2013-04-09 FCST: CO, NE, KS, OK

[Jeff Snyder]

What on earth is going on? This system is bizarre to me. Forcing is clearly present but the front is undercutting updrafts? Is there still a chance that isolated cells could develop in the warm sector out ahead of the front? It seems like some CU pops up and then dissipates in Eastern OK. Cape high, low level winds adequate and not terribly strong cap.

The cold front is considerably faster than previous NAM forecasts (this bias is often seen with these types of cold fronts, however). The 17z-18z soundings at LMN and OUN showed pretty strong capping in place (OUN had -207 j/kg MLCINH and LMN had -80 j/kg MLCINH). The latest 20z FWD sounding also has a strong cap in place. The convective temperature at OUN was 91 F; it was 88 F at LMN and 98 F at FWD (20z special sounding). With current temperatures in the upper-70s in OK and lower-80s in northern TX, the cap has inhibited more widespread initiation thus far, except for some recent attempts at serious initiation N of Abilene. [Update: And now initiation along the southern OK portion of the cold front.]

Since the front is moving quickly and mid- and upper-tropospheric flow is relatively backed (SSW @ 500 mb on 18z OUN sounding), the "cross-front" component to the storm motion thus far as been less than the speed of the front. As a result, there is a "front to back" relative motion of the storms to the front, resulting in persistent "undercutting". The triple point appears to be N of Abilene at this time, and something can try to come up off the dryline south of the front enough to have some residence time in the warm sector before the front sweeps by. Even at that, however, the 20z FWD hodograph is pretty nasty, with a significant weakness in the flow around 800 mb. The current Purcell profile also shows a veer-back-veer vertical profile, which creates the dreaded S-shaped hodograph.

My record chasing events like this one is abysmal. In fact, I don't think I have *ever* personally witnessed a nice supercell when the hodographs have looked like we have now and when the cold front sweeps in like this. The high-res models showed more widespread storms developing close to now, but the probability of surface-based supercells (as opposed to elevated storms behind the cold front) appears to be quite limited IMO.

Edit: Keep this slow bias in mind if you ever see model forecasts of a strong cold front with isobars that "bulge" into the high Plains (e.g., strong northerly winds in E CO, W KS, and the OK/TX panhandles associated with a strong cold front). Anecdotally, in my experience, the models are almost always too slow with such frontal passages, and we've seen many busted forecasts (including unforecast freezing rain / sleet storms in addition to busted chase forecasts) in these types of situations.

 

[Dan Robinson]

I am very concerned about the icing impact from this event across the Plains states. I believe that this event will have a very high death/property damage impact from car accidents. Much more so than if a tornado outbreak had happened today. People will not be expecting icing from a thunderstorm event in April, and freezing rain icing is not visually apparent. Drivers will be hitting this at highway speeds. Please help get the word out if you have the means.