Severe forecast — Illinois — 8 April 2008

Oh goodness, I haven’t done a severe forecast for a while.  Please be gentle…

The NAM and GFS are in pretty good agreement bringing a surface low through Illinois — of course Illinois is a big state.  The GFS brings the center of the low through the southern part of the state, while the NAM forecast brings the low further north, more towards the Quad Cities area.   The upshot of this is that the NAM brings more moisture into the area.  Moisture is traditionally the main concern this time of year.

Tomorrow’s setup bears this out pretty well.  Mid-50s surface dewpoints don’t look to arrive north of I-70 until at least mid-afternoon.  Currently, the surface Tds are around 32F through most of Illinois.  It’ll take some serious advection to get respectable Tds in time.  NAM forecased skew-Ts show some reasonably deep moisture, but the models seem to overdo moisture in these setups.  At best, we’re looking at LCLs around 1400m, which is generally too high for tornadogenesis.

Skew-Ts do show a happier picture when it comes to wind.  From 21 to 0Z, the low-level winds are very conducive to severe development.  The 1km and 3km helicity values are over 200 and 300 m^2/s^2 respectively.  If isolated storms can form, they’ll spin pretty well, but getting that rotation to the ground will be the hard part.  The NAM storm motion is a surprisingly slow 15-20kts, so it will be chasable.  Isolated storms won’t last long, though.

With the very strong LLJ and the low freezing levels, wind and hail will be the primary threat.  If you do plan on chasing, I expect the most fun to occur after 20Z in an area bounded by PIA-DEC-IKK-DNV-SPI.

Wow, that was total crap.  🙂  Here’s to getting more chances to practice!