Hi all,
I arrived in the US on Monday after a few (interesting) days at work last week in Vancouver, Canada and am chasing with Brad Hannon, Andrej Matko and Marko Korosec. We have been cruising for a few days watching as the models have toyed with us about tomorrow (Thursday here). We are starting to get into the timezone of more confluence between the models so its time for a forecast.
There are a few areas which have been suggested as potential targets. The warmfront in Iowa, the triple point (warm front vs cold front) in north eastern Nebraska, or the dryline through eastern Kansas and Oklahoma.
A surface low is deepening over western Kansas/eastern Colorado in response to a strong upper level trough. This is expected to deepen further overnight and move eastwards along the Kansas/Nebraska border. As the pressure falls (and has fallen over the last 24hrs) over the southern plains, strong southerlies have returned and moisture is gradually flowing northwards from the gulf of Mexico. At 8pm Wednesday DP's are currently 56f over a reasonable portion of central Oklahoma with 60-65f DP's through southern Texas. The LLJ will continue all night tonight and tomorrow with 30-45knt southerlies which will push this moisture northwards. How far this goes, nobody knows (seriously...there is substantial debate about this). In addition to this, a strong warm layer (cap) is coming in across the top of this moisture from about 870mb and above with 850mb winds of 35-50knts from the SSW. So the moisture depth is not HUGE (but ok IMO), the moisture (amount) is questionable as to how far north it is going to get, and the cap is strong so lots of quetions.
The Iowa target is being suggested to be a potential tornado producer with DP's forecast to get into the high 50's-60f (13-15C) as the pool up against the warm front and the directional shear is nice with lower level shear being south east to easterly upper level flow being south westerly, however, the speed of the shear is in question with only 25-30knts at 500mb. The best upper level shear is not due to arrive in this area until much (much) later. With the lowish DP's (and the quality of moisture being in question as well),
instability may not be that great. There is also the cap which looks to be relatively strong in this area too. So...IF the cap breaks and IF the DP's can reach the upper end of the scale (ie 60f+), shear *might* be good enough for supercells and possible tornadoes (and hail to 2 inches).
The Nebraska target is under similar conditions to the above, however, there will be stronger convergence as the front surges eastwards so the issue of breaking the cap will be far less in this location (as the convergence between the cold front and the warm front will be quite nice). The issues with this area are that it is even further for the moisture to travel (although pooling "in" the triple point could see DP's being ok) and also the surging of the cold front could see storms being rapidly undercut by cold air. In addition to this, this area's location in relation to the upper low will see upper level shear being more S'ly in orientation and as such, directional shear will be less with SE'ly's at the surface and
S'lys in the uppers. This is the "guaranteed storm" target. There will be storms here and they are likely to be severe but I would suspect these will be shorter lived and unlikely to produce tornadoes. Once the front moves through and the upper trough moves over this area, there is also the chance of snow in the early morning hours on Friday...rofl!!
The Oklahoma/Kansas target is the most interesting of them all in that there are serious questions over the cap breaking....but IF it breaks, it is likely that there will be some very severe storms with strong tornadoes possible. As this is the southern most of the three areas I have mentioned, the moisture is probably going to be best in this area. The dryline is set to extend south of the cold front from north central Kansas through just east of central Kansas and Oklahoma ("the I-35 corridor"). DP's are forecast by various models to get into the mid 60's (18C or so). There is a lot of debate at this stage as to whether this will actually verify (but as I type, the 9pm data is in and DP's are looking healthier in southern Texas with some nice 64-68f DPs). Also, as you head further south along the dryline, you head into more strongly capped air. 850mb temps are pushing just on 20C (thou vary between 17 and 20C depending on which model) which is my cut-off for chasing although i have been known to test 850mb temps exceeding this on various occassions - all unsucessful. Convergence along the dryline doesn't appear that it is going to be strong at this stage and models are showing the cap to be virtually/completely eroded by 00z (7pm - late!). The area I would expect to have the best chance at breaking the cap would be the intersection of the dryline and the cold front. As noted above, this is set to be somewhere around north central Kansas by late tomorrow (around Emporia, Kansas). So...lets assume the cap breaks (just for fun). With surface winds from the south at 35knts, the LLJ in this area tomorrow evening is forecast to be around from the SSW at 35-40knts and the 500mb jet is nosing into this area with SW'ly winds of 50-60knts. With temps in the low 80's and DP's in the mid 60's, CAPE is approaching (or even exceeding) 3000j/kg and with shear like that and lowish LCL's, supercells and tornadoes would be a definite possibility. The cap *could* break further down the dryline (as far south as central eastern oklahoma), in which case, any storms which do develop would likely have very large hail (to 3.5 inches) and tornadoes.
So...after all that...what is our plan. We are currently in Wichita, Kansas (on I-35). We are going to see how the moisture is recovering tomorrow and also how the sat pic looks during the afternoon. We can head NE towards Emporia quite easily (about 1.5hrs away) and also eastwards easily if need be. At this stage, I could see us heading towards the Emporia area simply as it should be the best chance at breaking the cap (with shear and instability being pretty much the same from there right down along the dryline).
Will post another update tomorrow. SPC is likely only to issue a slight risk for much of the area due to the uncertainty surrounding initiation but they will probably "hatch" the dryline so as to say that if storms develop, they will likely be severe. (Currently a slight risk and 30% chance of severe over much of the area/s mentioned above).
Macca (& crew).
