are designed to be called from a procedure and will generate an error if run interactively from the Edit Actions dialog. A third SmartTool, MakeTmpGrid, is a generic SmartTool for creating a temporary grid from the active grid. This is used in this procedure to copy and fragment the QPF and SnowAmt grids. The procedure deletes these temporary grids when they are no longer needed within the tool.


How the Procedure Works

All of the floating PoP, SnowAmt, and QPF grids have been created before running the procedure/tools. The PoP12hr, SnowAmt6hr, and QPF6hr elements exist in GFE.

After creating and saving the floating PoP, SnowAmt, and QPF grids, the forecaster need only run the Collaborate_PoP_SnowAmt_QPF procedure once to generate the PoP12hr, SnowAmt6hr, and QPF6hr grids needed for intersite coordination.

When the forecaster runs the procedure, he/she is presented with a dialog box that allows a choice of "All" or "Selected Time". If All is chosen, the procedure will generate the collaboration grids over preset time ranges. These time ranges begin with the current time and extend out to 192 hours for the PoP12hr grids, 60 hours for the SnowAmt6hr grids, and 96 hours for the QPF6hr grids. These ranges are longer than the required minimum collaboration times to account for the different times during the day when new grids may be added on to the end of the forecast database. If Selected Time is chosen, only grids that intersect the active time selection in the grid manager will be used to compute PoP12hr, SnowAmt6hr, and QPF6hr grids. Regardless of which option is selected, the procedure checks for the existence of the source grids so errors will not be generated if grids are missing from the end of the time range. Errors are generated if the floating grids are missing within a selected time range.

Notable Methodology/Assumptions Used
When the Collaborate_PoP_SnowAmt_QPF procedure is run by the forecaster, the floating SnowAmt and QPF grids are first copied to two temporary elements, tmpSnowAmt and tmpQPF. The tmpSnowAmt and tmpQPF grids are then automatically fragmented before being used by the getSumGrids Smart Tool to create the SnowAmt6hr and QPF6hr grids. The usage of the temporary elements in this process allows the original floating SnowAmt and QPF grids to be preserved; the fragmenting ensures that no tmpSnowAmt grid overlaps multiple SnowAmt6hr grids/no tmpQPF grid overlaps multiple QPF6hr grids prior to running the getSumGrids tool. This latter condition is important because such overlapping grids would cause the tool to produce inaccurate SnowAmt6hr and QPF6hr grids. Once the getSumGrids Smart Tool has executed and the SnowAmt6hr and QPF6hr grids have been produced, the temporary elements are automatically deleted.

To better illustrate the significance of fragmenting the tmpSnowAmt and tmpQPF grids, let=s consider a simple example utilizing 3 forecast tmpSnowAmt grids. For the sake of simplicity, we will assume that the default duration for the tmpSnowAmt grids is 3 hours, and that both the tmpSnowAmt and SnowAmt6hr grids are based on Greenwich Mean Time (GMT). Note that the following also applies to tmpQPF even though it is not discussed here. The forecast tmpSnowAmt grids are as follows:

Grid 1 - Valid 12-15 GMT, amount = 0" throughout
Grid 2 - Valid 15-21 GMT, amount = 4" throughout
Grid 3 - Valid 21-24 GMT, amount = 0" throughout

If these grids are not fragmented before running the getSumGrids Smart Tool, the tool will see a tmpSnowAmt grid with 4" accumulations that overlaps both the corresponding 12-18 GMT and 18- 24 GMT SnowAmt6hr grids - and will therefore assign 4" accumulations to both SnowAmt6hr grids. This in turn would imply an event total of 8" - not exactly what had been intended by the forecaster.

On the other hand, fragmenting the tmpSnowAmt grids before running the getSumGrids Smart Tool results in the following grids as input to the tool:

Grid 1 - Valid 12-15 GMT, amount = 0" throughout
Grid 2 - Valid 15-18 GMT, amount = 2" throughout
Grid 3 - Valid 18-21 GMT, amount = 2" throughout
Grid 4 - Valid 21-24 GMT, amount = 0" throughout

Note that the 4" tmpSnowAmt grid valid from 15-21 GMT has been split into two three-hour grids containing values of 2" throughout. This is because GFE assumes a constant snowfall rate when fragmenting the grids. Naturally, this isn't always a valid assumption, but its currently the only one available for the software to work with.

Executing the getSumGrids Smart Tool after fragmenting results in 2" accumulations in the corresponding 12-18 GMT and 18-24 GMT SnowAmt6hr grids, which would imply an event total of 4" - thereby matching the original intent of the forecaster.