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VIL Of the Day

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thunderman

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Hail can be a weather enthusiast delight during the Summer time (perhaps it reminds them of snow?). Unfortunately, forecasting hail size can be a bit of challenge.

One method that I use is VIL Of the Day (VOTD). It was developed by the NWS back in the 90s. It purely relies on temperatures aloft and while simple, does a surprisingly good job. Obviously there are some limitations, but I will get to those in a minute.

The calculation is straight forward and is as follows:

VOTD = 750 / [(h5T+h4T) / 2]

where h5T and h4T are the absolute values of the 500mb and 400mb temperatures respectively.

The VIL value given from this equation is the approximate value at which you can expect 0.75” diameter hail stones to fall. 0.75” was chosen because this used to be the hail criteria for a severe thunderstorm.

For operational purposes, the lower the expected VOTD the higher the large hail potential you can expect. I like to use 40 g/m^2 as a rough baseline in the Summer time.

For those that are not familiar with VIL (Vertically Integrated Liquid), it is a radar product that operational meteorologists use to locate areas of heavy precipitation and hail. It is available in real time in all GR-LevelX products and some places on the internet (Weather Underground, etc). See the wiki page for a brief overview.

Now back to the limitations of the VOTD approximation. It is best used for regular thunderstorms and not those with established mesocyclones (i.e. supercells) … although honestly it still works fairly well on most supercells we see around here. In storms with well-organized and established mesocyclones, there are additional mesoscale lifting mechanisms at work that can work to produce huge hail more efficiently than what you would tend to find in your average non-supercellular severe thunderstorm.

Keep in mind that the VOTD can change as the thermal profile aloft changes. Check forecast soundings (remember, there is more than 1 model) for the afternoon ahead instead of relying solely on the 12z observed soundings.

Lastly, VOTD gives you the VIL value for 0.75” hail stones and tells you nothing about maximum hailstone size, etc.

I have attached a calculator that I wrote to help me compute VOTD quickly. I wrote it a while back, it’s ugly, it’s written in FORTRAN, but it does the job!

votd.zip

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VOTD used to show up in AFDs back in the day, I guess when the VIL product was new, I rarely see it mentioned anymore.

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That's interesting. I'll have to try using that method...compared to the one I used as a student volunteer when I was at DTX and GRR. The way that I was taught was to take the height (in meters) of the freezing level...and multiply by 0.013. I still use this today...but just add 5 to whatever value the VOD was and it seems to work good. At my office we pay more attention to the height of the 50 dbz echos and have certain thresholds that we pop the SVR warning on.

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That's interesting. I'll have to try using that method...compared to the one I used as a student volunteer when I was at DTX and GRR. The way that I was taught was to take the height (in meters) of the freezing level...and multiply by 0.013. I still use this today...but just add 5 to whatever value the VOD was and it seems to work good. At my office we pay more attention to the height of the 50 dbz echos and have certain thresholds that we pop the SVR warning on.

Yeah the 50 dbz rule works well.  I use it often as well when trying to to determine severity.

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