Monday, May 17, 2010
Saturday, May 8, 2010
I generally look at the trio of the NAM, GFS, and ECMWF models when making my forecast 2-3 days out. The consensus with all three, is that there is a high likelihood of a tornado outbreak SOMEWHERE in OK or KS. There are several factors that will determine WHERE:
1) Speed and amplitude of the upper trough - The EURO has been the farther East solution, showing the dryline along or maybe even just East of I 35 in KS and OK. The NAM, on the other hand, has been farther West, with the dryline across Western OK.
2) Northward extent of the moist/unstable air mass return (warm front location) - Tornadoes cannot occur where the surface air is too cold. This may be the case initially North of the warm front which is forecast to be over Southern KS. But, strong warm advection, and cooling aloft, may allow storms to "be happy" and reside along this boundary for a while before crossing into the cooler air.
3) Capping issues with Southward extent - While there is significant cooling aloft over KS...it begins to dwindle around I-40 and points South. This is GOOD news for those of you who do NOT want to see strong tornadoes over SW or CNTRL OK...but it does NOT PRECLUDE the possibility of them. Some capping will also be in place across the entire moist sector much of the day, which is probably a good thing. Hopefully, we can sufficiently heat the entire warm sector so that if a big supercell plows into it, it will be sustained.
To me, the main issues that will determine HOW violent the tornadoes will be will be:
1) Moisture quality - I have some minor concerns, but even with a 63-64 dewpoint (low end of the progs)...it will be more than enough for strong tornadoes. In my opinion, 66-68+ dews will almost surely result in violent tornadoes.
2) Width and quality of the warm sector East of the dryline and South of the warm front - Given a rather dry air mass, and morning rain over ERN KS/OK...there is a possibility that it is too cold just a few counties E of the dryline. At worst, supercells with tornadoes form on the dryline but do not keep producing large, destructive tornadoes except for a 50-100 mile wide area. At best (for chasers), they get better and more self-sustaining with time, and we are able to observe mile wide wedges for hours.
3) How rapidly cooling aloft overspreads the dryline - "Lapse rates over 8, tornadoes will be great" I always say, referring to the temperature lapse rate between 700 and 500 mb. They are indeed forecast to be steeper than that over KS, and into far NRN OK. It will be so cold aloft into KS, that even a relatively cool / temperatures in the 60s / air mass MAY sustain tornadoes with -17 ish 500 mb temps.