2014-06-03 MISC: NE/IA

[Stan Rose]

So, now that the verdict is in, would be curious to hear thoughts on the general tornado bust yesterday, especially from those who chased. I haven't had a chance to do a real post-mortem, but it definitely seems like the early convection played a role with all that outflow. The HRRR was amazingly on-target (again) in forecasting the two dominant supercells, but did not capture the speed of the upscale development, which also seems to have spoiled things.

 

[Mark Ellinwood]

The setup had good bust potential written all over it from the beginning. High LCLs south of the warm front, tons of convective precip north of the front, and only a small area along the front for storms to get rooted and produce tornadoes that would likely be HP in nature before storms evolved into a MCS. Looking at it from purely a tornado development perspective, there were a lot of ways for yesterday to go wrong, and it played out pretty much as expected, minus the fact that I thought 1 or 2 storms would have perhaps been more robust along the warm front without starting as far north of the boundary as that main central NE cell did. Also agree that the upscale development was earlier than expected, but not by a whole lot.

 

[Royce Sheibal]

I got out of Omaha fast to try and escape the wind/hail monster that attacked. Made it to the central Nebraska cell just as it lost it's tor and got hit by a DRY RFD. Caused a massive duststorm just North of Seward. Got some photos of a dust-nado? but other than that it sucked yesterday. Dust storm in June in Nebraska....thats a new one for me.

 

[Bob Schafer]

Wanna know how fast the storm went HP? I and chase partner William Robertson were on it before VIL's reached 40. We watched the features sharpen, then it got on top of us and we skedaddled E back to Burwell. It was about 8 miles back to Burwell, during which time the storm was directly behind us, so if there was anything interesting to see during that time we missed it. Back at Burwell I went into the gas station--where we had been hanging out all afternoon--to warn the employees there.

It was already HP by that time, with nothing to see but rain. I would guess that that total elapsed time was less than 30 minutes. We never saw so much as a wall cloud.

 

[Jeff Duda]

A few related negative factors that didn't help things yesterday were high downdraft CAPE (1500+ on the 18Z LBF sounding) and cold downdraft temperatures. Cold air is more dense and will always push through warm air no matter how strong the warm air is moving into the cold air. Also, with high precipitation storms, downdrafts are likely to remain saturated for longer during their descent, which promotes the realization of all that downdraft CAPE. Strong downdraft full of cold air -> expect outflow dominant storms. The 50-60 kt 500 mb flow probably contributed to the faster storm motions. Also, since most storms went up north of the warm front, they were probably not entirely surface-based, and thus the strongly backed near surface flow probably didn't factor in very much to the effective storm motion.

 

[Derrick Herndon]

Jeff your comment on storm motion is interesting. We made the long drive from Madison, WI partly based on the idea that storm motions would be manageable. They were not really (roads not ideal and chaser convergence). Forecasted storm motions were 15-25 knots and instead they were more than double that at times. I think you are right that the storms never became completely involved with the lower level flow and that along with the outflow dominance lead to faster motions. A weak storm tried to get going southwest of Broken Bow in the warm sector and we initially headed for that but it looked soft and weak so quickly left it for northern storm.

 

[Stan Rose]

Thanks for the insights, folks!

 

[Mark Ellinwood]

Jeff your comment on storm motion is interesting. We made the long drive from Madison, WI partly based on the idea that storm motions would be manageable. They were not really (roads not ideal and chaser convergence). Forecasted storm motions were 15-25 knots and instead they were more than double that at times. I think you are right that the storms never became completely involved with the lower level flow and that along with the outflow dominance lead to faster motions. A weak storm tried to get going southwest of Broken Bow in the warm sector and we initially headed for that but it looked soft and weak so quickly left it for northern storm.

The general mid-level flow on the forecast soundings had 40-60+ kts, so I don't know why the models had such a slow storm motion. I'm not exactly sure how the models calculate it, but just by looking at the mid-level flow on the forecast soundings, it's no surprise that storm motion of 40-50+ mph occurred. I knew it was REALLY off on say the NAM when it had the storm motion vectors going to the northeast, but it was clear on the mid-level flow and on the QPF/simulated reflectivity that storms would be pushing east to southeast.

