Monday, February 4, 2008 - 11:03PM   
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Tuesday forecast and chase plans
Tonight's model runs are out, and the NAM/WRF and GFS are in remarkable agreement as to the values and placement of tomorrow's parameters. 60F dewpoints will be pulled as far north as southern Indiana/Illinois and western Kentucky. The main low pressure center will track northeast through the Mississippi Valley as a deep mid- and upper-level trough spreads very strong winds above the surface moisture. At the surface, winds will be generally southerly under the southwesterly mid and upper-level flow - resulting in OK, but not great, low-level shear. The precip output suggests that rain and clouds could be widespread over the threat area before things can get going, limiting instability. A small amount of CAPE (instability), however, looks likely as far north as southern MO and western TN - on the order of 500 J/kg. Higher CAPES will result anywhere the clouds can clear out during the day. Winds aloft will be screaming, meaning any storm that can go up and tap into those winds will have the potential for very damaging winds at the surface. The classic supercell/tornado threat is conditional on areas of higher CAPE and locally backed (more southeasterly) winds - but a widespread straight-line wind event with embedded tornadoes looks nearly certain. The new SPC outlook should be out shortly, and a high risk is not out of the question.
If I had the ability to be anywhere tomorrow, it would probably be northern Mississippi where the combination of better shear and higher instability will be maximized. I would also stay on the east side of the river to avoid getting trapped by the few bridge crossings, and stay well ahead of the system until dominant cells became established. My secondary area of interest is north-central and western Kentucky and southern Indiana/Illinois/Ohio, which may be the 'sleeper' target if clearing and instability can develop there. With that area being closer to the vorticity and lift of the low center, low-level shear would be excellent for tornadoes, along with slower-moving storms.
Compare the two models' output from this evening for 00Z (7PM EST, 6PM CST) tomorrow:
NAM/WRF model:
GFS model:
As for chase plans - with my web design job competing with my weather schedule these days, I have to weigh the cost of postponing client meetings and project work sessions against the 'cost/success probability ratio' chase potential of any given system. I don't see incredible chaseable tornado potential with this system. That is, the chances for slow-moving, photogenic (or at least decently visible) tornadoes are not that high tomorrow. More than likely we'll see fast-moving, rain-wrapped tornadoes (some strong and violent) within a monster squall line - which is nearly impossible to chase successfully. Add to that the threat area being 1.) in an area of trees and poor road networks, 2.) more than a 7 hour drive from Charleston and 3.) conflicting with an important web meeting I have in the morning, means I will not be traveling to the highest risk area tomorrow.
According to those model outputs, the squall line will not make central Kentucky until after midnight and not arrive in Charleston until dawn on Wednesday. Of course, we'll have to watch the actual trends tomorrow to get a better idea of what to expect. If I lived closer to Lexington, Kentucky, I might be more gung-ho on trying an aggressive chase in TN, MS or western KY. But right now I don't see a great success potential for the cost outlay for even a Kentucky trip, which would yield little more than some nighttime lightning and wind. Since I might get a little of that here at home, it makes more sense to stay here and save out-of-state chase funds for something with a higher 'catch' potential.
All things considered, I decided not to postpone my morning web client meeting - so I'm tied to Charleston until around 11AM. At that point, I can re-visit the forecast and still make it as far as central Kentucky by mid-afternoon if it looks worthwhile.
