May 15, 2012


This is the well-known Great Plains tornado pattern.  Identified in the 1950s, it consists of a well-developed surface wave cyclone surmounted by strong jet stream winds.  Ahead of the dry line, a deep moist air mass moves northward under these jet stream winds, which create the wind shear that contributes to rotating updrafts, when storms develop in that moist flow.

Supercells are most likely to form at A, multi cells at B and common thunderstorms at C.  The models are able to anticipate this type of pattern well in advance, which is why it is also known as a "synoptically-evident" pattern.

As an example I have provided the SPC outlook for April 14, 2012 (the day of the major tornado and severe thunderstorm outbreak in the central and southern Plains.  SPC had nailed down the region of greatest risk seven days in advance.  The last graphic shows the verifying tornado and severe thunderstorm reports.

What is key about the pattern, however, is that it brings together the ingredients for rotating deep moist convection.   It turns out there are many other patterns that can bring these ingredients together and there are local factors that can make up for the factors commonly observed with the "synoptically-evident" pattern.

The trouble with assessing those is that the patterns are difficult to anticipate long in advance because the phasing of the surges of the different ingredients is so critical.

So many chasers, including non-meteorologists, were out to observe the storms in KS in mid April partially because it was such an evident pattern, and the SPC website was advertising it long in advance.    On the other hand, there are subtle factors at work with the patterns next weekend and beyond.  While cyclones are emerging from the Rockies, the influx of moist air will be difficult to time and anticipate.   Contrast SPC's current 4-8 day outlook for this upcoming weekend and beyond, with that shown above.  Note "Predictablity Too Low".  This means that the pattern will be favorable for severe storms but the phasing and magnitude of the ingredients are too uncertain at this time.   

Severe weather meteorologists who storm observe and chase like this kind of challenge.  But it also means that people depending upon synoptically-evident patterns to be advertised by SPC won't be making plane reservations because they will be less likely to be aware of the potential.