 

[Derrick Herndon]

I believe most model pages use the Bunkers method of motion though that is not always explicitly stated. The method assumes that the flow in the 0-500 meter levels is contributing to the storm motion. The SPC archive pages shows motions of ~ 25 knots which is consistent to the other sites for the event.

 

[Jeff Duda]

The general mid-level flow on the forecast soundings had 40-60+ kts, so I don't know why the models had such a slow storm motion. I'm not exactly sure how the models calculate it, but just by looking at the mid-level flow on the forecast soundings, it's no surprise that storm motion of 40-50+ mph occurred. I knew it was REALLY off on say the NAM when it had the storm motion vectors going to the northeast, but it was clear on the mid-level flow and on the QPF/simulated reflectivity that storms would be pushing east to southeast.

I believe most model pages use the Bunkers method of motion though that is not always explicitly stated. The method assumes that the flow in the 0-500 meter levels is contributing to the storm motion. The SPC archive pages shows motions of ~ 25 knots which is consistent to the other sites for the event.

I'm pretty sure the standard is indeed the Bunkers method which, from his 2000 paper in WAF, takes the shear between the mean wind in the 0-500 m layer and the 5500-6000 m layer, then 7.5 m/s to the right of the midpoint between those layers. However, I think that method has since been updated a little. In fact, a new article in NWA's JOM just came out by Matt Bunkers discussing a big update to the method. I doubt the current operational models use it, though.

If you looked at the hodographs leading up to the event, many of them were semi-circular, so the method would have depicted very slow storm motions. However, those were more limited to the warm sector. Also, remember that the method uses the mean wind in the 0-500 m AGL layer. An elevated storm will not see any impact from the wind in that layer. Also, for the most part, the actual observed hodographs were more quarter-circle than semi-circle, which definitely pushed the storm motion away from the origin.

Finally, the NEly storm motion was actually observed in storms in the region that developed in the warm sector. I recall seeing a lone supercell that developed right on the Missouri River between SE NE and SW IA that crawled east for awhile before weakening later in the afternoon. Also, later in the afternoon there was a storm that did manage to develop south of the warm front near North Platte that also slowly moved northeast. Even so, a boundary can take over control of a storm's motion by forcing the updraft to stay near it. In the case of a warm front, if the storm motion vector had a significant perpendicular component to the front, the updraft would have a tendency to just fly off into the cold air and die. If there is not as much of a significant component of the storm motion vector across the boundary, the updraft can preferentially grow/maintain as long as it stays on the boundary. If it strays off the boundary, the storm dies. This gives the appearance of a storm with a strongly deviant motion.

In summary, a day like yesterday shows why it's a good idea not to chase storms that are north of a warm front.

 

[Rob H]

Having chased yesterday, I saw three problems:

1) Jeff already touched on this a little, but 1.8+ pwat and 40-45kt 9-11km SR winds pretty much meant everything would be far on the HP side of the scale. Anytime you have something HP, you have weaker updrafts and better chance for outflow dominant storms. This in turn will mean a larger cold pool, and consequently higher chances for an MCS to develop. If there had been slightly less moisture in the air, or if the upper dynamics were a little stronger, those supercells riding the front could have been prolific tornado machines. If storms popped further south like the 13Z HRRR progged, I think the upper air support would have been better in this area as well, so they would've less likely been as HP. They all ended up being prolific rain shaft and gustnado machines - I'm still wondering if there was even a single legit tornado from those specific storms.

2) Crappy surface pattern. Your prime tornado area was in between the two boundaries (wf and ofb/wf hybrid thingy):

http://i.imgur.com/4ayF5i6.gif
http://i.imgur.com/0cIfPnf.gif

Warm front plays tend to work well if you have a nice closed surface low 100 miles away, and a fat instability axis nuzzling up to the front. We didn't have either of those yesterday which already makes the warm front play problematic. Now if the 13Z HRRR was right, and cells popped NW of Kearney like they were forecast, there would have been isolated cells with no nearby cold pool marching through that 10+ STP. Low level lapse rates suggested two deformation axes and focal points for convective development - one west of North Platte, and one near Kearney (you can kind of see this on the STP image above). I'm *guessing* this had something to with the early convection outflow and the two surface lows back on the western Nebraska border, but would love to hear a proper explanation. The end result is that there wasn't enough "focus" for storms to go up NW of Kearney and march into the good air, which leads nicely into...

