April 28, 2010

It's the most wonderful time of the year...

April 29th Forecast:
The placement of the secondary sfc low is going to be pivotal in whether or not tomorrow is a tornado day. Most of the recent model runs (including the new GFS) have developed it further down the front, in the Panhandles, but the new 0z NAM puts it in wcntrl KS. If this actually happens, it would really help out with low-level convergence, and also (hopefully) slow the forward speed of the cold front which would extend the tornado window.

This initial shortwave over the nrn Plains will pull moisture rapidly northward throughout the day but will also act to slightly veer the LL winds in KS/NE. If the secondary low does develop further north, as the 0z NAM suggests, hodograph shapes in the lowest 1km will be much more favorable for tornadoes before dark from cntrl KS northeastward into wrn IA.

Marginal low-level moisture will be a limiting factor for tornadoes but, regardless, there is going to be some WICKED structure out there somewhere. My initial target will be in southeast NE, but I will adjust west from there if initiation appears likely at the cold front/dryline intersection north of Salina. Storms will be moving east rather quickly so my options will be open if something fires to the west.

April 30th Forecast:
Feel free to call it a geographic bias and stay away, but I'm really liking the ern KS target for Friday. Some precip forecasts (including the 0z NAM) have been clearing the squall line out of this area by late morning, which combined with strong moisture advection will allow for rapid destabilization. Storms would fire first along the surface front in northeast KS into southern IA in late afternoon and then possibly southward, along the dryline as the cap erodes. +80 kts (!) of effective shear along with moderate instability will promote the rapid development of rapidly moving supercells. We've got climatology and synoptics pointing to a big tornado threat over a big area, we'll just have to wait and see how the mesoscale plays out.

Again, the placement of the secondary surface low along the front will dictate the amount of low-level turning and the overall tornado threat. If the track is similar to what the NAM is thinking, it could be a big tornado day in the KC area.