What makes the cold core outbreaks so difficult to forecast? Are there some good papers to read on this subject?
There are a couple of relatively recent publications focused on cold-core setups:
Preliminary Climatology of Tornado Events with Closed Cold Core 500mb Lows.(Davies and Guyer)
Guyer, J.L., and J.M. Davies, 2006:
Enviroment Characteristics Associated with Tornado Events near Closed Cold Core 500 MB Lows. Preprints, 23nd Conf. Severe Local Storms, St. Louis MO.
I think the biggest problem lies in the fact that the parameters of "cold-core" setups are far from what we've come to expect from tornadic supercell cases. In addition, there's even some debate as to whether these events are nonmesocyclone tornadoes or mesocyclone tornadoes associate with relatively short-lived mini- or low-topped supercells. I'll be presenting a poster at the upcoming SLS Conference that focuses on the 3-20-06 NW OK cold-core tornado, which had dewpoints below the common lower threshold set forth by a previous Guyer and Davies paper (~50F).
From a chasing perspective, I don't think they really are that difficult to forecast. The cold-core setups that seem to produce are those with a surface low <150km from the center of a 500mb low, a distinct pseudo-occludded front, and steep low-level lapse rates that yield some decent low-level CAPE. The boundary involved yesterday was well-developed and clearly evident on local 88Ds. Earlier this year, on 3-20-06, we parked ourselves along a similar, well-defined surface boundary in NW OK, and waited for a moderate convective cell to approach, interact with, and cross the boundary. Well, not entirely unexpectedly, the cell quickly developed substantial rotation, took on supercell characteristics (RFD clear slot, wall-cloud, etc), and produced a tornado; all the while it, was snowing 30-40 miles to the northwest.
The last line of my last post in the FCST thread:
" There may be a risky cold-core play across far nw OK and eastern OK panhandle if the 12z RUC verifies."
This makes my choice even more frustrating, since I looked at the cold-core play (in the mid-morning, it appeared as though the front may stay in OK, which is why I mentioned far nw OK... Obviously, the front ended up in sw KS, so adjustment would have been needed!). So, why did I chase south of ICT yesterday instead of south of DDC yesterday? The same reason why I chased the warm-sector several times during the spring of 2005 while the cold-core play was lighting up to my northwest... It's tough for me to drive hours to a location, hoping to see tornadoes, with temperatures in the 50s and dewpoints even lower, when I can play a more "typical" warm-sector setup (dewpoints in the 60, good veering wind profile resulting in substantial low-level shear, etc). This will change next year, as I've realized that the cold-core target has been remarkably consistent the past 2 years, particularly since many of the cold-core days in the past couple of years have feature warm-sector setups that have had to deal with marginal moisture.
It is very important to determine, however, if yesterday's tornadoes really were associated with low-topped supercells with (relatively) deep, persistent mesocyclones (a little redundant, since supercells, by definition, contain deep, persistent mesocyclones). I haven't looked at radar data from the event, but there's the possibility that these were nonmesocyclone tornadoes caused by intense stretching of ambient vertical vorticity not associated with deep, persistent mesocyclones. This is a very important distinction to make! Many landspout tornadoes on the Front Range are caused by strong low-level vertical acceleration associated with building TCu stretching vertical vorticity in or near a misocyclone. A "cold-core event" typically refers to situations in which convective cells "feed off" the ambient vertical vorticity along a boundary and develop relatively persistent, deep mesocyclones. On the 3-20-06 case, the cell near Putnam indeed did develop a deep, persistent mesocyclone, along with other supercell "features" (RFD clear slot, etc, as noted earlier). I suppose much of this comes down to the depth and duration of the rotation in the cell. Were they really tornadoes associated with supercells / mesocyclones, or were they more shallow, landspout-ish tornadoes? It's not entirely a clear-cut distinction in the first place, and I haven't looked at any radar data to determine either way.
Did the local offices forecast this event?
The only mention by DDC that I read in yesterday afternoon's AFD was this: " VERY LIMITED CAPE SHOULD KEEP STORMS BELOW SEVERE LEVELS."
Did the SPC update their 20z outlook to account for higher tornado probs when they issued the tornado watch for SW KS?
