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April 24-30th Severe Potential


Chicago Storm

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4K NAM was pretty interesting. It appears to show CI around 22/23Z slightly west of I-35, eventually develops numerous discrete supercells(?) with some clusters as well. Impressive parameter space from about 23Z to 05Z. Wind-profile is not exactly amazing-- but thats really a given at this point with this event.

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Just looking at current obs on the mesonet vs hr3 on the 18Z models, the GFS is doing a much better job at initializing and handling DPs.  Not particularly surprising given the NAMs history but it does have me leaning towards the GFS when it comes to CAPE and DP tomorrow.  This holds especially true when you go look at a couple model runs prior 06Z for example and compare it with current obs.  

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Just looking at current obs on the mesonet vs hr3 on the 18Z models, the GFS is doing a much better job at initializing and handling DPs.  Not particularly surprising given the NAMs history but it does have me leaning towards the GFS when it comes to CAPE and DP tomorrow.  This holds especially true when you go look at a couple model runs prior 06Z for example and compare it with current obs.  

 

That's not surprising haha...

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Can anyone shed some light on the differences between WRF-ARW, WRF-NMMB and WRF-NSSL?  They're depicting considerably different solutions.  

I sure I'm not the best person to answer but WRF is just a weather model. The big difference is between WRF and say the GFS is the WRF has two different "builds" or the dynamic cores (the core of the model is the equations used to model the dynamics of atmosphere) and when running WRF it is either ran using ARW or NMM. Both have their own bias like NMM has a higher 2m temperature bias during the day and ARW almost always over does precipitation, however, averaged over a long time and large area both models have pretty similar skill. Doing a little research as I am not very familiar with these models the WRF-NSSL seems to be a ARW core and is ran by the national severe storms lab at 4-km resolution. The WRF-ARW and WRF-NMMB are ran by NCEP at 4-km resolution as well. The reason there are differences between each of these models is the same reason there are differences between the GFS and EURO, they use slightly different ways to approximate how the atmosphere works which leads to differences as the model goes out from the initial conditions. Also in this case there is going to be convection which each other these models handles very differently so that will cause even more change between them. I hope I answered your question and if you want learn more I can send you some links to good papers about all of this. 

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Just looking at current obs on the mesonet vs hr3 on the 18Z models, the GFS is doing a much better job at initializing and handling DPs.  Not particularly surprising given the NAMs history but it does have me leaning towards the GFS when it comes to CAPE and DP tomorrow.  This holds especially true when you go look at a couple model runs prior 06Z for example and compare it with current obs.  

I would actually say that they are doing a similar job as of right now with what was forecasted on the 12Z solutions. With both models showing the best dewpoints--near 70-- over S OK then propagating northward. Both the GFS and NAM are slightly overdoing DP's in KS right now. Both of them show widespread ~65-degree DPs across SE/EC KS, and obs are not quite that high yet. 

 

Either way, all models show significant moisture/instability across the plains tomorrow afternoon into the night. 

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As someone who is coming from pretty far south, I think I might end up trying to play the dryline near the exit region of the 850 mb jet in north Texas. It's far from a guarantee, but if a storm that fires off the dryline can stay isolated for just a little while, better low level shear should exist just to the east. Instability and lapse rates should be more than sufficient, and deep layer shear looks adequate. Guidance has also been consistent with convective initiation in the area (maybe too much so even). Concerns with backing in the vicinity of 700 mb still exist, and shear vectors off the drylline aren't ideal, but hopefully being further south (where the flow is somewhat less meridional) and the quantity of instability will help take some of the edge off these concerns. Besides, these same concerns exist pretty much along the length of the dryline, and potential targets still in my driving range further north honestly don't look too much better to me.

