HRRR forecast for N IL.... what happened?

[Elaine Spencer]

I'm not a meteorologist or storm chaser so I might be missing something that is obvious to a lot of you, but I have to ask:

What the heck was the deal with the HRRR on Friday spewing out sig tor parameters of 25 or higher for N IL on Saturday? If I'm not mistaken the highest sig tors in the 2011 Super Outbreak were in the 16-17 range. What was the HRRR "seeing" that could possibly have prompted it to go that far off the charts? I know that later models did back off of those predictions, and I gathered from some of the online discussions I saw that HRRR does tend to overperform (or should I say, underperform), but I'm trying to grasp why it would ever go that far off the rails. If this is a known fault of that model has anyone tried to correct it?

 

[Dan Robinson]

The HRRR (and other models) were not handling the morning convection, clouds and precipitation well yesterday. This isn't that uncommon. Sometimes models do well with an evolving situation like that, sometimes they are just terrible. When they are botching realtime conditions, you just have to throw them out as a reliable source.

 

[rdale]

And this is where being a meteorologist helps, especially with the 00Z HRRR run, because the PivotalWeather UH map that made the rounds was clearly unrealistic. If we had that many right-mover and left-mover cells simultaneously, well... textbooks would need to be rewritten

But this event got a lot of people caught up in the act, as I didn't see any NWS office downplay the SPC MDT (to high) risk. Most broadcast mets and storm chasers went along, so that's telling you something. On the other hand, I did see several mets openly post that the threat was being overdone due in part to the muddied cloud cover / convection concerns.

A lot of this is just philosophy - I had a good chat with Rich Thompson about this yesterday morning. Once SPC put the "we might upgrade to high" it's a dog whistle We know they can ALWAYS upgrade to a high risk, but saying it makes everyone think this is going to be upgraded. It's sort of like the way I deal with snow. If most likely is 2-4" but there's a chance I'll upgrade to 4-8", I'm going to stick with 2-4" but reduce my confidence in it staying in that range. I won't make mention of 8" until my confidence increases. The SPC crew yesterday went the opposite direction - preferring to go all in at the onset just in case. I'm not saying which one is right or wrong, because theoretically it could have been bad, but...

 

[Jackie Mann]

Just curious if the lack of observations from the airliners may have had anything to do with the forecast models being a bit off? BTW, I believe this is my first post on this forum, so go easy on me...lol

 

[rdale]

No harm in these questions There’s no easy answer about models that don’t get things right. But in this case, missing airliner data is more of a medium-long range impact on modeling.

In realtime though the aircraft soundings are quite valuable to judge how the model is doing.

 

[Jackie Mann]

No harm in these questions There’s no easy answer about models that don’t get things right. But in this case, missing airliner data is more of a medium-long range impact on modeling.

In realtime though the aircraft soundings are quite valuable to judge how the model is doing.

Awesome, thanks for the insights!

 

[Randy Jennings]

The formula for STP is: STP= (MLCAPE/1500) *((2000-MLLCL)/1500)*(ESRH/150)*(ESHEAR/20)*((200+MLCIN)/150) with some special rules for MLLCL, MLCIN, ESHEAR that I will not go into here. My point in giving this formula is to show that it only considers a few things. You can get high STP without surface based storms even forming. It is only a piece of the puzzle when forecasting severe weather.

 

[Jeff Duda]

I'm not a meteorologist or storm chaser so I might be missing something that is obvious to a lot of you, but I have to ask:

What the heck was the deal with the HRRR on Friday spewing out sig tor parameters of 25 or higher for N IL on Saturday? If I'm not mistaken the highest sig tors in the 2011 Super Outbreak were in the 16-17 range. What was the HRRR "seeing" that could possibly have prompted it to go that far off the charts? I know that later models did back off of those predictions, and I gathered from some of the online discussions I saw that HRRR does tend to overperform (or should I say, underperform), but I'm trying to grasp why it would ever go that far off the rails. If this is a known fault of that model has anyone tried to correct it?

The best plan is to examine the background fields that go into the STP calculation, as Randy laid out in his post. Then find out why those background fields are what they are. Then you'll be closer to figuring out your answer. Just openly asking "what was wrong with parameter XXX?" leaves out a lot of easily-discussed and available material that could help answer your question first.

As to why the event in general seemed to "underperform" (relative to human expectations)...I think @rdale hit a lot of important points. But from a more meteorological perspective, it looks like clouds hung around much longer than expected (and the cloud coverage throughout the daylight hours was too high). Therefore the expected levels of destabilization never occurred, and thus the buoyancy was very limited throughout the IL portion of the risk area. Inadequate destabilization is common in the cool season and early spring season events since the sun provides less total energy to the surface during the day compared to in late April - July, when it doesn't necessarily take as much heating to get a sufficient amount of buoyancy for strong updrafts and severe storms.

 

[Kevin R Burgess]

That was a 'hole-lota-nothen'

 

[TJKLECKNER]

That was a 'hole-lota-nothen'

Iowa happened

 

[Elaine Spencer]

Thanks for all the background information.... apparently STP is like any other model or index that relies on a few key factors only, it is only part of the picture and not the whole. But seeing STP numbers almost double those of N AL on 4/27/11 had me really freaking out for a while there. I suppose that IF there had not been any early morning crapvection and IF the skies over N IL had cleared and IF temps and dews had gotten well into the 70s early enough in the day, we could have had an epic outbreak, but there are lots of epic outbreaks that "could" have happened but for one or two ingredients or triggers that weren't there.

 

[SBonesteel]

Thanks for all the background information.... apparently STP is like any other model or index that relies on a few key factors only, it is only part of the picture and not the whole.

I like to suggest bringing up several of the components that make up STP composite and look at them separately and also remember that where the STP gets plotted on the map needs to have some type of convection to tap into those parameters. STP can get pretty inaccurate via the HRRR when it comes to 1-3 hours out.

As for the event itself, the early morning convection that continued to pop into the afternoon stunted development and then when things did pop, the forcing factor didn't arrive until after some of the organized cells had already lost their vertical development and had been essentially blown apart before taking off and proceeding into where the rest of the good parameters were expected, but ended up only being mediocre at that point.