As many people are now aware, this weekend looks to have some potent severe weather in the southern Plains. The SPC has a day three slight risk for a wide area of Nebraska, Kansas, Oklahoma and Texas on Saturday:
One method that I've occasionally mentioned for predicting the eventual outcome of a particular forecast is to look for analogs in the historical record. That is, to go back in time to when the atmosphere was in a similar state (or, better yet, when the model forecasted a similar state) and see exactly what ended up happening. There are groups that do this sort of thing in real-time. One is the Cooperative Institute for Precipitation Systems (CIPS) group at St. Louis University. With every model forecast they go back through the North American Regional Reanalysis archives and try to find days from the past when the forecasted pattern was similar. We can then look at what actually happened on those days to get a sense of what may happen now.
Take this Saturday, for instance. If we go to the CIPS southern plains sector, we see that their number one analog for the NAM forecast for that evening is 00Z, April 26, 1984. Here's the same four-panel plot as above, but showing what happened at that time:
Let's look at the second-best analog identified by CIPS: 00Z, April 10, 2011. Here's the pattern:
There were still hail reports in eastern Nebraska with tornado reports in western Iowa. There's another whole area of wind and hail (with a few isolated tornado) reports located across the southern Appalachians. It seems that subtle shortwave in the upper-level analyses over West Virginia-Virginia may have played a bigger role than one might have thought.
Finally, the third closest analog---00Z, May 4, 1999. That's the evening of May 3, 1999. You know what's coming. Here's the pattern:
The CIPS site also breaks out the analogs by individual variables and how closely they "match" the forecasted pattern. For instance, if we're interested in moisture return, the May 3-4 event has the highest correlation of these three in the surface dewpoint field. The 1984 event, however, has the higher correlations for overall upper-level patterns and winds. So it's a mix and match sort of deal...
How much can we trust these analogs to tell us what's going to happen? You can see that in just the top three closest events there are already notable differences between the patterns on those days and the current NAM forecast. Plus, as each new forecast run gets out, the closest analogs change. We also have only a limited record (some 40-50 years) that we can go back to look for close matches. Furthermore, in just three top analogs we've identified very different regions as areas of high impact. What we CAN say is that this sort of pattern definitely is conducive to severe weather, and in roughly the area the SPC has outlined. Beyond that, you still need your expert forecasting skills to help narrow down the exact areas favorable for severe weather with this particular setup.