 

Yeah I'm definitely starting to take note of this area. With more zonal flow aloft, this area should have less issues with the kinks in the wind profile that are causing issues further north (although there still are some on a few of the models). I would if we see more localized backing E of the dryline here too, with a 40+ kt LLJ setting up via the Euro by 00z Wed in response to the southern vort/significant 500 mb height falls. Playing somewhere S of the metroplex might be a pretty good idea for those who don't want to get suffocated by the hordes at the triple point. I like the amount of venting as well with 80-90+ kt H3-H2 flow, which may be able to counter some of the HP issues that will likely be generated by the high columnar PWATs.

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Hodographs along 35 in spots aren't too bad, but still contain some VBV.  Really anxious to get into HRRR range, and still feel we're likely to  get a line, but 18z runs have me more optimistic about the Central Texas area.  Right now, I'd see no reason to drive to Oklahoma over the southern target.

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Hodographs along 35 in spots aren't too bad, but still contain some VBV. Really anxious to get into HRRR range, and still feel we're likely to get a line, but 18z runs have me more optimistic about the Central Texas area. Right now, I'd see no reason to drive to Oklahoma over the southern target.

FWIW, 4NAM didn't show a line at all. Numerous supercells, with a few clusters. I'd even say the GFS didn't really show a line... Regardless of if certain things get better or not for the higher-end threat, we are going to get tornadoes--possibly a couple strong/long track, giant hail, and damaging winds tomorrow.
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FWIW, 4NAM didn't show a line at all. Numerous supercells, with a few clusters. I'd even say the GFS didn't really show a line... Regardless of if certain things get better or not for the higher-end threat, we are going to get tornadoes--possibly a couple strong/long track, giant hail, and damaging winds tomorrow.

 

I would argue that it doesn't really matter what models show in terms of storm mode... this is definitely an event where we have to go back on fundamentals and look more at things like wind profiles. If the wind profile is unfavorable, storm mode will be messy, and that will put a lid on how high this event will go. Agree that at least a few tornadoes will be in the cards, however.

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I saw earlier on this forum the talk of VBV being a limiting factor. As a younger generation weather enthusiast I was just wondering what VBV stood for.

Thanks, Stormgeek

Veer-Back-Veer. Leads to messy storm modes that can fail to stay organized for extended periods of time. Think of it like this, as someone else said, you are stirring your coffee, then all of the sudden, you stir violently in the other direction. The nice rotation you have is now demolished. While it isnt to that extent, that's the general idea.

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HRRR's having a little bit of a problem with the cap right now. It has supercells popping in Oklahoma in 1 hour. Visible satellite loop shows clouds have formed over the area in the past ~30 minutes... but a far cry from supercells popping.

Use the HRRRX. It's 10x better than the regular HRRR and has nailed about everything this year.

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I saw earlier on this forum the talk of VBV being a limiting factor. As a younger generation weather enthusiast I was just wondering what VBV stood for. 

             Thanks, Stormgeek

Veer-back-veer

Have you seen a skewT?  Are you familiar with forecast soundings and the wind barbs?  

A veering wind is a wind that turns clockwise with height, this is easy to see on forecast soundings which is why I asked the above questions.  

For example, at the surface winds are out of the SE, at 850 MB they are out of the South, at 700 MB SW and at 500 MB they are westerly....so as you gain in altitude the wind direction turns clockwise.  When you see VBV profiles mentioned it means it turns clockwise up to a point, then becomes more of a backing wind, so the direction with height begins to go counter clockwise, then back to veering as the height increases.  Hence the VBV

These sort of profiles disrupt the formation of strong and long-lived updrafts capable of producing strong to violent and long lived tornadoes...or so goes the theory.  It makes conditions less conducive to long lived discrete supercell.  My personal opinion and I'm not a meteorologist is that backing winds can play hell on sig tor potentials.  While there will likely be tors tomorrow the combination of VBV and only adequate SRH has me thinking we'll see a fair number of tors but not sure we'll see a lot of long track significant tors in OK for example.  Perhaps more up by the triple point.  

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I saw earlier on this forum the talk of VBV being a limiting factor. As a younger generation weather enthusiast I was just wondering what VBV stood for. 

