Friday, April 23, 2010

The day after tornado season started

An interesting severe weather day. Big threats for supercells in multiple regions.

The models gave us false impressions of the cap. This is a case for extensive numerical modeling of a null event in Nebraska and Kansas. The water vapor imagery combined with multiple soundings available for analysis make this case worthwhile to explore numerically.

The instability and moisture were well predicted. It appears the shear was well predicted. The timing in the NSSL and NCAR WRF models was too early and too widespread. I want to say that the models ejected the long wave trough out too quickly...but is a few meters per second really too quickly in phase speed? Or perhaps the models did not capture the vorticity evolution on the back side of this large rossby wave?
anyway the cap issue is nicely shown by the Topeka sounding (where the action was predicted by 21-00 UTC.
Note the low level cloud layer. There appeared to be shallow cumulus. Does parcel theory collapse when a cloud layer is already present? To speculate is poor science, but ideas emerge from speculation. Perhaps the cloud layer is so dynamic (entrainment at the PBL top, less than well mixed below) that parcels can only emerge or organize with the thermodynamics of the cloud layer. In that case, a parcel lifted from the cloud layer would immediately have convective inhibition.

Dr Adam Houston has published some work indicating that the lapse rate above the LCL and LFC might be very important here as well. The steeper the better which is not really observed on this sounding. Another factor may very well be the path that parcels take. That is the forcing for ascent locally may not have been favorable despite the PBL circulations.

Clearly though, the dry layer alofyt at TOP was associated with a dry intrusion of very limited extent as the inversion is not present just 4 hours later at any station. water vapor loops indicate the invsersion may have been present from 1845 to 2045 and was weakening. It was oriented north to south ... probably why SGF showed only a hint of it and why it wasnt seen elsewhere.

As the event unfolds now, Iowa and Nebraska are just now seeing the late evening initiation I was expecting if the afternoon stuff didnt go. I have some anecdotal evidence that when the warm front in Iowa is involved, chances are the cap holds until night time. I wish I could explore that issue further.

Not too much actual severe weather to report on. 8 tornado reports in 4 states.