Moderate Threat Risk for Eastern NE 6-26-2011

[Jeff Duda]

Its easy to criticize from the bleachers, but really the SPC forecasters are some of the best in the world. When you forecast you are working without all the variables and some that are misleading. Was a moderate risk for tornadoes warranted? Several of the numerical models indicated storms going up in an extremely favorable environment for tornadoes. However, if you switch up a few of the ingredients, you wind up with nothing. It was a good guess given the data we had to work with.

I have to disagree with you here, Skip. Anyone who works with NOAA pretty much gets access to every possible model that any world wide organization is cranking out. Thus there are no missed variables to them. You are right that they are among the best forecasters in the world and that's why they work there, but that doesn't mean they won't be off from time to time.

I think SPC was about one update too late in removing the moderate risk. I could tell by the time I got up and started looking at things (so by about 10 AM) that there was not going to be a tornado outbreak. I've seen setups almost exactly like this go cap bustola many times in the past. Also, I've noticed it becomes tough to trust the HRRR, especially over the Midwest (i.e., Iowa) and especially later in the season (i.e., from late June through at least August). Last year I saw several times the HRRR progging supercells somewhere in or very near Iowa and that pretty much never happened. I don't know what it is about the HRRR and Iowa, but it's something to watch. Also, you could tell the HRRR couldn't be trusted yesterday when midday runs were not capturing the W-E line of showers across E NE/W IA and were still developing supercells over that exact area a few hours later. Other than that, I didn't see much high resolution model data that indicated tornadic supercells. I did see the Nebraska MCS in pretty much all of the runs, though, so they nailed that part of the forecast.

Rob, the link Skip provided is good. Generally 12-14 C at 700 mb at this time of year is getting into the realm of an unbreakable cap. I've sat under 95/79 in IA before under 12 C with an outflow boundary and had a cap bust. Likewise I've seen tornadic storms form under 14-15 C in western Kansas and eastern CO. However, the 700 mb temp cap guidelines generally don't apply west of about U.S. 281 (being very rough and approximate) due to the higher terrain.

One more thing about the composite parameter bullseyes: lots of times, storms will fire along the gradients. So don't go to the CAPE max axis, go to the gradient. This is usually the case because higher CAPE is allowed due to extensive sunshine during the hottest times of year because there is such a strong cap that no clouds can form. Thus you want to get along the edge of the cap because that's the place where there's still a lot of instability, but just enough lack of capping to get storms to fire.

 

[Adam Simone]

Yeah I knew yesterday was going to be a major bust when I crossed the Nebraska state line and it was nothing but clouds and was in the upper 70's temps wise or low 80's at best in southern NE. I hung out in Auburn, NE for an hour or 2 then moved a little farther south near Humboldt. Like Jeff said the HRRR model kept showing convection starting in SE Nebraska around 5:00 or 6:00pm but it also never showed the showers in the Lincoln/Omaha area so I really was questioning the accuracy of the model all day. I did see towers going up in NW Missouri but I had to go all the way south to US-36 Highway and through St. Joe and then back north to try and intercept the storm. It was tornado warned for a little while but as I got about 10 miles from the storm it went from tornado warned to severe to not even dropping rain drops.

 

[Andy Jackson]

Rob, the link Skip provided is good. Generally 12-14 C at 700 mb at this time of year is getting into the realm of an unbreakable cap. I've sat under 95/79 in IA before under 12 C with an outflow boundary and had a cap bust. Likewise I've seen tornadic storms form under 14-15 C in western Kansas and eastern CO. However, the 700 mb temp cap guidelines generally don't apply west of about U.S. 281 (being very rough and approximate) due to the higher terrain.

I agree with what all of you are saying about H7 temps and low level lapse rates. I just was wondering if anyone out there knew specifically any dates where severe surface based storms fired in high H7 temps in the east of 281 region. It just seems as long as the skew-t is not bulging that convection can be possible. Just thinking out loud.

 

[Jeff Duda]

I agree with what all of you are saying about H7 temps and low level lapse rates. I just was wondering if anyone out there knew specifically any dates where severe surface based storms fired in high H7 temps in the east of 281 region. It just seems as long as the skew-t is not bulging that convection can be possible. Just thinking out loud.

I can't think of any off the top of my head (I'm pretty sure such events are rare), but you are welcome to check the SPC SVR event index for cases yourself. Each event includes 12Z and 00Z upper air maps as well as pretty much all of the soundings from 00Z and 12Z.

Regarding temperature profiles "bulging" representing a cap: the 700 mb temperatures are probably used as a good proxy for cap strength because 700 mb is one of the mandatory levels in rawinsondes (850 mb is another). Thus, anytime you get upper air data, you're going to get 700 mb and 850 mb analyses. You'll probably never get areal analyses at any level in between, which is where the maximum capping usually resides. When I say "maximum capping" I'm referring to the level at which the difference in temperature between an air parcel coming from below and the ambient environment at a given level is the greatest. This usually appears to be the point on the temperature profile that is farthest to the right once you get above the boundary layer, or it's the level at which the lapse rates dramatically increase (like at the bottom of an EML). CIN is the best way of quanifying how much "bulging" and capping is going on, because it represents the vertical sum of all of the different layers of capping and an air parcel MUST make it through all of those layers before it can reach its LFC and explode into a thunderstorm. For example you can get the same resistance to convection (i.e., CIN) by having a shallow layer with a sharp inversion (i.e., much warmer temps compared to the levels immediately above and below) as you can by having a thicker, still warm layer, but not as warm as it is in the other example. H7 temps in the 12-14 C range are almost always associated with a strong capping inversion between there and 850 mb. Also, consider the fact that the convective temperature associated with H7 temps of 12-14 C (assuming you aren't extremely moist below and also don't have a capping inversion below) are in the 100 - 106 C range, so the atmospheric column in general is usually just that warm when H7 temps are that large. Think of it as warm H7 temps being associated equally with other features that are more responsible for resisting convection than H7 temps themselves causing convection not to fire.