3) The EML. That was a hot, hot EML nosing into Nebraska. If surface features were more focused, or the cap was slightly weaker, maybe some storms could have popped in central NE before dark. They didn't, so the only chaseable option left was the HP junk on the warm front.

I'd say the high risk derecho scenario busted even harder, but I don't know much about derechoes or why that happened.

 

[JeremyS]

Here's a little something different than what has been discussed so far. I use GRLevel 3 for my radar and use GPS to plot my location on the radar. We were on the storm as it was near Taylor developing, and when it suddenly shot off like a rocket on it's forward motion. We went back to Burwell and then went southeast towards Ord. At some point we tried to stop to get out and take some pics, etc, but the storm was already on top of us again. This was surprising because according to the radar the storm should have been north of us by several miles. Also the tornado warning that came out was quite a bit south of even the hook of the storm. I believe at that time the storm motion was still east. The radar was updating at 3 minutes too so it shouldn't have been a matter of the image being old. The next update still showed the precip in the storm north of our position but at that time we were getting rained on. We continued southeast on 11, then east on 22, dropping south a bit to then continue east on 22 at Walbach, where there was a report of a tornado. Here we continued east where suddenly from the north we were blasted with strong outflow winds from the north that were strong enough to take down branches into the highway. Again the storm looked like it was nearly on top of us but yet the storm on radar showed it was north of us away. I know there was a very strong outflow boundary that eventually blasted south all the way to Hastings creating 70 mph winds, but no precip. We made it to Fullerton and then headed south where there again we were taken over by strong winds and zero visibility dust. One of the scariest moments driving I've had and I grew up in South Dakota with blizzards and the visibility was never as bad as last night south of Fullerton. Also saw a possible landspoutish tornado which I have a picture of. At this point the storm was moving east at 50-60 mph so we just went down to the interstate and drove on home to Omaha, not wanting to deal with the constant winds and especially visibility issues on a storm that seemed to only be able to produce rain wrapped tornadoes.
So the point of all of this was, did anyone else seem to have radar images that weren't matching up with what they were seeing/experiencing? The winds I could understand being well out away from this storm that was almost always outflow dominant and had a very cold outflow at that. However, many times I could see the rain flying towards us or it was actually raining at our position when the radar said otherwise. Hope this rambling makes sense!!

To touch on the speeds- Hastings WFO did say the day before the event that storm speeds would only be 15-25 mph. However, any forecast sounding/hodographs I looked at always seemed to be 35-40 kts. The night before/day of, their discussion changed to faster storm motions

 

[Kevin Walters]

I'm pretty sure the standard is indeed the Bunkers method which, from his 2000 paper in WAF, takes the shear between the mean wind in the 0-500 m layer and the 5500-6000 m layer, then 7.5 m/s to the right of the midpoint between those layers. However, I think that method has since been updated a little. In fact, a new article in NWA's JOM just came out by Matt Bunkers discussing a big update to the method. I doubt the current operational models use it, though.

If you looked at the hodographs leading up to the event, many of them were semi-circular, so the method would have depicted very slow storm motions. However, those were more limited to the warm sector. Also, remember that the method uses the mean wind in the 0-500 m AGL layer. An elevated storm will not see any impact from the wind in that layer. Also, for the most part, the actual observed hodographs were more quarter-circle than semi-circle, which definitely pushed the storm motion away from the origin.

Finally, the NEly storm motion was actually observed in storms in the region that developed in the warm sector. I recall seeing a lone supercell that developed right on the Missouri River between SE NE and SW IA that crawled east for awhile before weakening later in the afternoon. Also, later in the afternoon there was a storm that did manage to develop south of the warm front near North Platte that also slowly moved northeast. Even so, a boundary can take over control of a storm's motion by forcing the updraft to stay near it. In the case of a warm front, if the storm motion vector had a significant perpendicular component to the front, the updraft would have a tendency to just fly off into the cold air and die. If there is not as much of a significant component of the storm motion vector across the boundary, the updraft can preferentially grow/maintain as long as it stays on the boundary. If it strays off the boundary, the storm dies. This gives the appearance of a storm with a strongly deviant motion.

