Monday, November 18, 2013

Severe, tornadic storms and upper-level forcing

Just thought I'd quickly share this one screen capture I grabbed yesterday during the unusual November tornado outbreak over the Midwest.  I find myself more and more often using Weather Underground's "Wundermap" tool to overlay weather model output on current observations, satellite and radar to try and both see how well the model is doing and make connections between model features and what's actually happening.  Here we see the composite radar image over the continental US overlaid on top of the ECMWF 300mb forecast for around 1 PM CST on Sunday.  The 300mb wind speeds are shown in color with the 300mb height lines shown as the white contour lines.

You can really see how these lines of convection in the radar imagery formed along the exit region of the upper-level jet streak.  This is a favorable location for divergence aloft, particularly when the jet streak has cyclonic curvature as it does here.  Furthermore, the surface cold front associated with this storm was actually back over central Illinois at this time.  It was upper-level support out ahead of the cold front---this strong jet streak coming around the base of the trough---that provided a lot of the forcing to get this convection going.  Of course, once these storms organize into line segments they can be self-reinforcing in generating enough lift to keep themselves going.  But once again, this emphasizes how understanding what's going on in the upper-levels (something our models forecast fairly well) can be both useful and important for understanding when and where convection is going to develop (something our models don't do nearly as well).

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