             Thanks, Stormgeek

Veer-back-veer. Go back a page or two to find links to some research on it. Long story short, the hodo looks like how Gary Larson drew seagulls in the background and is not optimal for tornadic storms.

Re: 03z chat, it's plenty relevant for people in the path of the storm, and people that don't mind chasing tornadoes at night. Some of my favorite chases have been setups that lasted into the night. It seems strange to me to be picky with 03z soundings but smooth over the issue of cherry picking soundings in general. If you look at the code for how SHARPpy possible hazard types are generated it's easy to "game the system" and pull out PDS TOR and TOR graphs that will never have their potential be realized.

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Use the HRRRX. It's 10x better than the regular HRRR and has nailed about everything this year.

 

Unfortunately, it's been down since last night -- something that sometimes seems to be a good indicator of severe potential. ;)

 

The HRRRx is a marked improvement over the current NCEP HRRR for our purposes, because the latter continues to use an older version of the MYNN PBL scheme. The result is appalling over-mixing of the PBL in most Plains severe weather environments. The fact that a few HRRR runs this afternoon tried to initiate supercells in C OK, where other models and vis sat show a solidly capped PBL, is exhibit A.

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Unfortunately, it's been down since last night -- something that sometimes seems to be a good indicator of severe potential. ;)

 

The HRRRx is a marked improvement over the current NCEP HRRR for our purposes, because the latter continues to use the MYNN PBL scheme. The result is appalling over-mixing of the PBL in most Plains severe weather environments. The fact that a few HRRR runs this afternoon tried to initiate supercells in C OK, where other models and vis sat show a solidly capped PBL, is exhibit A.

 

That's really interesting, esp. because the current HRRR tends to overdo dewpoints during the day, which would tend to make me believe the exact opposite.

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HRRR's having a little bit of a problem with the cap right now. It has supercells popping in Oklahoma in 1 hour. Visible satellite loop shows clouds have formed over the area in the past ~30 minutes... but a far cry from supercells popping.

It's on crack or something for sure and is just having a good time popping storms up wherever. It has a blob of storms west of DFW at 11z tomorrow morning. Then again you never know with this system, that could be the fly in the ointment....

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That's really interesting, esp. because the current HRRR tends to overdo dewpoints during the day, which would tend to make me believe the exact opposite.

 

Hmm. I'm sure it overestimates 2 m dew points in some cases, mainly outside of peak heating or when there's cloud cover/precip during peak heating. But in a typical Plains environment with a moist PBL beneath a dry EML and under full afternoon sun, my observation for the past 3 years has been that it ends up mixing out moisture far too aggressively. The drought conditions in 2013-2014 masked the problem somewhat, since the LSM scheme was probably being too generous with ET (therefore offsetting some of the bias). And then last year, on a lot of the days where you had massive morning MCSs, it also masked the problem because there wasn't vigorous mixing going on in the wake of those complexes.

 

Here's a cherry-picked example from yesterday's 10z HRRR (first image) and HRRRx (second image) valid at 20z. Moisture was underwhelming yesterday, but nowhere as bad as the HRRR predicted over C and E KS:

 

http://rapidrefresh.noaa.gov/HRRR/for_web/hrrr_ncep_jet/2016042410/t5/dewp_t52m_f10.png

 

http://rapidrefresh.noaa.gov/HRRR/for_web/hrrr_jet/2016042410/t5/dewp_t52m_f10.png

 

Sometimes it can be entertaining in situations like the above to do a dProg/dt loop in the afternoon. It takes a few hours for the PBL scheme to mix out all the moisture, so I've found it's not uncommon for the 2m dew point forecasts for, say, 21z to rise by a couple degrees each hour around lunchtime, and then skyrocket even further by the time the 19-20z HRRR comes out.

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I have a question - one of the local OkC head weathermen just claimed the surface low will be in SW or West Central Oklahoma tomorrow.
Now, I don't have access to all the models, but the ones I do all show the low in Kansas or Nebraska.
Is there any models showing the surface low in Oklahoma?

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