Day 3 Moderate Risk: Reasons Why It Could Be Huge … Or, Hugely Disappointing

Day 3 Outlook from the Storm Prediction Center.  Valid: Wednesday, April 17th.

Day 3 Outlook from the Storm Prediction Center. Valid: Wednesday, April 17th.

As most chaser types already know, the Storm Prediction Center has issued a moderate risk for severe weather for Wednesday.  A significant severe weather event is expected, but the kind of event is still uncertain.   Will it be a wind and hail day with a few tornadoes?  Or, will it be a full-scale tornado outbreak?  Or, more likely, will it be somewhere in-between? It’s still too early to say, of course.  And, as usual, many will dogmatically assert their favored position, but the truth is that we just don’t know yet.  Here are a few reasons why Wednesday could be huge, and a few reasons why it could be a dud.

Reasons Why Wednesday Could Be Huge

  • Strong upper-tropospheric trough entering the Plains – this will set the stage for robust moisture advection, strong low-level shear, and the transport of the elevated mixed layer off the High Plains (and associated steep lapse-rates).

    500 mb forecast for 00Z 18 April from 12Z 15 April NAM.  Twisterdata.com

    NAM 500 mb forecast.  Valid: 00Z 18 April.  Initialized: 12Z 15 April. Twisterdata.com

  • Strong instability – the elevated mixed-layer looks primed – as it has all season – to generate strong instability.  Finally, rich moisture – almost 200 mb deep – will exist with it!  You can see this when toggling between the forecast CAPE and the forecast mixed-layer CAPE: they’re virtually the same.  Of course, this assumes that morning storms will not interfere with daytime heating.

    CAPE forecast for 21Z 17 April from 12Z 15 April NAM.  Twisterdata.com

    NAM CAPE forecast.  Valid: 21Z 17 April.  Initialized: 12Z 15 April NAM. Twisterdata.com

  • Incredible hodographs– in spite of the screwy nature of the upper system (from a pattern-recognition standpoint), the hodographs look bodacious.   Very strong low-level shear should develop as early as noon, owing to the development of a 40 – 55 kt low-level jet.  As a result, the classic “sickle-shaped” hodographs – associated with most major tornado outbreaks – are forecast to develop.

    NAM forecast hodograph for Gotebo, Oklahoma.  Valid: 00Z 18 April.  Initialized: 12Z 15 April.  Twisterdata.com

    NAM forecast hodograph for Gotebo, Oklahoma. Valid: 00Z 18 April. Initialized: 12Z 15 April. Twisterdata.com

  • Chaser-friendly storm motion – normally, storms move quite fast in 50 kt flow at 500 mb.  However, the low-level flow will be quite backed, which will slow the storm motion some (read: quite chaseable!).

Reasons Why Wednesday Could Be Disappointing

  • Weak cap / instability – the potential is there for some big CAPE, but will it materialize?  The cap is forecast to be weak, which could lead to the development of too many storms and a significant reduction in instability.
  • Poor phasing of parameters – generally, you look for negatively-tilted troughs for big tornado outbreaks, since the low-level shear is usually stronger in those cases (because of backed low-level flow) and the storm mode tends to be more favorable (because of the favorable orientation of the shear vector to the axis of the convecting boundary).  This system will be positively-tilted, which is generally associated with lesser events.  Will it matter?
  • Unfavorable storm mode – will a weak cap allow storms to develop early, leading to the early development of a mesoscale convective system?  Or will storms tend to “seed” each other, prompting the development of chaser-unfriendly high-precipitation storms?  Or will storms find optimal spacing, allowing for the development of multiple classic tornadic supercells?  These questions probably won’t be answered until the day the event occurs.

What do you think is the most likely outcome?

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About Gabe Garfield

Hello! My name is Gabe Garfield. I am a research meteorologist from Norman, Oklahoma. In addition to my work, I am interested in storm chasing, sports, philosophy, theology, and culture.
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5 Responses to Day 3 Moderate Risk: Reasons Why It Could Be Huge … Or, Hugely Disappointing

  1. “Weak cap / instability – the potential is there for some big CAPE, but will it materialize? The cap is forecast to be weak, which could lead to the development of too many storms and a significant reduction in instability.”

    That would be my biggest concerned and one which looked curious to me when I made a quick glance at the models this morning. Many chases have involved me getting rolled by MCSs because of a weak cap causing things to blow up way too soon and they become HPs with merging cold pools (nothing worse than trying to find tornadoes and you feel the cold shelf cloud passing you by…LOL). If the cap can hold off a bit longer and the highly backed flow is for real, regardless of the trough orientation, then it might just be a good day, at least for a some window. One thing I’ve learned chasing is that as darn helpful as the models are, nothing ever goes 100% according to the forecast plan…surprises abound.

    • Absolutely! This is also my biggest concern. As you mentioned, there’s a chance you could get a huge, rolling MCS. On the other hand, a weak cap may allow storms to fire well-ahead of the primary initiating boundary. In that case, all bets are off!

  2. Bill Deedler says:

    Yes; it looks great as far main set up with low level helicity & hodographs & instability but then you are relying on the NAM. NAM usually has doom & gloom and tends to be too “hot” with dpts & avail moisture, espcly further north. Jet core/axis also nice; GFS as wicked? Little inhibition also a prblm which you both have stated…she’ll blow too quick and get cluttered. Lastly; wait for next few runs; undoubtedly they’ll oscillate ;-)
    http://weatherhistorian.blogspot.com/

    • Thanks for commenting, Bill!

      Yeah … the NAM tends to over-do moisture/instability early in the season, when moisture depth is problematic. However, that doesn’t seem to be the case here, since deep moisture isn’t far away (even now).

      Personally, I would prefer the GFS solution for “doom and gloom”, as it keeps the cold front much further north and spreads things out a bit (hopefully, keeping chasers spread out as well!). I posted the NAM images because it was the most “conservative” model, and it is a mesoscale model (which the GFS is not).

      Yeah, the runs will certainly change! I’m thinking that the big picture will probably still stay interesting, but the details will (no doubt) change.

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