Question about the estimated EHI for St Louis EF-4

[Rob H]

Looking back at last night's tornado and the archived data from the SPC Mesoanalysis page, I came across something that doesn't make sense:

It looks like a small dot where the 0-1km EHI is 10-11.

EHI is CAPE * SRH / 160,000, so given roughly 2000 J/Kg CAPE, and 400 m2/s2 0-1km SRH, you get an EHI of 5, which seems completely plausible given the events that took place and the forecasted parameters.

Let's be super generous, and say that there was perhaps upwards of 3000 J/Kg CAPE - that would still require SRH to be almost 600 m2/s2. Any thoughts on whether this was an accurate estimate of EHI, and if not, where bad values were introduced?

 

[Jeff Duda]

By looking at the contours on the map,it looks like the 0-1 km EHI on the north side of St. Louis is about 9. I'm sure EHI is computed by gridpoint using the formula, so for it to be 9 - 11, so there is probably no error in the computation of EHI. RUC analyses are not perfect, however, and can contain errors. The actual gradient of EHI along the warm front is probably tighter than what is shown on the map due to the grid spacing of the analyses (13 km right now, I believe). It sure seems to me that an EHI of 9 - 11 would correspond well with conditions that produce significant tornadoes. What you might want to look at as well is 0-3 km CAPE. I believe that the lower the LFC and the lower the CAPE the more likely strong low-level SRH is to result in a strong tornado.

 

[GPhillips]

In the area just south of STL, where the EHI maxed out at between 10 and 11, the SBCAPE was between 3000 and 3500 and the 0-1km SRH was about 500. This gives approximately (500 x 3250) / 160000 = 10.15. So, it looks correct.

 

[Rob H]

Thanks for the replies, I hadn't thought that the regional maps would show that much more detail - from the CONUS maps it looked like 2000 and 400 respectively. A good lesson in using indexes, model output, and view. Interesting note about the 13km resolution - I wasn't really thinking that would matter in this case, but when you're that close to the warm front, 13km could almost be the difference between southerly and easterly winds!

Where do you find the archived images by sector, I could only find CONUS?

 

[GPhillips]

Where do you find the archived images by sector, I could only find CONUS?

Go to, for example, http://www.spc.noaa.gov/exper/mesoanalysis/new/viewsector.php?sector=14

Choose the parameter you want to view. Click on "Recent Image Archive & Loops". Under "Loop Options", choose the ending time of the archived loop you want to view.

It's not simple to switch among the various parameters for the same archived time. You may need to "Return to Real Time Viewer", then choose another desired parameter, then go back through what you went through to view the previous archived loop of that parameter.

 

[Jeff Duda]

You really can't view archived images by sector from this site once they're pushed off the backlog that GPhillips mentioned. You can only view CONUS images.