Limitations of WarnGen polygons Part II: Slide to the right.

Let’s address the second and third shortcomings of WarnGen, (b) There is no way to adjust the motion vector for uncertainty.  This must be done as a guess by manually editing the polygon vertices; (c) There is no way to change the default choice for the initial or projected threat areas.  The same parameters (10km, 15km) are always used, and a square area is always the default.

In WarnGen, the forecaster can only choose one motion vector, and is limited to only one expression of uncertainty in the motion – the increase in the size of the warning area to a square with an area 225% larger than the initial threat area square.  What if the forecaster determines that the storm is moving into an environment that is supportive of an evolution to a supercell, and the storm slows its forward speed and turns to the right (increasing azimuth in the motion vector)?  The practice that is typically being suggested is to edit the vertices of the polygon to include more area to the right of the motion vector:


But how does the forecaster determine how far to the right to re-position the vertex?  It’s another guess!  Just click the vertex, drag it until it “looks about right”.

What would be the more robust way to determine how far to stretch the polygon?  By being able to input information about the motion uncertainty in the form of a possible variance in the motion vector.  For example, if the forecaster thinks a storm has a chance to slow and move to the right of the current motion vector based on the “30R75” technique (30 degrees to the right, 75% of the speed), then that information could be input to determine the possible locations of the threat area at the extrapolated final position of the threat.  Here, I’ll only show the default threat position, and the 30R75 threat position:


This looks similar to the “skewed coffin technique” offered by Nietfeld, but in this case, the position of the second solution is based on actual motion uncertainty numbers input by the forecaster, rather than a guess.

This is still not the ideal solution for determining the swath shape based on storm motion vector uncertainties.  We’ll get to that soon.

Greg Stumpf, CIMMS and NWS/MDL

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