Effect on forecast — A quick look

For a quick sense of forecast impact, a loop of 700 hPa absolute vorticity is shown below. The maximum vorticity resulting from grid-scale convection with vertical motion representing a 22 km x 22 km area is near 700 hPa, though you'll see signatures at 500 hPa too for cases like this. (You may also view a very detailed explanation of why it's at 700 hPa).

The maps are every three hours, with precipitation accumulating during that three-hour period contoured at 0.01, 0.10, 0.25, 0.5, 1.0, 1.5, and 2.0 inches. These data are from output on the 40-km AWIPS grid #212.

The first night, the model generates an intense vorticity maximum in the location of heaviest precipitation over South Dakota and a weaker vorticity line associated with the model precipitation axis in Nebraska. During the succeeding twelve hours, these weaken, though an area of precipitation and residual vorticity continue to move northeast across North Dakota and northern Minnesota. The second night, a smaller strong vorticity maximum develops with the heavy rain over Kansas and weakens as it tracks across Missouri. Generally, we are seeing the forecast impacts confined to a mesoscale region around and downstream of the event and fading to background values by 12 to 18 hours following the end of the event.

The scale of the event, which is tied to the model resolution, determines the scale and duration of the forecast impact. An isolated grid-point blow-up in a 10-km Eta "threats" run causes a dynamic response primarily consisting of gravity waves, resulting in mostly a local and short-lived forecast impact and some minor noisy fluctuations extending over a broader area. A blow-up over several adjacent grid boxes in the T170 AVN, with its physics grid at roughly 80 km spacing over the CONUS, causes a more balanced dynamic response resulting in longer-lived impacts on surface wave development and synoptic systems. What we see here in the 22-km Eta model is a mesoscale forecast impact of less than one day.