In summary, a day like yesterday shows why it's a good idea not to chase storms that are north of a warm front.

As a rule I agree, there are exceptions. If there is decent CAPE present north of the wf storms can remain rooted at the surface and produce tornadoes well North of the wf. Doesn't happen often but it does happen. 4/8/99 is an example, strong tornadoes occured North of the warm front well into the more stable airmass.

 

[Sean Ramsey]

Here's a little something different than what has been discussed so far. I use GRLevel 3 for my radar and use GPS to plot my location on the radar. We were on the storm as it was near Taylor developing, and when it suddenly shot off like a rocket on it's forward motion. We went back to Burwell and then went southeast towards Ord. At some point we tried to stop to get out and take some pics, etc, but the storm was already on top of us again. This was surprising because according to the radar the storm should have been north of us by several miles. Also the tornado warning that came out was quite a bit south of even the hook of the storm. I believe at that time the storm motion was still east. The radar was updating at 3 minutes too so it shouldn't have been a matter of the image being old. The next update still showed the precip in the storm north of our position but at that time we were getting rained on. We continued southeast on 11, then east on 22, dropping south a bit to then continue east on 22 at Walbach, where there was a report of a tornado. Here we continued east where suddenly from the north we were blasted with strong outflow winds from the north that were strong enough to take down branches into the highway. Again the storm looked like it was nearly on top of us but yet the storm on radar showed it was north of us away. I know there was a very strong outflow boundary that eventually blasted south all the way to Hastings creating 70 mph winds, but no precip. We made it to Fullerton and then headed south where there again we were taken over by strong winds and zero visibility dust. One of the scariest moments driving I've had and I grew up in South Dakota with blizzards and the visibility was never as bad as last night south of Fullerton. Also saw a possible landspoutish tornado which I have a picture of. At this point the storm was moving east at 50-60 mph so we just went down to the interstate and drove on home to Omaha, not wanting to deal with the constant winds and especially visibility issues on a storm that seemed to only be able to produce rain wrapped tornadoes.
So the point of all of this was, did anyone else seem to have radar images that weren't matching up with what they were seeing/experiencing? The winds I could understand being well out away from this storm that was almost always outflow dominant and had a very cold outflow at that. However, many times I could see the rain flying towards us or it was actually raining at our position when the radar said otherwise. Hope this rambling makes sense!!

To touch on the speeds- Hastings WFO did say the day before the event that storm speeds would only be 15-25 mph. However, any forecast sounding/hodographs I looked at always seemed to be 35-40 kts. The night before/day of, their discussion changed to faster storm motions

We had the exact same experience. We were on highway 22 south of Ord heading east thinking the storm was to the north more than a few miles, but got blasted with rain and winds and zero visibility just to our north as radar showed nothing on top of us. We eventually decided to drop south on 281 instead of heading east to Fullerton because we didn't feel like we could trust what the radar was indicating, and the hook was already closing in on highway 22. Possibly everything was so low and the outflow so strong that radar wasn't picking up on it? Wasn't sure what was going on, but we weren't going to chance it and visually there wasn't a whole lot to go on.

 

[Rob H]

Yes, we noticed the exact same thing. We had a GPS failure around the same time, so coupled with the phantom tornado warning we ended up just ignoring technology for 30 minutes and chased by eye. David Drufke reported his GPS failing in the exact same spot as well. That area seems to be the Bermuda Triangle of chasing equipment. It could have been a blip in NEXRAD/Allisonhouse/who knows. It's a good reminder to trust your eyes over data.

Also, depending on which NEXRAD site you used, it was at least 75nmi away, which would be 8-9k ft. AGL. That combined with extremely strong gusting could explain how features seemingly several miles away on radar could sneak up on you.

I can't speak to the storm motion. I'm always horrible at storm motion and get confused by how TD/Earl/CoD/etc all show different values, so I just classify into "CO slow, normal, Dixie Alley fast". When you expect a range like 25-40kt you tend to not be caught off guard when they're at either end of the range