May 15-16 Forecast

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Above is my best guess at where the threat area will be on Friday and Saturday. There is still a fair amount of spread among the models regarding the exact target area, especially for Friday, so except that threat area to get moved around a bit and focused in a little more.

The 00Z NAM threw us a bit of a surprise with an extremely potent set of parameters coming together Friday afternoon in north central Kansas into south central Nebraska. I’m not buying it, at least not to the extent the NAM is currently selling. I think the NAM tends to over shoot dew points and instability a bit and juice setups. This morning’s GFS showed a much more modest setup, but still a respectable combination of CAPE/shear coming together. Anybody who has read my blog for very long knows that I’m a directional shear junkie an hence these day before the day setups are always my favorite. I think most times they tend to be the better pound for pound tornado setups, especially when you are dealing with a trough like this that will likely take on a strong negative tilt as it fully ejects into the plains. Typically the problem with day before the day setups is moisture return and the cap. The trough that starts to move through the plains tomorrow should help to start advocating quality moisture back into the plains ahead of this system, so realizing quality moisture in the plains by Friday doesn’t seem like much of a concern. What does seem like it may be a bit of a concern and is also a result of this weak midweek trough is the veering low level winds in it’s wake that pulls the higher dew points back east and will widen the moisture gradient along the developing dryline. That is one of the bigger differences between the NAM and the GFS. The NAM has the mid level jet nosing well into the plains Friday and as the dryline mixes east the dryline sharpens up a lot through the day packing mid 60 dew points right up along the dryline bulge. The GFS keeps a wider moisture gradient along the northern portion of the dryline due to the midweek trough drawing moisture east in its wake. Whether or not the dryline can mix far enough east and we can get mid 60 dewpoints packed up against a sharp dryline will play a big role in the extent of the tornado threat Friday I think so it’s something I keeping a good eye on with the models. A sharper moisture gradient isn’t a problem farther south along the dryline, but the area I’m focused in on for Friday is where the mid level jet noses in and stronger mid level winds overlay the strong LLJ. Shear profiles should be good for tornadoes Friday (assuming the timing of the midlevel jet is somewhat accurate and we get AOA 45kts or so) I think, it’s more a question of thermodynamics and storm coverage. I leave those details until tomorrow’s forecast, but Friday may be the best tornado day from a chaser perspective this weekend. Storm motions should be slower, directional shear should be better and the odds are much lower that morning convection will screw things up on Friday. The threat area may end up being fairly concentrated if we get a dryline bulge like the NAM is showing, which seems plausible. I can only begin to imagine the shit show of chasers that would pack into southern Nebraska if the 00Z NAM verifies. I think there is a pretty solid chance for tornadoes Friday. Until I see a little consistency out of the NAM or see the GFS come into better agreement with it, I’m not going to start hoisting the strong tornado threat flag, but the potential is certainly there if the things I mentioned above play out favorably. I’m keeping a close eye on it and I plan on chasing that day. Below is the hodograph from south central Nebraska with tonights NAM run. If that were to verify it’s got strong tornadoes written all over it. CAPE AOA 3000, good deep layer shear, excellent low level shear ahead of a dryline bulge with a breakable cap in mid May. Not buying in just yet, but that’s fairly impressive.

nebraska hodo

I’m tired so I’m not going to say much about Saturday, but I think the tilt of the trough and how much morning convection compromises afternoon destabilization is key to the tornado threat. With all the models showing this trough taking a negative tilt as it enters the plains you can expect a meridional component to the mid/upper level winds. How meridional is a question mark and will have a big influence on shear profiles and convective evolution, both of which will play strongly into the tornado threat. The ECMWF is a bit slower and has a little more veering to the mid level winds relative to the GFS. I trust the ECMWF a little more on timing and the GFS has been less than consistent with these details in previous runs so I’m leaning towards the ECMWF. That’s good because the more westerly mid level winds are the better for tornado potential. It helps to keep shear profiles more normal to the boundary which favors more discrete storms and (more favorable convective evolution for tornadoes). Anyway I think how earlier precipitation influences the warm sector and the tilt of the trough is going to drive the location and extent of any tornado threat on Saturday, so I’ll start sorting those details out in coming days. At the moment I’m not terribly optimistic about this setup since the trough may have a pretty hard negative tilt by Saturday, but if we maintain a decent westerly component to mid/upper level winds through Saturday afternoon and morning storms don’t both destabilization then it could be a moderate to higher end event. Below is a pic of the ECMWF and GFS 500mb charts for Saturday afternoon. Note the GFS being faster and the stronger meridional component to 500mb winds relative to the ECMWF. We don’t want the GFS solution. The slower ECMWF solution with more of a westerly component to mid/upper level winds will be more favorable for tornadoes.

GFS Saturday

ECMWF

There is no doubt in my mind we’ll see tornado in the plains this weekend. Just like I said last week, you don’t get strong troughs with a favorable tilt and decent moisture that don’t produce tornadoes when they come through the plains in May. Just doesn’t work that way. The question is the extent and exact location of the tornado threat, but both days could have the potential for strong tornadoes so it bears watching. I’m certainly keeping a close eye on it and plan on chasing. I’ll try to update in the morning if I have time. If not I’ll get another post up tomorrow night.

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