Show Walter THIS Severe Index!

Programmer and meteorologist Dave asked me yesterday if I had seen the Showalter Index. So I said "Show Walter what now?" because we used to have a forecaster here named Walter.

Turns out Dave was talking about the new Showalter Index (SWI) maps that he recently added to the Pro Site under the NAM, NAM-WRF, RUC, and GFS models. This severe weather index is one of many tools that you can use to forecast thunderstorms and tornadoes.

There are various interpretations of the numbers (see below) but the more negative, the better. This morning's NAM model forecast Showalter Index for 21Z tonight (late afternoon, typical severe weather outbreak time) shows a minimum of -10 in northeast Texas:

And the GFS model shows a -12 in south-central Oklahoma:

Both of those disagree with the Moderate Risk area outlined by AccuWeather...

...and the one by the NWS's Storm Prediction Center, though they are within the Slight Risk area of both. But remember, this is only one of many indexes, so those meteorologists were probably looking at other indexes or data that they believed held more weight for today's particular situation. UPDATE: Henry just pointed out that the Moderate risk may have only been for this morning's system that was moving through Arkansas, and in fact the SPC has dropped the moderate risk this afternoon.

We have a listing on my severe weather indices page showing what the numbers mean.

Ohio State points out on their Showalter help page, which has a slightly different interpretation of the numbers:

Meteorologist Jeff Haby goes in-depth about SWI on his site and offers an alternate list of values and what they mean.

The National Weather Service offers a fourth interpretation of the numbers in their glossary.

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