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April 12th/13th and 15-16th Severe Weather Thread


andyhb

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Now that the NAM has Friday in its range, it shows a roaring 40-50 kt southerly LLJ ahead of the dryline for OK/KS. Although forecast hodographs look slightly weird due to a relative weakness around H7, the pattern and overall look of the kinematics screams dryline tornado event, possibly significant. The biggest concern continues to be moisture, with "only" lower 60s owing to the deep trough off the East Coast.

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As brettjrob has mentioned before, you don't often get multiple, consecutive days of significant severe in the Southern Plains, especially in April. It's been a while since I've seen such a strong, continuous upper-level flow under a constant supply of moisture, for what the 6z GFS was showing as 3 days in a row (Fri, Sat, and now even Sun). Leftover boundaries, convection, etc. will all play into how this eventually unfolds, obviously. If a sharply defined dryline does in fact remain over western OK 3 days in a row, under that strong of an upper-level flow, it would be the first time in my memory for April in this region.

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Im debating on canceling my trip to Nebraska Friday/Saturday and going chasing instead. Is it worth it to chase instead of going on the trip?

I'd plan on it if at all possible, though I do see this is a rather low-confidence scenario for being only 3-4 days out. Both Friday and Saturday have the potential to be outbreaks, but they also could each turn into rather marginal days, pending the moisture situation and timing of individual impulses. If you aren't busy Thursday, there very well may be a decent event not too far from your neck of the woods (I'm eyeing roughly LaCrosse to Hill City as of now). Either of the latter two days could be big within an hour or two of MPR, too.

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One thing I'm noticing is that the GFS is continuing to trend more progressive with the evolution of this system, this could be a lot more widespread than I first thought, especially considering the strength of theta-e inflow that is available each day from Friday on, although I still believe Friday and Saturday hold the biggest outbreak potential given the very favorable jet trajectories aloft and excellent SLP position. The 12z Euro is pretty disgusting on Saturday, somewhat like the 00z run...with an absolutely monstrous LLJ overspreading the warm sector.

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That said, I would like to see a bit more of a vigorous/deep SLP in order to be a bit more confident on the resulting LL wind response, but I've noticed over the past few threats that the GFS and Euro have been underestimating the strength of resulting LL cyclogenesis.

The Euro actually has a line of discrete supercells ahead of the boundary on Saturday on its precip maps.

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That said, I would like to see a bit more of a vigorous/deep SLP in order to be a bit more confident on the resulting LL wind response, but I've noticed over the past few threats that the GFS and Euro have been underestimating the strength of resulting LL cyclogenesis.

The Euro actually has a line of discrete supercells ahead of the boundary on Saturday on its precip maps.

It's also been remarkably consistent on initiation along the DL in SW OK for Friday afternoon, accompanied by stronger cyclogenesis and more backed low-level flow than the GFS. In fact, I'd say both days look darn impressive on the EC, more so than the GFS.

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Honestly it looks like it's the GFS vs. everything right now regarding the slightly weaker solution (but still impressive nonetheless) vs. the more dangerous verbatims. The 12z Ukie is just as impressive as its 00z counterpart.

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That said, I would like to see a bit more of a vigorous/deep SLP in order to be a bit more confident on the resulting LL wind response, but I've noticed over the past few threats that the GFS and Euro have been underestimating the strength of resulting LL cyclogenesis.

The Euro actually has a line of discrete supercells ahead of the boundary on Saturday on its precip maps.

I totally agree with the underestimation you are talking about. with a low of that strength, pressure falls should be sufficient enough to back winds across a good portion of the warm sector. GFS on its 18z run had a more elongated, banana like low with a horribly veered sfc wind profile. whereas other models have a much more compact sfc low.

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I totally agree with the underestimation you are talking about. with a low of that strength, pressure falls should be sufficient enough to back winds across a good portion of the warm sector. GFS on its 18z run had a more elongated, banana like low with a horribly veered sfc wind profile. whereas other models have a much more compact sfc low.

Except we learned on March 2nd that even this type of low can create a major problem when paired with strong upper level support.

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The 00Z WRF/NMM suggests activity near Midland N to SW KS tomorrow afternoon. My hunch is this severe weather episode will linger into Monday for NE/E TX, SE OK, AR, LA and S MO as well as points NE.

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Except we learned on March 2nd that even this type of low can create a major problem when paired with strong upper level support.

Actually, the strongest tornadoes of the day occurred in areas where the sw were backed or at least more southerly than swly. Take a look at the Henryville tornado for example. Occurred on the warm front where sw were backed. Also, early on, supercells fired but struggled mightly to produce in Western Kentucky due to the awfully veered sw. In fact that original line became more linear before new cells began to fire out ahead of the line in Central and Eastern Kentucky where your sw were more southerly.

The one exception on the day was the EF3 that occurred that morning in the Huntsville, AL area.

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Actually, the strongest tornadoes of the day occurred in areas where the sw were backed or at least more southerly than swly. Take a look at the Henryville tornado for example. Occurred on the warm front where sw were backed. Also, early on, supercells fired but struggled mightly to produce in Western Kentucky due to the awfully veered sw. In fact that original line became more linear before new cells began to fire out ahead of the line in Central and Eastern Kentucky where your sw were more southerly.

The one exception on the day was the EF3 that occurred that morning in the Huntsville, AL area.

I meant more about the generalization about those types of banana shaped sfc lows that SC4L mentioned, generally mesoscale features will serve to create areas with at least some backing in these scenarios, which models tend to be poor at picking up.

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I meant more about the generalization about those types of banana shaped sfc lows that SC4L mentioned, generally mesoscale features will serve to create areas with at least some backing in these scenarios.

Not to mention the OFB's that the storms earlier that day produced. I was telling others to watch the OFB from the storm just East of STL early that day.

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