Virtual ISC Weather Elements

The Show ISC capability simply compares one weather element against another. For certain elements, a virtual weather element is calculated and that is what is displayed against the Fcst database. For example, QPF is a rate-dependent weather element. It doesn't make sense to display your 12-hr QPF Fcst grid against a 1-hr QPF ISC grid, since the time periods are not equal. Using the virtual weather element capability in the GFE, a new weather element is calculated that will display your 12-hr QPF Fcst grid against a virtual 12-hr QPF ISC grid.  The Virtual 12-hr QPF ISC grid was calculated from the multiple ISC grids in the database. For ISC, there are virtually calculated weather elements for QPF, SnowAmt, MaxT, MinT, and PoP as shown in the following table:

Weather Element
 Algorithm
QPF
 The virtual ISC grid is always the same duration as the Fcst QPF grid. The Virtual ISC grid is the rate-summation of the ISC QPF grid.
SnowAmt
 The virtual ISC grid is always the same duration as the Fcst SnowAmt grid. The Virtual ISC grid is the rate-summation of the ISC SnowAmt grid.
MaxT
 The virtual ISC grid is always the same duration as the Fcst MaxT grid. It contains the maximum MaxT value gridpoint by gridpoint from the ISC MaxT grids valid over the same period. Normally there is only 1 MaxT grid, except in cases where the office is in a different time zone or adjacent offices have configured their MaxT time constraints differently.
MinT
 The virtual ISC grid is always the same duration as the Fcst MinT grid. It contains the minimum MinT value gridpoint by gridpoint from the ISC MinT grids valid over the same period. Normally there is only 1 MinT grid, except in cases where the office is in a different time zone or adjacent offices have configured their MinT time constraints differently.
PoP
 The virtual ISC grid is always the same duration as the Fcst PoP grid. It contains the maximum PoP value gridpoint by gridpoint from the ISC PoP grids valid over the same period.

The following illustration shows the purpose of the Virtual ISC weather elements. In the case on the left when the virtual weather element is not used, the forecaster will be comparing QPF from Fcst and ISC for different time periods. In the case on the right, the virtual ISC grid is calculated from the rates of all overlapping ISC QPF grids. The duration of the Virtual ISC grid is programmed to exact match the duration of the Fcst grid.


The illustration below assumes a constant rain rate of 0.10"/hour over the entire ISC and Fcst grid. The Fcst grid duration is 7 hours, thus that grid will show 0.70" of precipitation. The three ISC grids are 2 hours, 3 hours, and 4 hours in duration, and at a constant rain rate of 0.10"/hour contain 0.20", 0.30", and 0.40" respectively.

The virtual QPF grid is programmed to match the duration of the Fcst grid, and thus it is 7 hours in length. The precipitation rate for each overlapping ISC QPF grid is calculated and that portion of the time period that overlaps is summed into the virtual ISC grid.

When the user enables "Show ISC Mode", they will be comparing the precipitation for the same duration as the Fcst grid. In this specific example, the Virtual ISC Grid will show 0.70" of precipitation due to the constant rain rate of 0.10".