6/26/05 TALK: Dakotas

[Glen Romine]

Although I do believe that mother nature always has some trick up her sleeve to go against our best forecasts, I dont think today is going to be a big tornado day.

With the benefit of hindsight, I'm not sure you can still call it a forecast. But anyhow, since you brought it up let's look at what was indicated by the SPC mesoanalysis graphics (SMG) and what was observed on area sounding at 00Z. DUring the late afternoon, the SMG showed MLCAPE > 3500 J/Kg, SBCAPE > 4500 J/Kg immediately se of the intersection of the weak surface low. Along the wf/ob extending northeast from the surface low, the SMG gave 0-1 km srh values >100 immediately north of the boundary, approaching 150, but RUC forecasts consistently showed a strengthening LLJ by early evening, which verified. COmbined with low-level instabilty, 0-1 km EHI values in the 3-4 range were respectable, with 5-6 values for 0-3 km EHI. A surface boundary near where developing convection was occuring at the time extended northeast (in the direction of motion expected from the storm), was aligned along the forward flank downdraft, and arched more eastward ~30 miles to the northeast, providing an anchor to encourage a more rightward propagation. With a 1.4 km LCL, there was some reason to be concerned, but SMG suggested steep low (7.5-8 ) and mid-level lapse rates (7-7.5) were also in place. The biggest fly in the ointment appeared to be weak storm-relative mid and anvil level flow which could lead to too much precip falling into the updraft, but it was simultaneously highly divergent, so it seemed like it was still feasible to be adequate.

So, when the 00Z soundings came in, ABR revealed a far less potent environment, with an unpleasantly located inversion in the 750-800 mb area, and with CAPE values of only around 2000, about half that anticipated by the SMG product. Also, there was some backing with height of winds in the 700-550 mb layer, which masked the otherwise reasonable 850 (S) vs. 500 (SW) mb crossover. Replacing the surface wind in the ABR sounding of SSE @ 15 with the winds observed north of the boundary (the ABR sounding was in the warm sector) still yields a shorter hodograph length than was anticpated by SMG. Since I didn't have the benefit of knowing this at the time, I had to rely on the parameter values estimated by the SMG, and based on those the environment looked competent to generate a few tornadoes. Given the products from SPC, apparently they also saw this potential, so I won't beat myself up to much over it. Also, thanks for hopping into the conversation - we need more participation in this forum.

Glen

 

[Scott Lincoln]

I was talking about the rest of the evening. Yes I know it was later in the day.... wasn't trying to be hindsight forecast.
I was talking about the supercells that were still ongoing at the time. Conditions were favorable for supercells... but I didn't see any big tornado threat from them.

What I should have said was that I didn't see a significant tornado threat during the rest of the evening.

The main problem that I was seeing with the atmosphere being favorable for strong tornadoes... well was alot of things. The changing wind direction with height was not exactly very favorable at low levels. Helicity is usually a good index... but some meteorologists don't think it means anything to tornado development. From what I have learned of it, its kind of an iffy one to watch.
LCLs of 1400m are not particularly favorable. They really should be under 1km for the best chance of good tornadoes.