We are now entering the cool season as a big trough will swing across the middle of the country with only a compact severe weather risk area in tow on Thursday. On either side of this, we’re looking cool and dry for the most part in the middle of the country. Also, stay tuned for an early look at the Eclipse sky forecast for Saturday.
If this kind of trough happened in April or May, we’d be screaming about a possible tornado outbreak — but this time of year, you need a lot more to go right for that to happen. Several things don’t seem to sync up: moisture and instability, shear, and lift are somewhat offset for this system.
The NAM has a more robust moisture return, but that is its tendency. We’ll show you why in a second, but if you shave off a few degrees on the dewpoints from this forecast, it is harder to see widespread storms until the cold front overtakes the dryline.
The Storm Prediction Center has outlined NE Kansas into SE Nebraska as a zone to watch for severe weather on Thursday.
I can’t stress enough how much these wind fields would otherwise support severe weather. But one key thing I notice is the SW direction of the low-level flow near/over the dryline. This will mean that if you are drier near the surface, I don’t see how you get widespread and robust storm development without mega sustained upper lift (which is why the risk area is currently so compact).
Using the NAM sounding over the dryline, you can see very puny surface heating owing to low clouds, which I don’t think will be a thing (the NAM has a tendency to overdo this). The instability on all models shows a shallow nature, which limits the overall CAPE values by quite a bit. The 3CAPE is impressive, though, at 118 j/kg. Shear also is similarly impressive over the dryline, but the unidirectional nature of the flow in the 2-5km layer is probably not helpful in getting robust, right-turning supercells. Another big caveat with this day is that the mean-wind SRH values are unspectacular to even negative.
Further north, nearer the surface low in Nebraska, the shear becomes pretty wonky, but 3CAPE looks extreme at 200 j/kg. This is a classic cold-core tornado type of setup where the environment only vaguely supports severe weather. Still, the lift in the upper atmosphere and the tilting/stretching will make for the possibility of pretty robust lower updrafts. This is especially true if you can get the near-60 degree dews wrapping on the low — which more than one model does project.
*Right now* the NAM forms storms as far south as the Oklahoma/Kansas state line. I suspect we’ll see some storms form in this corridor, but the exact extent of storm coverage is definitely in question. Other models, notably the FV3, keep storm development isolated to far NE Kansas and SE Nebraska owing to the drier air behind the dryline further south being shoved over the moist sector.
The Bottom Line
We will see some severe weather out of the mid-late week system, but it should be shunted north and east in the Central Plains. There will probably be a meager chance of a tornado and a risk of hail and damaging winds. Storms should move along at a very rapid clip, especially given the more unidirectional profiles aloft. The right-moving motion vector is over 40mph on most models. It’s a risk, but I’m not fired up.
Now, About that Eclipse
Listen, I’m trying to remain optimistic, but the upper-air pattern is highly reminiscent of those that produce widespread clouds in the western U.S.
Here is the eclipse’s maximum coverage overlayed with forecast cloud cover from one weather model:
This will need to be monitored for skywatchers — there will be some cloud breaks to take advantage of, but my best advice is to keep an eye on your local forecasts and ensure you are flexible with your plans.